i have a 99 zl 500 efi and finally got to ride yesterday, ran great except for every few miles. if i am on the throttle varying between 5000 and 7000 rpms and then gun it, it acts as if i fouled a plug if i let the rpms come down to say around 3 or 4000 then get on it, it's fine.

it acts like a fouled plug only when i have it on full throttle, if i let off and gun it, it will pull strong untill about 6000 rpms then act like a fouled plug

i checked the throttle saftey switch and that is good.
anything else i can check?

don't know if it helps, but i have a d&d clutch kit and a pipe/chip combo other than that it's stock

please help!!
thanks

update:
found out that one cylinder isn't firing every so often
i warmed up the motor and at around 5000 rpms, i gunned it and let off, over and over again and then just gunned it and got it to do it again. i then killed the engine and checked the spark plugs. the plugs looked great, but the one was wet.

i then changed coils with my friends zr 600 and it still did it.
i also checked the the light on the cdi box and it is giving me one blink (which i believe means everything is good in the cdi box)

i will compression test it tomarrow, but i don't think that would have anything to do with the plug being wet.

does anyone have anything i can try or any ideas of what it is?

thanks for any help!

Are you running EYA plugs? How rich is your oil mixture? I will however mention that modding an EFI in many cases produces the exact type of issues your going through. Another thought is to have your fuel pressure checked, yup high or low PSI can cause it to go lean or rich.

Happy hunting
Rick

it`s the throttle safety switch or bad wire connection on the coil .let me know what it is.i think if you put the kill switch bypass on it will solve your problem . M

Do you have fresh gas??? Mine did that alittle in the begining of the seasson. Not much, but after all the bad gas got ran threw, she was running good. And I unpluged the saftey switch in the throttle. Did you un plug the whole thing? or just take the kill switch out???

My first guesses would be throttle safety switch and loose plug cap to wire connection. Both are easy and free to check. The washers for the throttle pivot pin should only cost a buck or two.

the throttle position sensor is good, i did unplug it all the way.
i was out of ideas and switch to the stock chip and couldn't get it to bog any more.

called d&d and they said to check compression, checked it and it's 110 in the clutch side cylinder and 125 in the one that's loading up.
called them back and they think that's it's compression but they feel the cylinder with the lower compression should be the one that's flooding.

i think i will just ride it with the stock chip this winter and in the summer when i install new rings then give it another try.

my question now is 15 psi alot to be off from each cylinder? it seems like it to me.
should i get the new rings now or wait till spring?
and when i do it should i just get pistons also?
and when you do pistons and/or rings, do you need new gaskets if the old ones are still good?

thanks for all your replies!

forgot to ask one other question.
d&d said that compression should be 140. this seems high to me but if this is true sounds like i should install new rings this season.
and there is 2400 miles on my sled now also

thanks again!

Compression should be closer to each other. Actual numbers are difficult to achieve with different gauges, but the 125 sounds pretty good and 110 is more than 10% lower. You should fix the motor now. I would likely replace both pistons and rings so you know you have a fresh top end. The cylinders need to have a fresh crosshatch for the new rings.

The throttle safety switch is not the Throttle Position Sensor (TPS). The throttle safety switch is in the throttle block and is activated by the movement in the throttle pivot pin.