carb base gasket keeps leaking gas - Impala Tech
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post #1 of 6 (permalink) Old 07-14-2011, 12:41 AM Thread Starter
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carb base gasket keeps leaking gas

My carb base gasket keeps leaking and pouring out fuel. I replaced it about 2 weeks ago as it started leaking. Got a new one and after about a hour of driving its doing the same thing. The engine wont idle, gotta put her to the floor to start it etc etc. I pulled the carb off tonight and the studs were a bit loose in the intake, but the nuts were tight and the carb didnt seem to be loose. Any ideas what could be causing this? Intake warped? Carb warped? I hope its not the carb as its only a year old... its a holley SA 670. what is the best way to check, just put a straight edge across it? The gasket I had the first time was the one that came with the carb and I replaced it with a similar one from Mr. Gasket. Thanks for your help!
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post #2 of 6 (permalink) Old 07-14-2011, 08:21 AM
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Probably the floats sticking open in the carb
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post #3 of 6 (permalink) Old 07-14-2011, 10:35 AM
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As mentioned with the remark about the floats gas can get out of a carb in a few ways. My first thought was worn bushings on the throttle shafts. Anytime gas gets out (with the exception of it flooding) more air gets in than is accounted for by the carb so your motor runs lean. People compensate for the lean condition by richening the mixture's idle circuit usually but some times by increasing the jet size which won't help.

To determine if you have a warped carb base put a machinist's flat across the base plate fore and aft and side to side. Then check diagonally from corner to corner. If it is flat then no gasket change will help. As to loose carb studs. Remove them and examine the threads. Are they new and unmarred? If not you have two options. The correct way is to have thread inserts installed (Heli-Coils). These are stainless steel threads that will not seize up the way a steel bolt or stud installed in a an aluminum manifold will (if you bought that manifold used this is probably your problem as the former owner took out part of the threads when he removed the old carb studs or bolts). The second way is to fill the treads with epoxy thread locker like Loctite's 9497 compound so they are glued in place permanently.

If you bought the carb used, it might be a high mileage carb. The only moving part in a carb that sees a load is the throttle plate shaft as you are always standing on the throttle to pull it open, and there are strong springs trying to pull it closed again. The brass bushings in the aluminum base plate that separate the steel shafts from the base plate wear. If they are worn find another used carb and rebuild that one as it costs about $50 for the parts and $230 for the tool to drill the base plate out to install them (and it also requires a floor standing drill press to have enough room for the tool and the base plate mounted in your two axis vise on the drill press's rotating table).

If you are having float issues then the carb is flooding out of the vent tube pressed into the top of the carb (a quarter inch round brass tube and cut at forty-five degree angle). What isn't ingested into the motor causing it to stumble and run pig rich dribbles over the carb and down onto the manifold. Another source is a fuel bowl screw that is not torqued to the correct 25 inch pound specification (which is why Holley switched from a screw with a slot to a hex head). Finally there is and always has been an issue with porosity in the pot metal that Holley uses to cast the parts. Holley has gotten much better about this in terms of QC (since Barry Grant started machining Holley's out of solid billet aluminum and calling them Demons), but statistically there will always be a leaker that gets by (which is why Holley offers a one year warranty).

Big Dave
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post #4 of 6 (permalink) Old 07-15-2011, 12:02 AM Thread Starter
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The carb is brand new last summer. I will lock tight those studs in and check out the floats and adjust etc. Thanks for your help and advice!
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post #5 of 6 (permalink) Old 07-16-2011, 12:22 AM
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This sounds familiar to a problem I had on my holley 770 street avenger. The carb gasket would be soaked with fuel on the drivers side, later to find out that it was leaking fuel passed the secondary throttle shafts when I shut the car off. I brought the carb back, got another one that doesnt leak, then I went further to add in a 1/4" spacer. So I'm saying it could be the throttle shaft
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post #6 of 6 (permalink) Old 07-16-2011, 01:05 AM Thread Starter
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ya my gasket is soaked on both sides and I am starting to wonder if its leaking from the shaft, each time it has happend tho is when im far from home and I pull over and theres puddles of gas on the intake and didnt really feel like burnin her down and fire it up again to see where its leaking from exactly lol but I will now watch for that! I think Holley has a 1 year warranty and of course its just past that now so ... I will have to look further into things and see if they will honor it, if thats the problem.
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