Intermittent low speed idle

[Argess]

I've got my engine running quite well now, cept for one little problem. Sometimes when I get off the throttle, my idle drops too far and seems to stay there. I gentle tap of the gas pedal clears it and then it's fine.

I've seen dashpots to slow down the carb return to idle position before, but not on a Holley, which is what I am using.

My idle mixture screws are set for max vacuum, so I'm unclear as to how the engine seems to be loading up with fuel when I get off the gas.

To complicate matters, my dist is actually a bit advanced at idle, so I'm wondering if that may be part of the issue, but seeing as how a quick stab of the throttle clears it, I am leaning towards a fuel issue.

I have one thought so far. Float bowls flooding under acceleration. Fuel flows to back of bowl, floats drop a bit, fuel flows into bowls, and when I let off the gas and car stops accelerating, resultant fuel level is too high and idle is rich. If this is the case, I'm not sure what to do about it as lowering the float level may cause other problems.

The problem does seem more common when I have accelerated up a hill, but I need to re-check that. And now that I think about it, I'm not sure, but I may have had this problem on and off before, but not as bad or as often. However engine is up on power (due to tuning) than it ever used to be, so car is probably a bit faster.

Oh...it's a twin Holley set-up.

Any thoughts?

[Barnsnake]

As I remember, you have your centrifugal advance starting below your idle speed. With that arrangement the motor will be challenged to find a consistent idle speed. Any little thing that drops the speed lower than normal will retard the timing and keep the motor from coming back to "normal". You may be spot-on with the temporary mixture issues and the timing retard prevents the engine from recovering on its own.

You might be able to reproduce the issue with a timing light attached. Do something to make the engine stumble (maybe choke the engine a bit with a rag or the choke butterfly) and see if the timing stays low and the engine fails to recover on its own. If so, blip the throttle and see if the timing is more advanced when the engine returns to idle.

Keep us posted.

[Argess]

Yes, I agree.....the dist should not advance until rpms exceed idle speed, but mine does. I should order a Mallory kit I guess. I do have some other springs...generic, but they seem to all be on the stiff side, so it may fix the idle, but then keep the advance rate low. Of course, it could prove wether or not the dist is the problem or not, even if it doesn't work quite right to begine with.

However, it only occurs after a hard WOT run. And even then, not always...which may be due to how hard the run was, or wether it was up a hill or not. Testing up a hill is always a good idea with these light cars.....chuckle.

Oh well, more tinkering and testing tomorrow...if the crappy weather gets better. I swear, every time somebody buys a convertible, it rains....LOL.

Thank-you for your input. The dist wasn't my priority to start troubleshooting, but you point is well taken and it's fairly easy to test.

[vector1]

if you have a wideband it will tell you pretty easily what is going on, mine would show lean with similiar characteristics as yours--letting off the gas from above idle say at cruise the rpm's would drop and the o2 would go lean and and the idle rpm would drop below what it was set at, i don't know exactly the cause but here is what i did. try richening the idle, it doesn't take much, just a slot width of all the adjusting screws and see what happens, if better try a little more, not much. next would be to set the idle up slightly. if it is dying under braking it is probably going rich, in that case drop the rear floats an adjuster flat at a time till it goes away.

[Argess]

Thank-you. I'll try your suggestions short of the wideband monitor as I don't ahve one.

These carbs are a bit different in that they have an externally mounted primary float bowl vent that is operated via a lever from the throttle bellcrank. I tired to find a net pic and provide the link, but no luck.

As the carbs are mounted backwards in a 2x4 situation, the primary bowls face the firewall, so these vents are on the rear floatbowls. I notice that there are signs of evaporated fuel around these vents. So if that's due to fuel slosh under hard acceleration, where does the extra fuel go from the front (secondary) float bowls? Out the float vent and into the secondary venturis?

I see Holley has some baffles, screens, whistles etc to help fix float bowl flooding, but they are mostly designed to work with the carb facing forward, not backwards like in the Ford 2x4 systems.

Anyway, no tinkering yet today. Must try a few things and see how many facts I can get to help figure this thing out. I'll try re-checking dist for sticking, richening the idle, waiting to see if the low idle fixes itself eventually, try lowering the float levels a hair, and whatever else I can think of.

Thank-you.

[Argess]

I managed 1 test today. Got the low idle after letting off the gas, and waited. After about 10 to 15 seconds (I'm guessing, but at least that long) the idle slowly crept up to normal. I would think lean would kill it, so I'm still thinking momentary rich after return to idle.

[Argess]

Keeping in mind the twin Holleys are mounted backwards, I checked the inside of the carbs and found evidence of evaporated gasoline around the air bleeds, directly below the vent tube for the secondary float bowl (which faces forward).

I must put a few drops of gas down those air-bleeds when the enigne is idling and see if I get the same low-idle symptoms.

My float vents all point up, but i've seen pics of some Holleys where a single tube connects both air-bleed ports and has vent hole drilled along the top of the arc of the connecting tube. Seems like a good idea, other than it could mess up the airflow a bit.

[Bobcat]

Argess... go to the site ... 4secondsflat.com and look in the technical section . There is a short video on setting up a Demon carb ... but the basics apply , and also a great section on reading your plugs and what to do about various problems . I got his tuning guide ( $20 US ) and one "tip" when adjusting your carb is don`t touch the throttle while adjusting it . If it starts to drop off , tap the accelerator primary pump arm and see what the engine does . If it drops rpm more , you are too rich .... turn the screws in . If it speeds up , you are too lean . Repeat process for the secondary .
I also got his bushing kit for my MSD distributor and installed it last night ( rich at idle ) and then bumped my initial timing to 24 degrees (900 rpm ) while keeping the max at 36 degrees and it really cleaned up my idle . Next is to try the next bushing ( 10 degree ) and go to 26 degrees on initial and still a max of 36 degrees .
The potential downside is possible run on if the idle is too high and listen for ping as you start off ... which I haven`t experienced yet . The other is it may be hard to turn the engine over if there is too much timing , but he covers how to handle that also .
I`m an old racer and thought I knew Holley carbs , but he burst my bubble .
His biggest point was to get the ignition correct before attempting to adjust the carb .

Bob

[Argess]

Actually, my Cobra has worked great for years, but I had a slipped balancer, which may have continued to slip a bit here and there. Of course every time I reset the timing, things got worse and worse and detonation finally took out a piston and now with my latest rebuild, I have to start over. And with no history of what worked, and what didn't as far as igniton is concerned.

However, through experimentation, I found that with a mech advance only carb, the engine runs very hot with the timing down around 10 or 12 deg like Ford specs. With my cam, the Barry Grant advice is an intial of 16-18. I am currently running 20 intial with 38 total and it is fine. However, I caution you.... I know when I had 28 intial, I was getting some knocking. I couldn't hear it, but I could feel it. It just feels like the engine doesn't want to rev freely, and if you go too far, you can really feel the engine fighting the rpms.

However, I feel there is a little mroe room for advance at idle as I did have better reponse before, and the reponse keeps improving the more advance I dial in.

Cleaning up the idle? I tried to find what advance gave me the best vacuum at idle. It was 38 Deg BTDC. But it will knock there under load until the revs get up. If I could fit a vacuum dist on my engine, I would.

The best thing would be to start quite high intial for a good idle, good response, cool idle, and then have the timing, advance more slowly than normal. This way, you get back on track (as the rpms build) to the best timing profile where you want more power (with a standard tranny).

It's been a compromise. Hot engine with retarded advance, and uncertain about detonation with too high an advance..

I tend to think you can get away with a high intial as you won't develop full HP at lower rpms in such a light car, plus who ever starts accelerating at 1000 rpm? Probably have it from 1500 to 2000 just to pull away from a stop sign.

Someone in this forum gave me this link, it's seems quite interesting:

http://www.kitcarmag.com/techarticle...ine/index.html

I will check out the website you mentioned too. Thank-you.

I'm pretty sure I am down to carburation for my low idel after a WOT run. I discovered that if I leave the low idle, it will come back up after about 15 seconds, like it was temporarily swamped with too much fuel. I also just noticed if I brake hard, the engine rpm will also drop (all this with clutch disengaged, of course), but comes back almost immediately, as if it were starved for fuel, vs too much. None of it is a big deal, but I like tinkering....er....most of the time.

[Bobcat]

Argess ... on my idle , I also went to the strongest springs on my mechanical advance so it wouldn`t come in too quickly ... but be all in by 2500 to 2700 rpm . Interesting what you said about 38 degrees giving you the best idle as my engine builder just dynoed a 482 with tunnel port heads and had the distributor locked at 37 degrees . He made over 700 hp and said he had a smooth idle at 700 rpm .... which surprised him .
One other point , I`ve been told that a smaller spark plug gap ( .035 to .038 " ) will give a smoother idle than a large gap .
You mentioned engine rpm dropping under hard braking ... you might try running a rubber line from the front vent to the rear vent of the carb(s) , but drill about 6 to 8 holes in the top of the line at the high point .

Have fun .

Bob

[Argess]

Originally Posted by Bobcat

You mentioned engine rpm dropping under hard braking ... you might try running a rubber line from the front vent to the rear vent of the carb(s) , but drill about 6 to 8 holes in the top of the line at the high point .

LOL...I was thinking of doing exactly that, although with my luck lately, the rubber hose will come off and dissapear inside the manifold.

Originally Posted by Bobcat

Interesting what you said about 38 degrees giving you the best idle as my engine builder just dynoed a 482 with tunnel port heads and had the distributor locked at 37 degrees . He made over 700 hp and said he had a smooth idle at 700 rpm .... which surprised him .

If I were to lock the dist at 38, I'd be darned sure to get the rpms up over 3000 before I loaded the engine by letting out the clutch. But it sure idles nice there. I've never ahd cranking troubles, even with very advanced timing, but with a locked dist, one might have to have a seperate ignition switch and turn on the igntion after the enigne is already cranking.

Originally Posted by Bobcat

Have fun .

Bob

Thanks Bob, I am. Building the car was the most fun, driving it is OK, but tinkering and tuning is better (although sometimes frustrating....LOL). The satisfaction from success is darn worth having a beer over !