Thought I would pass along this tidbit to maybe help others avoid what I have been through over the past year with a backfire problem in my 1988 Sunrunner 302Ultra with twin Volvo-Penta AQ271C dualprop motors (Chevy 350)
A year ago while cruising, one of the motors started a backfiring noise through the carburetor anytime the throttle was cracked or over 1700 RPM. Sounded like a timing/spark issue, so started with replacing the plugs/wires, then proceeded through swapping the coil and distributor with the other motor without success.
Rebuilt the carb next, thinking that perhaps since I had just topped off the tanks, something got plugged up in the carb (the other engine still ran fine). Also the fuel filter.
Still unchanged, I pulled the heads and had them fluxed for cracks and stuck valves. Also checked cam and lifters - All OK. New gaskets and seals - still no change.
Busy summer, so just recently picked up on this again. Next in line was a new timing chain and sprocket set. The old chain was stretched quite a bit, but certainly still within range.
Replaced Exhaust manifolds and risers since #1 cylinder appeared to have some water intrusion based on the clean piston top relative to others. Still no resolution of backfiring.
At this point I was convinced that I had overlooked something stupid, so basic. So I reread my manuals, and found a passage that held some hope regarding checking for voltage drop to the coil.
Armed with this, I went through the electrical diagnosis chart and found I had indeed a 3.8V drop in voltage between the battery and the coil terminal. I went through every electrical connection between the battery, alternator, starter, relay, ignition switch and grounds, cleaning and tightening each connection. I was able to recover about 1.5 to 2 V of the drop.
Started the engine, and after revving it up a couple of times, the residual carbon from the misfiring was blown out, and then she purred like a tiger once more. Could have saved a grand if I had started from the basic electrical diagnosis instead of jumping in at changing parts...lesson learned.
Whats frustrating is that I had searched high and low on this and other forums and google and talked to service experts, and not one reference was found to highlight the voltage drop possibility. A potential clue that I overlooked was the timing instability. Due to the weak voltage, the spark was inconsistent, causing incomplete combustion to be blown back up the carb when the intake opened up as well as the timing mark to fluctuate. Now, it is as steady as a rock.
So now I am completely humbled, but wiser when it comes to engine diagnosis...thought I would share my pain to help others avoid this potentially overlooked problem/symptom.
steve
A year ago while cruising, one of the motors started a backfiring noise through the carburetor anytime the throttle was cracked or over 1700 RPM. Sounded like a timing/spark issue, so started with replacing the plugs/wires, then proceeded through swapping the coil and distributor with the other motor without success.
Rebuilt the carb next, thinking that perhaps since I had just topped off the tanks, something got plugged up in the carb (the other engine still ran fine). Also the fuel filter.
Still unchanged, I pulled the heads and had them fluxed for cracks and stuck valves. Also checked cam and lifters - All OK. New gaskets and seals - still no change.
Busy summer, so just recently picked up on this again. Next in line was a new timing chain and sprocket set. The old chain was stretched quite a bit, but certainly still within range.
Replaced Exhaust manifolds and risers since #1 cylinder appeared to have some water intrusion based on the clean piston top relative to others. Still no resolution of backfiring.
At this point I was convinced that I had overlooked something stupid, so basic. So I reread my manuals, and found a passage that held some hope regarding checking for voltage drop to the coil.
Armed with this, I went through the electrical diagnosis chart and found I had indeed a 3.8V drop in voltage between the battery and the coil terminal. I went through every electrical connection between the battery, alternator, starter, relay, ignition switch and grounds, cleaning and tightening each connection. I was able to recover about 1.5 to 2 V of the drop.
Started the engine, and after revving it up a couple of times, the residual carbon from the misfiring was blown out, and then she purred like a tiger once more. Could have saved a grand if I had started from the basic electrical diagnosis instead of jumping in at changing parts...lesson learned.
Whats frustrating is that I had searched high and low on this and other forums and google and talked to service experts, and not one reference was found to highlight the voltage drop possibility. A potential clue that I overlooked was the timing instability. Due to the weak voltage, the spark was inconsistent, causing incomplete combustion to be blown back up the carb when the intake opened up as well as the timing mark to fluctuate. Now, it is as steady as a rock.
So now I am completely humbled, but wiser when it comes to engine diagnosis...thought I would share my pain to help others avoid this potentially overlooked problem/symptom.
steve
