carb change

[nathanglasgow]

I have a 1979 20hp 207H9B with a wrecked carb and i have just swapped it with one from a 65hp(needed some minor mods to fit).They both are basically the same except for the 65hp having a larger intake opening.Does anyone know would it be wise to swap over the main jet from the old 20hp carb into the new 65 hp one?The engine seems to run well with just swapping the carbs but as the 65hp has a larger jet will it run rich or increase fuel consumption if i leave it as it is?

 

[jerryjerry05]

Re: carb change

Change the jets.Jerry

 

[Frank Acampora]

Re: carb change

Dont change the jets! You can NOT arbitrarily decide to change jets between carbs. These carbs are designed, air-bled, and vented to use a certain sized jet while maintaining a 16 to 1 air to fuel ratio throughout the operating range. The only acceptable jet change (for a novice) is to compensate for operating altitude above sea level.

If I remember correctly, the jet on the 20 carb is smaller and if you swap it into the 65 carb you WILL run lean and melt a piston (or two).

I changed carbs on my 20 a while back except I used a carb off an 85 HP engine--1 5/16 venturi just like the 65 carbs-- and I left the jet alone. The 20 ran fine (but with almost no increase in power) for several weekends before crapping-out. It has been a while, but I think I remember it did melt a piston.

So, if you continue to use the 65 HP carb, go RICHER on the jet--I would say about two steps. ---Better to change fouled plugs than melted pistons.

 

[nathanglasgow]

Re: carb change

I guess i'll leave the 65hp carb alone with its original jet.The motor does run rich like this as the new plugs i put in are black within a very short space of time and are wet with fuel when i pulled them.As you say better to run slightly rich than melt a piston.Thanks for the input