C2 Stumble-Long Message, Sorry

[Les Jacobs]

327/340 with Carter 3461 has two symptoms. 1. The engine Balks (to the point of almost stalling) in low gear, moving from dead stop. The problem is mimimized when I slip the clutch with the rpm over 1200 but even then it sometimes still acts up. It occurs with both extremely light to moderate acceleration. #2 Above 1500 it runs fine as long as there is at least a very light load. But when coasting at 1500 or under (in any gear), there is an engine uneveness which resembles an overly advanced ignition timing. ((Ive tried retarding it, but w no improvement). The kicker is ocassionally the car runs superfine. Usually it occurs when it has been sitting for a while (weeks). But after running for 20-30 minutes the symptoms gradually begin to reappear. (the choke is fully open within 5 minutes). It starts and idles perfectly.The vacuum and timing checks out according to the NCRS archives(static,vacuum,centrifugal all ok). The spark plugs have been replaced and the carb rebuilt -problems with float level,accelerator pump found and corrected. The only other possible related symptom is that the fuel bowl still seems to drain within a couple days, even when the carb isn't hot. Suggestions? Thanks Les

[Wayne K.]

Re: C2 Stumble-Long Message, Sorry

Les,

I think you've already discoverd one of if not the problem. In your last sentence you tell us that the fuel bowl drains within a couple days. This is not normal and the cause should be found and fixed. If your not seeing the leak externally then the fuel must be leaking internally which can cause flooding and performance issues. That's where I'd start. Keep us up to date on your findings.

Wayne

[Jim T.]

Re: C2 Stumble-Long Message, Sorry

Les your post saying that a problem exists that resembles overly advanced ignition reminds me of a problem I had with my 64. The centrifical advance was sticking in the advanced position when the Vette warmed up. I disassembled and cleaned the centrifical advance and used Blue Streak point lubricant on the contact points of the moving metal parts which cured this problem. I always perform this maintenance when servicing the points.

[Les Jacobs]

Re: C2 Stumble-Long Message, Sorry

Jim How did you determine that the advance was sticking?

[Jim T.]

Re: C2 Stumble-Long Message, Sorry

Les I am not sure of the details. I had driven my 64 from San Diego to Dallas and something made me pull off the distributor cap and and rotor and discovered the problem. This was in 1969. That summer I drove my 64 air conditioned convertible from San Diego to Glynco,GA on the east coast.

[Les Jacobs]

Update-No Cigar

Well I was convinced that the sticking centrifugal weights were the cause of part of the problem, until I took it apart and found they had already been cleaned and lubricated (as you suggested). ((By the way, Ive owned the car for 6 mo, and been chasing the problem the entire time)). So in the process, I double checked the timing (especially the centrifugal advance)) and it checked out perfectly. I keep coming back to the notion that there's a problem with the off idle circuit, but after checking the rods,jets, and changing the metering springs, I'm running out of ideas. I do have a slow leak internal to the carb, and Im getting ready to tear into it (again), but Im not hopeful that thats the root of the "bog" problem.

[Patrick H.]

Re: Update-No Cigar

Les,

I agree with the above comment on the bowls emptying. A couple of days is way too quick.

Anyway, my only thought is regarding your vacuum advance canister. Which one is installed? Do you have the number off the top of it? Why I ask is that if the vacuum advance is varying at all (i.e. advancing and then unadvancing) with light engine load, this might cause the problem. I'd wonder if the vacuum can is matched to your engine characteristics.

My Dad's car has a very similar problem, and I've discovered it's that the can is giving advance at times, but with a small change in vacuum (i.e. slightly less vacuum) will stop advancing, and the car falls on its face. As soon as it warms up and the car comes out, I'm replacing the can with one that matches the engine.

Patrick

[Les Jacobs]

Re: Update-No Cigar

Hey Patrick I changed out the original for the NAPA Echlin #1410 (or something like that)-in any case it was the unit suggested by Duke (of this board) for the HP engine. It gives 14degrees at approximately 8 inches. Re the timing stability, all I know is that when using the timing light mark is very stable at different speeds (in neutral) with about 12 inches of vacuum. In the back of my mind is the thought that if the vacuum drops below 8 inches while Im acclerating then the timing could retard and the engine might stumble. One of my planned tests is to hook up a vac gauge while test driving. But Ive also tried accelerating at a snails pace to hold the vacuum near constant and it still bogs. So my sense is the problem is not with the vac advance unit. Les

[Duke W.]

Re: Update-No Cigar

From what you say, the timing map seems to be okay. When you open the throttle and manifold vacuum drops, the vacuum advance backs off. That's how vacuum advance works - no manifold vacuum, no vacuum advance.

I'd suggest you pull the carb, empty out the fuel, them rig up a siphon to fill the fuel bowls with mineral spirits through the fuel inlet. This will allow the floats and needles and seats to establish the fuel level. If fluid spills out the discharge nozzles in the venturi clusters the float level is too high.

If the float level is not excessive, set the carb up on some blocks in a drain pan, let is sit for a couple of days, and inspect frequently for leakage into the pan.

I don't know how an AFB can drain the fuel bowls by leakage because the bowls are integral with the main body with no wetted gaskets. The idle/off idle fuel is pulled through the main jets and through various passages in the booster venturi cluster and main body. The idle air bleeds are drilled into the primary booster venturi clusters.

All I can figure is that there is some leak path due to a crack in the main casting, or somehow a siphon action is taking place.

Duke

[Bruce Boatner]

Re: C2 Stumble-Long Message, Sorry

Les,

This is likely not the sole source of your problem, but would be something you could at least rule out. My '64 327/300 stumbled badly off-idle from the day I bought the car. Drove me nuts trying to find the problem. As it turned out, the s-link connecting the accelerator pump rod to the pump lever was installed incorrectly--very easy to do. Be sure that the link forms an "S" when viewed from the front of the car and not a "Z". It made a huge difference for me.

Good luck.

Bruce

[Les Jacobs]

Re: C2 Stumble-Long Message, Sorry

Hi Bruce I wish it were that easy. The S link looks like an S, not a Z. By the way, Im questioning the fuel delivery from the accelerator pump: When I open it slowly (taking about 5 seconds to go from closed to to WOT I get NO gas from the accelerator pump nozzles. When I go twice as fast, it sprays a nice stream. Does that sound like a problem to you?