Rough running '65 L-75

[Mark F.]

I've got 300+ miles on my rebuilt '65 300 hp small block, and things have been going well, until last week.

Due to a malfunctioning (since repaired) fuel gauge, I ran out of gas near home. I filled the tank and immediately found that, on right turns, the car would stumble or die outright. It's an M35 car, so I set the idle a bit faster, thinking that that might cure the problem.

The faster idle did cure the problem, at least for a short time. Last week on 95 degree day, the car started and ran fine for about two hours, remaining at 190 degrees temp. After a couple of hours of great running, mostly below 3000 rpm, it began running poorly. I had a hard time getting it home without stalling. It didn't seem to want to run below 2500 rpm.

During the rebuld, I blocked the carb heater holes in the manifold with 1/2 inch plugs in an attempt to increase the longevity of the carb base (original AFB). The carb has been rebuilt, and was running fine untll lsst week.

Symptoms: Extremely rought idle,even when warm. Doesn't want to idle below 2500 rpm.

You advice is, as always, appreciated.

Thanks,

Mark

[Duke W.]

Re: Rough running '65 L-75

Assuming you are absolutely, positively certain that there are no vacuum leaks and EVERYTHING in the ignition system is to spec including the dwell, initial timing and vacuum advance, and the idle mixture screws are close to 1.5 turns out from their seats and the choke system is set to all the correct specs and operating nominally, the first thing I would recommend is to pull the bowl cover and check the float level and float drop.

I don't think completely blocking both sides of the heat riser passage is a good idea. When I tried it long ago my engine would stumble on start up from a dead stop until the engine had been running for at least 20 minutes. You need some heat on the base of the manifold under the carb. to promote rapid fuel vaporization for even mixture distribution. Usually blocking the heat riser valve open will provide satisfactory operation in all but very cold weather and will not overheat the base of the carb. Another alternative is to just block one side of the heat riser passage. This will create a "dead end" that allows exhaust heat to migrate under the carb, but won't allow cross flow, so total heat input should be reduced.

Since you report your problem occurs after considerable running time this may not be the issue, but I thought I'd mention it.

I recall the idle spec for '63s with PG is 475 RPM in drive, but check the specs for your year. When checking the ignition system verify that the initial timing is to spec (vacuum can disconnected) then connect the vacuum can and verify that total idle timing is initial plus maximum vacuum advance.

Duke

[Joe L.]

Re: Rough running '65 L-75

Duke and Mark----

I think that Mark is saying that he blocked the holes in each end of the carb heat groove on the front side of the carb manifold flange. This groove was used on many Corvette and other Chevrolet engines with cast iron manifolds prior to 1970. Doing this does not block exhaust heat from passing through the manifold cross-over passage---it just keeps exhaust gasses from passing through the groove in the manifold and seperated from the carb base by only the thin stainless steel baffle. I don't think that this mod is in any way related to the problems that Mark is experiencing.

[Mike McKown]

Check for dirt/debris lodged in your needle/seat

Especially if your rough running is accompanied by black smoke out the exhaust. Sometimes when you run one all the way out of fuel, on the restart with the sudden surge of gasoline through the system, you will dislodge something and it will hang up in the needle/seat.

[Clem Z.]

easy way to clean the dirt from the N/S

without taking the carb apart. remove the fuel line from the carb,plug the line so no fuel can come from the fuel pump,start the engine and run the float bowl dry,this will allow the float to drop all the way opening the N/S fully. next blow thru the the fuel line opening in the carb with 10/15# of air pressure. reinstall the the fuel line and you should be OK. i have done this many times and it sure saves a lot of work taking the carb apart. if the carb has a in the carb full filter remove it first and install a clean one after you blow out the dirt.

[Duke W.]

Re: Rough running '65 L-75

Duh!!! Now I get it. Yes, I recall posts about plugging the holes in the manifold with quarter-inch pipe plugs, so the exhaust gas can't get into the slot directly under the carb. Aluminum manifolds don't have that feature so it didn't register at first. Agree that this is likely not the problem.

One other thing to check is that the heat riser valve is not sticking and is open fully when the engine is at operating temp. Beyond this, assuming there is nothing amiss with the ignition system and timing map, I suspect something is not right with the carb - dirt or something out of adjustment.

Duke

[Bob R.]

Re: easy way to clean the dirt from the N/S

Another suggestion. If you ran the car out of fuel. You may have sucked alot dirt into the fuel filter. Try changing the fuel filter. If the car was running good before you ran out of gas I would think the fuel filter is the problem.

[Mark F.]

Thanks, gentlemen. You're great! *NM*