Carburetor jetting

[biggiescott]

1996 gtx with 657x and twin carbs
I’m going to rebuild my carbs this winter and I’m thinking of re-jetting the carbs while I’m in there.
Note I have oversized piston 1mm over.
How much more will I get out of my machine it already does 47 gps speed? Is it going to be worth it?

 

[AKnarrowback]

No, not worth it at all. Run the stock specs for your carbs and leave it there. 47 for that machine is really good.

Jetting any leaner risks burning a motor, any richer will waste fuel.

You will not gain any more speed either way.

 

[POPPS]

Listen to AK.,,leave it alone.

 

[JeremyD615]

Yep. Sea Doo actually did a fantastic job with the stock jetting on the carbs, so there’s nothing to gain by messing with it.

 

[Utahhillz]

At what elevation? If I am consistently above 3000' should I rejet?

 

[JeremyD615]

You’re good with stock jetting up over 5000’. At 3k feet, you should be just about spot on with factory settings.

 

[Utahhillz]

You’re good with stock jetting up over 5000’. At 3k feet, you should be just about spot on with factory settings.

Thanks good to know. I am occasionally above 6100 but usually 3000-4600

 

[JeremyD615]

Stick with stock settings and you’ll be fine. At 6100, you’re still safe, but you’re going to see some reduction in performance. In those really high elevations, I actually recommend dropping your fuel octane before making jetting changes. In Utah, you should have 85 available, so I’d use that when you’re riding in thinner air.

 

[Utahhillz]

Stick with stock settings and you’ll be fine. At 6100, you’re still safe, but you’re going to see some reduction in performance. In those really high elevations, I actually recommend dropping your fuel octane before making jetting changes. In Utah, you should have 85 available, so I’d use that when you’re riding in thinner air.

Thank you for the advice. I will run regular. Instead of my usual premium. Great info.

 

[AKnarrowback]

I was going to say give me through the weekend to answer the higher elevation question. I'm going to be riding around 3000' for the first time this weekend, my normal riding is 440' roughly.

I will pass on some background on rotax rotary valve motors that I have observed from years of riding sleds. The one brand, of the major four, that was the least finicky to elevation change was the Ski-Doos with the twin rotary valve motors. We could run sea level jetting up to 6000' without having the motor "bog out" enough to require a jetting change. We lost performance but the machine still moved when all the other fuji/suzuki/yamaha motors would not get off idle without a rejet and needle drop.

I'll tell you that my 94 XP runs so much stronger in the cooler conditions, air 55-60, water 45-50. I did all my tweaking for cooler conditions and was amazed at how sluggish it ran the one time I ran it at 80F in a lake with 65ish water temps. A night and day difference. If I was going to run in warmer conditions, normal conditions for you guys, I would be looking into carb tweaking.

Basic rule of thumb is an NA carbed motor looses 3% of it's rated HP for every 1000' of elevation, with rejetting to keep the fuel air ratio as close to ideal as you can.