Vintage Dirt Bike Q & A

1979 CR 250. Brand new top end with bore and sleeve, Wiseco, etc.
Bike runs great in low and mid-range, but when I get to top end I start losing power. It's not a matter of red-lining it, it didn't do this before, I know when to shift and know what it feels like wide-open - fast!. I've tried new top end (as mentioned), re-built carb, tried main jets ranging from 165 to 200 and eveerything available in between. and still problem persists. Have new air-filter, reeds look great. I'm monkeying with needle jet settings with no luck thus far.
Problem might have existed before but it only became evident on long straight-aways when the YZ's would just start pulling away.
I bought another CDI/stator on eBay, but haven't tried that yet as I want to try one thing at a time.
Anyone had/have this problem? Solutions?

Leak down test .. You can monkey around with everything else until you turn blue in the face (trust me I have) and if you got a leak it won't matter.

Ok. I understand what a leak-down test is, it checks your crankcase seals.
How is it actually performed? Do I have to remove the motor just for the test?
I'm about $7-800 deep into this issue ( a few problems along the way) and that's about what I paid for the bike.

Yes it detects leaks not only crank seals but base, head, crankcase and reed valve gaskets, reeds everything that can leak air even the sparkplug washer/ threads etc .. I bought a 83 that had a pin hole in the case! You can buy one (MotionPro) make one or take it to a shop..

Hey folks. The thing you want is called a pressure test kit, not a leak-down tester. A leak-down tester is used on a four stroke to measure how much leakage there is past the valve seats and the piston rings thereby measuring engine wear. A two-stroke pressure tester is different. The pressure tester uses plugs to seal the motor at the intake boot and exhuast manifold. Then, you screw an air fitting into the spark plug hole. From that air fitting on out you'll need some tubing, a low pressure air guage, an on/off valve, and a Miti-Vac air vacume/pressure hand pump. When you get that all hooked up you pump air into the engine until you reach a pressure of 6psi. You have filled up the entire motor above and below the piston with pressurized air. If it appears to be holding that pressure, then lock down the shut off valve and wait. If it loses more than 1 psi per minute, you have a significant air leak that needs to be addressed. If it looses nothing, then you did a good job rebuilding a nice air-tight engine. Leaks are easily found by spraying soapy water along the gaskets, crank seals, intake and exhaust areas, and the dreaded engine casings themselves. If it's leaking a ton, don't tighten the valve, just keep pumping in air and spraying the soapy water. Thankfully most leaks will be where you tried to jury rig that tricky exhaust opening.
This 6 psi is anoyingly strong too. If you plug your intake and exhaust with tapered rubber plugs, the pressure will pop them out when you least expect it. The best thing I've found is rubber freeze-plugs that you can get a good professional auto parts store. They come in all sorts of sizes.
It's also a good idea to do this procedure before and after you do any top end work. Before let's you know whether you need to do the lower end seals BEFORE you finished your top end. After makes sure your work is good.
Whew, hope that helps.
Enjoy!
Jon

Like Jon described, Leak down or pressure testing a two stroke is a pain-in-the-butt without the right equipment.
If you are going to restore and ride vintage bikes, I would highly recommend the purchase of a quality two stroke
leak down tester. I bit the bullet and purchased the Motion Pro tester and it has saved me a tremendous amount
of time, effort and money by indicating or eliminating air leaks as the source of the problem. Like others have
posted, air leaks are a common and vexxing problem especially in a 30 year old engine. The final step in any two
stroke build should be a leak down test.

The nice thing about a quality kit, is it will fit a variety of engines and has everything neccessary to insure good
repeatable results. I noticed that Motion Pro recently raised the price of their tester but I've included a link to a
site that still sells it for the old price If you are interested: http://www.hvccycle.com/tools.htm

Good luck with the problem, I will be following your progress as I am building a '79 CR250 as well.

dogger

_________________
1973 CR250M
1979 CR250R
1993 CR250R
2008 CR500R
2009 CR250R

I agree with getting a good tester!! When your up to your armpits in WD-40, soapy water or whatever you use to check for leaks, having a proper one is a must! I purchased the Motion Pro model also and while its pricey it performs well.