carb difficulty, could someone help please

[Buzzmobile]

Ok, let me first state that I am NO carb mechanic...and that I have 2 of them on my cobra Here is what is happening. When I am driving, I come to idle and it settles down in a nice idle for maybe 30 seconds. Then, it drops to a second, lower RPM idle, which nearly causes the car to stall. If the car is all the way warmed up, it will still idle at this lower RPM barely, but if not totally warmed up...I have to ride the gas a little at lights. I have checked the linkage over pretty well and have good springs to return the mechanism to its idle position, so I do not think it is the linkage. It seems like the car settles on the first idle with fuel from one part of the carb and when that runs out, it starts using a different circuit....if that makes sense.

Anyway, any help is greatly appreciated. I am also trying to dial in the idle mixture, which I think is getting pretty close to where it needs to be. Could that be part of this problem. Maybe a step by step instruction to set all this would be good. I plan at some point this year, to take it to a chassis dyno and have it really dialed in, but right now, I need other things for the car more...so that will have to wait


thanks again
Buzz

[392cobra]

I had the same problem a couple of times.

It turned out to be the centrifugal advance.It slighly hung up and then would come free losing a few degrees of advance,idle would then lower.

[rdorman]

The advance is one this to check. The other common cause is an idle curcuit that is not set up correctly. You didn't say what kind of carb you have. Do you have a off idle or tip in hesitation?

[Buzzmobile]

the carbs are both holley with vac secondaries. I am not sure the model numbers. I do not have any off idle hesitation that I feel. I have an MSD with mech distributor with about 16 degrees initial and 36 total, all in about 3K RPM per keith craft. I have been struggling with this problem for a while. what is the procedure for checking the idle curcuit? thanks guys...really appreciate this

Buzz

[Buzzmobile]

Just one more thing...I used to have an off idle hesitation, but that went away after I turned the idle mixture screws to a little more rich. But idle curcuit sounds like something that is probably responsible for what I am feeling because everything I am experiencing is either very low cruising speed or at idle.

[Clois Harlan]

Take it out and drive it NOW. It's beautiful outside. Someone should be enjoying this day that doesn't have to work today. Put it on the freeway for a few minutes at a little more than speed and I bet it all goes away. Maybe drive over to the speed shop and buy some 110 octane (take a can with you-you know the laws).

Clois

[Buzzmobile]

I am about to drive it (jobless has its advantages!), but it has been doing this since I got it back a couple summers ago, so I don't think romping on it will make it go away....but I will still keep trying that (smiles).

BTW, there are pics of the carb in my gallery if that helps determine what they are.

Buzz

[cosbytnkid]

Holley's are a good carb. I had a 780 CFM on a 390 powered '67 mustang. From what I'm reading here I would definately check mechanical advance under the plate the points ride on and make sure the weights aren't hanging up. I used to take a shot of carb cleaner in a spray can to clean 'em up. Every so often I would take the sight plug off on side of the fuel bowl. First I would check to see if fuel level was at bottom of the hole making sure my fuel level was OK. Then using same carb cleaner with spray nozzle shoot a goodly supply into fuel bowl and hurry to replace sight plug before I lost everything leaking down side of fuel bowl. I would then set the engine on high idle manually about 1900 RPM or so and just let the car run for about 5-10 minutes this way. What I was doing was letting carb cleaner mix with gas and running it through the carb to help keep it cleaned out. Also do you have a filter between fuel pump and carbureator? If not get one installed. Good insurance against a slug of dirt getting into your carb. I don't know about the newer Holleys but the older ones had your mixture screw on each side. What I did was using 1/2 turn increments turn the adjusting screw in leaning out the mix until the motor started pulling down. Noting the position of screw, like at 12:00, 3:00 oclock position I would using 1/2 turns start backing the screw out until the engine pulled down again. Taking toltal of 1/2 turns I would divide by 2 and set my mixture screw in that position so I was exactly half way between it's leanest setting and carbs richest setting and the engine usually ran pretty well idling. But that was from a long time ago. I don't know if things have changed in the last 30 years or not. Also I'd get my hot idle screw set to where the engine seemed to idle at the smoothest. I hope this might help you out.

[rdorman]

Do you have a surge at low speed cruise? Was is your hot idle vacuum reading? How far out are the idle mixture screws? Do the idle mixture screws seem to have a lot of effect on the idle quality or very little?

[rdorman]

Oh... and clean the fast idle linkage (if any) and the secondary throttle plate. Also check that the 'closer link', for lack of a better word, is bent just right so that is touching the slot and fully closes the secondaries. Sometimes the springs are really like in vacuum secondaries and that combined with a little dirt or interference will hang them a bit. Eleminate a linkage problem by disconnecting it and rev the engine a bit manually and see if it returns to the proper idle. You can also manually 'rev' the secondaries at the same time and note the results.

[c6ae]

Your description sounds like the engine may be running slightly rich at idle. I would first rule out timing changes using a timing light (a broken advance weight spring can cause erratic timing at idle) and then I would suspect a slightly leaking float valve (common) or sinking float. Renew the float valves and adjust them very carefully. This can be done externally with the carbs in place.

[SuperHart]

Perhaps some of this is reiteration but here goes: there are usually two possible problems to check out with the situation you describe assuming that your distributor has a vacuum advance unit. First, as previously mentioned, a sticky mechanical advance system could be the culprit. Rather than disassemble the distributor to check it out, check the vacuum advance module first. Unplug the hose to the vacuum module from the carb and pulg the hose. Check and adjust the idle speed to around 800 RPM. The plug the vacuum advance back in. The idle speed should increase maybe 200-500 RPMs and stay there. Try recreating the scenario you describe byt giving the motor some throttle and then returning to idle. A faulty or sticking vacuum module might initially hold vacuum keeping the idle high but lose it due to a faulty diaphragm and cause the idle speed to drop. Replacing the vacuum module will correct such a probolem. The purpose of the vacuum module is to provide advance at idle which supposedly reduces oxides of nitrogen emissions and helps the motor to idle cooler

If the vacuum module checks out OK, the it's time for the timing light to check the mechanical advance. Connect your timing light and disconnect the vacuum module and plug the hose. Try to recreate the scenario you describe about the dropping idle. You should already know what your initial timing is (10-12* approximately). If you seem to have more timing than that when you first get to idle. and the timing decreases back to initial setting when the idle drops low then you probably have an advance weight that is sticking or a faulty mechanical advance spring. Either problem is easy to correct but will require partial disassembly of the distributor, maybe even while the unit remains in the car.

[RedCSX1]

I am experiencing the very same issue. I set the idle after its warmed up so the problem I have is the idle takes a while to drop down from 1800 to 1100 rpm. Had the MSD checked and confirmed that its hanging up. I have moved to a stronger spring in the curve. THis past weekend I replaced the throttle base plate on the Holley 830 DP with a quick fuel billet base plate. Still need to dial in the carb.

[CHANMADD]

I have had similar problems,but I am sure that they are mostly camshaft related.If you are running a pretty wild cam its going to run rich at idle because of the overlap.A large cam and a low idle don't go.My car has an iski Z35, 274 at .050 with .575 lift and won't idle under 1000 rpm

[Tommy]

Buzz -
Here's a good web site on "How to tune your Holley carburetor":

[Buzzmobile]

thank you all, very appreciated. I think I may have a look at some stuff this weekend and get back with you guys. I am learning a lot everyday I hang out with you guys

Buzz

[Rick Parker]

Morgan hit on a good point. If the flyweights in the distributor are retained by the very light springs (MSD's are the small diameter wire, silver) they will be slow (weak) in retracting the flyweights and retarding the timing at lower RPM. They will allow a faster advance rate but the flip side is the timing "Hunts" (fluctuates) and consequently the RPMS are unstable at idle.

[cosbytnkid]

Hi Buzzmobile,
I was just wondering how you came out with getting your engine to settle down at lower RPM's? There was a lot of good information out there and I'm just curious to see if anything help resolve your problems.

[Buzzmobile]

just after the time writing that stuff, the weather turned cooler, so have yet to really check it out. I can say that I did spray a little carb cleaner on the advance mech of the distr and it seemed to help. The one thing I noticed, it still does it when it is not FULLY warmed up....but when the oil and the water temp are all the way warmed up, this behavior stops. does that help with any clues?


Buzz

[Dominik]

good morning,

I had similar hickups with a wild cam and the power valve intermittantly open causing the mixture to run rich at idle.
It might even be a blown power valve, but the mechanical advance would be my first bet.

Do you also run vacuum advance?

Dominik