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| Joined: | 05-01-2005 |
| Location: | Minneapolis, Minnesota USA |
| Posts: | 575 |
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11602 is running 45DCOE9 (that came off an Alpha Spider and had that cars OEM jetting in them when I got 11602, it was getting 9 MPG).That is poor fuel mileage for a 907. There is no standard 'factory' set-up for Webers on a 907, but I suspect your Idle Jets and Idle Air Correctors are too rich at 55f8. I'd be looking at something more like 50f9. The main jet is also a bit rich at 140 (try 135), but since the DCOE's idle circuit feeds the engine up to around 4000 rpm, it's probably the real culprit guilty of the 15 mph. These aren't hard recommendations that 'will work!' It's just my humble opinion, and I'll bet two cents against a donut that they'll work better for you.
Since October of 2017 the jetting is as follows:
Chokes: 36
Idle Jet: 55f8
Pump Jet: 45
Emulsion Tube: F2
Air Corrector: 180
Main Jet: 140
Float Height 8.5 mm
Having said that, altitude matters. As you go higher, the air gets thinner, so the Air-Fuel Ratio gets richer... how high are you? It's possible you could be running a Sea Level jetting set-up at altitude, where it's way too rich.
Try the following to driveway-analyze the jetting. Properly balance the carbs, and adjust the Idle Mixture Screws for peak manifold vacuum. If you're working by ear, then peak rpm corresponds to peak manifold vacuum. Finally, set the idle speed to 900 rpm. Only then...
1) If the Idle Mixture screws are out 2 or more turns, then the idle jet is too lean - Go up one size.
2) If the Idle Mixture screws are out 1 ¼ or less, then the idle jet is too rich. Go down one size.
3) If the Idle SPEED screw is turned IN more than 1/2 turn after contact with the throttle lever, then the idle circuit is too lean.
If the throttle speed screw has to be turned IN more than 1/2 Turn in order to achieve 900 rpm, then you are cheating the Idle Circuit by opening the butterflies to expose additional progression hole area in the carb's throat. A need to turn the Idle Speed screw in more than 1/2 turn is another indication that the Idle Circuit is too lean.
4) With the engine up to full operating temperature, set the Idle Speed as slow as possible, consistent with smooth running. Open the throttle slowly enough that the accelerator pump isn't much of a factor. If the engine hesitates off-idle, then the Idle Air Corrector is too small, go a step larger, repeating until the hesitation just goes away... no larger. When you're done, set the Idle Speed back to 900 rpm. Personally, I prefer 1000-1100 rpm, but let's not confuse the issue now... stick with 900 rpm.
5) Find a quiet stretch of road where you can make a full throttle run in a lower-mid gear without attracting the wrong kind of attention. You'll rip through first too quickly to be helpful, and 4th is to great a risk of getting a speeding ticket. After a rolling start, short-shift in to 2nd or 3rd at as low of an rpm as the engine will tolerate, then accelerate at full throttle. If the engine stumbles at or a little before 4000 rpm (the Idle to Main transition point), then the Idle Jets are too small... go a step larger and repeat. If the engine does not stumble, then you don't know where you are. Go a step smaller with the Idle Jets and repeat until the engine just stumbles at or before 4000 rpm. Then go back one step larger until the new-found stumble just disappears... no richer.
Any changes made to the Idle Jet and Idle Air Corrector affects the other one. So repeat the above procedure until the last round produces no further change to either the Idle Jet or Idle Air Corrector.
When you're done, any Idle Air Corrector change made sitting still in the driveway may prove to be too lean while driving. If the engine hesitates as the clutch is released, trying to get the car rolling from a standing start (ie, if pulling the choke out a bit helps), then go a step richer on the Idle Air Corrector until the hesitation just goes away... no more.
*~*~*~*
In some racing applications where high “G” forces are experienced in turns (ie, you autocross), the “Inlet Needle travel” (float droop) is limited to 1.5 mm of movement. This creates more of an on / off effect in the fuel inlet needle valve, and controls the fuel level in the fuel bowl in a more consistant, operable condition. Just something to play with.
*~*~*~*
I also have a 45DCOE-9... a single on my Lotus Europa. It's tiny car at 39" tall across the coupe roof, just 1400 lbs, with a 1565cc engine, and it would deliver only 19.8 mph at a steady freeway cruise. The jetting wasn't very different from what you listed above, but the chokes are smaller.
Last Spring/ Summer, I made a series of Idle Circuit changes following the above procedure. I ended-up at 45F9 with the float height set to 8.53 mm 0.336”, and the Idle Mixture Screws at about 1 3/8 turns out at peak manifold vacuum. The engine now sounds & runs better than it ever has in the time I've owned it, it has more pep, it no longer stinks rich, and it delivers 32 mpg during freeway cruise.
I'm not saying your J-H will ever deliver high fuel mileage, but I do think your current 15 mpg is pretty poor, and that I'd start tuning by playing with the idle jets and idle air correctors.
Regards,
Tim Engel
Last edited on 01-14-2020 08:43 pm by Esprit2