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2 stroke sustained rev after throttle chop

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I posted this last Oct...

My son races an 02 cr250, mostly stock, fmf pipe, recently added a shorty silencer.  15 hours on rebuild...has always been a little slow to come back to idle but seems to have gotten worse.  When riding hard it takes 3 - 5 seconds to come down to idle.  I don't know what it could be!  Jetting, reeds, air leaks...?? 

The response I got was to change the left side crank seal.  It made a little bit of difference but didn't completely solve the problem.  Over the winter it seemed to be less of an issue and into the spring racing.

We did a fresh top end this summer and had the normal slow to come back to idle I expect for the cr250.  It was running great and very strong.  We have 8 hours now on the top end and the problem is back in a huge way.  Almost un-rideable two weeks ago at Muddy Creek MX.   When he clutches in the corners I can hear it at a high rev.

 

My thoughts and questions...

1. If it's the left seal leaking air it would cause a lean mixture right?  I pull the plug and it's dark.  Wouldn't the leaking lean situation cause a lighter plug?  I see a lot of threads pointing to air leaks and leak down tests, but no one tells what the plug looks like.

2. If it's the right side seal wouldn't it leak oil into the case and smoke?  This would not cause a sustained rev would it?

3. Reading the plug dark, I moved the needle down thinking I have a rich mixture and it could be loading up on fuel.  No change!  Should I go farther or smaller main jet?  The plug is dark but not dark enough that I think it's a mixture problem...and it runs smooth and no missing.

4. Someone said warm it up and spray starter fluid around the engine to see if I can find an air leak.  But again wouldn't it cause a lean plug?

5. Throttle cable is new and very crisp...always has been through the good and bad times.

Frustrated and ready to tare it down if needed but I don't know what to look for or what to replace.  I'm looking for some been there done that wisdom.  I've noticed a lot of threads state the problem, list suggestions, but never state what the solution was.  My name is racer dad, and that's what I am.  I'm not a pro engine builder but we do race 3 bikes and I have done about 6 or 7 engines now and this is the only problem I've not been able to solve.  Any help would be greatly appreciated!

 

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Do a leak down test.  Sounds like an air leak to me.  Everything else would just be guessing and chasing your tail.

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Thank you for the post!  I have been told that before but...A dog chases it's tail because it's there, he never stops to think why it's there or why he can't catch it.  That's why I'm asking what is the problem and how do I catch it.  Gearing up for a leak down test isn't cheep so I would like for the evidence to point toward a leak before chasing it.  Can you tell me what my plug would look like if I had a significant air leak?  I've been told it would show lean mixture (white plug)...mines not white...it's dark leaning toward a rich mixture.  If someone could tell me from experience it could be a leak (say from the right side seal) pulling in oil (dark plug) but running lean enough to over rev then I would be ready to tare it down.  Spending $ on a leak down setup just because may be a step I have to take but it just sounds like spinning in circles to me.

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About 25-30$ in parts from a hardware and auto parts store is all that is needed to build yourself a leakdown tester,

a must for anyone trying to troubleshoot and tune a performance 2-stroke.

Anytime you suspect an air leak, doing a 30 minute test is simpler and cheaper than later on fixing a seized engine.

Pressurized at no more than 6psi, your engine should have ZERO loss in a minimum of 15 minutes under pressure.

 

Running too lean on the pilot jet circuit will also 'lean surge' then engine when letting off the throttle at high rpms,

with the rpms & vacuum still high and the throttle slide closed, the engine is only getting fuel from the tiny pilot jet.

 

Since the 'overly lean' condition is only happening when you let off the throttle, it is not long enough to effect the plug's coloration.

Having a richer than ideal main jet could mask the leak under acceleration and still show a dark plug.

Edited by mlatour

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When the housing market crashed and everything sucked for a few years, I went to car school and thought I wanted to be a tech. In one of my power and performance classes I learned how a 2 stroke engine functions and maybe it can help your thought process to figure out the problem.

Since there are no valves moving I didn't know exactly how the air/fuel mix got into the combustion chamber. I even thought the oil in the fuel coated the cylinder walls and that's how the engine was lubricated but I was wrong. One time when so was a kid I took an expansion chamber off and tried to start the bike just to see how loud it was. It wouldn't start and fuel was just dripping out of the exhaust port and the engine never fired. I figured it was just because of the back pressure and never gave it a second thought but I was wrong again.

The easy version to understand is that every engine is simply an air pump. The motion of the 2 stroke sucks air/fuel through the reeds into the bottom end of the engine. This is where the oil in the mix lubricates the bottom end bearings. From here the mixture gets sucked into the expansion chamber(ever notice the exhaust port has a divider in it?) where it becomes more of a mist(since the vapors are what are flammable and not the liquid fuel) before being sucked back into the combustion chamber and igniting in the cylinder.

If the engine keeps running after the throttle is chopped, it not only is getting extra air but has a fuel supply as well.

I don't have a solution for you because a lot of diagnostics is figuring out something you haven't seen before. It's just about looking at the clues. Is there a black oil dribble trail coming from the silencer or pipe anywhere?

Somehow excess fuel could be building up in the expansion chamber giving the engine a fuel source after chopping the throttle.

Good luck and keep us posted.

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Another thought I just had... Does this happen when chopping the throttle from all positions or just WOT(wide open throttle). Try having him ride the bike at 1/4 and 1/2 throttle and chop it. Does it have the same problem? If it's only after WOT the carburetor could be your culprit of dumping too much fuel. If it shows those symptoms you might try dialing back the main jet.

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The revs would not pick up with too much fuel but rather bog.

Unless there is a mechanical issue with the throttle grip, cable, spring, slide preventing it's return,

increased rpms and idle surging is a sign of a lean condition.

Edited by mlatour

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I don't think Racerdad said the revs pick up on its own but rather maintains extended RPMs after WOT. Those are 2 different symptoms but could be misunderstood as the same.

One more thought. It's quite possible you have more than one issue. Anything that isn't restricting the flow of air/fuel properly could contribute to your issue. Worn out reeds would be something else I would look at.

You have to look at the clues. The plug definitively tells us there is plenty of fuel in the combustion chamber when ignition occurs because it's black. A lean mixture burns much hotter because of the excess oxygen and in turn burns away the oil residue leaving a light tan color. A rich mixture burns cooler and doesn't ignite all the fuel/oil once the oxygen is used up. This leaves a darker colored plug.

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The more I think about it the more I'm thinking reeds. Are they original? If so they are 16yrs old and probably deserve an upgrade anyway.

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Here's another thought... Throttle return spring inside the carb could be worn out and not as stiff as it used to be. To test this I would remove the air box and boot, put the bike on a stand, start it and run the bike near WOT in 5th gear to put a little load on the engine. When you chop the throttle look inside the carb with a flashlight. Does the slide come down immediately or hesitate and come down slowly?

 

 

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Worn/damaged reeds create a 'rich' condition, again bogging the engine rather than increasing rpms.

 

Edited by mlatour

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Maybe both? How about worn reeds with an air leak at the exhaust port for example? The plug color is what confuses me.

This is the exact reason I don't work on cars anymore. Took me 3 weeks once to figure out a stretched timing chain was causing a catalytic converter to burn out. Flat rate my ass right into the dirt!!

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Some great thoughts...thank you!

We replaced cracked V-force reads last year sometime.  I should check them again!

We have not noticed anything leaking at the exhaust port.

I tend to run things a little rich to protect rings but maybe I should check the main jet and drop a size.  It runs really strong and smooth though.

It happens at all throttle ranges.  Sometimes he will come off the practice track and it's at high rpm 5-7 sec after a small twist of the throttle.

Throttle spring seems strong...return seems smooth, fast, and a nice solid tink at the bottom can be heard on return.

I'm gearing up for a leak test to check for air leaks...then I guess I will tear it down one part at a time and check reeds, seals, boots...and I don't know what else.

 

 

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Add to the mix checking the fuel filter on the inside of petcock.... could be clogged not allowing enough fuel flow at idle. I'd also simply replace the pilot jet as it could be partially clogged or has varnish build up if it's a few years old. 

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Unfortunately VForce products often suffer for design and/or quality control issues,

there has been numerous threads on this forum about warped cages and petals not sealing properly on BRAND NEW assemblies.

Exchanged by the vendor, the replacement parts were no better.

 

Re-installing the stock reed cage assembly and simply buying new stock equivalent reeds has many times cures tuning problems.

Seal the mating surfaces with a proper Yamabond type sealer (fuel resistant) as regular RTV/silicone will be gradually eaten away.

 

Don't drop the main jet size until you've confirmed there is no air leak,

the extra fuel has perhaps saved you from a seized engine.

Edited by mlatour
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Note that if the old OEM reed cage is worn out then a replacement from Boyesen could help. They have a factory second sale right now. I got a Boyesen Rad Valve for my KX250 for $117. That includes reeds, reed cage, and carburetor holder aka intake boot. This was only $12 more than new OEM and should last longer AND has a metal intake which also should be more durable than the OEM rubber which cracks over time. 

http://www.boyesen.com/overstock.php?machine_type=MX/Offroad/Mini&make=Honda&class=&product_category=&pic=

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Listen closely, everything that's been said here so far is in all likelihood pure hokum. A 2002-2007 CR 250 is extremely temperamental about its air screw setting. With that kind of idle speed inconsistency, your idle mixture is ridiculously lean. Turn the air screw in to the bottom and back it out to one turn and see if that starts to remedy the problem. 

All of this is assuming that you have left a tiny bit of slack in the throttle cable and you are capable of setting the actual low idle stop screw which is the miniature Philips screw with a jam nut on it on the left side located inline with the bottom of the throttle slide. With slack in the throttle cable and the airscrew set at 1 turn out from bottoming to begin with, begin to turn the Philips idle screw out counterclockwise until you get the desired idle down where you want it, while revving it a bit on the stand. Lock that jam nut and ride the bike to determine if you need to go slightly leaner or richer on the airscrew to deliver proper low end torque and throttle response. If you make a pilot mixture adjustment and then don't like the idle speed re-trim the actual idle speed with the idle stop screw.

Based on your description it is so lean that the low end torque has been feathery and almost non existent. When you get it working right he will likely have to relearn how to ride it properly and quit just making noise. Those particular bikes need a pilot screw adjustment for varying conditions at almost every event. You'll get it figured out and love the old bike. 

As a simple test in its present state, pull the choke on a certain amount, or full on, while its in this crazy rev mode being discussed, if the choke idles it down, its idle screw setting or its actual pilot jet is just too darn lean. 
.

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15 hours ago, Team Buggerall said:

Listen closely, everything that's been said here so far is in all likelihood pure hokum. A 2002-2007 CR 250 is extremely temperamental about its air screw setting. With that kind of idle speed inconsistency, your idle mixture is ridiculously lean. Turn the air screw in to the bottom and back it out to one turn and see if that starts to remedy the problem. 

All of this is assuming that you have left a tiny bit of slack in the throttle cable and you are capable of setting the actual low idle stop screw which is the miniature Philips screw with a jam nut on it on the left side located inline with the bottom of the throttle slide. With slack in the throttle cable and the airscrew set at 1 turn out from bottoming to begin with, begin to turn the Philips idle screw out counterclockwise until you get the desired idle down where you want it, while revving it a bit on the stand. Lock that jam nut and ride the bike to determine if you need to go slightly leaner or richer on the airscrew to deliver proper low end torque and throttle response. If you make a pilot mixture adjustment and then don't like the idle speed re-trim the actual idle speed with the idle stop screw.

Based on your description it is so lean that the low end torque has been feathery and almost non existent. When you get it working right he will likely have to relearn how to ride it properly and quit just making noise. Those particular bikes need a pilot screw adjustment for varying conditions at almost every event. You'll get it figured out and love the old bike. 

As a simple test in its present state, pull the choke on a certain amount, or full on, while its in this crazy rev mode being discussed, if the choke idles it down, its idle screw setting or its actual pilot jet is just too darn lean. 
.

This makes sense to me as I was messing with the air screw yesterday on my 07. I was getting a bunch of spooge dripping out of the exhaust joint so I leaned out the air screw and I ended up with a hanging idle. It's time for me to get a Keihin PWK and put an end to this grief!

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2 minutes ago, Dklassen said:

This makes sense to me as I was messing with the air screw yesterday on my 07. I was getting a bunch of spooge dripping out of the exhaust joint so I leaned out the air screw and I ended up with a hanging idle. It's time for me to get a Keihin PWK and put an end to this grief!

This fix all carburetor also has an air screw that needs adjustment tho less sensitive. Also, the fit is not so perfect -just my opinion.

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Sucking air somewhere, I have an old Husky that did what you describe, the cable fitting on top of the carb was loose and the little boot was incorrect so it was sucking air there.

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