Backfire through carb?
- Thread starter GOBR
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Jims86
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Backfire usually means its lean....or worse. Bit if its sudden, I would take the carb apart and blow the accel ports out with compressed air.
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crazy4offroad
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Some carbs just have a "flat spot", it has to do with the transition between cruise and step-up on the primary needles, or the spring weight of the needles. Basically you have a pair of needle valves in the primaries, the needles have a large and small diameter on each one, large at the top, small nearer to the end. Engine vac at cruise pulls the needles down against the springs, pulling the needles into the jets to the larger diameter of the needle. When you increase throttle, the engine vac drops allowing the springs to push the needles up to the smaller diameter of the needle, allowing more gas into the mix.
A slightly heavier spring would suddenly step up, causing a bog or rich condition. A lighter spring could cause delayed step up causing it to lean out. This has been my experience tuning on Edelbrock carbs anyway, trying to get that sweet spot. A 30 year old Q-jet could just have weakening springs causing it to be lean on step up. It could also have slight vac leaks making it run leaner. Also elevation can play a factor. A carb from an area with high elevation will run rich in the lowlands, and vice-versa.
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fallguy
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this one was TOTALLY me!
I called Edelbrock tech support directly on this one. EXTREMELY helpful! The spring issue was my problem, think i had to go to the pink springs and change the metering rods as well. They told me exactly what to use.
My 45000 mile engine had the timing CRANKED to compensate for the flat spot, unbeknownst to myself, so when the cam shaft on that engine blew going down the highway, i put in a new 350, and set the timing where it belonged. It fell flat on is a** under acceleration the first time i tried it...in the middle of traffic...at a corner! So i called Edelbrock....and the rest you know.
I called Edelbrock tech support directly on this one. EXTREMELY helpful! The spring issue was my problem, think i had to go to the pink springs and change the metering rods as well. They told me exactly what to use.
My 45000 mile engine had the timing CRANKED to compensate for the flat spot, unbeknownst to myself, so when the cam shaft on that engine blew going down the highway, i put in a new 350, and set the timing where it belonged. It fell flat on is a** under acceleration the first time i tried it...in the middle of traffic...at a corner! So i called Edelbrock....and the rest you know.
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dpra
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My own special problems
i have a 77 c20 with the 454. it is carburated. it stumbles under acceleration from a stop. especially when cold. if it's warm, its a lot less noticeable. i just did plugs and wires, looking to replace cap and rotor since i just replaced the vacuum advance on it. helped my problems out quite a bit. now its just the initial start off from a stop. Possible accelerator pump issues? or is it just that lean?
i have a 77 c20 with the 454. it is carburated. it stumbles under acceleration from a stop. especially when cold. if it's warm, its a lot less noticeable. i just did plugs and wires, looking to replace cap and rotor since i just replaced the vacuum advance on it. helped my problems out quite a bit. now its just the initial start off from a stop. Possible accelerator pump issues? or is it just that lean?
ScottyB
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I have the same issue with mine when it is cold. When it's warm it will do what I ask but cold it will stumble. I suspect either a bad carb setting or my distributor is shot.
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New thread created. Moved posts from TBI discussion.
GreaseDog
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You guys know that a carbed engine won't run right until it is warmed up and the choke is fully open, right?
