New piston in stock bore?

[Moded86R]

Alright guys I bought an 02 400ex a couple months ago.


Since then I have been getting smoked by my buddy's bike.His is just rejetted with a K&n filter and no airbox lid.He is already beating me on top-end badly.When I shift to 4th he putts about 2 bikes on me.The thing is he runs a K&N without an outerwear and no airbox lid.I tried telling him that's not good.(for filtering purposes anyway)And now he is getting a t-4 slip on.


Something has to get done to my bike.I am thinking about going in the motor first and then get a pipe,carb,etc,etc.


I have been thinking about a high compression piston.Something around 10:1 compression.And a cam.Probably from a motor builder.A drop in is prefered.



Also my bike has fairly low hours.Probably less than 60.Do you think I will be alright with a standard bore piston.Or should I get it bored.


Thanks.

[JOEX]

A few other things will make a difference in top speed.....

What are tire size and make/model, gearing, rider weight and other accessories that may add overall weight to each quad.

I still go by the thought that there are many more important things to consider than HP to get a higher top speed:

Tire diameter
Tire weight
Overall weight of quad and rider
Spocket sizes/ratios
Engine RPM

The key is to get the balance of all those components to work together along with the HP. JMO:)

Joe

[cals400ex]

yes, listen to everything joe said. i have a feeling its not because of that filter and airbox lid that he is beating you. first, make sure that throttle cable is adjusted. there should be about 4 or 5mm(a little more than 1/8 of an inch. go a little less than 1/4 inch for sure) of play in the throttle lever. just make sure when you turn the bars fully to the left and right, the engine does't pick up rpms'. if it doesn't, your not too tight on the throttle lever. this makes a big difference in top speed.

by the way, a higher compression, even stock bore, with a nice cam will make your bike eat his, even with the stock exhaust on yours. you might just need to do a little jetting.

[JOEX]

And to answer your question, as long as the cylinder is in good shape a stock bore piston will be fine but you will need to get the cylinder honed.

Your buddies quad is probably breathing better and able to use the upper rpm's better.

Joe

[hawiianpwr]

Weight is also a factor on these small machines. Every 10lbs.= 1hp. How do you and your buddy compare weight wise? Take that in consideration, just for kicks trade quads a race and see what happens.
Perfect example is my buddy and I. I got 40lbs on him and he is pulling more hp than I am and I can still get him by a quad everytime.

[wilkin250r]

You can certainly do a new piston into the stock bore, as long as you hone the cylinder.

My next question is, why do you want to stay with the stock bore? Not that there is anything wrong with it, but a big-bore doesn't add all that much cost. The piston is basically the same price, and it only costs about $75 to bore and hone the cylinder.

[cals400ex]

Originally posted by wilkin250r
You can certainly do a new piston into the stock bore, as long as you hone the cylinder.

My next question is, why do you want to stay with the stock bore? Not that there is anything wrong with it, but a big-bore doesn't add all that much cost. The piston is basically the same price, and it only costs about $75 to bore and hone the cylinder.

i agree, hone that cylinder if your changing pistons/rings.

wilkin, how much power will be lost with the stock bore over like a 406 or 416?? i talked to colby and he said the difference in power would not be noticable on the same two bikes one with a 406 and the other with a 416. i always thought the compression ratio was the most important??

[Tommy 17]

i have a 400ex mods are

k&n filter
tc mx grind cam
406 10.8:1 piston
full race porting
stk pipe
mod stk carb...

with 18 tires on and stock gearing (slows u down alot) i'll still pull 4 or 5 quads on a stk 400ex...


a cam and a stk bore piston will help u out ALOT!!!!

[cals400ex]

Originally posted by Tommy 17
i have a 400ex mods are

k&n filter
tc mx grind cam
406 10.8:1 piston
full race porting
stk pipe
mod stk carb...

with 18 tires on and stock gearing (slows u down alot) i'll still pull 4 or 5 quads on a stk 400ex...


a cam and a stk bore piston will help u out ALOT!!!!


who did your porting?? did you get it ported at the same time you did other motor mods??

[Tommy 17]

local motor builder (grumbling performance) did it at the same time...


220$ for full race porting

[Moded86R]

I knew I would have to hone the cylinder.Which is no big deal because I have a nice one.


The reason I want to stay stock on the bore is because I don't plan on getting rid of this bike anytime soon and want to keep it reliable as possible.I know boring it a little doesn't hurt the reliablity very much.But why bore it when I can just hone.


Also I plan on running a JE piston.Also I am not sure on the cam.I was looking at the sparks aggressive drop in cam.The 300+ dollar price tag.(On there web site)Kind of sucks though.I can't imagine why a cam would cost that much.I can get a brand new cam and lifters for my truck for 100 bucks.Why does a cam for 4 valves cost more than one for 16?




Anyway he got his pro circuit t-4.Right now it's jetted at a 162 main.We are going to try going bigger.And doing a jetting check.But we haven't had time.I am not really impressed with his pipe at all.Very little gains for the performance.It sounds alot better.He still smokes me up top.


I weigh 160 and he weighs 130.I have been riding for 14 years and he has been riding for 2 or 3.if we are on dirt or grass I can get him.But on the blacktop he gets me.


I planned on getting the piston and cam this month.But my car blew a head gasket and I got a new girl freind.So who knows how much money I am gonna have.



BTW my tires are 20/11/9 6 ply razr's and his a 20/11/9 4 ply razr's.Gearing is stock on both bikes.