Lacking top end power?

[Iliketogofast]

I rebuilt my 310R not too long ago but I haven't really gotten around to riding it much. It dragged down in the top end and I dismissed it as a clutch issue and put it back in the garage until I had the money to try and fix it.

I got it out today and I don't really think it's the clutch. I was riding around the yard and I actually thought that it had somehow fixed itself. It has great midrange power, and I didn't notice that the topend was lacking.

When I got it on the road though I noticed the problem. As soon as it gets to the top end, it's like some type of power light switch goes off and it just starts dragging. It's almost literally like the brakes are dragging. It still accellerates but it feels like a slow 4 stroke instead of a 2 stroke.

Any ideas?

[headache]

maybe the piston clearance is too tight?? when the motor gets good and warm its trying to lock up... Just a thought. Did you put in a new piston when you rebuilt it?

[86 Quad R]

have you completely dialed in the jetting consisting of a WOT plug chop on it? sounds like your jetting may be off.

[gandgracing]

what carb do you have and what is the jetting? Does the plug look dark?

[Iliketogofast]

Plug is dark, not black but dark. I think I may need to go a little slimmer on the main jet? Also the threads on the plug were kind of wet with gray stuff.

Entire motor was rebuilt top and bottom, new piston, bored and honed, bottom end bearings, has a 38MM A/S carb.

Actually the jetting hasn't been touched because ESR said they jet it for your setup, and when I first tested it myself the plug was tan but now it is dark.

[Iliketogofast]

Brought it down to 180 from 185 main, plug color is better but it runs no better. It sounds pretty healthy, it only misses every once in a while on the top, but there is still no power up there. It's a rocket through the mid but once it gets to the top it's just gone. It still sounds pretty healthy running, clean and crisp but it misses here and there in the top.

I changed the gap because it looked a little tight but then it cut out and would stay running but it ran like crap so I brought it back down.

I'm thinking maybe weak spark? What solution is there to that even besides CR ignition (which I can't afford)?

Also I want to note that this is a new bottom from someone online. I don't have a way of knowing for sure if it had this problem in the first place, so it isn't like it ran fine then I rebuilt and now had this.

Also, I haven't screwed with the ignition timing. Maybe the timing should be advanced on this motor?

Thanks for the help so far guys.

[Iliketogofast]

Bump because I still haven't figured it out.

[deathman53]

how was the cylinder ported? what about the pipe on it? Those 2 can have a big effect. Also how much compression, really high compression makes awesome low-mid, but is fighting itself in the top and doesn't make such great power.