305 backfires through the carb under accel since swapping it

[Pender1]

I pulled the 305 out of my '84 K10 and stuck it and the SM465 attached to it into my '86 K5. Other than burning some oil and not having a ton of HP, it ran great in the K10, but since I've put it in the K5 it's not doing so great.

Under accel it backfires bad out of the carb. I've got ~17" of vacuum and can't find a leak. I've tried with and without a fuel regulator in line. I've tried anywhere from 8-18 degrees of initial timing (without the vacuum advance hooked up). I've tried running it with and without the vacuum advance connected. It all does the same. I've adjusted my carb as best as I can using a gauge (Edlebrock 4bbl). I also had the Holley 4bbl that was originally on it and it did the same thing.

I've seen a couple of posts that say it may be a wiped cam, but this engine ran great before, so I don't see why it would have all of the sudden killed a cam.

Any suggestions? Ideas?

 

[bucket]

Bent pushrod or broken rocker can make it run exactly like a wiped cam.

But there is more than likely an exhaust valve that's not opening.

 

[1987 GMC Jimmy]

Your vacuum gauge's behavior should give you some clues. Specific valve train issues make the gauge act a certain way. I would also consider an issue with your timing set. That timing seems like a lot to me for a 305. I wouldn't ever go more than eight myself, and for mine, that's in the premium gasoline range. Did you have the distributor out at any point?

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[yevgenievich]

Timing does sound a bit off if it is a stock motor.Verify firing order as well.

 

[bucket]

Ok, are we talking rapid repeating backfires, or just a single backfire when you crack the throttle?

I was under the impression it was of the rapid repeating variety.

 

[Pender1]

Ok, are we talking rapid repeating backfires, or just a single backfire when you crack the throttle?

I was under the impression it was of the rapid repeating variety.

Single booming backfire.

 

[bucket]

Single booming backfire.

Ahhhhh, I see now. Please disregard my previous post.

 

[Pender1]

Your vacuum gauge's behavior should give you some clues. Specific valve train issues make the gauge act a certain way. I would also consider an issue with your timing set. That timing seems like a lot to me for a 305. I wouldn't ever go more than eight myself, and for mine, that's in the premium gasoline range. Did you have the distributor out at any point?

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I pulled the distributor during the engine pull. What should I have initial set to? This is me learning engines, so don't think you are insulting me by correcting me.

 

[bucket]

8-12 is usually a happy place for them.

 

[1987 GMC Jimmy]

I'd normally agree with that, but the 305 I run pings at 8* unless I run Premium. Granted, it has a not so good torque converter, and I just pulled off a bum harmonic balancer. It shouldn't be doing what it's doing through at least 12, though. I'd retard yours back to factory setting just to make sure it's not running way too lean if you don't find a conventional fix, but if those higher settings work for you after you fix it, then go for it. Before I spent time doing that, though, I'd go back and make sure the firing order is correct because even if two are mixed up, that can cause issues. I don't think the distributor's stabbed in there wrong at this point because I feel like that would cause more severe issues. I'd really check that firing order very carefully first.

 

[shiftpro]

A stuck valve backfires and also power starts to flock off and heat begins rising if you keep working it.

 

[shiftpro]

Your vacuum gauge's behavior should give you some clues. Specific valve train issues make the gauge act a certain way. I would also consider an issue with your timing set. That timing seems like a lot to me for a 305. I wouldn't ever go more than eight myself, and for mine, that's in the premium gasoline range. Did you have the distributor out at any point?

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I like #14... you know you're screwed when the needle gets blurred!
What does a vacuum gauge read like when stuck on one of those 1000+ hp diesels you see blowing up on the dyno?
Which BTW, cheers me up to watch when I'm feeling down, serious. haha BOOOM! Even my little stinker likes those vids.
But now when I fire up Big Blue to keep it, you know, felt loved, Enzo kinda runs away... (BB and bumper nerfs).

 

[Rusty Nail]

Engine ground to chassis / frame.

 

[Snoots]

If it's a Holley 4bbl and backfired through the carb, the power valve in the carb is shot. No fix, only replace. IMHO, I'd set the initial timing at 10 and leave it...assuming you haven't crossed any sp wires. Let us know.

 

[bucket]

The power valve situation isn't so bad anymore. Lots of power valves have taken a good backfire and lived to tell about it.