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mercruiser trim problem, backfire thru carb

E

effiezima

New member
one of the outdrives on my 67 Bertram won't stay up. It works fine but over several hours goes down. both trim cylinders have been replaced, and there is no sign of external leakage. it is a older style prestolite pump. problem number 2. the boat has twin straight 6 mercruiser engines. both start and run good, idle smooth, rev up in neutral ok. one however, as soon as you put it foward and try to accellerate will spit back thru the carb. then usually die. it will start right back up. even when it doesn't backfire therv is a huge hesitation. it has a rebuilt Qucksilver Rochester 2 bbl. any input greatly appreciated.
 
Has to be a 25ft model....

does the accelerator pump work on the backfiring engine?

If there are no external leaks from the trim system and you have replaced both cylinders, all that is left are the check valves on the pump assembly...you can try flushing the system or replace the valve assembly (if it is still available...)
 
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Yes it is a 25. the pump/valve is hard to come by, but I know a place that can rebuild them. Just wondering if there could be something else before I go thru the expense and hassle. As for the backfire, I initially suspected the pump(haven't yet changed it), but it seems to rev normally in neutral, happens only in drive. Also, although it would not be unheard of for a rebuilt carb to not function correctly, for what that cost I would like to give it the benefit of the doubt and look at other possible causes too. thanks for your help
 
Rev'ing in neutral is significantly different than under load...if the issue has been around since (or right after) the carb rebuild, its likely there. That said, like Guy suggests, any maintenance history on the engine would be useful in narrowing down the possibilities...lots of "potentials"...
 
Just another area to look at before any other work is done,

When a motor backfires thru the carb on acceleration under load but not in nuetral I would suggest removing the valve cover and look at the rocker arms while it is running and see if one or more of the rockers are not moving very much compared to others.

A rounded cam lobe will cause this issue.

It may not be this but I have seen this issue more than once and considering the age of the motors, (assuming original cam shaft) this could be your source.
If not than more than likely the accellerator pump circut is a miss.......
 
Really haven't done much to the engine other than routine maintenance but I know both engines are old and tired. I'll check a few things out this w/e and go from there. Difficult for me to get too involved as the boat is a few hrs. from my house and I need it to get to my cottage on an island before I can do much and can't disable it or I won't get back home. Thanks for the ideas.
 
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