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Discussion Starter #1
After putting the new 360 in and discovering that it had a broken pushrod, we started it. Adjusted the timing and it now backfires through the carb. The carb is new so i was told that it has a bent valve. Sound right? How do i test to see what valve? Could i just assume it was the one that had the broken pushrod on it?And do i have to get the new valve at a machine shop or will the local parts store carry it? Thanks!
 

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Discussion Starter #3
New cap, rotor and plugs. Wires are in good shape. Didn't backfire at all with the old motor, just with the new. Can i use a valeve from the old motor if it's in good shape? Never re-used used parts before.
 

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somethings not firing at the right time, or a valve is not in it's proper place when the firing occurs...
the easiest way to diagnose this and many other engine probs is to start from one end of the process, check it, and move on along the line.

when redoing caps & wires, sometimes even a small crack in the cap can throw everything off. check for hairline fractures. ( and even check the firing order... could even be as stupid as a mixed up wires) then recheck points and gaps... since is a good first step since this doesn't cost anything. It never hurts to do a valve lash on an older motor. this will make shure the valves seat correctly...
then check compression.. this should tell you if your valves are sitting correctly or if something major is wrong. and will tell you which cylinder to focus on.

this should be a good start before finding out you have to rebuild the motor.

- but then again, you already knew all this from performing online searches.:flipoff2:
 

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Brawler said:
New cap, rotor and plugs. Wires are in good shape. Didn't backfire at all with the old motor, just with the new. Can i use a valeve from the old motor if it's in good shape? Never re-used used parts before.
If you bent a pushrod, you could have very easily bent a valve. Does the backfiring happen through the entire engine speed, or just a low or high RPMs? You could put the old valve in, but since you are going through that process you might as well put a brand new valve in.

You should be able to get a valve at a parts store, but more than likely they'll have to order it. You can get them in various internet and mail order places too.
 

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texas blake sounds like the winner..
since im in the process of puttin all new push rods and values in my 96 4.3 blazer then its gettin traded off.. pos.. bent push rod slightly fawked entire motor
 

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Discussion Starter #7
Not at idle but mostly when you rev it up. Will check all the necessaries before i tear the head off. One thing after another i'll tell ya!


TexasBlakeWFO said:


If you bent a pushrod, you could have very easily bent a valve. Does the backfiring happen through the entire engine speed, or just a low or high RPMs? You could put the old valve in, but since you are going through that process you might as well put a brand new valve in.

You should be able to get a valve at a parts store, but more than likely they'll have to order it. You can get them in various internet and mail order places too.
 

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The simplest method of checking out the valve is a compression test. If the valve is bent, it won't seal completely. If it is an exhaust valve, it will burn quite quickly. Do the check before doing anything else. If you run it too long you can screw up the valve seat and heaven help you if the valve decides to break and the head bounces around in the cylinder. If you get good compression, you problem can be one of the following:

1. Bent pushrod
2. Collapsed lifter
3. Flattened lobe on the cam

LOL

Tom Houston
Loveland, Colorado
 

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Ok... let's thing about this logically:
1. You've got backfiring through the carb.
2. Check to make sure the distributor is timed,
3. Check to make sure the timing gear/chain set is intact and not worn.
4. Then IF all that's good... then,
5. Buy a mechanics stethascope.
6. Pressurize one cylinder at a time and hold the pressure.
7. Listen for the compressed air excaping into the intake manifold.
8. If you hear it... then there's a vavle problem.
9. If you don't.. then likely the problems in the timing.
10. A collapsed lifter wouldn't cause the problem, but a sticking one would.
11. Is there any tappet noise?
 

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I had a very similar prob.

My vacuum advance on the dist was TU. Try pulling off the vacuum line and plugging it.

I hope that's it cuz it's very easy and cheap!!
 

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Had the same problem with my AMC360 when I first put it in. You check the timming with the vacuum disconnected. Then Reconnect it!!!!!!! If you don't it will backfire thru the carb! My guess if you don't have any tapping noise and it idles ok. It is a Vacuum leak.
 

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Compression Test It like Leve said. Then look at Ignition. After that check out the Carb? Is your float level too high? Fuel might be overflowing the bowls into a vent when you goose it and therebye cause it to backfire. If it's a Holley it could be a blown power valve. More than likely it takes a bad backfire to blow the powervalve though which means you have another problem. My money is on Ignition. Does the rotor point to # 1 when the engine is at TDC?
 

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Discussion Starter #13
I',m going to check all the timing stuff tomorrow and let you know. The carb is a newly rebuilt factory two barrell. Will also check the vaccuum. We did have some confusion on one line.
 
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