300ex big bore kit

[Honda_exrider]

what exactly is a big bore kit and is that all i have to buy and is it a lot of trouble to do. Cause mainly i just want more top speed,(as long as i can keep up with a warrior or maybe a 400ex im good) and about how much money does it take to get this done. please help!!!

[bwamos]

A Big Bore kit is exactly what it implies.

It is nothing more than a larger diameter piston and sleeve bore. It increases the displacement of the motor. On a 300ex the stock piston is 74mm (282cc). A 78mm piston and bored sleeve will give you 297cc's.

This alone will not make you "faster" than a 400ex or warrior.

If you're just worried about top speed, a bore kit is not at all necessary. You just need to adjust sprocket gearing and increase the rev limit on the CDI.

However if you want to out accelerate a stock 400ex, you have to at least have the 297cc bore with a higher compression piston 11:1 for example, but add in head work (port, polish, cam, oversized valves, etc.), a pipe, aftermarket filter, and rejet the carb.

Cost to have it done by a major engine builder will be in the $1000 dollar range not including the cost of a full system pipe (a slip on is not good enough) & filter.

[bwamos]

If you since you jsut seem to be worried about keeping up once hitting top speed, I'd reccomend getting your CDI modified to increase the rev limit. The rev limit on the 300ex is horribly low.

You could also reduce the rear sproket to a 36 tooth. Those 2 should get you to their top speed, if not a little faster.

22" rear tires (stock size) will give you the highest top speed, where 18" will give you the most acceleration.

[Honda_exrider]

help me out i don't understand the concept of a rev limiter and cdi stuff oh yea i've got 20'' holeshots, what do you think the top speed will be. cause all i really want to do is be able to be near or close to the top speed of a warrior or 400ex and i dont want to spend a lot of money. what u think?

[whiteflash]

Originally posted by Honda_exrider
help me out i don't understand the concept of a rev limiter and cdi stuff oh yea i've got 20'' holeshots, what do you think the top speed will be. cause all i really want to do is be able to be near or close to the top speed of a warrior or 400ex and i dont want to spend a lot of money. what u think?

A rev limiter or CDI, just allows you to rev higher in the RPM's. it's a little black box you plug in, on a 400ex its under the headlight, but i dont know where it is on the 300ex.

[Honda_exrider]

BUT WHAEN IM PINNED IN 5TH GEAR IT ALREADY REVED UP SO HIGH, DO YOU MEAN THAT IT REVS EVEN HIGHER, CAUSE IF IT DOES, THAT SOUNDS LIKE A BAD IDEA. MABEY IM CRAZY
PLEASE EXPLAIN , CAUSE I NEED SOME HELP

[whiteflash]

Originally posted by Honda_exrider
BUT WHAEN IM PINNED IN 5TH GEAR IT ALREADY REVED UP SO HIGH, DO YOU MEAN THAT IT REVS EVEN HIGHER, CAUSE IF IT DOES, THAT SOUNDS LIKE A BAD IDEA. MABEY IM CRAZY
PLEASE EXPLAIN , CAUSE I NEED SOME HELP

ya it revs higher

[wboy62417]

the rev box is behind one of the shocks in the front (not sure which) I have one of the high rev boxes it is ok to keep it pinned for a lil bit DONT HOLD IT WIDE OPEN FOR VERY LONG if you get one that just has a higher rev and not one NO REV LIMIT(not sure if this exsists) it should hit a point where it wont let you rev any more

[bwamos]

Originally posted by Honda_exrider
BUT WHAEN IM PINNED IN 5TH GEAR IT ALREADY REVED UP SO HIGH, DO YOU MEAN THAT IT REVS EVEN HIGHER, CAUSE IF IT DOES, THAT SOUNDS LIKE A BAD IDEA. MABEY IM CRAZY
PLEASE EXPLAIN , CAUSE I NEED SOME HELP

It does sound high, compared to a car. The ATV's have a smaller pistons and cams so they can rev a lot higher than a car can.

The 300ex I beleive stock has a 7200rpm rev limit. You can go up to a least 9000 rpm's without any worries.

Your 20" tires will reduce your top speed.

A stock 300ex with stock gearing w/ 22" stock tires has a top speed of ~55mph.

Stock w/ 20" tires has a top speed of 50mph.

Change the rear sprocket from the stock 38 tooth to a 35 tooth and you will get back up to 54mph.

Increase the rev limit from 7200rpm to 8600rpm and you will top out at 64mph. (very close to a 400ex)

[Honda_exrider]

man thats awsome thanks for your help but i do have one more question, if i get a k&n air filter and a pre filter that replaces the air box lid, will that help me at all?

[2muchquad]

any time you can help the motor breathe better it will be benefitual but you might not add any top speed though.you will increase the potential to make more power if you add a slip on or cam etc.:)

[bwamos]

What he said.

It will require a slight rejet. Not much if you're not replacing the pipe. Go up 2 sizes on your main, and you should be set. (1 size since the 300ex is a little lean stock for emmisions. And, 1 size for the filter change and removal of the lid (128 keihin jet below 2000 ft). Shouldn't need to adjust your needle or idle circuit. Start there and adjust as necessary, every quad is different.

You will feel a crisper throttle response. Top speed is limited by, gearing, tire size and rev limit. HP wont make you go faster. It will just get there sooner.

[Honda_exrider]

thanks for the help

[rooster300ex]

I was thinking about getting an aftermarket rev box to increase revs, but when i rebuilt i had to take the balancer out(long story). anyways do you think it would cause some serious problems if i went ahead and got one.