Jump to content
Sign in to follow this  

Uncle Alpo or others with older with RM/RMX power valve help needed

Recommended Posts

My last post was creaping on idle, many thought the idle was to high. I tried to adjust the idle, but to know help (idle). So I made a leak down tester to see if I had an air leak. I hooked it up to my compressor and set it to 4 PSI. You could "hear" the air passing thought the engine. Got out the soapy water and couldn't find the leak.

I then just went by sound and it seemed like it was coming from the ign. cranks seal. I am thinking :banana: I just changed that but once I got the mag off and hit it with the water, no bubbles.

I then noticed the vent tube that comes from the power valve area. All the air was passing through the vent tube. I put my finger over the vent tube and I ended up with a leak around the power adjuster.

The bike is a 90 RM 250, But the PO had put an RMX cylinder and head on it . It had also started to sieze up around the exhaust port, (thats why I got it so cheap. The bike runs real strong once you are riding it, but no Idle. It did seem to make a funny noise around the power valve wheb riding.

Where and what should I be looking for? Thanks

Share this post


Link to post
Share on other sites

There's no difference between a '90 RM250 cylinder and head and a '90 RMX cylinder and head. All of the differences are in the power valve and the pipe (and the wide ratio RMX trans and the external flywheel with a lighting coil). The head and cylinder in this model year are straight up ~ the same.

If you pressurize the cylinder (with the trans in gear and the piston at TDC) with only 4psi, it will leak past the ring end gap (there's ALWAYS some "leakage"). That air would then work it's way out the exhaust. That is the path of the least resistance.

If you pulled the pipe and completely sealed off the exhaust port, then air could leak past the power valves and out that breather. Again, that would be the path of least resistance. Yes, it would push against both main seals but past the power valves and then out the breather tube is the easiest exit.

Bottom line? A creaping idle could be an air leak or the air screw on your carb is not adjusted correctly, or your bike is running lean and needs the carb cleaned out. If the cylinder is showing signs of fatigue and a light seizure have you fixed the cylinder and installed a new piston yet? If not then there's no reason to worry about your idle condition until you have fixed any other known problem.

And how long are you letting it idle before it starts to climb?

Share this post


Link to post
Share on other sites

How fresh is the top end? If you have not installed a fresh piston & ring I'd start there. Rule #1 after buying a used two-stroke: install a new top end. While you have the cylinder off clean and inspect the exhaust valves; "funny noises" worry me.

If the engine won't idle either the carb is malfunctioning, or you have an air leak. If the crank seals are good it could be a leak around the intake manifold.

Share this post


Link to post
Share on other sites

The motor does have a new top end with about 2 hours on it. I plugged the breather hose upped the pressue to 8 PSI and noticed a small amount of bubbles at the 3 arrows on the cylinder.

cyl4.jpg

However, most of the air was coming out of the Kickstarter seal. I had replaced both crank seals when I did the top end. I always thought that if the clutch side leaked it would smoke like a banshee, but still idle.

Could I be looking a an internal case leak? I sprayed soap all around the case seams and multiple times around the intake, but no bubbles.

Thanks

Share this post


Link to post
Share on other sites

Through the power valve linkage there's a direct path into the clutch and transmission, so a few air bubbles comming out the kick starter is not a surprise.

Personally I think that doing a leak test with a 2 stroke is a waste of time, there's way too many paths that air can go especially when you have a motor with a powervalve. Just do a simple compression check and then move on to cleaning out the carb.

Share this post


Link to post
Share on other sites

Its pretty hard to kick, but how much pressure should I see with the tester? I'll also double check the carb.

Thanks

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×