High Speed Orifice Jet Size

[Jose67]

When an engine has been bored oversized (.020) should you increase the high speed orifice jet size? By model number my engine calls for #59D jets with other models listing #69D and #70D. Should I stick to what is recommended for my model engine?

 

[Joe Reeves]

There is no need to change the high speed jets even if the engine was bored to .040 (or higher). Stick to what the engine calls for originally.

 

[F_R]

There is no need to change the high speed jets even if the engine was bored to .040 (or higher). Stick to what the engine calls for originally.

Agreed, with one exception. It is possible that the jets have been changed to smaller ones, causing a lean running condition and resulting in damaged pistons. If that is the case, do what was just suggested--use what the book says.

Well two exceptions. In rare cases, the original jets were too small and a service bulletin was issued to change them. This happened mostly in the 1970's with the advent of crummy low octane unleaded gas.

 

[Tim Frank]

....and one other. If you are not the original owner, you should verify what jets are actually installed....and if you can, that the carbs themselves are appropriate.
There are a lot of on-the-fly modifications done with good intention but poor results. IMO.

 

[Faztbullet]

I disagree as anything over .030 needs up 1 size.....

 

[oldboat1]

2 threads on this motor again.

 

[F_R]

I disagree as anything over .030 needs up 1 size.....

I know your qualifications are unquestionable and I respect that. However do you know of anything in print from OMC that states that? I would be interested in seeing it for my own knowledge.

We had our own boring bar back in "The Day" and I've bored many cylinders, both for our own shop and also all the other outboard shops in town.

 

[Faztbullet]

 

[Faztbullet]

The above is from OBR which a lot of sellers on sites use. I go up 1 size at .030 due to crappy fuel and it makes pistons/cylinders happy!!!