SJW

Hi all,

Background:

Girlfriend's 1981 Vette. We've been dating ~6 months, four of which the car was sitting in a body shop, so I've spent relatively little time with this car so far.

Automatic trans (rebuilt original trans, I believe). Engine has been recently (~2 years, but not a lot of miles ago) rebuilt to non-stock specs. Has more-aggressive-than-stock cam but nothing extremely radical (actual PN and spec are not yet known to me). Emissions equipment is limited to PCV valve. No EGR, no AIR, no cats, etc. Car has headers, dumping into a Y-pipe. Edelbrock Performer intake, I'm pretty sure. Electronic ignition distributor (stock HEI, I think, but not certain).

Has a Holley carb -- I don't yet have the LIST number, but I think it's gonna turn out to be a 4160 0-80457S. I'm told it's a 600 CFM, and it definitely does have an electric choke on it. This carburetor is nearly new. It was installed (by somebody else) a few weeks ago, and has since seen only a few hundred miles of service. I'm reasonably certain that a fresh in-line filter was installed between the pump and carb at this time.

Cooling system seems to function quite well. Temps hover a bit below 200*F, if the gauge on the IP is to be believed, and I've seen nothing that leads me to doubt it.

After the carb was installed it seemed to run well for a few hundred miles under all conditions, but began to stumble a bit at idle/off-idle a coupla weeks ago while I was driving it (engine was fully warmed up), but cleared up at some point. When it was acting up, I bumped it into neutral and fed it a bit more throttle to keep it going. Nothing severe, but definitely not right, either.

Last weekend, while we were on a two-day cruise with our club, it began to misbehave much worse at nearly the end of the trip (again, engine fully warmed), and she had to fight it a bit to keep it alive at a stoplight. It wasn't right pulling away from the stoplight, and stayed not-right for the remaining few miles to her house.

During this most recent episode, I was following in my car, and it was quite apparent that this was not an over-rich condition. I tasted no exhaust, and saw not a trace of black from the pipes. Although I'm no longer a professional wrench, I was in a "previous life", and have continued to shade-tree wrench during the past 25 years. Based upon its behavior, I'm pretty confident that this was not an ignition-related failure. I'm nearly dead-certain it was a lean/fuel-delivery related misfire. I can say with certainty that it was not a dead miss such as would be caused by a dropped plug or wire. I had the cap/rotor off a month or so ago, and those looked fine at the time.

At the end of that trip, my girlfriend was disgusted/hot/tired, I had no tools with me, and we made no attempt to diagnose the problem at that time, beyond me having her re-start the car in the parking lot at her place while I stood behind, re-checking for any trace of an over-rich condition (again, I saw nothing to suggest such was the case). We didn't lift the hood.

I don't think this is being caused by an obstructed fuel filter, as it seems worse at idle/low-load than under heavy load, and I believe the filter is almost new.

She's asked me to try to get it straightened out for her this coming weekend. My experience with Holley carbs has been pretty much limited to float level and idle mixture adjustments, but I'm no stranger to carbs in general, having extensive rebuild experience with, and a solid understanding of Rochester 2G series & Q-Jets, Carter AFBs, and lesser experience with assorted other carbs on vehicles and small engines. I've been boning up on the basics about Holleys today, with a crash course of digesting what I can find on the web about them.

My plan, as of now, is to proceed as follows:

1. Verify timing, mechanical/vacuum advance, etc just to make sure nothing's getting goofy with the advance at idle/off-idle, and to see if #1 is firing consistently.

2. Verify spark quality with a spark tester.

3. Check fuel pump pressure.

4. Check fuel level/float settings.

5. Monitor manifold vacuum while doing all that follows.

6. Check PCV valve for obvious problems.

7. Look for any vacuum leaks.

8. Adjust idle A/F mixture.

9. Dip a stick with Water-Finder on it into the fuel tank.

10. Wing it from there.

My girlfriend seems to think there's a good chance that the fuel tank may be providing more than good clean fuel to the carb. That wouldn't be my first thought on the matter, but it should be checked into. With a fresh filter in-line, I'm doubtful that foreign matter has gotten all the way to the carb so soon. but I suppose it shouldn't be dismissed without at least a cursory check. (I've posted another thread asking for advice about how best to deal with tank sedimentation problems).

If the problem's in the carb and can't be cured by tweaking the idle A/F mixture and float levels as required it seems to me that it's likely to be an obstruction that has developed in the idle and/or transfer circuit(s). Any advice about how to deal with such a fault would be appreciated.

I'm reluctant to dive too deeply into this carb right away, because it may turn out that the most logical course of action here is to get it back to the folks that installed/tweaked this carb a few weeks back, and as we all know, no mechanic wants to have anybody else monkeying around too much with a come-back job before it returns. Having done my time in the business, I'm keenly appreciative of this, and will be as respectful as possible.

The goal at this point is to, at a minimum, get it running well enough to get it either back to the installer or to my garage where I'm better equipped to dig into things (obviously, it'll be her call on this, not mine). She lives 30 miles away from me, and around the same distance from the place that installed the carb, so it's gonna need to run better than it did last weekend in order to make it very easily to either place. I'll be carrying a bunch of tools and supplies out there this weekend to see what I can do with it there for now.

If y'all can point me toward any other possibilities that I may have overlooked, or furnish any other sage advice, I'd be grateful.

One other unrelated item about this car:

Last time I was under the hood, I noticed a puddle of coolant on either side of the intake manifold, up front. Seeing no signs of leakage at the T-stat housing/upper hose, I pulled the two forward-most intake bolts on either side, and applied some RTV to their threads, then reinstalled them. I left the system dry overnight, refilling it ~24 hours later. My girlfriend told me today that she now sees the same coolant puddles on the intake. Thoughts?

Many thanks in advance.

Be well,

SJW

68coupe

Get the Lars paper and set the timing accordingly.
Then go after the carb with the Lars carb tuning paper.
I have both, but they are on the web. I think on BarryK’s website.
If you need them, shoot me an email @ joecalo@gmail.com and I will send them to you.
I would give you Lar’s email, but he is traveling internationally this year, so he may not have time to respond.
Thanks,
Joe

SJW

Thanks, Joe. I pulled down those papers by Lars (and a couple by John Z) from BarryK's website a few weeks ago, and read them. I'll review them again before lifting the hood on this car. The goal at this time isn't necessarily to fine-tune either the timing curve or carb A/F, but to get the car back to running as well as it did a week ago before something went south. Fine tuning will only occur later, if/when she decides she wants either me or somebody else to dive into that.

'Preciate the advice.

Be well,

SJW

Kalway

Just want to remind ya that 95% of problems people relate to the carburetor are actually ignition problems. The distributor bolt may not have been tightened enough and timing could be super retarded for all you know.

SJW

Right you are. I'm relying on decades of wrenching experience when I say that I'm 95% convinced that, in this case, it actually is a fuel-starvation fault. I'll definitely check out the basics with the ignition system before I dive into the carb, but I'm very doubtful that I'll find anything of any consequence with the sparks. I saw/smelled no evidence of unburnt fuel at the tailpipes, and it just doesn't run like a SBC with a spark-related misfire.

I doubt it's a slipped dizzy, because it started to exhibit this misbehavior a few weeks ago, then cleared itself up. In any case, the timing light will quickly lay this to rest. We shall see...

Thanks for the advice, Kalway. Muchly appreciated.

Be well,

SJW