backfires and carb adjustment

[2500sierra81]

so my truck will start and run fine, but it always has trouble turning off. sometimes nothing happens but other days, like today, it backfires every time i turn the ignition switch off. it doesn't just back fire right away, it fires to keep firing, and then rocks back and forth, once its settled it backfires real loud.

i stopped by the only local mechanic that knows how to work old cars and who has adjusted my truck before, and he said its probably the carb having an idle speed thats too high?

anyone else think that could be the problem?

if so how do i adjust it?


yesterday i also found my starter had come loose. is it possible its because of these backfires and rocking thing?

 

[crazy4offroad]

Timing too advanced, and possible vacuum leak.

 

[2500sierra81]

so is it regular engine timing or ignition timing?

i don't have the equipment to check for vacuum leak. guess i just need to take the truck in for an adjustment

 

[Boone83K10]

so is it regular engine timing or ignition timing?

i don't have the equipment to check for vacuum leak. guess i just need to take the truck in for an adjustment

they are one in the same. you don't need "equipment" to check for vacuum leaks. get some wd40. spray around the intake, bottom of carb, all the lines etc.

the engine will speed up when you spray into the vacuum leak.

as for the timing, you don't need equipment for that . follow the #1 spark plug up to the cap. make note of where it is on the distributor. Think of it in reference to a clock. Most #1 firing post on a SBC is at 5 oclock when you are standing at the front of the truck. Loosen the hold down clamp bolt on the distributor.

When you turn the distributor clockwise - you are retarding the timing. if you turn it counter clockwise you are advancing it. Turn it ever so slightly either way. I mean hardly at all because a small turn is a lot in degrees. If it backfires less, then continue until it is gone. Check the response of the engine by revving it a little. A delay is bad, quickly revving is good. That is the "poor mans" way of timing the engine.

 

[chengny]

Make sure your choke butterfly is not sticking in nearly closed position.

That will result in a high rev and a rich mixture - perfect for huge backfires when shutting down.

 

[2500sierra81]

since it only backfires when it turns off and never backfires while running i think its the choke butterfly problem mentioned above.

how much damage can backfires do other than pissing off your neighbors and other people in a parking lot?

 

[crazy4offroad]

It can blow your muffler apart. Running lean (which is what the closed choke butterfly is trying to compensate for) can make the engine run hot, burn up spark plugs prematurely and in extreme conditions burn a hole in a piston or crack a cylinder head. What brand of carb are you running? I'm betting it's an Edelbrock with a plugged idle air circuit which makes them run extremely lean, idle high, backfire, etc.

 

[chengny]

Maybe you could determine if the choke plate is not fully opening.

With the motor running and warmed up, remove the top of the air cleaner housing and look down into the carburetor.

If you see that it is closed, check for a hang up in the linkage.

If the linkage is free and the choke plate seems to be trying to hold itself closed, the electric assist choke pull is probably out of adjustment.

Assuming that you have a stock Rochester Quadrajet with electric choke assist, adjust the thermostatic assist as shown below.

1. Loosen the three clamping screws indicated by the arrows (one is hidden in the image)

2. Rotate the adjustment plate a couple of notches in the CW direction

3. Also be sure that the power connection to the assist coil is attached and tight (circled).

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[cooter55]

I had a similar problem and found the 12v battery wire on the starter solenoid had loosened and was just touching the ignition wire post, which kept the truck running rough when turned off. Check that because you mentioned the starter had come loose. Weird situation but it happened.