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| DHLA 40 vs. 45 | Rating: 
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| Posted: 09-15-2005 04:13 pm |
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1st Post |
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andrewo Member
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I have noticed that 40s seem a lot cheaper than 45s. On other cars with Webers (similar design) I actually found that it was better to undercarb than overcarb. What are the tuning and performance implications of using 40s? Thanks - Andrew
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| Posted: 09-15-2005 05:55 pm |
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2nd Post |
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Greg Fletcher Administrator 
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I think the 40mm Dellortos are great on the JH (it's what they came with from the Factory, of course, on non-Federal cars) and tend to be underrated for no good reason. If you want to build a performance motor, and perhaps add a big valve head, you will want to move some air through that and to move air you will need larger venturis. The 40mm Dellortos go up to 36mm for the chokes, but a performance 907 will want 37mm or 38mm plus and so 45mm carbs would be prefered. If you want to go nuts, a set of 48mm Dellortos might even be desirable. Parts-wise, all the jets and mis. bits are close in price, so I wouldn't hesitate to use a set of 40mm on regular road car with a stock engine or even a tuned engine. If you're looking for getting the most out of the 907 engine, you'll be wanting the 45mms. The Dellortos are an improved version of the Weber (which don't offer the same progression) and I'd avoid Webers for this reason.
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| Posted: 10-28-2005 04:39 am |
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3rd Post |
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Esprit2 Member
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andrewo wrote: I have noticed that 40s seem a lot cheaper than 45s. On other cars with Webers (similar design) I actually found that it was better to undercarb than overcarb. What are the tuning and performance implications of using 40s? Andrew, ... and 45's are by far cheaper than 48's. And 52's are pure gold. For an early, near-stock, street 907 (especially low compression) 40's are adequate. Not optimal, but adequate. Compared to the Strombergs, they're great. The performance improvement offered by 45's (45E's are easier to adjust accurately) is probably not worth the added expense if you intend to keep the rest of the engine stock and don't flog the car. On the othe hand, Lotus never used 40's on their own versions of the 907. Only 45E's. The best choke compromise for a 907, IMHO, is the 37mm, and that's really just beyond the maximum that should be used in a 40. 37's in 45's work well for a street engine. I just helped a local owner of a new-old J-H install Lotus Spec-5 DHLA 45's with 36mm chokes on his 907. The only not-spec setting is bumping-up the idle jets from 50 to 55. I think he'll tell you that the car runs great and is very driveable on the street. I have Lotus Spec-9 Dellorto DHLA 45E's on two 2.0 liter 907's (one with 107 cams, one with 104's) and a hot-rod 2.2 conversion. They display no temperment in traffic and pull hard all the way to red line. 40's really tend to drag-out that last bit of the climb to red line. If you aren't one to use red line often, then maybe that's not an issue. However, if you enjoy a good flogging, then consider going with the 45's. Greg Fletcher wrote: If you want to go nuts, a set of 48mm Dellortos might even be desirable. (Snip)... The Dellortos are an improved version of the Weber (which don't offer the same progression) and I'd avoid Webers for this reason. 48's are track-nutz. Over the top for a street 907/912. Not all Dellortos have the same progression hole pattern, and most generic Dellortos have 5 progression holes. Lotus worked directly with Dellorto to develop a 6-hole progression pattern for the 907 that really helps off-idle and mid-range performance. If you can find a set of Dellortos from a Lotus engine, take them over a generic pair. Regards, Tim Engel
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| Posted: 10-28-2005 04:52 am |
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4th Post |
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Esprit2 Member
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Spec 5 is about right if you are running a near stock 907 with DHLA45's. Front ............................ E907E0789F Rear ............................. E907E0790W Tag Number Front ............................ 5295A Rear ............................. 5294P Carb Type .................... DHLA 45E Choke .......................... 36 mm Main Jet ....................... 160 Main Air Corrector Jet ... 230 Main Emulsion Tube ...... 7772-8 Idle Jet .......................... 50 (Too lean! Try 55-58 range) Idle Jet Holder ............... 7850-7 Float Weight ................. 10 gr Float Setting Height ...... 16.5 - 17.0 mm Float Needle Valve ........ 170 Pump Jet ....................... 38V 42H Starter Jet ..................... 70 Starter Emulsion Tube ... 7482-1 Power Jet ....................... --- Slow Running Speed ..... 900 - 1000 rpm Idle CO Level (hot) ........ 2.0 - 3.0% Spec 9 is about right if you have a mildly warmed up 2.2 liter. 9.5:1 or 10:1 compression, up to 104 cams, headers. Carb Type ..................... DHLA 45E Choke ........................... 37 mm Main Jet ........................ 160 Main Air Corrector Jet ... 230 Main Emulsion Tube ...... 7772-8 Idle Jet ......................... 55 (maybe a little lean, try up to 58) Idle Jet Holder .............. 7850-9 Float Weight ................. 8.5 gr Float Setting Height ..... 14.5 - 15.0 mm Float Needle Valve ....... 170 Pump Jet ...................... 50H Starter Jet .................... 95 Starter Emulsion Tube 7482-3 Power Jet ...................... --- Slow Running Speed ..... 900 - 1000 rpm Idle CO Level (hot) ....... 0.7 - 1.5% Jensen Healey... 8.4:1, C-cam, 2.0 liter. Carb Type ....................... DHLA 40E Choke ............................. 35 mm Main Jet .......................... 130 Main Air Corrector Jet ..... 160 Main Emulsion Tube ........ 7772-5 Idle Jet ........................... 52L (56L on 4-speed) Idle Jet Holder ................ 7850-5 (aka, Slow Running Air Jet) Float Weight ................... 8.5 gr Float Setting Height ....... 16.5 - 17.0 mm Float Needle Valve ......... 170 Pump Jet ......................... 45 Starter Jet ....................... 70 Starter Emulsion Tube ..... 7482-1 Power Jet ........................ 70 Needle Valve ................... 7180-15 Slow Running Speed ....... 950 - 1000 rpm Idle CO Level (5-spd) ...... 0.2 - 0.3% Idle CO Level (4-spd) ...... 1.2 - 2.0% Default Dellorto Factory setting Carb Type ..................... DHLA 45E Choke ........................... --- Main Jet ........................ 165 Main Air Corrector Jet ... 170 Main Emulsion Tube ...... 7772-6 Idle Jet .......................... 50 Idle Jet Holder .............. 7850-1 Float Weight ................. 10 gr Float Setting Height ...... 16.5 - 17.0 mm Float Needle Valve ........ 170 Pump Jet ....................... 33 Starter Jet ..................... 70 Starter Emulsion Tube .. 7482-1 Power Jet .......................... --- Idle Jet Holders (Idle Air Corrector Jets)..... Weaker ................................. Normal ........................ Richer 7850.5, .10, .9, .4, .1, .3, .6, .7, .2, .8 Idle Jet Holders in common useage 7850.1, 7850.6, 7850.2, 7850.8 Regards, Tim
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| Posted: 10-29-2005 02:35 pm |
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5th Post |
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Judson Manning Member 
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Tim, I don't know where the Spec5 (160mains) came from, and it's exactly what I received from DBE years ago, but the jets are WAY off. The 2.0 listing you have below with the 130mains is A LOT closer to what I eventually settled on and we just put in Sander's car. Thanks as always for posting these specs! Judson
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| Posted: 10-30-2005 01:53 am |
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6th Post |
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Esprit2 Member
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Judson, I was quoting "Lotus" Dellorto set-up's. Lotus numbered their carb set-ups (Stromberg & Dellorto intermingled) sequentially as set-up changed through the years and models. Spec-1 through Spec-10 cover the evolution of the Lotus version of the naturally aspirated 907 and the 912... Elite, Eclat, Excel & Esprit. Lotus only used DHLA 45E's on their N/A 9XX's, never 40's, and their Dellorto set-ups are covered by... Spec 1, 1974 DOM 2.0 907, 38 mm chokes (too big) Spec 3, 1975 DOM 2.0 907, 35 mm chokes (too small) Spec 5, 1976-1980 Euro 907, 36mm chokes (just right) Spec 9, 1980+ 2.2 912, 37mm chokes (Lo-CR, 9.4:1) Spec 10, 198?+ 2.2 912 HC 37mm chokes (Hi-CR, 10.9:1) I run Spec 9 on two almost stock 907's and on a hotrod 2.2 907, and they run beautifully. Tim
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| Posted: 10-31-2005 03:09 am |
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7th Post |
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Judson Manning Member
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Tim, Running the Spec9 on my 2.0, I couldn't get the exhaust temp over 1200^F. I also had terrible throttle response, and was black-flagged at Roebling Road for fogging insects down the front-stretch! (WAY too rich) I started experimenting with different jets that got to the more ideal 1500-1600^F range. Ironically, this jetting was very close to the stock Euro JH jetting even though I had 9.5:1 pistons, a ported head, and higher lift cams. Likewise, Sander's car was delivered to me with a Spec9ish set-up, and ran like a dog. We leaned the mains closer to JH specs, did some other fiddling, and the car really came alive! When I built the 907/910, I did have to fatten things up a bit, but still nowhere as rich as the Spec9. Undoubtedly, we have a mystery if three of your cars run well on Spec9 and two of mine run poorly on the same set-up. Have you ever run an EGT or O2 sensor on any of your cars? Judson
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| Posted: 10-31-2005 10:14 am |
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8th Post |
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Harkes Member 
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Judson, to confuse things a bit more my new 2.2L high compression engine (10.9:1) doesn't have 37mm chokes but 38 mm chokes! my new 45E's came with 36mm chokes. On the dyno we have tested the 36mm, 37mm and the 38mm. The winner by far were the 38mm chokes in every rpm range. don't know if they have altered the other jets etc, but i'll check with Garry. Garry by the way already predicted the 2.2L engine would perform much better overall with the 38mm... but since my carbs came with the 36mm they started with these on the dyno. you know the performance specs of my engine. very torquey. erik, Netherlands (my engine arrives coming thursday...i'll make pictures and post them on the jhppg.com)
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| Posted: 10-31-2005 02:00 pm |
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9th Post |
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Judson Manning Member
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Erik, The choke 'mystery' is easily solved, and it's based more on cam lift and engine breathing efficiency. 38mm chokes correspond to a theoretical maximum HP level achieved for a 2.2 at ~6300rpm, which matches the dyno results posted for your engine. 36mm chokes would have cut your HP peak to ~5900rpm. What Tim and I are at odds with is the jettings. My observations suggest Spec9 with a 160main is WAY too rich, while Tim's observations suggest it's right on the money. Tim was kind enough to post Spec10 & Spec11 for me last year on the Lotus Forum while I was jetting the 907/910 hybrid. According to my notes, both Spec10&11 show 135 mains vs 160 mains for Spec9. The real question is: Why does the JH have 130mains, Spec9 has 160, then Spec10 &11 shift back down to 135mains?
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| Posted: 10-31-2005 09:32 pm |
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10th Post |
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Harkes Member
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a good question i certainly won't be able to answer for you, but i am interested in the answer. I will try and find out what the rest of my jettings are and see if my mains are 130/135 or 160. thanks for the explanation on the chokes. I'm starting to understand the engine a little more each time this way. cheers erik
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| Posted: 11-01-2005 02:09 am |
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11th Post |
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Esprit2 Member
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Judson Manning wrote: What Tim and I are at odds with is... (Snip)... Judson, I know, just words, and I really am tracking with you. But "mutually curious about..." might be more descriptive. ;-) Tim
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| Posted: 11-01-2005 04:29 am |
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12th Post |
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Esprit2 Member
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Judson Manning wrote: Tim was kind enough to post Spec10 & Spec11 for me last year on the Lotus Forum while I was jetting the 907/910 hybrid. According to my notes, both Spec10&11 show 135 mains vs 160 mains for Spec9. Judson, There are differences between the carbs other than jetting. I haven't played with these things on a dyno along with an exhaust gas analyzer, so I can't talk smart about what the differences mean to mixture individually, or in concert. I'm just noting that the differences are there. Specs 1, 3, 5 & 9 carbs do NOT have a power jet. Spec 10a, 10b, 10c (Excel manual, Excel automatic, Esprit... there is no Spec-11) all DO have a power jet (as does the Fed. carb-Turbo). Quoting a Lotus manual, "At high engine speeds, an additional amount of mixture flows through the needle jet (52) upstream of the inner venturi (50); mixture is composed of fuel drawn from the float chamber through channel (56) and metered by the power jet (55), mixed with air metered by the calibration passage (54) and drawn from the float chamber top (53)." In other words, the power jet circuit dumps additional fuel through an extended needle-orifice right in front of the inner venturi's inlet. With the Power Jet carrying part of the load at high speed, the Main Jets can be set a little leaner to favor the mid-range mixture requirements. So, comparing idle and main jetting combinations between Power Jet and NON Power Jet carbs isn't valid. I tend to conclude the fact that jetting similar to a Spec 10 carb (Power Jet) used in a Spec 9 carb (non power jet) worked for you is coincidental. Any attempt to figure out a correlation will just strain a neuron. Spec 1, 3 & 5 have relatively rich Idle Air Correctors (Idle Jet Holders) Spec 9 & 10 have very lean Idle Air Correctors Weaker ................................... Normal .... Similar .......... Richer 7850.5, .10, .9, .4, .1, .3, .6, .7, .2, .8 .1 is midrange and used on Spec 1. .6 & .7 are virtually identical, upper-midrange rich, and used on Spec 3 & 5 respectively. .9 is the third leanest and used on Spec 9 & 10. That "may" be of more significance when you consider that... Spec 1, 3 & 5 have 10g floats set to 16.5-17.0 mm height. Spec 9 & 10 have 8.5g floats set to 14.5-15.0 mm height. Float weight and height must be coordinated. A heavier float naturally rides lower in the pool, and a lighter one rides higher. To achieve the same fuel level, a heavier float must be set deeper (16.5-17.0) and a lighter float set higher (14.5-15.0). So... I don't know what the real fuel level difference is between those two combinations, but it's not 2mm as one might conclude by casually reading the specs. However if one fuel level is higher, that carb will run richer overall. Leaner jets could be used with a higher fuel level to achieve a similar mixture. Tim Last edited on 11-01-2005 04:33 am by Esprit2 |
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| Posted: 11-01-2005 02:23 pm |
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13th Post |
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Judson Manning Member
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My neurons are exploding!!!! lol But seriously, now I think we're finally getting somewhere... So if I understand what you're saying, there is power-jet and non-power-jet version of the Dellorto? If so, that would explain why the jettings are so different. Apparantly both sets of carbs I got from DBE were Spec10 w/ Spec9 jetting, and the carbs on your cars are all correct Spec9s. I guess the lesson here is that all DHLAs are not the same. Tim, you are a wealth of information. Thanks for help clearing this up! Last edited on 11-01-2005 02:26 pm by Judson Manning |
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| Posted: 11-01-2005 05:54 pm |
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14th Post |
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Brett Gibson JH5 20497 Member
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Tim, how do you know which spec Dell's you have, mine are 40'E's, on a stock engine, would it show on the Dell tags, the best match I come up with from your listing is below, I have tried running them both ways (stock/changed) and still have had no luck tuning them in, so I went back to Stromberg's for now, but they are on my winter list of to-do, and would very much like to get them running smooth. It was recommended I use anti-spit back things on the intake, plus I dont have pump jet filter/springs installed, do I need to get them ??? Jensen Healey... 8.4:1, C-cam, 2.0 liter. Carb Type ....................... DHLA 40E Choke ............................. 35 mm changed to 33 Main Jet .......................... 130 changed to 145 Main Air Corrector Jet ..... 160 changed to 190 Main Emulsion Tube ........ 7772-5 Idle Jet ........................... 52L (56L on 4-speed) Idle Jet Holder ................ 7850-5 (aka, Slow Running Air Jet) changed to 7850.7 Float Weight ................... 8.5 gr Float Setting Height ....... 16.5 - 17.0 mm Float Needle Valve ......... 170 Pump Jet ......................... 45 Starter Jet ....................... 70 Starter Emulsion Tube ..... 7482-1 Power Jet ........................ 70 Thanks Brett
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| Posted: 11-02-2005 02:58 am |
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15th Post |
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Esprit2 Member
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Brett Gibson JH5 20497 wrote: Tim, how do you know which spec Dell's you have, mine are 40'E's, on a stock engine, would it show on the Dell tags... It was recommended I use anti-spit back things on the intake... ...plus I dont have pump jet filter/springs installed, do I need to get them ??? Brett, First I should clear up this "Spec" business. Lotus used "Spec" numbers for the engines used in different years/markets/models. When I talk about a Spec-X Dellorto, I'm referring to the Dellorto configuration used on the Lotus Spec-X 9XX engine. Call EuroCarb and ask for the jets for a Spec-X Dellorto and they won't know what you're talking about. Spec 1 1974 DOM 907, Dellorto DHLA45E, 38mm choke (too big) Spec 2 1974 USA 907, Zenith Stromberg Spec 3 1975 DOM 907, Dellorto DHLA 45E, 35mm choke (too small) Spec 4 1975 USA 907, Zenith Stromberg Spec 5 1976-80 Euro 907, DHLA45E, 36mm choke (good) Spec 6 1976-79 Calif / 1976-77 49-State USA, Zenith Stromberg Spec 7 1978-80 49 State USA 907, Zenith Stromberg Spec 8 1980 California, USA 907 , Zenith Stromberg Spec 9 1980 onward, Euro 912 Lo-Comp, DHLA45E, 37mm choke Spec 10 1986 onward, Euro 912 Hi-Comp, DHLA45D, 37mm choke Nothing on the carb body itself tells you what Spec it is. If the tags are still intact, then a good Dellorto source like EuroCarb may be able to tell you what car/engine they are from and what the original jetting was. If the carb happens to be from a Lotus, then you could relate the jetting combination back to the Spec-number via the data Lotus published in it's manuals. I don't have all the relevant tag numbers, just a few for Lotus applications. However, If you have 40E's, then they are NOT Lotus and hence, are Spec-nothing (the Spec-X being Lotus-only terminology). Lotus used DHLA40's only on the J-H 907's, but didn't refer to them as a Spec-something. All "Lotus" naturally aspirated 9XX engines got DHLA45's. Some model-years of DOM/ROW carb-Turbo 910's got 40M's, some got 45H's or 45M's. All Fed carb-Turbos got 45M's. The best Lotus manual to refer to for a 9XX jetting history is the 1983-87 S3 & Turbo Esprit. It includes the carb data for those years plus all years prior for "Lotus" carb-9XX engines (1974 onward Lotus, but not J-H). Section TDA (Technical Data, A) covers the naturally aspirated 907/912 engines, and TDB covers the carb-Turbo 910's. BTW, I think every naturally aspirated 9XX owner (Lotus or J-H) should have a copy of TDA (TDB optional) to supplement their Shop Manual Technical Data Section. I've posted them in PDF format on my son's website: http://www.tc.umn.edu/~enge0214/lotus/ (TDA.pdf and TDB.pdf) 33mm chokes are too small. Stock J-H 35's are too small to start with (IMHO), and with 33's it's like having both hands firmly gripping the poor thing's throat. You wrote, "Anti-spit back things…" ?? Anti-reversion soft mounts, maybe? Lotus' metal soft mount spacers have a tapered bore with a lip. Like a funnel pointing down stream (looks like something from Dr. Kamm's theories). At first glance it appears to be a flow restriction, but it actually helps. There are some really weird flow patterns taking place in an intake manifold including reverse flow/ reversion waves. The anti-reversion spacers block the reversion wave pressure pulses that would otherwise screw up the steady vacuum (depression) the carb needs to meter properly. Weber took a different approach on some of it's carbs (mostly downdraught). An extension tube from the venturi can be seen sticking out of the carb throat/ air horn. That's so the venturi can breathe reversion-free outside air. But I digress… Anyway, anti-reversion soft mounts are a good thing if you can find them. They're also a Lotus-thing and you won't find them just anywhere. I've not seen the reversion lip on any aftermarket soft mount spacer. You wrote, "…plus I dont have pump jet filter/springs installed, do I need to get them ??? :-/ ...Huh? The springs on the accelerator pump linkage? Absolutely you need them ! They're what operate the pump, not the pushrod directly. Without the springs the pumps won't work, the engine will stumble/ fall on it's face whenever the throttle is opened, and the car will be a bear to get off the line. Or are you talking about something else? 7850.7 Idle Jet Holders (Idle Air Correctors) are about right. I use them with a 55 Idle Jet as a starting point with naturally aspirated 9XX's. The Holders are usually about right, but the Jet sometimes needs tweeking to suit the individual engine. 52's or 53's might be big enough for a stockish Federal J-H 2.0 907. To jet the idle circuit, start with the Holders. Open the throttle gently. If the engine hesitates off-idle, the Idle Air Corrector Jets (Idle Jet Holders) are too lean. Install a step or two richer and try again. When the Holders are set, move on to the Idle Jets. If the engine stumbles under full throttle somewhere in the 3200-4000 rpm range, then the idle circuit is running out of capacity before the main circuit takes over. Increase the Idle Jet size until the stumble just disappears, no more. If a large change in jet size is required, then the change may also affect the air corrector. Go back and repeat the full cycle of checking the air corrector, then jet. For the same air flow (ie, feeding the same engine), smaller chokes result in higher air velocities through the carbs. Higher velocities mean a stronger vacuum in the venturi which can in turn, pull the fuel over more quickly… ie, better throttle response. With smaller chokes a slightly richer overall condition exists and it's usually possible to go with slightly smaller mains. You went smaller chokes and larger mains. I can't tell from where I'm sitting if your engine needed that, but it's a combination I wouldn't normally have expected. With 8.5g floats, I usually run the float heights at 14.5-15mm. Lighter floats need to be set higher to maintain the same fuel level. Light floats set lower (16.5-17.0mm) will cause an overall lean condition in the carb. If I were going to play with your carbs, I'd probably start by putting the missing springs on the accelerator pump (if that was what you were talking about) and set the pump stroke. Then put the chokes and mains back where you found them (well, okay, if it were me, I'd go with larger 36mm chokes… but lets not introduce that debate here). Keep the .7 Idle Jet Holder and go with a 53, 55 or 56L Idle Jet. Then set the float height to 14.5-15.0mm to go with your 8.5 gram floats, balance the carbs with a 4-tube manometer and adjust the Idle Mixture Screws to give peak vacuum. Balancing is key. Good carbs will run like crap if the balance is off just a little bit. And the balance will be off when you install the carbs… almost guaranteed. Then see how it runs and go from there. Good luck, Tim Last edited on 11-03-2005 12:52 am by Esprit2 |
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| Posted: 11-02-2005 03:19 am |
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16th Post |
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Esprit2 Member
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Judson Manning wrote: Apparantly both sets of carbs I got from DBE were Spec10 w/ Spec9 jetting, and the carbs on your cars are all correct Spec9s. Judson, Until you open the carbs, it's just speculation as to what DBE sent to you. Des Hammil's book doesn't discuss the Power Valve. Or at least I didn't see it during a quick scan the other day. However the Lotus Service Notes (aka Workshop Manual) for the 1983-87 Turbo Esprit covers it in the carb section. I could sweet-talk a friend with access to Adobe Acrobat to scan it into a PDF, but don't hold your breath waiting. Euro-stock OEM J-H Dellorto DHLA40 carbs were equipped with a Power Jet... size 70. Lotus Spec-10 2.2 912 and the 1988 Esprit NA (naturally aspirated 912) DHLA45's used a 70 Power Jet. Lotus Spec's 1, 3 & 5 did not have a Power Jet. Tim Last edited on 11-02-2005 03:21 am by Esprit2 |
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| Posted: 11-02-2005 03:46 pm |
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17th Post |
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Mark Rosenbaum Member 
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Wow. The data in TDA.pdf is an enormously valuable supplement to the JH shop manual -- the ignition timing and cam timing areas, in particular, caught my attention in a brief and casual scan. Doubtless there's a lot more information there that I haven't yet noticed. Although I think I'll stick with Strombergs, as I can tell myself that I understand them, I now know more than I'd ever expected to know about Dellortos. My thanks to Tim for providing this information.
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| Posted: 11-03-2005 01:55 am |
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18th Post |
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Esprit2 Member
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Mark Rosenbaum wrote: The data in TDA.pdf is an enormously valuable supplement to the JH shop manual -- the ignition timing and cam timing areas, in particular, caught my attention... (Snip)... My thanks to Tim for providing this information. Mark, You're welcome. Glad to help. With regards to other data in TDA: There's a bit of a caveat for J-H owners using Lotus data... cam timing specifically. I'm probably getting off-topic for this thread, but... UPDATE 05/08/2007: The info I posted originally was wrong. It was deduced from the belief that the published MOP values were accurate, but one is not. The 100 MOP is really 97 MOP. No one seems to know why it was rounded up before publishing, but it was. All the other MOP values are accurate as published. Using 97 MOP instead of 100 MOP, the numbers all fall in line, confirming there is no keyway location difference between the J-H and Lotus versions of the camshafts and pulleys. They're interchangeable. There is a small difference in cam pulleys after (?) 1979 that changed the hub offset to better center the pulley on the belt; however it's not a works/ doesn't work sort of change. J-H and Lotus pulleys of the same MOP can be interchanged. I'll leave the original next paragraph, but don't get sucked into the bad info. It's marked. *~*~*~*~* There is a difference between early J-H cams & pulleys and the Lotus versions of the same parts. (No, there's no difference) In matched combinations (JH/JH & Lotus/Lotus) the nominal timing events are the same; however, the hardware is different by a small change in the angular placement of the keyways (nope, same-same). The same difference is applied to both the cam & pulley in each pair so that the J-H pulley & cam together give the same nominal cam timing as the Lotus cam & pulley give together (nominal being 110°). But installing a Lotus pulley on an older J-H 907 cam will increase the MOP 7.5°, and installing a J-H pulley on a Lotus cam will reduce the MOP 7.5°. (no, you can interchange the J-H and Lotus cams and pulleys of the same MOP) *~*~*~*~* On either engine, Increasing MOP Advances the exhaust and retards the intake. Reducing MOP retards the exhaust and advances the intake. J-H used a dual MOP pulley that gave 110° or 115° MOP depending upon which side of the pulley faced forward. Similarly, the Lotus dual MOP pulley gave 110° & 100°. In order to produce those different combinations, different keyway locations were required on both the cams and pulleys. Later J-H engines also used the 110° / 100° pulley and from that point on have full cam/pulley compatibility with Lotus. Prior to that time, care is required not to mix and match similar looking sibling parts. The early J-H cam/pulley's timing events match those of what Lotus called it's C-cam. The Lotus C-cam and D-cam both use a nominal 110° MOP (the alternate 115° & 100° MOP's are both for playing emissions games and not desireable) and would both be at some risk of an MOP-error if used with early J-H pulleys. All other Lotus cams listed in Section TDA have different MOP's and require different matching pulleys (E-cam/ 102.5° pulley, 107-cam/ 104° pulley, 104-cam/ 104° pulley), so there's little chance of mixing-n-matching there. UPDATE 05/08/2007: The 115° MOP reduced the overlap 5° per cam for a total of 10°... from 52° to 42°. That reduced HC emissions readings but cost power. The early test engines met Federal specs up thru 1974, but just barely passed '74. The change was made to give a larger margin for error/ wear, not to meet specs. In later 907's, Lotus switched to a 100° Intake MOP (really 97° MOP) for emissions engines. The point there was to increase the overlap. That allowed the intake and exhaust charges to co-mingle a bit more, giving a result much like Exhaust Gas Recirculation (reduced oxides of Nitrogen). The extra advance also increased the horsepower, replacing Hp lost to overall emissions tuning. Unfortunately the high end power came at the expense of low end torque, and the overall effect was a weaker, more difficult to drive engine. "Horsepower sells cars", but the net performance was way down. Neither emissions cam timing MOP (115° or 100°/97°) is desireable. The engine will run better with both of the stock cams timed to 110° MOP. Other than that, there shouldn't be any land mines hiding in Lotus Section TDA waiting to pounce on J-H owners. Indulge in good health. Tim Last edited on 05-09-2009 06:39 am by Esprit2 |
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| Posted: 11-03-2005 11:22 am |
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19th Post |
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Brett Gibson JH5 20497 Member
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My neurons are exploding!!!! gotta agree with Judson on that. Thanks for all the info Tim, it helps clear up some issue's for me, as for the pump jet filter/spring, I was not talking about the main fuel pump with the large spring over the connecting rod on the outside of the carb, but the much smaller pump jet thats on the top side of the carb under a brass screw cap , 2 per carb quite a small item, Euro Carb shows it as item #53 on there blow up, someone told me that it didnt really matter having them, but seeing I was having troubles I thought it maybe an issue. I printed off the TDA and TDB and will go thru it tonight but a quick glance show's alot of again great info, Thanks Brett
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| Posted: 11-03-2005 12:17 pm |
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20th Post |
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Esprit2 Member
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Brett Gibson JH5 20497 wrote: ...as for the pump jet filter/spring, I was not talking about the main fuel pump with the large spring over the connecting rod on the outside of the carb, but the much smaller pump jet thats on the top side of the carb under a brass screw cap , 2 per carb quite a small item... (Snip)... Brett, It was the "filter/spring" thingie bit that threw me. The part you describe is the Pump Jet. And it is definitely required !! There aren't a lot of extra parts in a carb that you can just leave out (like... none). They ALL need to be there in the proper size combination/ adjustment. If the carbs aren't fully assembled by the book, then you may have found the key to your poor running issue. The accelerator pump is NOT driven by the linkage and your foot. When you lift your foot/ close the throttle, the linkage pulls the pump lever up against the small, concentric spring, compressing it to pre-load the mechanism. Basically, "cocking" the pump mechanism. When you open the throttle, the linkage simply releases the pump lever so the spring can drive it forward with a calibrated spring rate force. The amount of "injection pressure" is controlled by the stiffness of the spring. The volume of fuel or "shot size" is controlled by the length of the stroke/ how far the linkage pulls the lever back/ a nut & jam-nut at bottom of linkage pull-rod. The time period over which the shot is delivered is controlled by the size of the Pump Jet. All three things must work in concert. With no Pump Jet installed, any spring (light or heavy) will immediately dump the full shot size in one big gush as the throttle is opened instead of gradually over a brief time as the shot pees out in a thin stream through the Pump Jet. Then as the throttle is opened, the engine will quickly go way too rich and bog down. And before the Idle Circuit or Main Circuit can establish a fuel flow and take over, the shot-gush will be flushed out and the engine will go way lean and stumble again. Sound familiar? Additionally, if you purchased used carbs, assume they are dirty inside. They are, even if the float bowl and throats appear clean. Even if you spritz the passages with aerosol carb cleaner and the little red tube. A modern carb shop will soak a carb body in an ultrasonic solvent tank for 3+ days or so, then manually clean passages as far as they can. Sometimes you simply can't get them clean by squirting an aerosol into the open end of those passages you can see, and Dellortos/ Webers are very sensitive to dirt and varnish build up. You can't get them "too clean". Tim
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