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Discussion Starter · #1 ·
I've read a ton about jetting but am still having problems. Here's the details:

2008 Raptor 250
GYTR pipe & slip on
GYTR air filter & prefilter
GYTR jet kit
Outerwears filter on air box lid

After the mods above I rejetted per the GYTR instructions. It ran rich at the top end (bogged & stumbled). I proved this by running without the air box lid & it ran perfect.

I rejetted again (many times!) reducing the main jet & the settings below are what I'm running to date. I'm concerned it may be too lean. Seems counter intuitive to reduce the main jet so far with the mods above but the engine stumbles with anything larger. It also now seems to run a bit hotter than normal & backfires occasionaly when letting off the throttle making me think it's lean. Hope to read the plug tonight.

Current settings:

27.5 pilot jet (GYTR)
130 Main (Stock was 133.8)
Fuel screw out 2 turns
Needle clip at 3rd position from the top.

Also, has anyone come across a fuel screw adjuster that works? I've tried to make one but the area by the carb is so tight I end up burning my hand before the adjustment can be made. Any & all advice is appreciated.
 

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From my exp. backfire when the throttle is off indicates a lean mixture on the low end (pilot and/or fuel screw). This can also make the engine run hot, as well as the header pipe to glow.

As for the top end, I would not worry so much about number and worry more about what the plug looks like. I suggest getting a new plug, warm up the quad on the old plug, switch to the new plug and run the quad up to full speed, pull clutch and turn off... let it coast to a stop. Pull plug and read it. Then go from there.

You started right by going with a larger jet, and then work your way down to where it does not stumble anymore, then you should be close. Remember, the needle has a different profile I assume from the stock, so it's going to flow different. I know on the 660, if you mod it and don't change your needle, your jet size will be huge compared to stock.
 

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You are running way to lean on the main right now for sure. On my sons raptor, i have the full ct exhaust, k&n filter with no lid. Im running a 165 main, the CT needle, stock pilot, and screw about 3 turns out. Runs perfect!! Im about 500 ft above sea level. You might wanna check out this article

http://blogs.atvrideronline.com/633...p-quadcross-limited-mod-race-class/index.html

Sounds like the same setup you have. Seems that full exhaust's with an open style lid seem to like mains around 160. Good Luck!!
 

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Discussion Starter · #4 ·
Thanks for the good info. I pulled the plug last night expecting to find it dry & white as would be indicated in a lean condition. Actually it was medium to dark brown with the electrode being black. I'll replace the plug with a new one over the weekend & do a plug chop to see where it's at.

For the record, I've had every main jet size installed that came with the GYTR kit. I've also experimented with changing the clip location on the GYTR needle. One change at a time to determine if the condition improved. There was only one pilot jet provided with the GYTR jet kit (27.5) so that's what was installed & still is. The engine starts fine & idles good. OK. Any change larger (bigger number) in the main jet & the engine runs worse.

When originally experimenting with the jetting the carb the engine would run worse if choked & better if the air box lid was off, indicating to me at least that it was too rich. I tried but could not adjust the fuel screw while the engine was running due to the tight quarters near the adjustment so I turned it out 2 turns & left it. I'll try to make a tool this weekend to allow adjustment while running. I think this might help correct any lean condition on the pilot.

Still seems counter intuitive to reduce the main jet size after adding a freer flowing pipe & opening up the air box.

I'll repost after my experimentation over the weekend. Thanks again for your input. Appreciate any other advice.
 

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I assume by the airbox lid you mean like the one I had in this picture? If so, and you put that little cover thing over the snorkle of the lid, that restricts the airflow big time. I had a full exhaust, K&N filter, and the lid on my 250 and was running a 138 DynoJet main in it (Dynojets are numbered higher than the Mikuni/GYTR jets so it was probably equal to about a 135). It had the symptons of running lean, but showed a black, wet plug. I later learned that a lean misfire can cause a black plug as well. If it's doing alot of popping then its got to be running lean. I don't think the pilot or fuel screw are causing much of a problem since it idles fine. It seems to me like more of an issue with the needle. I'd try putting a washer under it or moving it up a clip.

 

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Discussion Starter · #6 ·
Thanks blue250. Yes, this is the same air box lid I have. I also do have the snorkel cover installed that came with the Outerwears lid. The air filter in your pic also is similar to the GYTR filter but I also have an Outerwears prefilter over it. You didn't say in your post if you used the snorkel cover. I would remove it but it seems better to have it in place as another filter for the incoming air. On the other hand, Yamaha didn't include a snorkel filter from the factory so maybe it's not necessary.

I plan to do a plug chop over the weekend with a new plug. Hopefully I can get a good read & go from there. I'll try moving the clip again to see if that helps & also maybe try a larger main jet or two.

It's not backfiring a lot so I think I am close. I'm just a little nervous about being too lean & burning up the engine. I'm also still struggling with the fact that it runs best so far with a smaller main jet than the stock factory jet before the pipe & filter change, especially after reading so many posts where guys are running much bigger main jets after similar mods with good results. I've yet to read anywhere that the main had to be reduced to run right. Just doesn't make sense from my experience.
 

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Discussion Starter · #7 ·
Follow up:

I did a plug chop with a new plug over the weekend. The plug was pure white with a dull white "soot" on the electrode. No hint of discoloration other than the whitish gray color on the electrode. Based on what I know this indicates a lean condition as suspected.

I then verified the fuel screw position with it running just to be sure. It runs best two turns out.

I also verified the clip position on the needle.

To help richen it up, I installed the largest main jet (140) from the GYTR kit. It seems to run OK but I was not able to make any extended high speed runs due to limited space.

Set up now is:

27.5 Pilot (GYTR)
Fuel screw 2 turns out
Clip on 3rd position from the top on the GYTR needle
140 main

I'll have to do some high speed runs on a warm engine to know if it's improved.

Also does anyone know what the plastic "cup" is that is installed over the main jet at the bottom of the float bowl? There is no reference to it in the Yamaha service manual. I forgot to reinstall it after changing the main jet. I don't think it's that important as I think I read in a Dynojet kit instruction that they recommend removing & discarding this piece.

I also found this in one of my internet searches. Interesting:

http://www.jtmc.co.uk/yfm250fi.htm

All comments & feed back appreciated. Thanks.
 

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I think they call it a splash guard. However, you don't need it, or I have yet to find a valid reason for it to be there. Mine is stock, and when I cleaned the carb, I left mine out by accident. It has ran fine since.
 
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