7 hp by changing a jet?

[huntxtrm]

I was looking in my repair manual for my 18 hp 1972 rude, and noticed the only thing different between it and a 25 hp same year is the fast idle jet size. Higher wide open rpm's are also higher for the 25. bore, stroke, compression, ect... are all the same. Does anyone know this to be true. Is it that easy to kick it up 7 hp? By just changing the carb, or the fast Idle jet?

 

[bgbass.1]

Re: 7 hp by changing a jet?

You have to change the intake as well as the carb. if you look at both the 18&25 intake and carb side by side you will see the difference in the hole sizes.

 

[ottertail]

Re: 7 hp by changing a jet?

I took a quick browse thru the parts books and the 2 engines do run the same carb body and same intake manifold and the same exhaust tube. go to the BRP on line sight and carefull compare the parts between the two. The difference very well be just the jet and rpm. In the johnson world that was a 20hp. You will not gain 7 hp but you will gain some.

 

[F_R]

Re: 7 hp by changing a jet?

Ain't no way you are going to increase horsepower by changing the jet. But what it will do is make it run lousy. Try it if you don't believe me.
Now, as for the real differences. I'm not going to waste my time looking it up, but if you care to, I believe you will find the pistons and cylinder heads are different.