Flat top-end on '97 cr250r

[plynn41]

After 30 years of being away from dirt riding, I've recently bought a '97 cr250r with a newly rebuilt engine including main bearings. I'm happy with my deal ($1125), and rode the bike before and saw the machine shop receipts before buying it so I totally trust what the former owner told me.

I've since researched everything thing I could find about this model and fully concur that the suspension is painfully harsh. However, I have read that the engine is a rocketship from mid to upper, and this is not what I'm feeling. My bike has great low to mid power but feels flatter on the top. It doesn't seem to have an upper rpm miss, which might indicate the coil. I wonder if I'm jetted too lean on top? I wouldn't be dissappointed, except that what I've read has led me to expect more top end. The bike easily wheelies in the first three gears, and can be made to pull the front wheel in fourth and fifth--but it doesn't keep stretching my arms into the over rev. Any ideas or suggestions?

Btw, the chain and sprockets are stock, reeds are stock, I don't know about the jetting, bike has an FMF "fatty" pipe.

 

[spark250]

Every bike is going to stop pulling eventually. Are you reving this thing to the moon?

Look in to the jetting though. I think FMF has some baseline recommendations for each pipe/bike on their website. Check the power valve as well. I think on the older cr's there is an inspection cover you can pull and while the bike is running it will indicate if the power valve is working. It could be stuck in the closed position?

 

[plynn41]

Thanks for the response! I'll check the power valve and I plan on verifying the jetting at some point. (bikes didn't have power valves when I last rode them :)). Thanks for the tip on FMF baselines.
No, I've got too much experience (old experience) to try and over rev it. Maybe I'm just expecting too much, and I confess I don't have much experience on modern mx bikes. But the bike absolutely rockets out of corners in 2nd gear, then eases up on the pull sooner than I'd like.

 

[dirt bike dave]

Yeah, it is very easy to check if the powervalve is operating properly by pulling the inspection plug on the left side of the cylinder. As you rev the motor up, the end of the shaft will rotate 90 degrees as the power valve opens. There is a slot on the end of the shaft, and markings; I think it will line up with H when it opens, and L when it is closed.

If the power valve is not opening, there are a couple of things to check. First, open the powervalve inspection cover on the right side of the cylinder and make sure all the little allen head bolts in the powervalve system are tight.

However, if the valve is not opening, there is another more likely problem that will require you to remove the cylinder. Basically, when the cylinder is installed on your CR250R, it is easy to misalign one part of the system that operates the powervalve. There is a little 'fork' in the cases that must align with a shaft that comes down from the power valve. If that fork and shaft don't align right when the cylinder is installed, the power valve won't open.

 

[plynn41]

Thanks, this is great. Can't wait to check it out. Previous owner told me they had done nothing in the way of cleaning or inspecting of the power valve when they rebuilt the motor--so this sounds like a definite possibility. I'll be happy with the bike if I don't find any more HP, but ecstatic if I do (if the stiff shock doesn't bounce me into the trees).

 

[dirt bike dave]

FWIW, when I had my '99 CR, the suspension was done by Factory Connection and set up for trail riding/off road, and it was sensational. Very plush.

As for the powervalve, it is quite easy to mis-align the fork in the case with the pin that comes out of the bottom of the cylinder.

It's not hard to put the cylinder on correctly, but you may have to manually position the fork in the cases before you set the cylinder on, so that the pin goes between the fork tines.

Of course if you do have to remove the cylinder, you will need a new base gasket and a new head gasket.

BTW, if your powervalve is not opening at all, you are going to be amazed at how much power you get when it opens! The '97 is known for very good top end power.

If your powervalve is opening and you are only a little down on top end power, your problem might be with the carb. Your carb has a 'power jet' solenoid. These occasionially go bad or the wires get disconnected. Basically, the solenoid clicks at high rpm and leans out the carb, and the leaner mixture provides more power. You can check the solenoid by removing it from the carb and hooking it to a 12 volt source. It should fire as soon as the juice is connected to each side.

 

[plynn41]

Good grief Dave, I didn't expect so much helpful info! Thanks again. I'll tell y'all what I find on the bike after tonight.

The bike has been alot of fun already, but I can tell I've got to get my forearms in better shape. I'm going to check out how the 40+ geezers ride this weekend at my local track. Hopefully they won't scare me.

 

[plynn41]

You nailed it Dave!
I pulled the inspection cover off last night and the power valve isn't moving at all. I tried to turn it with a screwdriver and couldn't, so I wonder if it's gunked up with carbon? I'll probably take the cylinder off this weekend to fix it, and do a little port cleanup and polish while I'm at it. (I was part owner of a snowmobile speed shop in '78-81 and I've got lot's of experience porting and polishing 2-strokes--still got the right grinders).
I can't wait to ride the bike when it's 100%!

 

[dirt bike dave]

Just to be clear, the power valve shaft should rotate open when the motor is revved up. Not sure what rpm it is supposed to open at, but it is pretty high up there.

FWIW, the power valve on your model CR250R tends to stay pretty clean compared to some other bikes, so they are less likely to gum up and stick closed due to carbon build up.

Seeing as how the motor was recently rebuilt, IMO the likely cause is something came loose in the power valve assembly (there are a couple of small allen head bolts in there) or whoever put the cylinder on did not align the powervalve pin with the fork in the cases. Either way, it's not a major repair and the bike will rip when you get it fixed.

 

[plynn41]

Good,
The engine was new when I took the bike and it came with a couple of tanks of extra rich premix which I diligently ran through the bike with conservative throttle (I know now that that's now how the pro's break in a new top end, but I promised the seller to do it). So I rode the bike for a while without pinning it either in gear or in neutral. Then when I got it out on an open field, like I said, it felt flat on the top end to me--but the last mx bike I'd ridden was a '79 RM125N "back in the day", so maybe I didn't know what to expect. The bike is real impressive (to me) at anything below half throttle.

Last night was the first time I tried pinning it for a couple of seconds in neutral, and the shaft on the power valve didn't move at all. Also, the engine sounded like it had a governor or rev-limiter on it. It didn't miss, but it clearly wanted to rev higher and couldn't do it. The non-functional power valve makes perfect sense--it's got a trail bike's exhaust timing. I'll check on loose linkages on the right side, like you suggested. But I won't mind taking the cylinder off if I have to for the aforesaid porting opportunity.

Thanks again and best wishes!