1967 Mercruiser 120hp Backfiring.......

[HOLIDAY120]

Greetings

I have a 1967 Mercruiser 120hp 2.5l 153cu that is backfiring through the carb between 2000-2400 rpms. Prior to it being torn apart and pressure tested, it ran perfect. The reason for testing it was due to water in the oil. That problem has been solved. However, not sure what to do with the backfiring. I've been caught off guard. I was assuming that everything would be good to go.
What could be the problem and how should I go about fixing it? I'm not all that mechanically gifted, but I have so far learned a whole lot from working with this motor.
Any suggestions would be greatly appreciated.

Kind Regards
HOLIDAY120

 

[Bt Doctur]

Re: 1967 Mercruiser 120hp Backfiring.......

might be cross-firing (2 plug wires touching)
burned intake valve or seat
weak valve springs/weak/bad lifter
carbons tracks in dist cap
water in the fuel,
just to name a few

 

[HOLIDAY120]

Re: 1967 Mercruiser 120hp Backfiring.......

Thank you for the feedback. One more question for you. How do you go about testing the valve springs and lifters?

Regards
HOLIDAY120

 

[Bt Doctur]

Re: 1967 Mercruiser 120hp Backfiring.......

Well, if a spring is broken you may visually see it or you should be able to push the rocker down with your hands and open the valve.
Carefully tap each rocker with a small hammer listening for a "soft" respopnse
Lifters dont usually just go bad, a leakdown test can be performed to test them, but if you can push the pushrod end dlown ,thats a good sign it`s bad/collapsed internally.
3A-13 and 3A-14 in this link explain it.
http://www.boatfix.com/merc/Servmanl/13/13A3R2.PDF

 

[cr2k]

Re: 1967 Mercruiser 120hp Backfiring.......

blown head gasket?

 

[HOLIDAY120]

Re: 1967 Mercruiser 120hp Backfiring.......

Thanks Bt Doctur - I'll check everything over and I'll let you know.

cr2k - not sure if that would really be it. I dont have water in my oil, plus the head gasket was just changed out, plus the block and head were just decked. I should be good there. I hope!!