weird jetting going on

S

snowman_959

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Nov 8, 2021
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2000 yamaha mountain max 700, all stock expect for mbrp trail can, slydog trail skis, and kimpex seat, and jetting. i ride sometimes -30°f to 44° above close to sea level and even drove to 1000° lol. The jetting on it right now has 128.8 on all mains and 62.5 pilots. i did get new jets. 147.5 MJ 1 and 146.3 MJ 2 and 3 with 45 pilots and the sled didn’t want to start. swapped back to old jets and started and ran fine like it did before. is the jetting correct or is there something i’m missing. thanks in advance!
 
i have yet to check the jet needles but i haven’t done it before. also i don’t think there was any porting done to it either
 
I'm wondering how you haven't burned down with your jetting. 62.5 pilots seem way fat for sea level. I couldn't even get my sled to start on 62.5s it was so rich. Dropped to 55's and it fires right up and idles clean.
 
I don't mean to hijack, but this brings up a question. What is the logic on the larger pilots for altitude? I read on a MX forum that at altitude there is not enough... negative pressure? to pull the proper amount of fuel through the smaller pilot jet, so they recommended going larger since less restriction would allow more fuel to flow. But it would not flow at the max capacity of the jet at altitude. I haven't read this anywhere else nor seen it reflected in jetting charts so I am wondering if this is true or not.
 
MNMead said:
I don't mean to hijack, but this brings up a question. What is the logic on the larger pilots for altitude? I read on a MX forum that at altitude there is not enough... negative pressure? to pull the proper amount of fuel through the smaller pilot jet, so they recommended going larger since less restriction would allow more fuel to flow. But it would not flow at the max capacity of the jet at altitude. I haven't read this anywhere else nor seen it reflected in jetting charts so I am wondering if this is true or not.
Click to expand...
It's true. Less vacuum created with the different air so need larger pilots.
 
That sled had safe jetting for around 10,000 ft ASL!
I would start with a complete disassembling and cleaning of the carbs...don't forget the little top hat filters hidden under the float needle and seats.
As stated check the slide needles clip positions as well as the size stamped on the needle tubes.
Put everything back to stock settings found in the tech section and you should be good to go. These Red Heads typically are very forgiving when it comes to carb settings as long as they are not too lean.
 
I will try and check the jet needles tomorrow. i was looking at info on jet needles and seen there is a screw on mag side. some said not to mess with it, so do i just check the other 2?
 
No you need to do all three. Just work around the brass screw. Should only tak 10 minutes to do all of them.
 
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