Runs good at idle and mid range but dies at top end. Not getting enough fuel. I have checked the obvious, fuel supply, fuel filter, air filter. The fuel bowl stays full, so it's not fuel supply. cleaned the carb twice it seemed very clean. No pipe restriction. If I slightly restrict the air inlet tube it will run fine. I the air filter was dry(no oil), so I added oil to it and that made it run pretty good but still a little lean, at times. I suspect that the high speed jet is too small, but I can't justify why it would go lean with out any mods. I beleive this quad is completely stock? The jet is #80. Does that seem small? Does the air filter require oil or is supposed to be dry? Any help would be greatly appreciated.

 

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Welcome to this forum.

If you feel its dying above 3/4 throttle, then you may need to adjust the float setting. RE: float bowl is running dry when going above 3/4 throttle. If you feel its float setting is ok and it isn't plugged with a grain of sand / junk, then proceed into reading its spark plug color for more details.

Surf the pictures at the middle to bottom of page: http://www.dansmc.com/Spark_Plugs/Sp...s_catalog.html

Perform a Spark Plug reading at 1/4 throttle, 1/2 throttle, 3/4 throttle and wide open. Ensure you're getting "in the good to great" range of colors. If too lean in the 1/2+ throttle range, then lower the carb's needle C-Clip setting. This will make the carb run "a little richer". Keep moving the C-Clip setting downward (which raises the needle). If you run out of C-Clip notches (on the needle), then upgrade to 1 higher jet, put the C-Clip at the top most notches and keep adjusting it downward - until its spark plug color is within the proper color range.

Size 80 does sound small to me. The Pred-90's 16mm main jet is an 82.5. But.... If your elevation is completely different then mine, then 80 could be right - for your altitude.

Hope this helps....

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Thanks fo the rapid response. I was thinking that 1 up size would do it. Could you tell me if the stock foam air filter element should be oiled? I don't want to rejet until I know that I have the air cleaner correct.

Thanks,
Jtbull

 

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Our '03 Pred-90 uses a "dry" filter material within its factory air box. Been running like this for 2 years. Last spring, I started to Oil our "new" material via "proper Air Filter Oil". To do this, I put 2" of oil in a large epmty coffee can and use my bare hands to dunk / soak the filter. Then, I use my hands to squeeze out the excess (back into the can). Same method one would use to squeeze one's socks semi-dry. Then, I install this oiled filter material and double check its spark plug via 1/2 throttle and full throttle readins. Most of the time, NO carb adjustments are required. Why do I now oil our filter??? Because I've read many times that an oiled filter works much better then a dry filter. It keeps out the small dust / crud much better and in the long run, it will make our engine last longer. Since many sports ATVs recommend their air filters oiled and they are on the same trails as our Pred-90, I figure oiling our air filter would be a "much better" thing to do.

If you do "average" trail riding (like we do), its up to you if you want or don't want to oil your air filter material. Some people follow their manual to a "T" and other people follow what make sence. To me, Oiling our Pred-90 air filter material makes good sence.... Especially on the dusty trails....

Hope this helps....


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Your cat 90 has a completely different intake system than a predator 90 with your under the seat airbox. The filter is a dry foam filter. If the spark arrester screen is taken out it will run lean with a 77.5 or 80 mainjet if totally stock a 77.5 or 80 is stock jetting. I would think you have a small peice of dirt in the carb or an air leak in the intake tube/snorkel/airbox. If no leaks are found then I'd rejet it up 1 or 2 sizes and see how it does. With a 90cc 2 stroke cat theres restrictors in the carb that only lets the throttle be opened 1/2 way, a CVT restrictor that limits top speed, and the endcap of the exhaust is restricted but needs a 2 mainjet increase when removed. The cat 90 is the most picky bike about jetting I know of. We have owned and raced the Cat 90s with great success but stock I hated them. If you cant fix it I have a 90cc cat carb I'd part with cheap enough. Let us know if you figure it out or need more help.

 

Opened up jet .001 in. Ran good until it got hot. Checked compression Only 70 psi. Looks like a new set of rings or bore is in order.

Thanks for all the help,
jtbull

 

Bore is probably good. I'd just get a new piston and rings not too expensive. You can PM me if you cant find one. I've got one in the garage.