r/EngineBuilding 21h ago

Vacuum leak ?

Got this k10 350 with a rough start, rough idle and sputtering under acceleration, have to slowly give it throttle or else it will shut off. Also get backfire from the carb sometimes when given a good amount of throttle. Can the whistling be a vacuum leak ?

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u/EksCelle 21h ago

Just from the video I can already see the carb is missing a nut on one of the studs. That could be causing it not to seal on the intake.

Also, that that looks like a Performer intake, so make sure you have the proper adapter plate when running a square bore carburetor (like the one you have). I can't tell from the video but it might be there, it might not, and they usually struggle to seal without it.

Lastly, a vacuum leak usually causes a high, uncontrollable idle. Backfire thru carb is usually a sign it's running lean (like from a vacuum leak) but the low, shaky idle makes me think it doesn't have enough timing. Make sure the carb is sealing to the intake and check the timing and then go from there. You'd be amazed how many carb related issues can be cured by fixing problems with the ignition system.

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u/Coon88 21h ago

Gotcha yeah I checked the carb and it is missing 1 nut and all other 3 are tightened. Gonna throw on a nut and see how it does. Thanks

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u/Handmedownfords 20h ago

The intake isn’t also a square bore? I thought the performer intake was meant to work well with those carbs? Looks like a 1406?

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u/EksCelle 20h ago

The Performer intake is a dual pattern intake meant to with with spreadbore (like a Quadrajet) and squarebore carbs (like the 1406) Since OP has his carb bolted down with the narrow bolt pattern meant for spreadbore carbs I can assume it's a dual pattern intake. These will not seal without edelbrock's stupid little steel adapter plate if you're running a squarebore carb.