4 stroke top end

[Paul_E_D]

Just did my first piston/cam chain replacement on the 250F today. I've had the bike 2 seasons, and I don't know how many hours were on it when I bought it, so better safe than sorry.

It was a LOT of work compared to what I'm used to with the 2 strokes. It wasn't hard, just involved many steps including removing the flywheel. I don't have those tools, so I had to make 2 trips to my local dealership to borrow the correct stuff.

The old piston was a wiseco (same as the one I bought) and it's in fantastic shape. Probably useable for a bunch more hours, but that's why they call it "preventative" maintenance. The cam chain was pretty stretched and is the real reason I did this job. The bike was running a little rough and it was down on power because of it.

Don't have time to start it up today, but there are no bolts leftover, so I know I did it right

[Tricky Mike]

Any head work too?
leak-down test?

[Paul_E_D]

No, I took care of the head last fall with fresh KW intake valves and cut valve seats. I didn't bother with a leakdown test for the piston, as it's pretty cheap to just replace it on schedule. I knew the timing chain was stretching because of how the marks were lining up when checking valves. Everything should be tip top now.

[Tricky Mike]

Nice I like.

[gregp]

I had a well worn KLX250/300 a few years ago, and the timing chain was also very stretched out, to the point where it started rattling. I replaced it, and left everything else alone. That change alone made the bike a one-kick starter again, and the bike was much more responsive and powerful. The cam chains on these newer high performance 4-stroke singles must really take a beating, and it does not surprise me that they (the chains) may need attention more often than the piston/rings.

[Paul_E_D]

Yeah, I thought about doing just the chain, but for only a little more work and 130 bucks, I have a fresh bike.

I think I might go to a 2 chains per piston schedule.

[typeone]

nice werk

[csmutty]

2 stroke FTW!

[Paul_E_D]

2 stroke FTW!

If only they didn't try to kill me on MX tracks!

[Paul_E_D]

OK, so I FINALLY put a heat cycle on the new top end. I kicked it 20 times before giving up and bump starting it on the dike. It fired instantly and then first kick after that. The bike is noticeably quieter without the camchain slap. It runs about the same with that raspy decel pop it has always had. It does have a fair bit more snap off the bottom, pointing the front plate towards the sun much more quickly than before.

The piston that came out was indeed the same wiseco hi comp model that I bought to replace it. It looked mint after 2 seasons (estimated 80-100 hours of MX) Zero signs of blowby or detonation. I'm no longer gonna worry about the decel pop. I'm not able to tune it out with the pilot/fuel screw/idle adjustment, and it shows no damage to the ring land on the piston, so eh, it sounds tuff. I like it!

Braaaap!!!!

[xsiliconkid]

Paul, how's the exhaust header gasket seal?
this can suck in air on decel and cause a decel pop.

[Paul_E_D]

Wirelessly posted

I replaced that recently. It appears to seal, but is suspect #1. I may try stacking 2 in there.

[breakdirt916]

great stuff! I think you're the only one I know who does proper "preventative" maintenance...if I still lived back out east, I'd buy any bike from you in a heartbeat

but with anything, the first top end will take the longest; and every subsequent one will be faster

now, why do the TT teenagers complain that 4t top ends are sooooo expensive??

[csmutty]

great stuff! I think you're the only one I know who does proper "preventative" maintenance...if I still lived back out east, I'd buy any bike from you in a heartbeat

but with anything, the first top end will take the longest; and every subsequent one will be faster

now, why do the TT teenagers complain that 4t top ends are sooooo expensive??

Because they are. 2 stroke piston kit and gaskets are like $160. 4 stroke piston kit, gaskets, cam chain, etc...more like $350.

Not to mention when something goes wrong (boom) on a 2T you don't have valves to worry about. Those get expensive fast.

[Paul_E_D]

Well, not true. My piston kit was 130, and there was one available for 99. gasket was cheap chain was cheap. I would say it was comparable. I did spend 250 on valve/head work last fall, so that is an added expense. If you DON'T do maintenance is when it gets expensive. A new head is over 500 bucks.

And thanks Bhavesh. You would be smart to buy a bike from me. How about a piece of crap ford ranger??? :-)

[csmutty]

Well, not true. My piston kit was 130, and there was one available for 99. gasket was cheap chain was cheap. I would say it was comparable. I did spend 250 on valve/head work last fall, so that is an added expense. If you DON'T do maintenance is when it gets expensive. A new head is over 500 bucks.

And thanks Bhavesh. You would be smart to buy a bike from me. How about a piece of crap ford ranger??? :-)

Where did you get that price? I was going by the wiseco kit retai pricing from parts unlimited. Top end kit was like $300 and when I bought a cam chain for the crf it was $50 after my discount.

[Paul_E_D]

I used the google. Vertex kits were available sub 100 bucks! I get a good discount at my local dealership, believe it or not, and the cam chain was 30 or so.

[typeone]

yup, same here... FBF top end for my 250, $134 shipped

[typeone]

Wirelessly posted

I replaced that recently. It appears to seal, but is suspect #1. I may try stacking 2 in there.

you could try two stacked or a little RTV copper to help it seal. used to have to do that for the Buell, worked great, never blew it out.

[Paul_E_D]

OK, A little frustrated. This bike has been running rougher and nothing I do seems to fix it. The new top end feels great and makes great power, but I'm still getting that raspy decel and there's a slight miss at steady throttle once in a while. Today at the track it was hard to start after riding hard sessions. These things usually point to valve adjustment, but they were in spec when I put the top end back together. The intakes were on the tighter side of spec though... Maybe I'll check them again and shim them to the loose side of spec.

Or maybe the plug is failing? or some other electrical component?

The idle does hang sometimes... which indicates a leak, but damned if I can find it!

[lrrs313]

Is the hot start plunger sticking? Only thing I can think of off the top of my head that might help.

[Paul_E_D]

Good thought. I was kinda checking that yesterday, but I can't really see it where it meets the carb. I'll look into that.

[typeone]

hmm, i'd be curious how it acts with the valves shimmed to the loose side of spec. seems the trouble starts when the motor is hot (?) after things tighten up even more.

[csmutty]

Ok so maybe the top ends have come down a little in price. 4 strokes still suck.

[Paul_E_D]

Well, I found one problem. Intake cam was one tooth out. It's funny, but it really didn't make a huge difference. It's still raspy on decel (though less actual backfires) and misses a little at steady throttle.

I talked to the guys at valley, and they suggested going way out on the fuel screw. I wasn't really sure what the range of adjustment is on these, so I was near stock settings. I went a turn and a 1/4 further out on the fuel screw, got a fresh plug, and some packing for the exhaust. I may try raising the needle one spot as well. These are all cheap things that might make a difference. We'll see.