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Discussion Starter #1
What's up guys! I recently purchased a CM250 that had been sitting for about a year. The bike is completely stock, engine wise. The bike does crank up, but the idle is surging quite a bit at idle. It will only stay idling consistently with the choke engaged(for the most part). I can get it to run with the choke disengaged, but as soon as I touch the throttle in either with choke or without, it will fall on it's face. If I VERY gradually increase the throttle, I can get it up to about 4000rpm, but it will eventually fall off again. I will state that when the bike was at 4000rpm is was vibrating itself across the garage floor whilst on the stand with me on it. Here are a few things I've noticed/completed AFTER I realized the idle was surging:

-The lower bowl drain was leaking from the drain tube. I noticed the cracked o-ring on the bleeder screw. I removed the o-ring and applied a VERY small amount of sealant to the threads only as a temp fix and it stopped. This did not solve the issue and I figured it wouldn't. Just got tired of fuel leaking onto the garage floor.
-I noticed the two upper intake manifold nuts were missing. Luckily I am a hoarder of fasteners, so I had two nuts that worked nicely. I had high hopes this would fix the surging issue thinking it might have been a large vacuum leak, but still no luck there.
-I removed the factory air cleaner and kept the box open to get a little better air flow, but it almost seemed to make it worse. This pointed me further towards fuel starvation.

Overall, my guess would be something related to the carb. I know the jets are extremely prone to clogging, but I want an outside opinion. I'm sure I'm not the first one to go through this, so I thought I would ask before spending the money on a carb rebuild kit as a repair. I will likely still get the kit, but I want to eliminate this issue first. I read that it could be related to valve lash, but I'm not convinced. If anyone has any recommendations on rebuild kits or aftermarket/different model carbs that will fit, let me know as well! Thanks! :)
 

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First, check the timing for variations; a loose advance mechanism can cause surging at idle, and springs that old ...

I do think you still have an air leak, combined with a problem with the float levels, so, if the timing is OK, then I would first take the intake off, and replace the gasket; being loose like that could have split or cracked it. While there, check the rubber boot for splits and cracks, and that the carb is fully seated. Next, check the float level. If the drain has a tube you can use to put a clear line on, you can do this both with the engine off, and with it running. Just connect the line and turn it up into a 'U' shape, without kinks, then open the drain screw. The fuel should rise to the level in the bowl. Somehow fix the line so it won't fall (tape works for temporary jobs), then start the engine. If the float valve and fuel lines are working properly, the level shouldn't change.

If you can't connect a line like that, you will have to turn the petcock off (unless vacuum operated), take the bowl off, then lift the float until it presses against the valve, plus a bit. Have a catch can ready for the next step. Turn the fuel back on (if vacuum-operated, turn it to PRIme), verify no fuel flows, then lower the float until it just starts to drip and measure the distance from the bottom of the float to the seating surface for the bowl. Verify against the setting in the manual. Next, let the float drop, to see how quickly the fuel flows into the catch can; it should be steady and quick enough to fill the bowl in less than a second - use your judgement for the flow rate.
 

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Discussion Starter #3
Today...

First, check the timing for variations; a loose advance mechanism can cause surging at idle, and springs that old ...

I do think you still have an air leak, combined with a problem with the float levels, so, if the timing is OK, then I would first take the intake off, and replace the gasket; being loose like that could have split or cracked it. While there, check the rubber boot for splits and cracks, and that the carb is fully seated. Next, check the float level. If the drain has a tube you can use to put a clear line on, you can do this both with the engine off, and with it running. Just connect the line and turn it up into a 'U' shape, without kinks, then open the drain screw. The fuel should rise to the level in the bowl. Somehow fix the line so it won't fall (tape works for temporary jobs), then start the engine. If the float valve and fuel lines are working properly, the level shouldn't change.

If you can't connect a line like that, you will have to turn the petcock off (unless vacuum operated), take the bowl off, then lift the float until it presses against the valve, plus a bit. Have a catch can ready for the next step. Turn the fuel back on (if vacuum-operated, turn it to PRIme), verify no fuel flows, then lower the float until it just starts to drip and measure the distance from the bottom of the float to the seating surface for the bowl. Verify against the setting in the manual. Next, let the float drop, to see how quickly the fuel flows into the catch can; it should be steady and quick enough to fill the bowl in less than a second - use your judgement for the flow rate.
Okay, so I did a few things.
-Put some fresh fuel in it with a little bit of Sea Foam.
-Got it idling with the choke and allowed it to warm up.
-Cut the choke off and adjusted the idle screw until it was somewhat smooth.
-Cracked the chain tensioner bolt loose and it seemed to smoothen out quite a bit. Not sure if it was just me. I kept my hand on the ignition switch just in case the chain started to slap around, but the auto tension feature either worked or it's frozen. Either way it seemed to help quite a bit. I will include that the chain was not making any noise prior and the rockers aren't making any abnormal noises either (That doesn't mean they aren't out of adjustment though).
-I let it idle for about 10 minutes without choke. It had a fairly smooth idle with a little bit of spitting/sputtering, almost like a slight misfire. Even with this spit and sputter it's really healthy with slow throttling up to around 3000 rpm then falls off. I was able to drive the bike around the neighborhood like this up 2nd gear, but still couldn't really twist the throttle.

I am having the lower bowl leak from the plastic line onto the floor. Even after i put sealant on the threads. I have the screw turned all the way in and it's snug, but not overly tight. Not sure what the function of this is other than to drain the carb, but it doesn't seem logical for it to leak like this all of the time. Especially when the valve screw is tight. Is there some sort of internal seal besides the one near the head of the screw? I am about to pull the carb out and start disassembling it. I'll let you know what I find. I purchased a seal kit that includes the bowl seal, main jets, slow jets, etc.
 

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I'd have to look at it, but it could be what you are seeing is the overflow, which will leak if the float valve is stuck open, or is set to fill the carb too full. Overfilling would create the running problem you have, as the main jet needle begins to rise, making the mixture even richer than it is near idle. Check the spark plug.

Also, most chain tensioners are used when the engine is not running, usually set to TDC.
 

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Leaking on floor is actually a good thing

Leaking fuel from the overflow can save your bike from disaster. If that fuel was not leaking, it could be leaking internally into your engine through the carb intake and thin out the oil in the crank case.

For how this leaking happens, look inside a toilet tank. notice the center tube [overflow tube] in the tank. If the water rises too far in the tank, it overflows out the tube, and the toilet 'runs' into the bowl, wasting water. There is an overflow tube that does exactly the same thing exactly the same way in your motorcycle.

So why is the fuel too high in the first place? Three possible reasons. Just as your toilet has a shut off valve that is actuated by a float, your bike has a float [some of the floats are adjustable and some are not]. The equivalent to the float valve in your toilet is a simple needle and seat that seals the fuel flow when the float pushes them together just like your toilet fill valve.

So in your motorcycle, the needle and seat can wear out or get some dirt particle in between them to stick open preventing the thing from shutting off. Also, the float itself can develop a leak and get waterlogged with gasoline and the needle and seat will not activate thus overfilling the bowl.

The third problem is that occasionally, the overflow pipe itself can develop a crack and your carb bowl will leak out even with the fuel height at the correct level.

to view some images that will make this easy to understand, Google 'motorcycle fuel overflow tube' and open the links related to the pictures.
 

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More

http://www.dansmc.com/carbs2.htm

Be sure to click on those pictures to make them big. [learned that after a while]

The link also explains about setting the float height for adjustable floats, but a repair manual for your bike will tell you the factory settings and how and where to take the measurement.

Good luck.
 

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Discussion Starter #7
Update!

Be sure to click on those pictures to make them big. [learned that after a while]

The link also explains about setting the float height for adjustable floats, but a repair manual for your bike will tell you the factory settings and how and where to take the measurement.

Good luck.[/quote]

So I pulled the carb and removed the bowl. LOTS of gunk and apple jelly looking stuff. Plus a little bit of oxidation on the aluminum. A couple of the jets were clogged up a bit, so I'm assuming this is where my fuel issue was coming from. I went ahead and ordered a new jet, needle, and seal assembly to freshen it up. It turns out it was the overflow tube and it is working properly. I'm assuming that the float is cracked, wasn't seating properly due to the small amount of trash in the opening or that the float seal is just very old. Either way, two out of three of those will be resolved with a good cleaning and some fresh parts. The float did not contain any fuel so I don't think it's cracked, but I'll double check. I'll keep you guys posted, but honestly I could probably put this thing back together with the original seal and I'm fairly confident that it will run great. I'm going to be waiting patiently for my package of seals, jets, etc to arrive though. I'd rather do it once and do it right.
 

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crazyrydawg

You found the problem. "Gummed up" fuel in the carb. The 'O' ring on the fuel bowl drain screw has nothing to do with the carb leaking fuel.
It's only function is to keep fuel from coming out at the screw when you drain the fuel bowl.

Your carburetor just needs to be cleaned. Putting in new parts is OK but many times you can just clean em.
You MUST clean the ports (passages) inside the carb body too.
Soak the disassembled carb and parts in some kind of carb cleaner. Get a spray carb cleaner and clean/check ALL ports inside the carb.

While the carb is off, it would be a good idea to check or replace (if needed) the gasket where you found the missing hardware.

Hope this helps,
SemiFast
 

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^ +1; if there is gunk you can see, there is gunk you can't, in places that just cause trouble. Some carb cleaners can damage the flexible parts, so choose wisely, or you will be taking the carb farther apart.
 

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Discussion Starter #10
Final Finished Carb

^ +1; if there is gunk you can see, there is gunk you can't, in places that just cause trouble. Some carb cleaners can damage the flexible parts, so choose wisely, or you will be taking the carb farther apart.
Sorry for the delay. I completely disassembled the carb. Cleaned out all jets, etc. with a small strand of bicycle brake cable wire. Cleaned the carb several times with aluminum wheel cleaner from Walmart. I covered the carb in this stuff, scrubbed with a brass wire brush, let it sit, and scrubbed it again before rinsed it with water. I did this about 10 times. I replaced all of the fasteners with anodized aluminum cap screws. It turned out pretty great I think! I'd like to post a link, but the rules wont allow it.
 

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Don't forget that there are small passages through a carb, which carry the fuel to openings that open into the throat of the carb. This is especially true of the idle circuit, which has tiny holes around the throttle plate. You can blow through the hole the idle jet fits into, to see if it comes out there.

This is not your carb, but it has all the same features:
 
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