Help with my idle circuit?

[Irish911]

I am having a problem with what I believe to be my idle circuit and hope others can help sort me out. I recently replaced my 40 IDTP’s with IDA’s. The throttle shafts were loose so I had the shafts, bearings and valves replaced (also all gaskets and cleaned everything). The Webers are set up as follows:
Venturi 30mm
Main jet 125
Emulsion F26
Air correction 170
Idle jet 55

I reinstalled the IDA’s and turned out the mixture screw 2 full turns as it says for setting them up. All float bowls are measuring the same fuel level and both sides are pulling the same air (reading 7 right now).

My problem is I can’t really set the mixture because my idle keeps moving between 1000 to 600 rpm. It will sit at 1000 and then drop to 5-600, almost sounding like it will die, then the idle climbs back up to 1000rpm. As it warms up the carbs will start spitting, to the point where the engine literally stops for a second and then continues to run. I put in new plugs (NGK BP5ES), and the plug wires are new. I took it for a drive and the car runs great on the highway, pulls nicely, that is why I thought it was the idle circuit.

I have tried adjusting the mixture screws and they didn’t have any effect. I turned them out 4 full turns. Turning them in increases the spitting. I put in 60 idle jets and that had no effect. I’m pulling my hair out as I don’t know what else to do. It has a Bosch low pressure pump putting out 750-800ml/minute.

I would appreciate any thoughts on where to go next.
Thanks

[M_deJong]

What engine? How is your ignition timing set? Vacuum advance/retard?

[gsjohnson]

Is this motor stock? What are the vacuum readings with your uni-sync on each cylinder at idle? You should see 4.0 to 4.5 on the gauge. This should raise your idle after they have been adjusted properly. Make sure they are all equal and in that range by loosening the jam nut and turning the air bleed screws. Then relax the idle screws for each carb from touching the throttle shafts. Check your vacuum with the uni-sync again and make sure the readings are still within range. If that doesn't cure the problem, I would next remove the air bleed caps, the idle jets and the air/fuel mixture screws on all 6 ports. These are all connected so spraying some carb cleaner, followed by some compressed air should clean out your idle and transition circuits without damaging your floats. Make sure that the needles on the mixture screws are in good shape. Then start the idle mixture adjustment process all over again. Hope this helps...

[Irish911]

Motor is stock 2.2. How do I check vacuum advance/retard? The timing hasn't been changed since the IDTP's were on the car. Should I recheck?

The readings with the unisync are around 7 on each cylinder at idle. If I use the idle adjusting screw to drop it to 4.5 the engine stalls just before the spitting reaches continuous. Should I be turning the air bleed screws out? Right now they are turned almost all the way in. Paul at Performance Oriented set them so I was reluctant to change his settings on these.

[gsjohnson]

Paul certainly is more knowledgeable than I in the Weber carb arena. I would suspect your vacuum readings to be higher with a stock cammed motor. Screwing the air bleeds in will lower your vacuum reading, but if Paul has already set them, I would leave them alone. You can set your timing statically if you don't have a timing light.

[edmayo]

Did you set float levels with engine running, with Weber gauge ? Are you saying Paul assembled the carbs,,,not you? Did you disconnect both drop links while you were setting the speed and mixture? You're going to need to start from the beginning, CHECK your ignition timing First! You have NO vacuum control on a 2.2 distributor, set for factory spec (right at about TDC I think, it will be on your information decal) Close all the idle air by-pass screws, now find which throat is drawing the most and bring out the other two by-passes to match the high one, now do the same for other carb. Now you have each bank with equal draw, now using the idle sped screw adjust each bank so both sides are drawing the same, and at roughly 1,000 RPM. NOW adjust each idle mixture ONE at a time by turning in to the stop and SLOWLY backing out. When you trurned in the screw the cylinder should have gone dead as you cut off the mixture, now slowly back out screw until cylinder picks up again. If you get no change that cylinder isn't working,,,,maybe dirt in jet, take out and blow clean. When you have all idle mixtures set correctly you'll find your idle speed will have changed, reset with speed screws to equalize both sides again. Do mixture adjustments AGAIN, reset idle speed as necessary. WHEN you are done then hook up drop links and adjust to snap on with out any pretension. This is all very straight forward, as long as you follow an orderly sequence.

[Irish911]

Thanks. I will check the timing tomorrow. Float levels were checked with engine running, meniscus for all at the top of the two lines shown on the gauge. When Paul replaced the throttle shafts etc he also set the idle air screws, which is why I am somewhat reluctant to tinker with them. I did disconnect both sides when checking the air volume. One side was at 10 and the other 7. I brought the one side down (using the idle screw) so all 6 cylinders were reading 7. I'll start with the timing tomorrow and report back.

[Irish911]

Timing I got was 5 deg ATDC at idle and 15-16 deg BTDC at 3000rpm.

When I recently installed the new plugs (NGK BP5ES) I set the gap at 0.35mm. I just read in one of the repair manuals that they should be gapped at 0.55mm, but that is for Bosch plugs. Do I need to widen the gap?

One thing I noticed is every few minutes I hear a soft thunk on the right bank, within the engine, which often coincides with a drop in the rpms, but not always. Don't know what it is or if it is related. Could it be one of the cylinders isn't firing properly and would that contribute/cause my idle issue?

[911s55]

First off that seems like a pretty hot plug to me, most I see use BP6ES or BP7ES. I believe the NGK site recommends the 7 and the 8 for an S model. For my carbureted 2.7 I have used both, the 6's are better for tuning, less chance of fouling.

Also that gap should be .028"-.032" or .71-.81mm

[Irish911]

Should have mentioned it at the start, I have a T, not an S. Are the BP5ES plugs OK for a T, that is what I read somewhere, if the compression was less than about 9:1, use BP5ES, and BP6ES for higher compression. Would that plug gap still apply for the BP5ES plugs?

The following link is a short video of my engine running and the problem hopefully apparent. At around 21 seconds you hear the idle drop way down, and you can also hear the thunk I refer to earlier. You can hear the thunk a few times after that. Hopefully the link works.

http://youtu.be/uAEb8Qn7Cl4

Thanks.