Engine Idling Dilemma

[jonward786]

Hey guys,

As some of you know ive got a fresh 302 in my 68, and its got 1200 miles on it after a few months of driving, runs great.  My only issue, and its a small one, is my idle speed.  Ill describe whats going on and maybe someone can provide some advice.

When the car is started cold, electric choke works properly, gets the idle up to around 1500 for quick warm ups.  After 3-5 minutes i blip the throttle and the idle in park drops down to around 900 in park which is perfect, right where i want it.  After about 30+ minutes of driving around, needless to say the engine continues to warm up even more.  so much so that when i pull back into my parking space after a drive around town and put it into park, the idle is up at around 1200, almost as if the choke is on again, which is far to high because it will diesel for a second when i turn it off .  i have looked at the carb butterfly when this happens and its fully open with the little red cam for the choke being fully disengaged, so it doesnt appear the choke is coming back on. 

I know idle speeds are supposed to be set with the car completely warmed up, but if i set it when its 100% warmed, the idle speed for those 15 or so minutes where you drive around without it being 100% warm the idle is so low, it wants to stall. 

its like i cant have both.  either i get the car to idle perfectly when 100% warm and then sacrifice idle ability during the 20-25 minute warm up phase, or the exact opposite. 

Thanks in advance for any ideas

[AzPete]

What carb and how are the idle mixture screws set?

[jonward786]

its a holley 570 street avenger, mixture screws are set to a point where idle roughens and then backed out 1/4 turn.

[Midlife]

I always thought the idle mixture screw settings are best when the car reaches the highest curb idle speed and smoothest running, with continual adjustment of the idle rpm screw to keep it in range.  This is typically 1.5 turns out for each idle mixture screw.

[AzPete]

The mixture screws should be gently screwed in to bottom out.....then back them out 1-1.5 turns. Then set for the highest vacuum from there. Make sure both screws are set the same. During this time, you may have to adjust the actual idle speed screw to keep the engine running but keeping the idle near the 700 to 900 rpm area.

[65fast]

Where do you have you electric choke wired to?

[jonward786]

its wired to one side of the fuse marked "ACCY".  it was one of the only ones that had 12v with the key on and 0V with the key off

[Starfury]

Set the idle speed and mix when the car is warm.  You're welcome to borrow my vacuum gauge if you need it.

Rotate the choke cap towards the "cold" position a bit, usually counter-clockwise.  This will keep the choke on for longer.  Then back the high-idle screw out a bit to compensate for it being on a different high-idle cam step when you first start up.

SLO sucks for choke settings.  The temp varies so much between 7am and 12am, and then one day and the next, that it makes setting the choke properly a pain.  I've been tempted to switch back to a manual choke, but I think I'll wait until I get a different carb.

[jonward786]

the funny thing is, i looked at where the choke housing was set, its already all the way to the cold side, meaning the choke should stay on for the longest.  but the little red choke cam seems to want snap down a notch within like 30 seconds, which i dont understand. 

one thing though, everyone is saying "oh your carbs idle mixture is usually good around 1.5 turns out of the screws, well, the engine wont idle at all at that setting unless the idle speed is cranked way up.  then when i put it in park it idles at over 2k haha.  the only place i can get it idling is about 3.5 turns out of the mixture screws and that gives it only about a 300 rpm jump up into park, which is still more than i want, but tolerable.

[AzPete]

If you have to have the idle mixture screws that far out, there has to be other problems. Timing or other carb problems like dirt of bad gaskets could effect things. It is all tied together...timing, idle, mixture, choke...a small adjustment on one may change another.

I always suggest at this point.....go back to basics and re-set everything. Even if you swear it is correct.....move it and make sure all it perfect...one step at a time. Once something is set correct...do not mess with it thinking something is wrong.

[jonward786]

im thinking there is dirt or something clogged in there, because ive read other places that excessive drops in rpm from park to drive indicate a lean idle and if i have a lean idle at 1.5 turns out on this mild 302 then there has to be blockage somewhere.  where to check first?

[AzPete]

Yep....It sounds like you have to have the idle up so much that you are no longer using the complete idle circuit. I would pull the carb and check for dirt in the bottom of the bowls and then blow gentle air thru the idle circuits. Also use a bit of carb cleaner. The spray can may provide enough pressure to open the blockage. The next would be complete tear down and rebuild.

[jonward786]

by idle circuits, do u think pulling the mixture screws out and spraying in there might work? or should i be spraying from a different spot?

[AzPete]

Normally, the clogged up area is not at the needles but further back in the channels. Trying a shortcut sometimes causes the plugged area to become tighter.

If you did get the blockage to move, you still need to get it out of the carb so it does not cause a repeat. Sometimes you never see the material causing the blockage since it is so small.

[jonward786]

well i pulled the bowl off and found the gasket to be pretty damaged.  maybe contributing to the problem im not sure, i figure the only purpose of this gasket is to prevent gas from leaking out of the bowl onto the intake

[AzPete]

Gaskets also seal each air passage from each fuel passage from the outside and each other......and on and on and on....

Looks like maybe a complete rebuild is in order if the gaskets are that bad. I know I would clean and rebuild if it was mine.

[jmlay]

IF an auto trans you should set idle in drive.

[Starfury]

Ya know, this is the 2nd carb you're having possible clogging issues with.  The expensive conclusion is that you could have a bunch of crap in the gas tank...

[jonward786]

Yeah im replacing the gas tank in a few days.  When i had it out last time i rinsed it out with a hose and there were these little sheet like black pieces being washed out.  no idea wtf those were. 

another thought it it needs more initial timing, because i have shitty vacuum at idle, the needle is all over the place.  im going to bump it up a couple degrees to 14 and see what happens.

[jonward786]

Well, this is strange.  somehow the initial timing was down at like 7-8.  i dont remember setting it there, and i thought the engine builder set it at 12.  but anyways, i put it up at 12 and idle quality is improved.  i no longer feel the need to shift into nuetral at stop lights to keep it from stumbling.  maybe that little bit of extra vacuum helped? who knows.

[Starfury]

Quite possible.  The low vacuum could've caused the transmission modulator to up line pressure, putting a load on the engine at idle.  Basically, the transmission thought you had the gas pedal pressed (which causes a drop in vacuum), so it compensated by trying to apply that power to the rear wheels.  But at idle you're not moving, and the rpm's are so low that you just bog the engine down and kill it.

[Starfury]

Update:

Jon came by and talked to me at work yesterday.  Said the car was still idling poor/shaky, but ran fine at higher rpm's.  Holley tech and I both said vacuum leak.  He then asked me about vacuum hose for the a/t vacuum modulator line.  Turns out, the rubber hose hooked up to the modulator is super soft and loose on the nipple, which is likely causing a vacuum leak.  After questioning, he also said he noticed an oil pool on the inside of the intake under the fitting where the modulator line connects.

I sent him off to one of our other local stores to pick up a new modulator, so hopefully he gets home ok:)

Edit: Apparently that didn't fix the issue.  He's going to take it to a local shop to get it checked out=/

[jonward786]

And the saga continues.  Since i head to my parents house for christmas, i just decided to trailer it home with a friends Yukon.  Started it up, sounded good, nice and smooth.  Let it warm up a bit then shifted into reverse and nothing happened.  Back up into park, and down into reverse again, still nothing.  Sigh, this had happened before when the pin that holds the shifted lever had fallen out, so i got under the car to see if that was the case again.  Nope, it was still there.

So i lowered the car down, started it up and put it in reverse again.  This time is shifted properly.  Cool.  So i decided to take a spin around the parking lot before loading it on the trailer just so see if it was still running well above idle speeds.  But now there are bigger issues.  Car is shifting into second and back into first and second and first and pretty much shifting however it sees fit.  I would give it throttle and it would rev up and then seemingly slam into first after a couple seconds. 

So the question is: did my torque converter just bite the dust?  or maybe the new modulator isnt playing well with the tranny?

[daveSanborn]

When you removed the modulator.... did you notice a removable "pin" within the hole?  This pin has to be inserted/aligned when installing the modulator.

[jonward786]

Yeah, when the old modulator was pulled out, the pin came out with it.  So i just kinda stuck the pin into the new one and screwed it in.  maybe the pin is not in there correctly?