[SIX8YAK]

I read a few of the recent threads that were calling to the same issue, of course I'm sure we won't have the same exact fix.

Background: Snapped the tranny modulator hose last night. Drove it a few blocks in 1st gear before I could inspect. Fixed that this morning.

I also topped off a low radiator (couple inches low) and replaced the fan clutch. Although cooler weather today, the car was rock steady at about 180 during extended periods of 50-65mph, even cooling off at times to 175ish. Before doing the fan clutch and topping the radiator, the temp would rise after coming to a stop, or after flogging the throttle for a few blocks. Prior to this, routinely over 190, touching 200 commonly.

Now, to the issue. Original year correct Rochester Q-Jet, with specified jets and other components to match up with engine work two years ago (parts spec'd via Cliff).

I had the idle set to around 600ish, and after warmup it would idle there just fine. Starting last night, I noticed it wouldn't come down to that idle at times when I came to a stop. Same thing today, after being well warmed up (30 minutes into consistent driving) it would hang around 800-900 at some lights, while at others it would come down to the correct idle. Pulled into my driveway and it stayed at 1000rpm in park.

Things I know: Small vacuum leak on passenger side base gasket, seen through the smoke test. Doesn't seem very large, but there. The choke is mechanical and the spring on top of the intake may not be holding it fully open, but that's not a professional opinion just something that seems off. The linkage doesn't seem to be binding on the throttle side, either.

Would vacuum issues be this intermittent during the same drive session?

Thanks,

Alec

[wbnapier]

My amatuer opinion would be to start by fixing the known vacuum leak, then see if the problem still exists... If you still have such an issue, check the new hose for leaks, then take it from there.

[Cliff R]

Check the ignition timing to make sure that it isn't hanging up and allowing some timing in at idle speed. I see this more times than not with inconsistent idle speed especially if any aftermarket parts (springs/weights) were installed in the distributor or if it is an aftermarket distributor.

Stock distributors also need to be taken apart and the old dried out greased removed and parts polished, new grease and re-assembled. If it's a points unit 99 times out of 100 the factory rubber stop bushing will be missing or rotten so it will also need replaced. I make them from stainless steel here and crimp them in place during the rebuild.

Another problem I see a lot with inconsistent idle speeds are the secondary throttle plates sticking or not fully closing and sealing off the bores. Even on untouched units it's a good idea to very gently loosen the screws on the secondary throttle plates and gently tap them into the bores with a screwdriver handle to make sure they are fully sealed then re-tighten the screws. About 2 in 10 will be off some allowing too much air past them and not completely closing to the same position causing idle issues.......Cliff

[ta man]

Did your idle issues happen the same time your vacuum line to the transmission modulator let go? I would be investigating that as well as fixing any minor vacuum leaks.

[SIX8YAK]

Thanks guys. I just ordered an HEI for it so ill be able to make sure everything with the distributor is functioning as it should shortly, obviously it will be getting cleaned and looked at before installation.

Noted on the secondary fit, thanks Cliff.

TA, I want to say yes but I do believe there were a few sporadic times before that. But now it's much more pronounced, if I can even get it to idle normal at all.

[Cliff R]

So this morning a customer comes in with a recently purchased 1972 Chevelle, 406 small block, Edelbrock carb/intake (probably sits on 90 percent of the ones I've seen in recent years). It doesn't do anything right and he's completely fed up with the carb. Can't get it to tune at all, engine runs-on when you shut it down, idle inconsistent, etc, etc.

He expects me to amputate the carb and fix all his issues, instead a throw a timing light on it, says 16 initial, rev it up and it moves to about 22 degrees all-in. I remove the distributor cap and the stock weights/spring are still there (unexpected but a good thing) but cheap POS silver springs from a distributor "kit". I removed the springs and installed much stronger factory type HEI springs on it, one of my 14 degree VA cans (hooked up to ported vacuum) and fire it back up. Timing is now at -4 degrees at idle speed. I move it to 12 degrees and it advances well off the scale now starting right at 1000rpm's and all-in around 2800 or so.

Idle is now steady and smooth and returns to the same RPM each time it comes back to idle. With the new found timing it also makes a butt-load more power now, and the owner is happy and we didn't even mess with the carb.........Cliff

[SIX8YAK]

Cliff, I think the carb needs a little tuning anyway based on how rich it's running at the moment (I dont have an AFR guage) and it also spit some fuel into the air filter yesterday after trying to start it.

I do believe the HEI I'm getting will allow me to fine tune the other side of the equation, being as the timing seemed pretty good as it was but I i havent double checked the dwell.

Cliff do you have records of what carb kits you've sent out? I dont want the carb guy changing the upgraded parts you specified for me back in 2018.

[Cliff R]

I should have records in Quick Books of any sales made back that far.....