Worshiping False Idles

[kcirtap1984]

Hi guys,

I have been searching for the answer to my problem for months, and I keep coming up short, so here it is:

I bought my '79 a few months ago. It had an old ADHA carb, with a dual plane manifold. The carb leaked like a sieve, and and ran like crap. Following the advice of everyone everywhere, I decided to go for a new carb instead of putting any time/money into the old ADHA. I purchased a new DFEV and a single plane manifold, installed it right away and removed every smog related item. It runs a million times better, and there was a huge performance gain, but I am having trouble getting the engine to idle below 1200rpm. I followed the initial setup procedure to the letter. During the setup procedure, you are supposed to back the idle screw out completely, and the engine should run rough. If I back the idle screw out completely, it will not run at all. The idle screw needs to be in about 1.5 turns in order for it to idle at all. Once it is idling, (at 1500rpm), I can adjust the idle air mixture screw accordingly. It sounds great, no misses, revs fine and returns to idle quickly. But if I turn the idle screw down, the engine will slowly lower from 1500 down to 1200...then 1199, then it shuts off. It doesn't stumble and bang, then stall...it just shuts off.ccAlmost like I'm turning the ignition off.

I have checked for vacuum leaks ad nauseam...nothing. I went through 3 cans of carb cleaner trying to find a leak. There is no vacuum leak. Has anyone ever encountered this issue? Am I crazy to think that this thing should be able to idle at 900rpm? Any input is appreciated.

Thanks!

[davidbruce]

That carb should idle at 900 RPM. I had a similar problem. I purchased a new Empi DFEV from a vendor, installed on a 1800 manifold that wouldn't idle well. I took the top off and found brass and metal particles in the bowl. I have heard of this on genuine Webers as well. Maybe pull the cover off and see if there is contamination in the bowl. Or try pulling the idle jets and see if any contamination is apparent. You say you have no vacuum leaks but do you have decent vacuum at idle? Is ignition timing correct?

[Slartibartfaster]

I found several QC issues with my DFEV out of the box, though it was weber not empi. Here's some of what I found in the bowls...oh and the little filter thing was misshaped

Things got really bad when the secondary idle jet fell out at speed...learned a bit about carbs with that one.

[kmoses]

You also need to confirm that you tachometer reading is correct, mine is 150 rpm off. When mine shows 1000 rpm I am actually at 850 rpm.

[kcirtap1984]

That carb should idle at 900 RPM. I had a similar problem. I purchased a new Empi DFEV from a vendor, installed on a 1800 manifold that wouldn't idle well. I took the top off and found brass and metal particles in the bowl. I have heard of this on genuine Webers as well. Maybe pull the cover off and see if there is contamination in the bowl. Or try pulling the idle jets and see if any contamination is apparent. You say you have no vacuum leaks but do you have decent vacuum at idle? Is ignition timing correct?

Thanks. I'll take it apart and see if something's in there. I haven't checked to see if I have good vacuum at idle. I'll try that too. Do you know what the range should be?

[davidbruce]

I believe vacuum should be in the 15-20 " range with little fluctuation at idle. Ignition and valve timing will affect this as will worn valves etc. While you may not have a vacuum leak at the carb or manifold, it could be at one of the vacuum operated devices like a ruptured vacuum advance on the distributor. Make sure the engine is in a good state of tune. Correct ignition timing, cam timing etc. If you find debris in the bowl you could have a plugged idle circuit . You may have Sen this but here's a troubleshooting chart that may help.
http://www.carburetion.com/troubleshoot.htm
Hope this helps.

[kcirtap1984]

Thanks for the help everyone. I’m still having trouble. I checked the vacuum below the carb. It’s at 17 Hg/in at idle and it drops to 0 when I blip the throttle and returns to 17. The needle is pretty steady. The A/F mixture is as good as I can get it. The bowl is clean and the timing is good. I though about the tach not being calibrated, but it’s pretty obvious that its idling high. What stands out to me is that the motor doesn’t stumble as I’m lowering the idle. It’s running great, until it gets below 1200rpm, then it just shuts down. It’s like I turned the key off. Sometimes, it does a puff back out of the carb after it stalls, like it’s igniting the fuel that’s still being delivered at idle. Any other suggestions?

[baltobernie]

This is a long shot, but have you temporarily detached and plugged the brake booster line?

[kcirtap1984]

I have disconnected it. Not so temporarily. It was leaking pretty bad when I bought the car. I detached it, plugged the manifold and ran it without the booster. I actually like it better. The pedal was real sloshy with the booster hooked up.

[blazingspider]

Sounds like you are idling on the primary main jet rather than the idle jet. Have you pulled the idle jet to see if it's plugged? Check the passages for it too.

[baltobernie]

Are you getting 17" of vacuum at the "Vacuum Advance Port" shown on this photo?



If so, you are indeed idling on the main circuit, not the idle circuit. You should have zero vacuum at this port, located below the butterflies, at idle. If the butterflies are open at all, you'll see holes of the Progression Circuit (red arrow) exposed.



First thing to check when installing a new (or old) carb is the ability of the linkage to allow the butterflies to close completely.

[kcirtap1984]

Thank you to everyone who replied. This has been haunting me for months. After going through the carb and intake looking for vacuum leaks, I kept coming up with nothing. I opened the carb up to make sure nothing was clogged with debris. It was clean. I couldn’t help but think that the fact that the car would shut off at a certain rpm was suspicious. Especially because it sounded great right up to the point that it shut off. I kept coming back to ignition. Since I recently replaced the cap, rotor, wires and plugs, I suspected the coil might be the culprit. I know that there are a lot of posts that say that coils don’t go bad, and everyone I talked to kept telling me it was air/fuel related, but I figured it was worth the $50 just to rule it out. I order an Accel 8145 and popped it in. The car idled at 850rpm. What a difference. I adjusted the carb to run well at its new idle rpm and its been running great since. I guess sometimes you just have to trust your gut.

[zachmac]

Glad you found the issue. Also just wanted to compliment your clever tag line.