popping through exhaust

3491 Views 8 Replies 2 Participants Last post by Steve340, Mar 16, 2007

79 kmino

Discussion Starter · Mar 15, 2007

I have been searching through back postings about the backfiring through the exhaust and have tried some of the fixes but still backfiring. 350 sb, new 600 holley carb, new wires, plugs, cap and rotor. The problem started before winter. I put on a new set of headers but had to take the camino to a repair shop for the exhaust pipes. When I went to pickup the car they said for emissons that they needed to make some adjustments. When I started it I noticed that there was a tapping noise from the pass side valve cover. Driving it home it started to backfire through the exhaust. Went back and they said that the tap was the problem. Talked to another person and their answer was the power valve was bad in the carb. Since then I put on a new carb and the other parts listed above. I have to check under the valve cover yet but after installing the new carb the idle was real low but it did not backfire nearly as much which makes me wonder if the problem is under the valve cover or something else. When driving at say a steady 40 the car jumps or bucks or launches something like that but it is not smooth. Sorry so long but wanted to explain as much as possible. Thanks

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Steve340

· #2 · Mar 15, 2007

First off
are you sure timing is correct, and are you sure firing orders correct
afterfiring is caused either by an exhaust valve still open on the compression stroke, or ignition firing at the wrong time, or a ton of fuel in the exhaust gasses which ignite from hot deposits

So considering the wires have been off a few times, make SURE firing orders correct, make sure the timings correct

thats a good start

Remember it started after you put headers on, so look at plug placement carefully

And sometimes a header leak can sound like a valve tap

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Steve340

· #4 · Mar 15, 2007

for a close set, assuming the distributer was indexed correctly
rev engine to 2000 2500 rpm, slowly advance dist until it starts to run rough, then back off a bit, thats semi close, But to find your timing mark, IM not sure on the engine you have, but thats a pretty important thing to be missing, Backfiring doesnt seem to be timing to me, unless its WAY out

facing the engine, 1 3 5 7 to right 2 4 6 8 to left
18436572

It seems firing order....

Maybe your off a tooth on your dist, bring up #1 on compression stroke, have someone turn the crank pully with a 1/2 drive breaker bar, slowly, have something in the plug hole, and slowly feel when the piston reaches its top most limit, when it hits TDC you will have a certian amount of crank travel before the piston starts moving down, but your looking for when it first hits the top, then pull your dist cap, and see if the rotor pointer points where #1 plug wire is, if not, move your wires one hole whichever way will put #1 right where the pointer is, IF you cant do it by moving the dist

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Steve340

· #6 · Mar 15, 2007

Only if the springs broke on your exhaust valve, or your exhaust valve ate something
a compression check would let you see that
usually a tap < valve noise > means excessive clearance, ie valve not opening enough
If its a serious tap, the spring could be broken, allowing the valve to stay open

OK try this

Start the engine when its cold

rev it to 2000 or so for like 1 to 2 min, get the header tubes hot, then spray each tube with liquid, water, windex, doesnt really matter, and see if one tube is cold, on a hot tube water should immediatly boil, if it doesnt. thats your problem cylinder

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