350 Engine extreme backfiring please advice

ivangf · Apr 17, 2026

Hi there, i own a Cheyenne 1991 mexican version (like Chevy truck 1987) with a 350 engine, a Rochester Quadrajet carburator and automatic transmission. The problem i experience are as follow.
-in Idle speed i can not adjust the gas intake moving the two screws in the front, i tried closing them at all (clockwise), then open them three and a half turns as recomended and tune as required, it can not stabilize the engine. The engine was at warm temperature and the choke fully open.
-if the acceleration pedal is pushed, the engine produces a lot of explosions or backfires.
I replaced the parts with the renewal carburator kit, the float level has been adjusted from 7mm to 4.5mm and seems to improve a little bit but still not stable.
Do i need to replace the carburator? Or any clue what i am doing wrong?
What may be the cause of the explosions?
Thank you in advance for the advices

75gmck25 · Apr 17, 2026

It sounds like you may have a significant vacuum leak, since you cannot adjust idle mixture.

First - get a spray can of carburetor cleaner, and spray it around the base plate and any other areas with gaskets. If there is a leak and it’s pulling in extra air, the carb cleaner will temporarily plug the leak and the idle will change.
You may also want to spray around the edges of the intake manifold, in case there is a crack or a loose bolt.

Further troubleshooting

- I would pull the carburetor and then adjust the curb idle screw on the linkage so that the throttle plates transfer slot exposure is approximately a square shape.
- unscrew both idle mixture screws and remove them. Use compressed air to blow through the passage and make sure they are clear. Reinstall the screws and set to 2 1/2 to 3 1/2 turns out.
- Put the carb back on, make sure the PCV and brake booster ports are connected and tight, and cap all other vacuum ports, including the vacuum advance port (don’t reconnect the vacuum advance). It should idle at about 650-800 in Park.
- now try again to adjust idle mixture to get best RPM. If you first gently turn the screws in, it should idle down slowly, and maybe even die as the screws are fully seated.

Ricko1966 · Apr 17, 2026

Check the ignition out. Was someone messing with timing,spark plugs or wires etc. And for your future knowledge 1987s in the United States were nit carbureted,so you might dial your comparison of your truck to American spec trucks a year or so,so you don't get wrong parts or advise. Have a great day.

ivangf · May 8, 2026

Ok guys sorry for late response (i spent not too much time on the truck since i work every day), thank you for your advises, let me list the actions i did on the issue,
-I pulled the carburetor, disassembled every part, cleaned all and blowed air in every passage making sure everything was clear, adjusted the curb idle screw on the linkage so that the throttle plates transfer slot exposure is approximately a square shape (it was not adjusted, it was as a rectangle shaped)
- I unscrewed both idle mixture screws and removed them. Used compressed air to blow through the passage and made sure they were clear. Reinstall the screws and set to 3 turns out.
- installed the carb back on and connecting all vacuum ports making sure they were tight. Actually i replaced the PCV hose cause it was a little damaged
-Since the vacuum advance did not work, i replaced the entire distributor with a brand new one, spark plugs, and cables. I was sure that the sync timing was OK putting my finger in the piston 1 spark hole method, clicking the starter until i felt a blowing compression on it and making sure the distributor pointed to piston 1, then of course put the spark plug back and the cable connected.
-i tested that every cable had spark when cranking (unplugging the cable and put it close to the engine so i saw the spark)
-it has gas

After all these actions performed the engine starts but it does not accelerate and then it turns off. (i feel a little frustrated)
How i can test that the carburator is OK? Or should i replace it?

Sad Sack · May 9, 2026

As @Ricko1966 said ignition should be checked, using the finger on the spark plug can work for finding TDC but the process can easily have the piston on the downstroke. I use a compression tester to find TDC. Easier when you’re by yourself.

Ricko1966 · May 9, 2026

ivangf said:
Ok guys sorry for late response (i spent not too much time on the truck since i work every day), thank you for your advises, let me list the actions i did on the issue,
-I pulled the carburetor, disassembled every part, cleaned all and blowed air in every passage making sure everything was clear, adjusted the curb idle screw on the linkage so that the throttle plates transfer slot exposure is approximately a square shape (it was not adjusted, it was as a rectangle shaped)
- I unscrewed both idle mixture screws and removed them. Used compressed air to blow through the passage and made sure they were clear. Reinstall the screws and set to 3 turns out.
- installed the carb back on and connecting all vacuum ports making sure they were tight. Actually i replaced the PCV hose cause it was a little damaged
-Since the vacuum advance did not work, i replaced the entire distributor with a brand new one, spark plugs, and cables. I was sure that the sync timing was OK putting my finger in the piston 1 spark hole method, clicking the starter until i felt a blowing compression on it and making sure the distributor pointed to piston 1, then of course put the spark plug back and the cable connected.
-i tested that every cable had spark when cranking (unplugging the cable and put it close to the engine so i saw the spark)
-it has gas

After all these actions performed the engine starts but it does not accelerate and then it turns off. (i feel a little frustrated)
How i can test that the carburator is OK? Or should i replace it?

Have you set the timing?

bucket · May 9, 2026

Ricko1966 said:
Check the ignition out. Was someone messing with timing,spark plugs or wires etc. And for your future knowledge 1987s in the United States were nit carbureted,so you might dial your comparison of your truck to American spec trucks a year or so,so you don't get wrong parts or advise. Have a great day.

From what I recall, the TBI was not standard in '87. It was optional, but most trucks got it. It may have just been the 305 that was standard with a carb, not the 350. But I don't remember. It might have also just been the early production 87's.

Ricko1966 · May 9, 2026

bucket said:
From what I recall, the TBI was not standard in '87. It was optional, but most trucks got it. It may have just been the 305 that was standard with a carb, not the 350. But I don't remember. It might have also just been the early production 87's.

I'm pretty sure,sure enough I'd bet money that 99% of 1987 1/2 tons had tbi and all 86s were carbureted. Look up some fuel pumps. BATM I will later. I'm back.

CalSgt · May 9, 2026

I’m with @Ricko1966 and @Sad Sack, I’d go back to checking the ignition timing. You could be anywhere from a tooth off to 180 degrees out.

Double check firing order and plug wire routing.

What was the truck running like before working on the carb and distributor?

Ricko1966 · May 9, 2026

Sad Sack said:
As @Ricko1966 said ignition should be checked, using the finger on the spark plug can work for finding TDC but the process can easily have the piston on the downstroke. I use a compression tester to find TDC. Easier when you’re by yourself.

Easier yet take the hose off the compression tester tape the whistle from a party noise maker on it,it will make a sound like a duck as you are coming up to TDC. Or twist tie a balloon to it,it will inflate on compression,deflate after you cross tdc. One more backyard way plug the spark plug hole with a cork,just like spit balls in grade school,the engine will blow the cork out as you are coming up on compression stroke.

bucket · May 9, 2026

Ricko1966 said:
I'm pretty sure,sure enough I'd bet money that 99% of 1987 1/2 tons had tbi and all 86s were carbureted. Look up some fuel pumps. BATM I will later. I'm back.

Yes, I'd bet that 99% of them were too. It also may have been a credit option to delete it. I know I've seen a couple small block '87 half tons that were carb. I vividly remember the first one I saw. It was a white C10 longbed, under 100k on it, no rust and straight from Tennessee. My boss at the time had bought it. That's wheen my boss (Owned the junyard I worked at and he loved GM trucks) schooled me on it. This would have been in '02 I believe.

Ricko1966 · May 9, 2026

bucket said:
Yes, I'd bet that 99% of them were too. It also may have been a credit option to delete it. I know I've seen a couple small block '87 half tons that were carb. I vividly remember the first one I saw. It was a white C10 longbed, under 100k on it, no rust and straight from Tennessee. My boss at the time had bought it. That's wheen my boss (Owned the junyard I worked at and he loved GM trucks) schooled me on it. This would have been in '02 I believe.

Well I sure won't argue the subject, not now that you have that big green badge under your name

350 Engine extreme backfiring please advice

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