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1989 5.8L Cobra misses under load

J

jackpine

New member
My 5.8L Cobra starts fine, shifts well and will push my 27' Bayliner 2755 all day long at an idle. When I try to advance the throttle (either rapidly or slowly), I can't get enough speed to plane and the engine seems like it is missing or shuddering. Replaced the points, set the timing, rebuilt the carb, carefully setting both floats. It will smoothly go up to 2000 RPM in neutral without a miss. Would a damaged rubber prop hub cause the same "shuddering" symptoms? Just bought the boat this Spring. Thanks!
-Al
 
Have you ever had it on plane?

a single 5.8 in a 2755? That engine is going to have to be making 100% of it's rated power in that boat. Start with a compression test of the engine and look for any differences in spark plug appearance. Clean flame arrester? Good gas?
IF compression good, Check advance weights and springs underneath the points plate in the distributor. Rusted stuck parts may prevent timing from advancing.
Check fuel system and fuel pressure to the carb inlet when attempting plane and compare to specs.
Any water in the hull or excess weight in that boat is going to be an issue too. Check how it sits in the water compared to other 2755's.
Then there's a few other things to look for like exhaust restriction (flappers blocking exhaust) or failing/clogged manifolds.
Prop pitch.
But worry about none of that until you are convinced the engine is in olympic condition.

That's what I hate about under powered boats. You can't get on plane when even the slightest thing is amiss.

Ever try to start off in a standard transmission car in 3rd gear instead of 1st gear? That shudder is what an engine will do when it is asked to deliver more torque than it can physically produce.
 
Last edited:
Thanks for the tips!!! The 2755 has only a 8.5' beam, only room for one engine. Ran well at slow speed last weekend, I was able to engage the ESA and it never missed a beat at the lowered RPMs. Pulled both carb bowls and reset the float heights again. Started the engine yesterday, rough running at idle and when I increased the RPMs in neutral. Just received my factory manual will check out your ideas, thanks again.
hystat said:
Have you ever had it on plane?

a single 5.8 in a 2755? That engine is going to have to be making 100% of it's rated power in that boat. Start with a compression test of the engine and look for any differences in spark plug appearance. Clean flame arrester? Good gas?
IF compression good, Check advance weights and springs underneath the points plate in the distributor. Rusted stuck parts may prevent timing from advancing.
Check fuel system and fuel pressure to the carb inlet when attempting plane and compare to specs.
Any water in the hull or excess weight in that boat is going to be an issue too. Check how it sits in the water compared to other 2755's.
Then there's a few other things to look for like exhaust restriction (flappers blocking exhaust) or failing/clogged manifolds.
Prop pitch.
But worry about none of that until you are convinced the engine is in olympic condition.

That's what I hate about under powered boats. You can't get on plane when even the slightest thing is amiss.

Ever try to start off in a standard transmission car in 3rd gear instead of 1st gear? That shudder is what an engine will do when it is asked to deliver more torque than it can physically produce.
Click to expand...
 
I have the same boat. My engine used to stall after 2000 rpm. It was starving for fuel. Carb filter had black flakes caught inside. Turned out fuel pump paint was pealing from inside. Cleaned fuel pump loose paint. Everything worked like a charm.
 
I had a similar problem on my 92 4.3 and it wound up being cracked spark plug insulators.
Couldn't see the cracks in the porcelain insulators until I took them out and ran a magic marker all around the porcelain and then wiped it off. The marker stayed in the cracks.
 
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