need help from mechanics and Supreme Mariners

[matth121]

Hey guys, got a frankenmerc here- thought it was a 25hp from 1981, turns out it was an 18 wearing a 25 cowl. Apparently, the 18, 20 and 25 from 80-82 i think, were all the same motor with the exception of the main jet in the carb and the tuning/exhaust pipe in the driveshaft housing. So, it should be easy to switch an 18 to a 25 right? So upon the advice of a well respected member here i decided to be sure my main jet was .067", what would be in the 25. Upon removing my jet, i found it was a .052" which is how i knew i had an 18Hp. So i reamed it out to .067 and figured now i have a 25(especially since i replaced my exhaust pipe a few months ago with a larger one than was there). Went out yesterday.

from lets say 17mph as an 18hp down to 7.5 with the newly reamed jet. I just don't get it, i opened that jet way up and it seems like now its not getting fuel. Or maybe thats not the problem. I'm open to any and all suggestions. I really dont want to scrap this motor, but if I dont get it running right soon, i'm thinking i might have to. New fuel pump and lines all over, carb rebuilt a month ago, new stator, coils, plugs and wires

it was ok until i reamed out that jet.Thanks guys
Matt:facepalm:

 

[Silvertip]

Re: need help from mechanics and Supreme Mariners

The only way you can tell what is different between three motors of the same displacement is to compare the parts lists. Just because an engine has the same displacement as another does not mean the porting is the same. So check the block part numbers. If they are the same then porting is not an issue. If they are different, there likely is some difference in the blocks. More fuel means more air and since an engine is basically an air pump, just changing jets means the engine must also breath better and that means at the intake as well as the exhaust. Ignition timing and the advance curves can also be different.

 

[CharlieB]

Re: need help from mechanics and Supreme Mariners

Increasing the jet size without increasing the airflow thru the carb only fattened the mixture and causes the motor to run poorly.

If you check the parts lists for both motors you may find a slight difference in carb part #'s, a larger venturi for more airflow as well as the larger jet, AND a corresponding change in the exhaust tuner, together allowing more flow thru the engine to create more power.

 

[matth121]

Re: need help from mechanics and Supreme Mariners

my seloc manual does show a definite timing difference between the 18 and the 25 so that could be it, I'm so far unable to get it timed, i can't do it myself, all the people i know who could don't have an inductive timiing light since they're pratically unused nowadays. that and it has to be running at WOT to set it.

The advice I'm following is from a retired merc mechanic on this site, I'm sure I'll hear from him soon, but from what I've been told, the blocks are identical and apparently carbs too. I'm going to double check all my part info right now. Thanks for your thoughts guys

 

[matth121]

Re: need help from mechanics and Supreme Mariners

there are 2 different carb numbers listed and apparently i have to go see whats stamped on the flange- either BC1A BC1B or BC1C. its a tillotson rectangular bowl double float with integral pump. I'll get back to you.

part number 13848498 is the carb for a 25(should be stamped BC1C)


part number 13848704A1 is for the 18HP

heres the thing- my seloc manual has a set timing procedure for the 18HP. primary pickup timing is 12deg ATDC
the 25 has a note that says carb BC1A is 12deg ATDC and carbs BC1B and BC1C are 5deg ATDC- which means the 25 could have any of those carbs on it, doesnt it? I'll have the number in 20 minutes or so. Thanks guys

 

[matth121]

Re: need help from mechanics and Supreme Mariners

BC2A:facepalm: uggghhh.
wtf.
any ideas?

 

[matth121]

Re: need help from mechanics and Supreme Mariners

so no new thoughts on this guys? What now? replace the main jet with what was there before i started i guess. Any ideas on that carb? Havent been ablt to find any info on it, is it even the right carb or was that frankenmerc'ed up too? It is the right style but the BC2A stamped on the flange has me seriously confused. Appreciate any thoughts guys, also if possible, would be willing to buy a BC1C carb for a 25Hp, missed one on ebay as he wouldnt ship to Canada

 

[CharlieB]

Re: need help from mechanics and Supreme Mariners

The differences in the carbs are venturi diameter and throttle blade. Larger for the 25 for more air flow.

Order new jets the correct size to replace the ones you drilled to restore the motor performance.

If the diameter is published you 'could' have a machine shop ream the venturi, but it is risking ruining the carb. A replacement would be a better choice.

 

[matth121]

Re: need help from mechanics and Supreme Mariners

Thanks Charlie, going to order the .052 jet for the 18 and worry about converting to 25 when i have the right carb.