Unstable idle in my '66 BB
 

My stock '66 427/390 starts poorly and stumbles off the line, even with light acceleration. This is true in both first and reverse, warming it up only helps a little. Taking off from a stop light, esp. with the AC on, also results in a stumble, hesitation or death. Once you're over 2,000 RPMs, the car seems to run well. However, when you get to a light, esp. with the AC on, the car won’t hold it’s idle and occasionally dies, or stumbles when you try to rev it up. It’s difficult to set the idle and have it remain consistent.

Specifics:

1) The carb is only 2 years old. I took it apart, and blew it with compressed air out about 3 months ago. The accelerator pump appears to have a good “pump shot”. The choke isn’t set right, but I don’t think that’s the major problem. I feel sure that the idle mix is correct. I don’t think this is a “fuel” problem.

2) The distributor was professionally rebuilt a year ago. I set the base timing at 4° BTDC, which is what is supposed to be. The advance mechanism works properly. I also don’t think this is
“spark” problem.

3) Thinking that this sounded like a vacuum leak, I used a propane torch to find one, but found nothing. Tried twice.

4) Most importantly, placing a vacuum gauge at either of two different “full manifold vacuum” sites gives a widely and wildly fluctuating reading. (Actually, you can’t read it!). So, the carb must be getting a very erratic vacuum signal and that’s why it runs crappy, I think. ??

5) I don’t think the car uses much oil and I don’t see any blowing. Nothing obvious, anyhow.

6) I took the car to a professional mechanic for a compression test and it was fine. He thinks this a carb problem.

What next?

Thanx again.


steve

Steve, do you have a Mity-Vac handy? If so, maybe hook it up to the vacuum advance cannister to see if the diaphragm is leaking by. They don't give up the ghost that frequently but I've had a few that leaked including one on a relatively low mile Mallory distributor. Long shot but easy to verify.
Mike T.

Yes, if you apply vac vis-à-vis the Mity-Vac, the RPMs do and timing both move up. So I think the can is OK, and, it was checked last year, but could've gone South since. It's the wild vac reading that are making me nuts. s

check out this link:

http://www.secondchancegarage.com/public/186.cfm

can you narrow down the problem from the way the vac gauge is showing your vacuum is acting. Could possibly be problems in the valvetrain.

Normally an indication of leaky valves but your "professional mechanic for a compression test and it was fine" statement tends to dismiss that.

Really, really worn valve guides will also make a vac guage dither wildly and give a good compression test read.

That link is excellent and a couple of those examples look like my Mity-Vac in it's spastic tremulous state. I'll bet it's in the valve train. Thanx again. s

Steve, I do think a cracked or split vacuum cannister being the cause was a long shot but instead of seeing if the vacuum pod moves the point plate of the distributor, I was suggesting you check the pod out by itself with the engine off because with the engine running ragged like it is now, it's kinda hard to isolate the cannister as the culprit and even a cracked diaphragm will exert some pull on the point plate. Most times I ran across a bad vac pod, it wouldnt' hold vacuum from the second you applied a vacuum pull on it but I have run across some that seemed to hold until I got up around the mid-teens for vacuum. I assumed the diaphragm crack was very small and didn't show with just a few easy pulls on the Mity-Vac.
Mike T.

Could those erratic vac readings be caused purely by a carb issue? The mechanic who looked it over thinks it's all a carb issue. thanx s

Did you check the metering block gaskets to see if they had shrunk after the extended non-use?

Mike, I personally replaced all the gaskets about 2 months ago and nothing changed. Cleaned the carb as well, etc. See my other post about "funky noise in my '66 BB". I think the problem maybe more serious. Thanx s

Do you have a dial-back timing light to check to see what your total intial and mechanical advance come up to when all in? I guess they ran alright back in the day but with today's gas I don't know how anything runs well with just 4 deg initial timing. I have to wonder if it wouldn't run a lot better with 8 or 10 deg initial advance if that wouldn't cause it to exceed 36 to 37 degrees total.

Also, did you put a rebuild kit in the carb? It may just need a complete rebuild and setting of all adjustments.

yep, I have a dial back light and the timing is set a 4° and all in at about 36. The carb was brand new 2 years ago, off the shelf, correct Holley 3370. I went through it 2 months ago and replaced all of the gaskets, blew out every orifice i could (hmmm...that sounds very provocative, eh?). Confirmed that the shooter is good and she runs great after about 1500-2K rpms. thanx for your help. s

I think it would help if you could put a bushing on the mechanical advance stop pin and limit it some so you can boost your initial timing up a little. Also, I don't think anyone asked if you have your vacuum canister hooked up to ported vacuum or full vacuum. If hooked up to ported vacuum, changing this will probably help. With a lot of Holley carbs the easiest way to accomplish this is to put a "T" in the vacuum line to the choke pull off and hook the distributor vacuum can to the other leg. this made a significant improvement in my engine's idle and low speed driveability. Good luck.

Dan, I've got the vac advance hooked to full manifold vacuum, so that's OK. I put a "service" Holley on the car yesterday and will check that out tomorrow...maybe it's the carb, like the mechanic thinks. However, there's no question about something being bad wrong on cylinder #4 (bad rod bearing, loose connectring rod, broken piston skirt, etc.). So I think i'm gonna pull the engine and fix that soon. thanx for your thoughts and help. s

Idle problems
 

I kept having idling issues with my 427/390 and this was the problem.. Stuff was getting in the carburetor from the gas tank.

The small gas filter in the carburetor was not effective.

We installed a gas filter in the line near the gas tank, and voila, the problem was fixed.. The gas quit dripping into the carburetor and flooding it..

I had been putting up with this for 4 years... Next I will replace the gas tank...

Cheers, Zale

good idea. thanx s

Steve I am having the exact same problem as what you posted on the forum several years ago. If you see this, would you drop me a message on what you found. I also have a BB with wide and wildly fluctuating vacuum and it's messing my idle. My center carb is NEW, distributor is totally rebuilt, canister is the low vacuum unit from NAPA and I've checked for leaks with propane twice. My dry compression test show all cylinders at the 230 level and all withing 5% or less deviation. Crapola problem. :cheers:

I'm so sorry I never posted the denouement! I never related a weird knocking noise I had with that car until one day it hit me that this noise I'd been hearing for a very, very long time might be important. You could only hear it at idle and it seemed to be coming from the left bank. I eventually found that the stupid noise went away when I pulled the #6 plug wire! I knew I was on to something then. It was 100% reproducible, so I knew something was bad wrong. Pulled the engine expecting a broken piston wrist, but it turned out that I had a broken
ring. Can't remember now which one it was. Complete rebuild with all new stuff and now the car runs and sounds great. Compression testing didn't bring home the bacon. Not sure if a leakdown would have or not.

I had a somewhat similar problem in my '67 SB 'Vette. Car would always, always hesistate from a dead start. New carb, accelerator pump, etc, etc were to no avail. Ultimately pulled the engine...two cylinder walls were cracked! Replacement block (argghhh!) and problem solved. These cars are so powerful that they'll run pretty damn well even when things are terrible. I still have both cars and they run great. In fact, I'm driving the '67 SB roadster to have breakfast with my son in the AM. steve