help! apex 90 rebuild

[trxHRC450r]

Did a top end on a 90cc apex atv. Runs great on idle and right off up to 1/4 throttle but breaks up right before it hits powerband. Any ideas. It's all stock w 118 mainjet and stock bore with wiseco piston. Running out of ideas. Please help.

[chunky0071]

Check your jetting jetting can change with top end builds!

[trxHRC450r]

I only had larger main jets. I'm thinking I have to go smaller. Didn't really ride the bike before so I don't have a baseline to go off of. Gotta hit the bike shop and grab some jets. Its got the 118 main in it now I'm thinkin 110 or 100?

[chunky0071]

Around a 110 main is were we run our 90.

[nastynotchback1]

Check for an air leak also.

[trxHRC450r]

Ok thanks. I will try that. Have u rebuilt your apex top end yet? If so, did u have any issues like this?

[fomospede]

there are always people here getting help sometimes its simple sometimes not
when you did the topend was the arrow on piston pointing towards the exhaust exit?
throw a compression check on it
did you check the squish?
when you gave your explaination for your problem you didnt mention if you were riding it or if it was on a stand?
answer these questions if you havent figured it out and we can go from there
:D

[trxHRC450r]

I sprayed carb cleaner around base gasket to check for air leaks and had no raise in rpms. I rode it and revved it up with the back wheels off the ground. As far as the squish I really don't know what that means.

[trxHRC450r]

O an the piston arrow was def pointed toward the exhaust. Would it run at all if the piston was in backwards??

[nastynotchback1]

You should check around the intake and crank seals for air leaks also.I have rebuilt alot of top ends and never had an issue like this.

[trxHRC450r]

Ok thanks. I'm gonna mess with the netting next week and ill let u know how it turns out. The motor just sounds so crisp at idle and when its reved up a little but at soon as u get on it it just breaks up

[selbygirl]

check your spark plug wire , you may of nocked somthing loose , i had somthing simaler , i cut about a 1/4 inch of both ends of plug wire and re atach it , and i zip tie my plug wire to the spark plug . sometimes they shake loose. your main jet is for 3/4 and up your c-clip is for aruond half range . and your pilot jet is from idle to 1/4 range on the throttle

[fomospede]

Originally posted by trxHRC450r
I sprayed carb cleaner around base gasket to check for air leaks and had no raise in rpms. I rode it and revved it up with the back wheels off the ground. As far as the squish I really don't know what that means.
squish
distance between piston and the cylinder head
should be around .027=.030
i have had a similiar problem it was the clutch bell out of round
the possibilities are endless
leave nothing overlooked!

[selbygirl]

a weak shoe spring will feel like carb problems also

[chunky0071]

Ok # 1 you changed your piston from a stock piston to the Wiseco or is that what was in it?
#2 If it is doing it on the stand then you have a jetting or electrical problem.
# The main jet does effect 1/4 to full throttle change it is just metered by the needle from 1/4 to 3/4 throttle.

When going to a wiseco piston it can and will effect jetting !

[trxHRC450r]

Got it running nice on a 100 main jet. Had a 118. U thing I should try a 105, 110, and 115 to make sure I can run it as rich as possible

[selbygirl]

i would go fat as you can on the main . till it runs good .check your plug color it should be a nice brown color

[trxHRC450r]

Got it running pretty good on a 112. Seemed like it was breaking yo slightly w me on it but sounded real good when my neighbors kids was riding it. U think if I used both base gaskets that were in the kit I woulda had these issues??

[chunky0071]

yes and no but you should have used both gaskets to get the cylinder to seat at the right height. with only one of those and they are different thicknesses you are not fully opening your ports!

[flooringit]

So your suppose to use both base gaskets?

[flooringit]

Ive always heard if you raise cylinder you get top end,if you lower get bottom end. I fixed the exhaust port. Was hoping that would really help top end. Didn't have the right bit to widen exhaust port. But that exhaust flange is off 3mm that's crazy

[mmsoup]

You already got a response on squish..........

If you add a base gasket and get too much gap you could lose performance, if you are too close, you could add heat by raising the compression too high and facilitate detonation and possibly shorten the service life. (Course you could raise octane to help to a certain point)

Make sure you are around 30 thousandths and you will be fine.

Take the spark plug out and place a piece of sauter inside the cylinder against the wall of the cylinder and rotate the piston until it smashes the sauter.
Measure the smashed area and if it falls between 27 and 31 thousanths go rip up the dirt.

The timings are already cut into the cylinder and they will work best at the settings above

[flooringit]

I put both base gaskets on and did a squish test came up with .080.lol so im taking the cylinder in for some maching