Chevy 350 back-firing?

This is a Mercruiser 350 Magnum (Chevy smallblock). 1996 with a Carter carb, about 350 hours use. I had the carb professionally rebuilt because it was very difficult to start cold, and after running with a load it would diesel after being shut off. After the carb installation, this thing ran like a champ. After the first 30 minutes I would get the occasional (10-20 seconds apart) backfire through the carb. This seems to only occur after running under load for at least a few minutes. It runs great and strong between backfires. I have not checked the timing yet, but I have never, ever had it backfire before. I have adjusted the idle jets so that I don't get any dieseling. Could a different carb setting reveal a bad timing setting? Is the float level wrong? What about adjustments on Carter carbs? I have not fooled with wiring, plugwires, distributor etc. so that can be ruled out. You guys who eat, breathe and live chevy...help!
Chevy 350 back-firing?
the needle might be sticking
Chevy 350 back-firing?
did the carter come with the motor and just develope the problem? have you tuned the carb? if not tune it, especially after a rebuild. check your timing. if niether of those help try swapping carbs to a different brand. it seems like you may be on the right track by looking at the carb.
Reply:I own a shop, and think this; The dieseling is caused by the idle speed being too high, and in some cases; the carb may have a solenoid that allows the carb to completely shut off when powerd down. The mixture screws should be adjusted with the use of a vacuum gage attached to the intake vacuum. Adjust them for the most vacuum while keeping the idle down to around 850 rpm's. Turn the mixture screws all the way in till they bottom out, but don't bottom them out hard, or it will damage them. Back them out 2 and 1/2 turns, attach the vacuum gage, and with the engine at normal temp, turn the screws one at at time (in) till the vacuum starts to fall, and back it out till it clears up. Keep going until the vacuum begins to fall again, and then go in with the screw till it reads the best vacuum reading you can get. You are trying to get the best vacuum reading on both screws at idle speed. The 2 and 1/2 turns is your starting place to begin with.


As for the backfire; It may have the vacuum line going to the distributor hooked to the wrong outlet on the carburetor. It must be hooked to ported vacuum. Meaning: No vacuum is on the line until the throttle is mashed. If it has vacuum on it all the time; it will backfire once in a while as its throwing the ignition timing off at cruise speed. A bad vacuum advance chamber on the distributor can cause this symptom too, so check to make sure its moving the advance plate, and it holds vacuum without leaking down. Remove the spark plugs, and if they are black with build-up from too much fuel, you will need to remove the upper portion of the carburetor, and adjust the float down about 1/8 of an inch. Keep in mind; your old carburetor may have fouled the plugs. A backfire can take place if you have a bad plug, and when it gets under a load, the backfire shows up. Clean the plugs good, and set the gap. If it keeps doing it; then replace the plugs with new ones. If this doesn't take it away, then start looking for some problem with the cam being worn down, or a lifter may have a tiny hole in it causing this problem. A good vacuum gage can tell you a lot about the health of the engine, and is a fine tunning tool to own. Also; if the ignition timing is out of adjustment; it can cause this problem.


Glad to help out, Good Luck!!!

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