Vintage Dirt Bike Q & A

Hello all,
I am working on a 1979 CR125R that I purchased. It was lightly used and left to sit for who knows how long. I was able to start the bike without doing any work besides changing the oil and gas. The air boot is all cracked up and was letting air leak in. I fixed this problem but am still having issues. The bike starts and will idle fine. Once I get on the gas, it starts to misfire through all ranges.

I changed the crank oil seals on both sides but still the same. I did get a few glimpses of it running correcly (1 or 2 sec at a time). Each time it ran ok was after making a tight turn with the bike at a steep angle to the ground. I checked the carb and it all looks good and clean. The float level is good and all jets are clean. I notice that there is a fair amount of fuel mixed with black crud from the pipe leaking out. This is my first 2-stroke so I don't know the signs very well yet - but I think I may be running too rich?

I just bought new spark plugs and fuel line today and plan to put them in tonight for a try in the morning. By the way, I broke down the top end and changed the gaskets. The chrome cylinder looks perfect and the old piston with one ring looks ok. I have a new wisco piston on the way. I did notice the piston feels pretty loose. I only have experience with 4-stroke bikes and don't know if the tollerences are different - but I am concerned that I'm getting ring blow by.

I'm in new orleans - so basically sea level (at best!)

Anybody out there have any advice to a 2-stroke newbie?

thanks,
nola_fiddy

1978 xr75 (original owner!)
1979 CR125r
1997 z50r (tricked out)
2000 xr70

It's tough to diagnose running problems on bikes that have been dormant for years.
Changing the seals was a good idea and I'm assuming you replaced the cracked
intake manifold instead of trying to patch up the old one. If you have the equipment,
I would recommend you perform a leak down test. If there is a leak, you will never
get it to run right.

You mentioned replacing the gas and oil, what type of oil are you using and what
ratio for the premix?

The possibility of blow by can be checked by measuring the ring gap and condition of
the ring. Also check the cylinder with a bore gauge for taper and wear. If either is
out of limit, you will need to replace or resleeve the cylinder. I'm assuming you have a
manual with all the tolerences listed. Another useful check is a simple compression test.

Some "spooge" is normal from two strokes. You usually see it dribbling out the exhaust
and to a lesser extent, oozing out around the cylinder to pipe flange. If it is excessive,
I mean it gets all over you and side of the bike, you could be running the bike rich
(too much oil in your premix or jetting problem or both).

One last recommendation, take the carb apart again and check that the float is not
sticking due to varnish. When you say the bike runs good after a big lean over, it
sounds like the carb float is hanging up. I would open up the carb and soak everything
in carb cleaner then blow out all the passages with compressed air.

Start with the easy stuff and work your way up.

dogger

_________________
1973 CR250M
1979 CR250R
1993 CR250R
2008 CR500R
2009 CR250R

Right turn eh? That would slosh fuel toward the left side wouldn't it? Maybe you have a clogged in-tank filter that's acting like a snorkle. Empty the tank, then remove the fuel petcock and check it out. While you're at it, take the whole valve apart and get the crud out. And, I hate to even say it, but make sure you have a bunch of fuel in the tank.

Two stroke pistons aren't shaped like a coffee can, they are shaped more like a shot glass. The top of the piston has a smaller diameter than the skirt. Direct exposure to the combustion flame gets the piston crown hot so it expands more. Always measure the piston to cylinder clearance at the skirt.

Enjoy!
Jon