300ex motor mod questions

[MX174]

Well the lil 300 just doesnt have enough power for me anymore. I was thinking about going with a stock bore high compression piston. How much will this affect its power? I was also thinking about going with a .20 over high compression piston. Which of those would be a more reliable piston to run? I plan on getting a TC cam too. What are the ups and downs on these? I would go with a web cam but I had a friend that had to get hardened rockers and other things since the cam is hardened. Thanks for your help.

[emaxxjeremy]

i am in the same boat. i have a 300ex that i would like more from. i am only going to a pipe,filter, and jets because i dont want to lose the reason i bought a honda....reliability!

[kd5sfq]

Going to an 11:1 piston with the right cam, pipe, air filter, cdi and carb results in a good gain in performance even at the stock size of 74mm. There is some gain going to 75mm or 76mm but it's not as much as you might think...

A 13.5:1 piston is even more power but it will tax your electric starter.

Higher compression also requires race fuel.

If you go with a larger size piston, I recomend that you use a big bore sleeve. I've seen several stock sleeves fail after being bored. That's what we did at 75mm though the major factor in replacing it at the time was the stock sleeve overheated and needing replacing anyway.

My son's 300ex must be around 30hp or more because it walks away from my stock 400ex (with pipe and K&N.) It's faster than most blasters he races against in the hole shot as well. The low end torque is awesome.

Our current engine mods to the 300ex are. 75mm 11:1 piston, all around webcam, hardened rocker, heavy duty valve springs, ported head, heavy duty clutch springs, +1000 RPM CDI, 400ex carb, shortened FMF power core 4, oversized header pipe, oil cooler, 400ex boot and modified air box with 400ex K&N air filter.

Then engine has been run hard since last November. Last weekend a cam bearing went out. ( Don't over look replacing them with a new cam like we did.) Other than that the engine is still tight... It's been very reliable.

Expect to have to replace clutches once or twice after those major mods. After that they seem to last awhile... Hardened rockers are a good idea with a larger cam no matter what brand. The stock rockers wear way to fast with a performance cam.

After this race season is over we are going to tinker some more to try to squeeze even more HP out of it.

Anyway I hope this answered your questions.

[dirtmomma]

I have about the same mods as above ^^^^ & LOVE IT it made a BIG improvement over stock, I did take the 400 carb off though due to it being cracked but once we put the stocker back on it STILL has GREAT pull!! Just re packed the pipe & WOW that made a difference too LOL ;):) no more messing w/ the jetting thats for sure, if I could find another one or get a 450r one forour 400 I'd put it back on probably thoguh :)