Logo

Mercruiser 260

Muller

New member
HI all,
I have a mercruiser 260 with 350 chevy enigine. Recently overhauled(last week) because of bearing problems. Now the motor won`t run propperly. If put under torque it dont plain at all, when outside with muflers on it only goes up to 4000 rpm. Did not had this problem in the past. Could it be that the valve timing is not 100% correct? Will the motor start if the valve timing is not 100% correct? It seems to backfire alot around 4500rpm and if it is getting too much fuel.
 
Ayuh,... My guess is a fuel restriction,....

Backfirin' is a sign of a Lean condition...

Rebuild the carb...Check the filters, 'n fuel pressure...

'n,... Why didn't ya post this in the Mercruiser forum,..??
 
............... Could it be that the valve timing is not 100% correct? Will the motor start if the valve timing is not 100% correct?
It seems to backfire alot around 4500rpm and if it is getting too much fuel.
The crankshaft to camshaft drive ratio is 2:1.
The chain sprocket teeth count is 22:44.

18246862-574-BBC-Chevy-True-Roller-Timing-Chain.jpg


360* divided by 44 = 8.1818* per sprocket tooth @ camshaft.

8.1818* @ camshaft = 16.3636* per sprocket tooth @ crankshaft.

(or you can divide 360* directly by 22 for crankshaft degrees)

Either way, being one tooth off would cause an approx 16.36* "out-of-phase" crankshaft indexing in one direction or the other.
I've never misaligned (mis-indexed) a SBC camshaft, but I suppose if you did, it may run... but certainly not very well.

Incorrect and/or too little progressive spark lead (progressive advance) will also cause what you are describing.
 
If you could only rev the engine to 4000 rpm while it was out of the boat unloaded (Assuming this is what you meant)

The problem is not going to be fuel related.

As Rick explained cam timing needs to be checked first.

If cam timing is correct, you will need to do a leak down test. Reason: I had a rebuilt long block from a known good builder that was giving me the same symptoms that you have. It would not rev past 4K rpm's, it popped back through the intake and the boat would not plain. Failed a leak down test, disassembled the engine to find a cracked head.

Your original post says Backfiring at 4500...Will it rev to 4000 or 4500?
 
Last edited:
If you could only rev the engine to 4000 rpm while it was out of the boat unloaded (Assuming this is what you meant)

The problem is not going to be fuel related.

As Rick explained cam timing needs to be checked first.

If cam timing is correct, you will need to do a leak down test. Reason: I had a rebuilt long block from a known good builder that was giving me the same symptoms that you have. It would not rev past 4K rpm's, it popped back through the intake and the boat would not plain. Failed a leak down test, disassembled the engine to find a cracked head.

Your original post says Backfiring at 4500...Will it rev to 4000 or 4500?

Checked the firing order carefully?
 
Back
Top