Weber 40 IDF Dual Carb: Tuning and Hot/Cold Problem

[kbay_11]

Hello,

I am trying to adjust my dual weber carbs (40 IDFs) on my 2150cc engine after I installed the engine in my '66 VW Bug and I seem to be having some issues. After using the "snail" tool to synchronize the carbs and get the same reading on both by using the idle speed screw (with linkage off), I then reconnected the linkage and adjusted the idle mixture screws on each carb and tuned the engine by ear. However, even though the engine seems to idle alright after the adjustments to the idle mixture screws are made, the head temps are not the same and one carburetor (cylinder side 1-2) is very cold while the other (side 3-4) is hot.

When I first synchronized the carbs with the snail tool, I had to turn in the idle speed screw on the cylinder side 1-2 carb quite a few times more than the other carb to get the same reading (a reading of 7). After reconnecting the linkage and checking the same reading, I made the adjustments to the mixture screws and the carbs seem to impact the engine idle almost instantly, but again, I cannot seem to find the balance with head/carburetor temp and I am not sure what the proper carb manifold temperature should be, but I am fairly certain it shouldn't be very hot or ice cold either.

Another note: When accelerating the carbs backfire/pop and the exhaust gets darker/black.

When we had the engine out of the car, we adjusted it to the point where it was running very well and accelerating nicely with only slight backfiring at high RPM. I am hoping to get it back to this stage.

If I could get some direction with this, that would be great. Thank you.

[67rustavenger]

IIRC, When you are syncing the carbs. You adjust the snail to match the lowest reading carb. Not the highest.
Doing it the way you have. May have exposed the progression ports on the 1-2 carb. If you can. Pull the carbs and check the clearance between the throttle plates and the carb throat. I adjust mine to be at .003" that gets them close to matched at idle.

While doing that have a look at the throttle plate position. Is a complete progression port exposed? It shouldn't be. It should look like this. Or slightly less exposed. Like half the first progression port.

That doesn't explain the cold carb. But it's a path forward.

Have you made sure that you have spark to the 1-2 cylinders? If no spark, that would make the carb cold.

Make sure to sync the carbs with the engine at running temps. Otherwise you might get fooled into making the incorrect adjustments.

After every carb, timing change. Go back and adjust the mixture screws for the best lean idle (LBI).

That's a start.

Good Luck
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[kbay_11]

Thank you both for your input! After re-checking the sync and the idle adjustments, it seems that I have everything working pretty well now and the carbs are both getting hot (instead of one being ice cold). After tuning the carbs again and getting good head/carb temps along with decent idle, it seems that one cylinder is misfiring slightly even when I try to make adjustments to the idle mixture screw (nothing seems to correct the misfire). I may have to check the spark plug and/or engine timing to see what the problem is, but if checking those two things doesn't solve the problem, I am not sure what else it could be.

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check idle jets for clogs, and remove idle mixture screws. Check for a bent or grooved needle.
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[1vwjedi]

Couple of things. I have found that the thinned cupped washer designed to hold the ring. On mixture screw is to thin and sometimes catches threads. Stopping spring from applying proper pressure to bring allowing air to mess up adjustments.
Another possibility. There should be a small deflector tin at bottom of heads. One may have popped out. Those are critical as far as head temp is concerned

[1vwjedi]

Couple of things. I have found that the thinned cupped washer designed to hold the ring. On mixture screw is to thin and sometimes catches threads. Stopping spring from applying proper pressure to bring allowing air to mess up adjustments.
Another possibility. There should be a small deflector tin at bottom of heads. One may have popped out. Those are critical as far as head temp is concerned