260 psi of compression!!! Need Help

[KFX400 RACER]

Well I bought this quad with hearing the motor was rebuilt by a honda dealership and that is all the guy knew. I just got done taking the head off to check piston size. It is at 67mm which makes it 254cc. Now what I should have done was test the compression before I took it apart. I decided to put in a thin base gasket mod and I put it back together. I then put in the compression tester and held the throttle wide open and kick the motor over till the needle stopped and did not move anymore. It read 260.

Is 260 psi way too much or what. I wanna at least run race gas but to have the motor reliable and not blow bearings cause of this high compression. I have no idea if the cylinder is ported or if the head is machined. I really do not know what to do?I am thinking since I have a new cosmetic stock gasket, I should put that in and check compression again with a stock size head gasket and see where it is at.

Any help will be appreciated.

Thanks,
Jake

[headache]

holy ****, take the sparkplug out, put in a glow plug and pour in some diesel!! Seriously though, i'd check your compression gage to make sure its working correctly. 180 psi is quite a bit with porting and a cut head. i've never heard of that much compression before.

[wilkin250r]

I've known people that routinely run 210psi compression, obviously using full race gas. But I've never known anybody to run 260, that's really high.

Like Headache said, first and foremost I would double-check with another compression gauge. 260psi is so hard to reach that I would have to second-guess whether or not it's real.

If it had high compression to begin with, like 205, then the thinner base gasket would push it up to 220 or so, it wouldn't get to 260.

Don't get me wrong, 260psi is possible, it's just not realistic. You would have to have a configuration that nobody in their right mind would ever create, like a 14cc dome or something equally stupid.

[SET THE STAGE]

what is the stock compression? i've never really knew this.

[deathman53]

my pv top end made about 220 psi and they are real hard to kick over. My friend built motors with 250 psi, but he only used them for about 3 races then rebuilt them again. I can't imagine how hard a 260 psi motor is to kick over. I complain abut 220 psi.

[KFX400 RACER]

Today I am going to use a different compression tester since we have like three or four. I will let you guys know what I find out.

If it is still the same with the other compression testers should I take that thinner gasket out and put a stock one in and test it again. But I am thinking even with the stock one in there it can't lower the compression that much.

Somone most have done alot of work to this motor to get that much compression. It does not run that fast. My kfx pipe and a airbox is about the same on top end. My bros stock pred tld is like 6 to eight bikes in front and keeps pulling away. But I think my gear ratio is messed up so that I am going to change since I am running mx tires.

Maybe some hardcore crazy horsepower guy worked on this motor. Hell I don't know.

I did not check to see if it is ported but it must be and I do not know if it is a stock crank or not.

Thanks guys for the help so far, I really do appreciated.

I will keep you posted on what happens,
Jake

[400EX QUADER]

did you check it?? It has that much compression and is slower than a kfx and a pred??:huh

[KFX400 RACER]

Yes I checked with three diferent compression testers and kept the throttle wide open and kept kicking it over till the needle stop. This time on all three tester it did not stop and kept building pressure even past 260. So I do not think that is the way you check compression like I have read on here since you are just building up the compression in the line of the tester more each time you kick it.

So I kick it over once with a good kick and got 180 and did it a couple times just to check again. I got mostly 160 to 170 by kicking it one with the throttle wide open.

So I guess I will see how it runs when I get my seat cover put on and get my new clutch perch since mine broke. Later on I need a new clutch but it can wait for now. If it seems to runs fine I will leave it as is or I will change it back.

[KFX400 RACER]

I think why my kfx and my bros pred is faster is because four strokes have top end and two strokes die off on top end.

Also since I have a stock carb and reed valve. I am sure this is not helping the motor since it is bored to a 254.

This is going to be a purchase for sure.

I am thinking of a pwk 39 mm and a cr250r v-force 3 ported rad valve with a ported aluminum intake manifold.

It should run alot better after I put that stuff on it.

[kiesta00]

yeah that stock intake setup will suck on top end...a bigger carb will definately help top end. 2 strokes don't suck on topend, you just have to build it for a specific application.

[SET THE STAGE]

since when do 2-strokes die off on top-end?

[250r4life]

Originally posted by SET THE STAGE
since when do 2-strokes die off on top-end?

i guess i missed the newsflash too

[BLACKeR]

thats all 2-strokes have...:rolleyes:

[KFX400 RACER]

I have owned some mx 2 stroke dirtbikes and this 250R. All of them die off on top end. Four strokes pull alot longer for sure.

You cannot just keep a 2 stroke pinned on top end for ever since that is way too much rpm and stress for the crank. I have seen some guys that live by me on two stroke mx bikes run around down our roads on full throttle and eventually the crank bearings are shot to sh*t.

On a two stroke you have to let off and get back in the powerband since on top end the powerband dies off like it hits a wall.

Yea you can port a two stroke for top end but most of those bikes or quads are for drag racing or something to that nature.
Who wants a top end port job when running mx. Usually you do not even see top end on a motocross track except if it has a long, flat straightaway.

This is my opinion but I guess everyone thinks everyone elses opinion stinks. That is a very true statement but think what you want to think.

Now you guys come on my post just to start crap and joke about what I said.

That is why I hate forums since people young or old run there mouth and no one can ask a question about there bike or whatever without it turning into a kindergarten sh*t talking session.

Good God this is a long post. If you guys have nothing good to say don't say it at all.

[Aceman]

If you post BS, expect to get called on it.:rolleyes: Saying 2 strokes have no topend is just silly, maybe you should make your way over to the dyno forum and look at some graphs.

You'd better be running some high octane race if you're going to try and get away with 260 psi. Me personally, I'd try different gaskets/heads until it was down around 220psi.

[BLACKeR]

just because we disagree with you doesnt mean we "have nothing good to say" these forums are about information. while modern 4-strokes do rev guite high the majority of their power is made in the lower RPM's 2-strokes dont make their peak power till the mid- upper RPM's this is why riders who arnt very agressive dont like them for MX and XC. your information is incorrect. a 2-stroke the is poorly jetted or set up for lowend will fall off on the power, but its not the norm. also there is far more of a reliability issue with running a 4-stroke WFO than a 2-stroke. as long as your 2-stroke isnt undergeared you can run it wide open all day. KFX's and predators are not topend 4-strokes. you may want to check your jetting and or get a different pipe and carb. if your falling off the topend as much as you say you are.