Backfire

[johnnynegaunee]

At idle in the yard with the muffs on the '72 120, pre-alpha Merc I/O Islander I recently purchased purrs nicely and sounds fine even when throttled up. Out on the big pond, under a load it backfires, and sometimes dies too. I've done a bit of investigating: the dwell was set at 48 degrees, I changed it to 32. Number 4 cylinder reads 70-90 psi, 90 if I'm careful about the seal with the test meter. The others are 120+/-. I'm planning to check the dwell change to see if it cures the backfire, would that be the likely source of the backfire? Or is it the low compression in #4. Can I get by like this for a while or will this motor strand me soon? Whats the best engine to re-power this 21 footer (with gas at 3/4 dollars )?

Any advice would be appreciated, thanks.

 

[Limited-Time]

Re: Backfire

At idle in the yard with the muffs on the '72 120, pre-alpha Merc I/O Islander I recently purchased purrs nicely and sounds fine even when throttled up. Out on the big pond, under a load it backfires, and sometimes dies too. I've done a bit of investigating: the dwell was set at 48 degrees, I changed it to 32. Number 4 cylinder reads 70-90 psi, 90 if I'm careful about the seal with the test meter. The others are 120+/-. I'm planning to check the dwell change to see if it cures the backfire, would that be the likely source of the backfire? Or is it the low compression in #4. Can I get by like this for a while or will this motor strand me soon? Whats the best engine to re-power this 21 footer (with gas at 3/4 dollars )?

Any advice would be appreciated, thanks.

Low compression could be a bad valve, head gasket, rings, or a cracked head. If its a bad intake valve it would cause it to pop through the intake. I think a leak down test would be my next step.
As for how long the engine will run as is roll the dice, no way of telling.

 

[firehog6305]

Re: Backfire

it might last 5mins or 5yrs, its hard to say, but my advice is get it fixed, and if its a starcraft islander???? I believe its either a 22ft or 19ft, I dont think they made a 21fter, but I could be wrong, have been before ha ha, when was the last time it was tuned up??

 

[Bondo]

Re: Backfire

purrs nicely and sounds fine even when throttled up. Out on the big pond, under a load it backfires,

Ayuh,..........

Change your Fuel Filters,...... It's running Lean causing the Backfires........