I'm hitting the rev limiter when I hold the throttle on my 300ex. I have 20 inches tires with a 14T countershaft sprocket and a 39T rear sprocket. What's up?

 

Buy a smaller rear sprocket,37 teeth maybe. Check into a new rev box, Powroll.com I think has them for the 300.

 

I just brought new sprocket+chain last week so I won't buy a rear sprocket again. I was having a 38T before and I never noticed this

 

14 and 39 is a good ratio, maybe try for a new black box. On the other topic try about a quarter turn at atime. But first turn it all the way in counting the turns. If it is more than 3 to 3.5 out then you need a bigger pilot.

 

If you went to a bigger rear sprocket, from a 38 to a 39, you raise the rpms at full throttle, so you hit the rev limit sooner. You will need to drop back to a 38 or a 37, or invest in a modified cdi with a raised rev limit.

 

I don't know why it's doing that. A stock 300ex is geared 13/38 with 22 inches tires and it's never going to hit the limiter in 5th

BTW, since people are reading this topic, check my 300EX DRAG RACING QUESTION TOPIC

 

Look at it this way, you've gone from a 22 inch rear to a 20 inch, this alone is the same as adding 2 or more teeth to the rear sprocket, add a freer flowing intake and aftermarket exhaust, and you will be hitting the rev limit way sooner than stock. You can't keep stock gearing with the setup you have, either that, or you will need a modified ingition to rev higher. I put 20 inch tires on, lost power hitting the rev limit, went from a 13 to a 14 tooth front sprocket, ran fine. I put a Supertrapp exhaust and Uni filter on, rejetted,ran great, but started hitting the rev limit near full throttle in every gear, because now it can rev freer without intake and exhaust restrictions. I got a cdi off ebay that has the rev limit removed. Now it revs great, full speed runs let me rev it out in each gear, and 5th wide open runs fine. You have 2 options: regear or buy a cdi ( a third would be put 22 inch tires back on, just to see the difference it makes.

 

Is putting on a new cdi great? Can I badly damage the engine? Is it faster with it?

 

If your hitting the limiter in 5th and it's not gearing you clutch could be slipping. Thats what happened on my friends 400.

 

A modified cdi will usually have the rev limit raised 1000 rpm to make the most of engine mods, the one I have has the limit removed, with one like that, you just have to use common sense and not rev the living daylights out of it. With a mostly stock motor,you will likely float the valves before you do any real damage, but with an aftermarket cam and valve springs,you could probably hold high enough rpm to do damage. As long as you use the cdi to just shift in the power band, and not hold it while the motor revs to the sky, it won't do any damage.

yellowexracer - that icon of yours sure looks painful, too much chilli and barbeque??????