engine pops thru carb

Hack · Mar 23, 2006

Hi,
I don't have any experience with diagnosing a problem like this, so I'm hoping for advice here.

I have a friend with a carbed 302 - happens to be in an '83 Capri. The thing starts and idles well, but when he steps on the gas it will backfire through the carb occasionally.

How should we diagnose this? The car was sitting in a field until recently and he just bought it, so I have no history on the thing. He put a new carb on it.

Timing?
Carb settings?
Worn out cam?
Bad lifter?

Anyway, I'm not sure where to start.
Thanks!

68torinogt · Mar 23, 2006

check the timing with timing light

69Rcode_Mach1 · Mar 23, 2006

Could be either timing, or something causing the valves to be out of adjustment.

classic boost · Mar 23, 2006

timing or too lean.
if it's been sitting in a field, then the entire fuel system should be given a once over.

blackjackbender · Mar 23, 2006

test everything else stated, then try and adjust the rockers.

BDT 1967 · Mar 23, 2006

I would say its a timing issue.

thehueypilot · Mar 23, 2006

Sounds like the engine is leaning out during a quick stab of the throttle which means you need a bigger accel. discharge nozzle or a accel. pump cam with a bigger cc shot profile or your accel. pump diaphram has a hole in it.

Hack · Mar 23, 2006

Ok, I'll start by checking the timing and get it set.

How do I diagnose if the brand new carb has a problem? The engine does pop even if you slowly feed the throttle. You don't have to stab it quickly.

If the cam has a worn lobe or a collapsed lifter, how do I diagnose that / differentiate between them?

I didn't realize that the stock rockers were adjustable. I thought you just bolted them down.

69Rcode_Mach1 · Mar 23, 2006

Oh I didn't realize that they didn't have roller rockers, they are just bolted down. It is either a valvetrain issue or a timing issue. Are you hearing any kind of noises from the engine?

SoCalCruising · Mar 23, 2006

Your timing is likely retarded. Time it first, then see how it responds. You didn't say it was a new carb, which is why many of us thought it could be a carb-related problem. If it still does it after timing it, get back on the board and let us know.

Does it have a stock distributor? I hope so 'cause they're easy to time.

D.Hearne · Mar 23, 2006

I agree with Socal. Timing's likely retarded too much. If not then adjust the accellerator pump arm ( if it's a Holley) to get rid of any free play between the arm and pump lever. Definately not a wiped cam, if it was, you'd hear a loud, loose rocker bangin in the cover.

Hack · Mar 23, 2006

D.Hearne said:
I agree with Socal. Timing's likely retarded too much. If not then adjust the accellerator pump arm ( if it's a Holley) to get rid of any free play between the arm and pump lever. Definately not a wiped cam, if it was, you'd hear a loud, loose rocker bangin in the cover.

Thanks, D. Hearne. I was hoping you'd swing by. Yup, it's a Holley.

.. and thank you everyone for the advice.

Hack · Mar 23, 2006

SoCalCruising said:
Your timing is likely retarded. Time it first, then see how it responds. You didn't say it was a new carb, which is why many of us thought it could be a carb-related problem. If it still does it after timing it, get back on the board and let us know.

Does it have a stock distributor? I hope so 'cause they're easy to time.

Yeah, I should have stressed the new carb part more. It's easy to miss the way I stated it.

Yep it's the stock dizzy. I will be going to help out on Saturday, and I'll let you know what happens.

Hack · Mar 25, 2006

Ok, we got the timing adjusted today. It wasn't too far off. Not sure if it's a little better now or not. I think it might be slightly better.

It does still pop, mostly only with a quick stab of the throttle. It seems to me as though it might bog very briefly also when the throttle is first depressed, then take off and pop.

So now we should adjust the accelerator pump linkage to take out any slack?

Or should we be fine tuning anything else first?

Please post any explanations or thoughts you have.

Thanks!

D.Hearne · Mar 26, 2006

Take the slack out if the accellerator pump linkage, that should cure the popping. What's the timing set at ?

Hack · Mar 26, 2006

D.Hearne said:
Take the slack out if the accellerator pump linkage, that should cure the popping. What's the timing set at ?

There was a sticker under the hood that said to set it 10 degrees BTDC, so we set it there. Would it make sense to advance it a little more?

D.Hearne · Mar 26, 2006

I run em anywhere from at least 10 BTDC to 14 BTDC. If it pings at what you set it at, ether run a higher octane fuel or retard it some. But i's been my experience that they'll only pop thru the carb when the timing's back down around 6 or less.

Dark Knight · Mar 26, 2006

You might check the firing order too, might have a couple wires crossed or the wrong order. You DID change the cap and rotor etc if it was sitting for awhile, right?

Sawdust · Mar 26, 2006

When giving it a quick jabs it seems to boug a little before it pops. If you slowly ramp it up runs fine. I set the firing order for H.O. motor. It had been sitting for 3 years and the guy I bought it from put gas in it but I'm not sure if he drained it first. I bought a brand new Holley and bolted it on but have not adjusted any thing. Would mixture have any thing to do with the poping.

D.Hearne · Mar 26, 2006

Sawdust said:
When giving it a quick jabs it seems to boug a little before it pops. If you slowly ramp it up runs fine. I set the firing order for H.O. motor. It had been sitting for 3 years and the guy I bought it from put gas in it but I'm not sure if he drained it first. I bought a brand new Holley and bolted it on but have not adjusted any thing. Would mixture have any thing to do with the poping.

Well, yea the popping is a result of a lean condition ( fuel/air ratio) but this isn't fixed with a jet change or a different idle mixture screw setting. The accellerator pump is the means of enrichening the mixture when the throttle is suddenly opened. This is normally adjusted by closing any clearance between the accellerator pump arm and lever. If that doesn't work, there's other things you can play with like different pump cams (the plastic thingy screwed to the backside of the primary linkage the pump arm rides on) or larger pump squiters and lastly a larger 50cc pump. The lean mixture when the throttle is nailed is also why mech secondary Holleys have accellerator pumps on the secondary side too ( double pumper). Water in the fuel will also make it pop and backfire. This is easily checked for using a plastic coke bottle bottom and unscrewing one bowl screw and using the bottle bottom to catch the fuel. Any water in there will be seen in the bottle.

engine pops thru carb

Hack

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