90HP - rejet of carbs after powerhead rebuilt

[berndie61]

I'm currently rebuild my powerhead with oversized pistons 0.02". I'm not confident if it's necessasry to use new jets for the carb.

berndie

 

[Frank Acampora]

Re: 90HP - rejet of carbs after powerhead rebuilt

With an .020 overbore, you are going from about 75 cubic inches to 75.25 cubic inches. The engine will never even notice it.

What you must do, however, is to determine the reason you need to rebuild. Did you have a lean run on one or more cylinders, causing detonation and scored cylinders? You do not need to rejet the carbs if they are the originals, but you absolutely must at a MINIMUM clean them and adjust the low speed needles correctly. A common mistake is to set them too lean in order to acheive the best idle possible. This invariably results in melted pistons at or near full throttle. Idle mixture needles on three cylinder engines must be set NO LEANER than 3/4 turns out from lightly seated, preferably 7/8. And a fuel filter must be installed between the fuel pump and the carbs so a piece of junk will not get into one and cause it to run lean. A cheap 3 buck plastic filter is sufficient.

 

[berndie61]

Re: 90HP - rejet of carbs after powerhead rebuilt

This is the second time I have to rebuild the powerheads. First time I rebuilt my Force 90HP (Year 1991) after 13 years. In 2004 I had jamming and melting of the pistons. I assume it came from a lean run, or poor oil or maybe carbon builtup. I bought Wiseco piston oversize 0.010". Unfortunately I used wrong piston pin spacers. The spacers were to small and made an abrasion from the piston. The small aluminum particle went in the combustion chamber and caused again jamming of the pistons.

Thanks for the advice to take care with the right mixture to have not a lean run.

The pistons looks like similar in the pics from this thread.
http://forums.iboats.com/showthread....&highlight=125

It seems that jamming of the pistons appear often after a run of more than 10 years.