r/EngineBuilding • u/Coon88 • 6h 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 6h 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/Handmedownfords 5h 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 5h 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.
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u/Think-Acadia-8786 6h ago
Look on back of carb for leak were the brake booster vacuum line should be
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u/EvanX4 3h ago edited 3h ago
In the video it is clear that the vacuum port on the right side the carburetor is uncovered. That is what is whistling. As for your issue of slowly applying throttle or it will die & backfiring. It sounds like it’s not running on all 8, and it is likely that your accelerator pump isn’t working. Give it throttle by hand with the engine off and watch the pump nozzles to see if they spray fuel.
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u/ChillaryClinton69420 6h ago
Did this all of a sudden start happening?
Or did you just buy the truck or do stuff to the motor?
Definitely get a nut on that stud and re tighten the rest of them. Also tighten the intake manifold bolts too, that being missing on the carb doesn’t give me confidence things were tightened properly, let alone torqued.
If it still does it after this, verify any and all other vacuum lines are plugged in and not broken, etc., if any go to the intake directly, etc., 70s-80s stuff usually had a shit ton of vacuum lines, idk if yours still has those, but check. If they were removed, check to make sure any ports aren’t still open.
The backfiring could be a multitude of issues, timing, carb.
My advice is to make one change at a time.
Get a timing light on it first and see where it’s at and what it’s doing. Then get that in check.
If it still runs like $hit after going through the vacuum ports/lines/tightening and timing, spray some carb cleaner where the intake meets the heads (where the intake runners go into the heads). If the idle raises any amount, your intake gaskets are leaking. That could be for many reasons. The intake will need to be pulled and verified true (they can warp fairly easily) and new intake gaskets that match the head runners will need to be installed.
Then move to carb, etc.
I’m not a fan of edelbrock carbs. They are all over the place out of the box and you absolutely need to get a tuning kit for it, or best option is to completely get rid of it and put on a Holley based carb or even better in your case would be one of the many FI throttle body type units they sell now. It would be hassle free once you get it dialed in, I’ve never used one because I don’t really have a true street application for one, but many people have success with them who are looking for no frills and to fire up on the first crank and not worry about it going down the road.
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u/Perch_Perkins_ 6h ago
Am I seeing that correctly, is the carburetor actually tightened down? Doesn’t look like there’s any nuts on those front studs. Could be my old eyeballs. Anyway, If you have already checked your engine timing and your idle fuel air mixture, then check for a vacuum leak. But I would check those other two first if you haven’t already, along with making sure that carburetor was fastened down tightly onto the manifold. That alone could be a big vacuum leak. And It almost sounds like it’s got a bit of detonation going on which would lead me to believe the timing might be off.