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Old 01-14-2005, 12:27 PM
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Backfire through exhaust

Ok, here's the situation: I just rebuilt the carb on my truck and now every time I punch the throttle or even just give it more than half throttle the engine backfires through the exhaust. Carb is a Holley 750 vac. secondaries on a stock 305. Now I know that this is WAAAY too much carb for this engine but it's all I have until I can afford something smaller. It didn't backfire like this before I rebuilt it, I rebuilt it because it idled and ran poorly when it was cold, figured the carb just needed a good cleaning. The truck revs fine in park or neutral, nary a trace of a backfire, only does it in gear. I've put it in gear and loaded the engine up and it only misses a little, read once or twice. I think it might be my timing ( I've been setting it by ear) or the small, TINY exhaust leak it has on the driver side bank. What do you think?

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Old 01-14-2005, 02:27 PM
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It sounds like it may be too rich and the timing is to retarded. Get a timing light, by ear is a waste of time. I'm not trying to be ugly here, but if you have a problem like this you need to know where you are. If it ran fine before try and see if there are some marks where the distributor position was and try that first. Check your float levels too..
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Old 01-14-2005, 07:41 PM
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I think you're probably right on the over rich mixture and stuck float. When I fired it up cold to drive home from work it backfired through the carb twice before it cranked, so I think the floats may be stuck or just have trash in the needle valve. Doesn't surprise me though, the needle/seat is the only thing I didn't replace that came in the kit, so I'll just do a more complete rebuild tomorrow. It has to be something in the carb itself throwing too much fuel in the mix. Thanks for the help. Any other theories.
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Old 01-16-2005, 08:10 PM
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Here's an update. Last night I re-rebuilt the carburetor, this time replacing everything that came in the kit. I set the floats to 7/64 clearance as per the instructions, and put it back together very carefully. Put it back on the truck and ran it, didn't help. Checked for vacuum leaks around the carb base and manifold gaskets, found nothing. I the pulled the plug wires one by one to see if it would change the idle. Turns out #4 doesn't affect it at all, idling or revving, just a dead cylinder. It's getting plenty of fire ( trust me, my hand tingled for a good five minutes after ) and the plug isn't fouled, so I'm thinking I've got valve problems. When I sat down and thought about, it seems like it has taken longer than usual to start, especially when it was hot. The motor would just spin over for a few seconds until it finally fired and ran. Seeing what's happened now, I figure that may have been a sign of weakening valve springs, and maybe one of them finally just gave it up completely. I'm going to do a compression test tomorrow, we'll see what that turns up.
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Old 01-18-2005, 10:24 PM
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I just wanted to say that its technically an afterfire when it comes out of the exhaust, a backfire is outa the carb
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Old 01-19-2005, 11:42 AM
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I've seen you say that in a number of old posts, you aren't going to let anyone forget the afterfire are you? LOL

Anyway, I'll give ya'll the rest of the story. I ran the compression test Monday, all cylinders were between 130 and 140 psi after six rotations, which left me completely stumped, as I was leaning toward broken valve springs ( they're over 30 years old ). Well, while I was doing the test, I noticed that a couple of cylinders on the left bank were fuel saturated, so I spun the motor a couple of times with the plugs out to clear the fuel out and replaced them with a couple of my old plugs. Put everything back and cranked the motor, idled better. So I went ahead and stuck all the old plugs back in and she runs like a top. Revs smooth, quick, no backfires at all, actually better than ever, so I guess my carb work wasn't for naught.

Here's what got me: those plugs were less than a month old. So what's the deal? Bad plug, cracked insulator, manufacturing defect? I have no clue.
They're AC Delco R45TS plugs, exactly what the truck is supposed to have, and the old plugs are the same thing. Just a bit of bad luck I guess. A friend of mine suggested I switch to an R44T. He said they're a non resistor plug, that they don't foul out as easy as the 45's will. Is he correct?
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Old 01-19-2005, 01:06 PM
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Did you gap the plugs when you got them? Just a thought but the gap can play a role is how the fuel burns. Sometimes the plugs can be pretty far off. Use a feeler gauge and get the gap on the old plugs and do that to all the new ones and try it again. It could also be a cracked plug some like a hair line that is hard to see you should with the lights off be able to see arching though. (not always though)

It could also be a bad set of plugs though I find it hard to believe that all the plugs would be like that.

Also in the test did you do both a wet and dry test?

Chris
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Old 01-19-2005, 03:28 PM
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Yeah, I gapped em all to .045 before installation. I figured it would be safer and easier to pull all the plugs back out so I wouldn't have to test them all. I'm thinking it was #4 that was causing my dead miss though, because at one point I started pulling wires off to find my dead cylinder and it seemed like pulling #4 didn't affect the idle quality at all, nor how it revved, so that one may have been the busted one. I plan on closely inspecting all of them when I have the time, and carrying the defective plug back for a new one. I didn't wet test the motor, why do you ask?
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Old 01-19-2005, 03:45 PM
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Yea I once had that problem I had a bad miss and couldn't figure it out and it turned out to be a little crack in the white part of the plug hardly able to see it only way iknew it was how you just said.
Also check your wires and for any cracks in the dis. cap it could be a crack in there at that one cyd. area. Things like that happen out of no wheres.
With the wet dry testing it's just another way to look and see if you lose by the valves or the rings. If you wet test it and the numbers go way up it could mean a problem with rings. If it does not good up it could mean valve problems as was said in another post. Look Here

Killer made a good point in there about wet testing too but I still don't think it hurts. It is really better to watch oil use and running condition. It can help in tracking down problems though.

Chris
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Old 01-19-2005, 09:27 PM
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Quote:
Originally posted by SAATR
I've seen you say that in a number of old posts, you aren't going to let anyone forget the afterfire are you? LOL
lol yeah I know! its just something that bothers me for some reason! like a pet peeve... I figure if enough people use the right terms we will not have to say intake or exhaust and talking about this stuff would be alot easyer. after all a meachanics vocabulary is his best tool when talking among other mechanics. it puts us all on the same page just my 2 cents...

-Leo-
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