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| Zenith Stromberg Carburetor Vacuum Lines Schematic | Rate Topic |
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| Posted: 03-30-2005 10:51 pm |
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1st Post |
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Mark Rosenbaum Member 
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A question came up elsewhere about how the vacuum lines work together on an engine with Strombergs. The shop manual doesn't really show this, so I created a simple sketch. Attachment: JH Vacuum Lines.gif (Downloaded 331 times)
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| Posted: 10-18-2013 10:29 pm |
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2nd Post |
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pjg55 Member
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Hi Mark, Thanks for creating this schematic. My question is what are the consequences of simply plugging off all these vacuum lines at the carb ? I have already converted the distributor to a non-vacuum assisted version and removed the carbon canister. It would seem these are purely for emissions purposes and could be eliminated.
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| Posted: 10-18-2013 11:39 pm |
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3rd Post |
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Jensen Healey Super Moderator 
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I'm sorry to inform you that Mark Rosenbaum has passed away. I don't know how removing all the vacuum lines would affect the performance of the engine.
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| Posted: 10-19-2013 01:26 pm |
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4th Post |
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Brett Gibson JH5 20497 Member
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I have removed most of the vacuum lines and plugged them off leaving only the inlet manifold to the brake booster, plus I blank off the by-pass valve on the side of the carbs and adjust the temperature compensator so that it does not move and seals that chamber as well. Having done that and driven the car thousands of miles and done the same for a few other cars all I can say is it works for me. Brett
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| Posted: 10-22-2013 11:37 pm |
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5th Post |
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pjg55 Member
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Thanks for the info Brett. That is what I was looking for.
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| Posted: 12-15-2017 12:08 pm |
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6th Post |
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chrisl Member 
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I have removed most of the vacuum lines and plugged them off leaving only the inlet manifold to the brake booster, plus I blank off the by-pass valve on the side of the carbs and adjust the temperature compensator so that it does not move and seals that chamber as well. Having done that and driven the car thousands of miles and done the same for a few other cars all I can say is it works for me Hi Brett I've acquired a set of Strombergs and am refurbishing them to fit on my GT. It currently has Dellortos on it and runs fine other than the stutter on flooring it but I think the Strombergs will make for smoother running. Anyway, a lot of the pipework on the Strombergs is missing so I am working through what goes where. The shop manual is no help; and I see from your post above a lot of the vacuum lines aren't really necessary anyway. I think I've got a good idea of what goes where but it would be good to confirm. Do you have a picture handy that shows which ones you've capped? Did you also ditch the carbon canister? It wasn't fitted to RHD cars as far as I can tell.
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| Posted: 12-16-2017 06:49 am |
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7th Post |
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answerman Member
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Hi Chris: if this helps, I went through the removal of most of the vacuum lines when I was sorting out some running issues a couple of years ago (turned out to be distributor related). Still, I agree that most of the lines are unnecessary. I removed the vacuum switch since it wouldn't be doing anything anyway, and also removed the vacuum retard line to the distributor (I ended up installing a different distributor that didn't have the retard). Here's a "before" and "after" diagram (the "before" diagram is actually Mark's). Before:  And after:  The original post with context is here: http://www.jensenhealey.com/forums/view_topic.php?id=1798&forum_id=15&page=6 (starts around post 107) Last edited on 12-16-2017 06:55 am by answerman |
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| Posted: 12-16-2017 11:39 am |
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8th Post |
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chrisl Member
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Brilliant; that answers the question. These Strombergs have more holes than a block of Swiss cheese, plugging them saves a lot of messing around. I'm all for originality but... Thanks a heap
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| Posted: 12-16-2017 06:21 pm |
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9th Post |
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Esprit2 Member
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chrisl wrote:I've acquired a set of Strombergs and am refurbishing them to fit on my GT. It currently has Dellortos on it and runs fine other than the stutter on flooring it but IIt's interesting to see how different people view and/or fix the same problems. I have 3 907 engines, and fixed all the carb issues by converting to Dellortos. If your Dellortos have an off-idle hesitation, or don't otherwise run smoothly, then they are not properly set-up and tuned. If the Strombergs aren't set-up & tuned any better, then they will give you problems as well. ... a lot of the pipework on the Strombergs is missing so I am working through what goes where. The shop manual is no help; and I see from your post above a lot of the vacuum lines aren't really necessary anyway. As far as removing vacuum lines goes, follow Answerman's simplified schematic. However, it's also important to defeat the devices those lines feed, rather than just disconnecting the hoses & plugging the ports. The Bypass Valves allow intake flow to continue around the closed throttle butterflies during closed-throttle over-run. The strong manifold vacuum that would otherwise result would produce a strong spike in oxides of nitrogen. The Bypass Valves prevent that. The Vacuum Switch (aka Vacuum Delay Valve) controls how long the Bypass Valves stay open after the throttles close. The Delay Valve and two Bypass Valves are all part of the same function. If you delete one, delete them all. In addition to disconnecting the hoses, also blank-off the Bypass Valves. Remove each valve, replace the gasket with one that is hand-cut and solid (no center holes and passages for vacuum), and re-install the valves. The 'solid' gasket blanks off the valve, making it little more than decorative. Separate, and not vacuum related... If the Thermal Compensator is suspect or not working, then don't bother fixing or replacing it. Simply adjust the plunger in all that way such that it can never unseat/ open the port. The Thermal Compensator is a 'seasonal' thing, providing a slightly richer mixture during the warm-up cylcle in cold weather. Most 907 powered cars get put in storage and aren't driven during 'real' Winter. If that's the case for you, then restoring the Thermal Compensator is a waste of effort. Regards, Tim Engel Last edited on 12-18-2017 05:07 pm by Esprit2 |
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| Posted: 12-18-2017 03:41 pm |
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10th Post |
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Brett Gibson JH5 20497 Member
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Chris, sorry for not getting back to sooner, I was out of touch for a bit, but I see you got the answers you need from the group. good luck. Brett
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| Posted: 12-19-2017 07:54 pm |
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11th Post |
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answerman Member
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Glad the diagrams helped. Tim brings up a good point, which I didn't mention above but was referenced in the link to my original post at the bottom of the diagrams. In addition to removing the vacuum switch, disconnecting the lines and capping the ports (by the way, the caps are shown in red in my diagram), I also did disable the temperature compensator as Tim mentioned. Additionally, I did remove the bypass valves and cut/install a full face gasket for each to ensure that they were truly disabled. I should have mentioned that above. I have to say: this is the first time I've ever heard of someone removing their Dellortos and replacing them with Strombergs. Usually it's the other way around. I'd love to replace my Strombergs with Dellortos, but it just doesn't make financial sense for me for the way I drive the car. If in the future I end up with some disposable income that I just can't find a use for, a set of Dellortos would be high on the list (along with a stroker kit). Last edited on 12-19-2017 07:58 pm by answerman |
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| Posted: 12-20-2017 12:00 am |
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12th Post |
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chrisl Member
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I also have a J-H with Dellorto's that runs well. It needs a bit of a tune up as it's running a fraction rich but doesn't have the 'stutter' on progression. My GT originally came with Strombergs but a previous owner put D's on it; it runs well and I've even (nearly) got the flooding/stutter sorted. I managed to pick up a set of Strombergs and an inlet manifold for not a lot of $$. I like to play around with various setting etc to see what works. The thought was to use the J-H as a speedster and the GT as more of a cruiser. It will be interesting to compare the pair if/when I fit the Strombergs. If I don't like it the Dellortos will go back on. The responses and advice from everyone is greatly appreciated, have a great Christmas and new year.
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| Posted: 05-28-2025 02:11 am |
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13th Post |
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vnavaret Member 
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Esprit2 wrote: Separate, and not vacuum related... Tim, Your description of the compensator function is a bit off. I do not recommend removing or disabling them for reasons I shall explain. The thermal compensator does not enrich the mixture, it LEANS the mixture when the carburetors heat soak. It does this by using a bi-metallic strip that moves a small pintle. When the pintle moves, it uncovers an opening, allowing a small amount of air to bypass the jet and join the air fuel stream as it enters the intake manifold. This additional air results in a leaner mixture. Why do we care about heat soak? When the carburetors get hot after a drive cycle, they will heat the incoming air. This reduces the air density and since carburetors add fuel based on the VOLUME of air (not the mass of the air), the carburetor is now over fueling. This can make the car hard to start, and can result in rough running for a couple of minutes until enough cool air passes through the carb to cool it, which allows the air density to return to normal. If you are experiencing difficult hot starts, or rough running after a hot start, the temperature compensator is not working, or is misadjusted (opening at too high a temperature). This issue manifests when the engine has fully warmed, and is then shut down for 15-30 minutes, and then restarted. It is particularly evident on hot days when the ambient exceeds approximately 90F. The Lotus engine does not have the carbs over the exhaust manifold like Triumphs or MGs. So this tendency is not as severe as it might be. If you live in a cool climate, I suppose you might never experience it. If you experience this issue, then restoring or adjusting the compensator is warranted. Vance Last edited on 05-28-2025 02:13 am by vnavaret |
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| Posted: 05-28-2025 02:10 pm |
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14th Post |
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noomg Member
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It's interesting that two fairly knowledgeable guys have two completely different opinions about the same thing. When this happens I usually ask Mr. Bentley. Also interesting on this or just about any other LBC website there's always someone hell bent for leather wanting to remove any and all smog equipment, the feeling being it's going to turn their car into a fire breathing monster. Unfortunately that doesn't work it either creates driveablity problems or no noticeable difference or both. Recently while cleaning the engine I unknowingly disconnected a vacuum line then when started it was idling at 2,000rpm once I found and reconnected the line idle went back to normal. As Tim pointed out it's not just a matter of removing lines and plugging holes you'll also have to come up with work arounds for those devices.
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| Posted: 05-30-2025 04:14 pm |
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15th Post |
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vnavaret Member
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noomg wrote: It's interesting that two fairly knowledgeable guys have two completely different opinions about the same thing. When this happens I usually ask Mr. Bentley. I suppose I am picking nits, but the compensator can only add air to the mixture, it cannot add fuel. In my book that means it can lean the mixture, but cannot enrich it. The starter box adds fuel, but not air. So it enriches the mixture. As to the latter point, I agree completely. People want to "clean up" their engine bay, or solve a real or imagined problem by removing emissions gear. Then they wonder why their garage stinks of fuel, oil leaks mysteriously appear, or the engine operates less than perfectly. <sigh> My perennial JH favorite: "I went to electronic ignition, but now my tach doesn't work." <whimper> There is no power to be gained, and in the case of removing an EGR (not on our Jensens, however), highway fuel economy declines slightly. I think much of this is driven by a lack of understanding as to what the devices do, and subsequently how to re-engineer them to avoid any detrimental effects. There is also the loss of assistance in solving problems, since much of the advice offered in forums assumes a more or less original configuration. Let's not forget the fact that the less original the car, the lower the resale value. Oh well. To each his own. Personally, I prefer a stock configuration. Vance Last edited on 05-30-2025 04:19 pm by vnavaret |
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