8valve

I have a 74, L48, 4 speed I am doing a resto on this winter. Finally got under the hood and notice that the Holley List 4777-3, 650 cfm double pumper is all black inside. Its not wet oily black, nor is it soot. I suspect carbon, it will clean off with a brush and carb cleaner.

Other than the black in the carb, the engine runs perfect, instant start, great idle, plenty of power for a L-48. The Edlebrock Performer intake is as clean inside as a new one. The spark plugs are a beautiful tan, PCV is clean and functioning properly.

Any idea what the cause of the black inside the carb might be ?

I bought this Vette late fall last year. Its also possible that someone put this carb on, from another engine in the present black condition....dont know.

Thanks
8Valve

Big2Bird

Black inside the venturis, or black in the bowls?

8valve

The entire inside, everything is flat black. The same as the carb inside was anodized flat black, but of course it was not anodized flat black

8valve

Rescue Rogers

since its an unknown to you, i would say clean it and run it and see if it comes back. All I can think is the pcv valve is sucking oil possibly, but you say it isnt oily, so it is weird

8valve

I agree its a wierd problem, that why I asked for any opinions/suggestions. I dont believe the PCV system is at fault as the residue is more like carbon, not oily and does not rub off on the fingers. I now have it cleaned well with brushes and carb/choke cleaner. The inside is now in Bristol condition. I will put it back on tomorrow.

I cant do a lot of engine running as we are in deep winter, Vette is on the Kwik Lift and deep snow outside. So any real testing wont come to spring.

With that said, I looked everything over again this morning and have come to this diagnosis;

I believe the Carb came off another engine that was suffering with Back Fires, thus the black carbon. If this black carbon had happened on this engine, then the inside of the Edelbrock Performer intake would also be black, it is as bright and clean as a new intake manifold. Some past owner took this carb off a back firing engine and put it on this engine....thats my best guess. If after a few hundred mile this summer it stays clean, we know this engine was not the problem.

8Valve

mikem350

Have to say its not weird, I have two carbs right now with same black coating! Both carbs came from engines with big cams with big overlap, raspy idle.

8valve

Thats interesting. To the best of my knowledge, the cam in this 74/L48 is stock. With a accurate vacuum gauge, I am pulling 19 inches of mercury at steady and smooth idle.

What do you attribute the black in your carbs in relating to the big cam ?

8Valve

mikem350

Well, maybe not everything. Should have mentioned its more related to blow-by sucked in by the PCV system. The vapors carry oil droplets into the intake system. The combination of oil, unburned fuel and other contaminants in the blow-by accumulate inside the intake tract and leave that black soot like coating.

Gets worse with age of the engine, with worn rings, valves.

Michael Roper

The black is most likely aluminum oxide from cleaning with acid. I know of nothing that dissolves it. I would remove it with blasting media.

lars

This thread is almost 3 years old. Problem is likely moot by now,..

Lars