Vintage Dirt Bike Q & A

I have to perform a leak down test on a 78-79 Cr250 motor and needed a little advice. I will be making a tester from instructions found on this site. The pressure will come from the intake manifold back into the motor.
My questions are 1) Do I need to take the reed valve out?? Seems that once I put a little pressure in the reeds would close and render my guage useless... 2) Piston position. I assume that the piston would have to be high enough to allow the pressure to act on the intake as well as the
exhaust???
thanks
PS any other advice would be nice

I would leave the reed in. If you are introducing pressure from up-stream where the carb would be, it is going to leak past the reed valves towards the crankcase. Leaving the reed block in situ means you can also test the effectiveness of the carb boot-reed block and reed block-cylinder junctions. I would have thought the piston needs to be at TDC so the transfer ports are closed. This will then exert maximum pressure on the base gasket and crankshaft oil seals. If the transfer ports are open you will be pressurizing the combustion chamber and exhaust port. The former should be tested with a compression tester and you will be able to see exhaust gases leaking from the exhaust sigot. Hope this helps....just being logical.

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF

In thinking about it further maybe it should be all the way down (BDC) Its on this down stroke when the fuel/air mixture is pulled into the crankcase??? Right?

Actually on the down stroke fuel-air mixture is pumped from the crankcase via the transfer ports into the combustion chamber. In the case of our CR fuel-air is also pressurized back towards the reed valve - which closes - and then it flows into the combustion chamber via the rear transfer port. On the '78 the intake and rear transfer are one, on the '79 they are separated by a small horizontal bridge. On the up-stroke the piston closes the transfer ports and then the skirt opens up the crankcase. The pressure differential causes fuel-air to rush in past the reed valve and into the crankcase. I think!

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF

This is a really good question. I've been running different scenarios through my mind. The problem I have with TDC is that the exhaust port is below the rings and it seems testing the integrity of the rings should be part of the test. But, obviously, I have no flippin' clue as to the correct answer.

the idea is you are testing the lower end for air leaks.that area is critical on a two stroke.the crank case is alternately under pressure and under vacuum.
as explained earlier when the piston is going up it puts the crank case under a vacuum to pull in the fuel-air mixture through the intake port when the piston is going down that pressurizes the crank case to push the fuel air mixture through the transfer ports.a conventional compression test can determine ring condition and sealing.

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(2) 1968 DT1 1970 RT1M,1970 DT1C 1971 RT1B Enduro 1974 MT250 1974 DT250A (2) 1975 DT400B 1974 DT100,1978 DT175
1975 CR250M1 1979 CR250R 1979 Kawasaki KZ400H1 Jesus is Lord!!

IMHO: You may be thinking too much => leak down telling you ring condition. The exhaust port and intake port should be blocked so you could even run the test w/o a piston!

If you want to test rings, just get a compression tester. They are relatively cheap and should give you some useful numbers. Just me but I use 120 PSI as a minimum before I do a ring job. I don't race, just recreational riding.

There's quite a list of things a leak down test will tell you. That a few other observations like burning oil or running lean even with a properly jetted carburetor will tell you just which parts you need to replace.

I don't think reeds will seal well enough to work with a leak down tester. By think I mean they may but you still want to test with the carburetor blocked off. It would/should be impossible to jet a bike that fails a leak down test so you want the crankcase seal to be the *only* thing you test. Get what I mean? If you do a leak down test using the reeds, did it fail the test because the reeds are worn or because you have a bad seal on a crank bearing? Can't tell because you have to many things going on at once.

Rick

This is why by instinct I started with TDC. This creates a chamber that includes the intake and crankcase. This is where you want to test; carb-reed block, reed block-cylinder, base gasket, crank seals. A compression tester tests rings and head gasket. And basically you don't really care too much about the exhaust side. It is the small but influential leak on the intake side that is going to screw your jetting and lean it out unpredictably....

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF

I guess what I could do is test it at both (piston) postions and check with soapy water/Simple green on all the seams and gaskets. Everything should be plugged so it should hold in both top and bottom ends of the motor. If it does leak and I can't find it on the outside then I'll take the clutch and and rotor off to check the main seals on each side..

Picture help?? This is a 1980 Cr250 cylinder with a piston almost TDC, I pushed it down so you could see the skirt...as you can see its a straight shot to the crankcase, don't forget about putting too much air pressure in so you don't blow the crankseals out where they contact the crankshaft.

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Garry Mainer
#33m (retired)
Honda Cr250r 1980
Honda Cr250r 1980
Yamaha TY250C 1976
Arctic Cat 650 H1 2007

well it certainly looks like performing the leak down test with piston at TDC will pressurize the cases... Another thing I have been thinking about is a compression test.. How does the pressure remain in the combustion chamber if your kicking the motor through its entire stroke??? would'nt the pressure be released out of the exhaust port??

Compression testers have a built in one way valve. You just screw them into the spark plug hole and kick the engine over a few times and read the value after.

right but what keeps the pressure from leaking out the exhaust port when you kick it over??? At some point when your kicking it the piston moves where the exhaust port should be open and I would think it would relieve the pressure???

The compression tester measures peak pressure - and retains that reading. So as you kick it over the pressure rises until the piston is at TDC. Then as TCD is passed pressure in the cylinder falls, but the one-way valve in the tester has closed and the reading on the gauge stays constant. You do the test when the engine is warm so the piston and rings are at optimum sealing.

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF

your right.. I'm thinking about this too much