CR250 - short surges when the the throttle is let off. Please help.

[g_man80]

Today I just finished putting a new top end in my bike ('00 CR250). It fired right up, idled fine, and I took it out to break it in. It was doing something it didn't do before the rebuild. When I would let off the throttle the engine would surge for short little busts (while moving, not idle). Seemed like little spurts of fuel or air. The bike ran great other than that. The power was fine and no misses while accelerating.

If anyone has an idea of what could be causing this let me know. Thanks for the help,
G

 

[NO HAND]

I had pretty much the same scenario with a 00'CR125 and it was an air leak on the left side crank seal.

 

[g_man80]

I had pretty much the same scenario with a 00'CR125 and it was an air leak on the left side crank seal.

Why would this just start after the rebuild? Could I have done something to damage the crank seal? Maybe some grit fell down there when I had the cylinder off.

Any other suggestions?

 

[Crash 142]

Maybe you re-routed the throttle cable wrong and it's binding/pulling the slide up a little? Holding it open just a little?

 

[john3_16]

Sounds like it's lean...Could be an air leak somewhere...Check everywhere there could be a leak..The head, cylinder base, carb connections, reed valve connections...Did you install new gaskets ?

May just need an air screw adjustment..But you stated that it ran fine before the topend, sounds like a small air leak...

 

[darringer]

g_man80, my 99 cr has done the same thing for over 2 years. I have never found the problem. Even after a complete engine rebuild and 2 top ends. I have played with the jetting numerous times (main jet, different needles, and pilot jet), as I am constantly adjusting for temperature variations. When I bought my bike it had a Honda piston, and it did not have the problem. I have been running Wiseco ever since the first top end, and it has done it ever since. I prefer the Wiseco over the stock piston, so I won't use anything else. I have lessened the problem with jetting a small amount, but have never gotten rid of it. I just got used to it.

 

[g_man80]

Thanks for all the replies. All new gaskets were installed and the mounting surfaces were pretty clean. The reeds also looked good although the gasket for the reed caged didn't fit the best (kind of small), but I still think it made a good seal. I'm going to double check the throttle cable.

At the same time as the rebuild I leaned out the pilot jet two sizes. I'm going to go back up one size and see if that helps. I'll also mess with the air screw a bit. Hopefully it's something simple with the carb.

Thanks again for the help.

 

[pro2k]

Sounds a lot like what my bike was doing after a rebuild. It was running perfectly before and surging and knocking after (when I would let off the gas and coast in gear). Turned out to be a cylinder gasket air leak. I built the DIY leakdown tester for about $30. But before you do that put the jetting back to where it was befroe you rebuilt and try that.

Here is a link to my old posts: HELP!

Good Luck!

 

[Mike_socal]

My guess is it's an air leak around the reed area.

 

[bikepilot]

Some surging on decel is normal for a two stroke, leaning the mixture will make it worse. I seriously doubt it had anything to do with the rebuild and everything to do with leaning the pilot jet.

good luck

 

[NO HAND]

Why would this just start after the rebuild? Could I have done something to damage the crank seal? ...

What happens often is the new piston has more compression and also draws a stronger vacuum in the crankcase. This added amount of pressure and vacuum is more taxing on the crank seals and may be the cause why you only discover after the rebuild your crank seal was almost worn out. It may also be an air leak around the reed cage, the stock rubber seal there is somewhat fragile...

 

[Trailboss11]

Air leak

I have a 2001 cr250 and I think all that is happening is you have your idle set to high. Adjust your Idle down and see if it goes away. The idle is set to high and it is right about the rpm that the powervalve is opening and this makes it act up. At least this is what I found out with mine. Thanks, Trailboss

 

[Idaho Charley]

I agree with the post that your pilot (or needle) is a little too lean. There are often minor changes in jetting necessary after a fresh top-end due to the increased ring seal and slightly different piston skirt designs. (Honda verse Weisco etc.) I would turn in my air screw 3/4 to 1 turn and see if it goes away.

 

[g_man80]

Thanks for all the great info! I'm going to start with just the outside adjustments (idle and air screw). After that I'll look at the pilot jet and so on. I'll let you guys know when I figure it out.

Thanks again everyone.

 

[pace]

The pilot jet is most likely the culprit. Whenever a problem like this arises, you should always first look to actions you've recently taken that can easily cause the behavior you're observing. Since you leaned the pilot jet, and lean AFRs can cause surging, that's an obvious and importantly, an easy check. After that, look to the gaskets that you disturbed/changed on the reed block and cylinder base.

It's generally good practice to change jetting components one step at a time. I'd go one step richer on the pilot since you went two steps leaner initially, and then fine tune with the air screw. If you have to turn the screw outside of the recommended specs, then you need to go another step richer or leaner as appropriate.