7.4 mercruiser backfire noise

[dausbury]

Looking for some help,I have a 7.4 mercruiser/bravo and have not had it on the water yet. It had some issues that needed attention before I hit the water. The water pump impeller was shredded and it appears that it was run hot because the inside of the one exhaust hose was burnt but not completely through and the exhaust tip shutter on that side was like a burnt piece of toast and the other one was out but I found it in the exhaust box and it was not burnt just came out. Upon inspection of the thermostat I found that it had been removed so I replaced it with the factory replacement. I noticed that the distributor locking bolt was lose and the distributor could be turned easily. Once I got the water pump rebuilt and the exhaust shutters replaced I got the motor started and reset the timing to 8 btdc per the spec. Now the motor starts and runs well at idle but when throttled up it starts making a backfiring noise but quits when throttled back down. There is no water in the oil and the compression is good (135 - 140) on all cylinders but their were two plugs on the side that got hot that were cleaner than the others. I am guessing that I have a cracked head or a head gasket bad. Any ideas on how to pin point the problem without an unneccessary tear down.

 

[6meter]

Re: 7.4 mercruiser backfire noise

Try running it and disconnecting one then the other then both "clean plug" wire. Does it backfire with one off? both off? either off? Is it backfiring or popping? If popping and with both wires off, yes running crappy, could be your coil. If it is more of a backfire and only stops when both wires are off, chances are the head gasket between cyl is blown. If backfiring stops with one wire off, its that cylinder somewhere, probably a valve. But 135-140 lbs comp says valves are good and head gasket good if "clean plugs" were adjacent. Were those pluge perfectly clean? Cylinder burning water will clean everything in the combustion chamber like new. Closed cooling system? If so pressure test it then take out those two plugs. Hold a white sheet of paper next to the plug hole and see if any water spits out while somebody turns it over.

 

[dausbury]

Re: 7.4 mercruiser backfire noise

Thanks 6meter ran motor on the hose and the backfire is only happening when reving the motor to about 2500 to 3000 RPM's otherwise it runs and sounds great. I noticed that as soon as the motor reached 160 degrees and the thermostat opened the exhaust on the one side started blowing steam. Checked the plugs again and the #3 plug was partially clean and the #5 plug was completely clean.

 

[6meter]

Re: 7.4 mercruiser backfire noise

Replace them with dirty plugs and run it. If they clean up after running a bit, you need to pull the head on that side.

 

[dausbury]

Re: 7.4 mercruiser backfire noise / water in oil

Re: 7.4 mercruiser backfire noise / water in oil

Still working on this project boat, had heads cleaned up and checked for cracks, replaced head gaskets and ran on muff sounded great. Put it in the lake and it powered out of the whole just fine heard intermittant popping noises but they quit. Came off plane and when I went to power back up not enough power to come out of the whole. Checked oil and found water in crankcase not alot but enough to make the oil milky. What is the process from here? I am thinking Cracked, exhaust manifold, Cracked intake, bad gasket seal, cracked block. What should be the order in the process of elimination be and what is the best way to locate a possible crack?