carb jet swap

[oldblue]

I was looking in to buying a larger motor when someone told me I could change my jets for the extra I have a 99' yamaha 40 w/o oil injection and when I looked online at part break downs every thing but the jets were the same between the 40hp and the 50hp of the same year model. Can any one tell me if this would work or why not or what else might need to be done with it to make it work right. I sure do like the idea of $100 in jets than $2000 used motor just for ten extra hp

 

[rodbolt]

Re: carb jet swap

try it,
both the idle and mains jetting are different.

 

[oldblue]

Re: carb jet swap

Would I have to adjust the pilot screw or would just changing all the jets be ok I dont want to lean it out I did see that the spark plugs were diff also prob. just a little hotter

 

[ondarvr]

Re: carb jet swap

Changing the jetting does not increase the HP, the jets would be different between the two motors only if there were other changes that require them to be different..

Jetting is either right or wrong, typically the factory jetting, while not perfect, is close. Using bigger jets would cause it to run rich, using smaller jets would make it run lean and both conditions can cause poor performance and damage. Even if the stock jetting isn't exactly right, you will not get a 10HP increase by fine tuning it.

If there are other internal differences between the two motors, larger carbs, different porting, different intake or exhaust, higher compression, etc, then these differences may require different jetting.