Vintage Dirt Bike Q & A

I'm still trying to get my stock 74 CR250M running again. When I pulled her out of the moth balls and decided to ride her again, I found there was fuel coming out of the stock Kein carb's overflow. At the advise of several here, I replaced the seat and needle valve to correct that problem. Now my problem is the bike seems to bog down, until I get her wide open. When I can get her wide open, it runs strong. When I twist the throttle, it acts as if it has a fouled plug. It does not rev clean, its just not running like it should. I've put in a new plug, fresh gas, but it seems to have no low to middle range, until I get her wide open, at which time it goes like hell.

Also, it seems to smoke more than I remember (of course I was alot younger, and memories, well they tend to fade...) It not black, but white smoke. Could it be the carb just is still gummed up from sitting. Should I soak the carb, and if so does the cleaner damage rubber parts?

Thanks for the help.

If you are getting black goo for you exhaust. The carb is not turned properly. The needle controls the low and middle range. These old bike are prone to needle wear. I had to replace the needle and the problem was solved. Also, The pipe maybe clogged. Good Luck

i had the same problem with the white smoke it was one of the main seals that had sprung a hole. it was sucking oil into the cylinder chamber. after i replaced the main seal it ran like a dream and didnt smoke.

Your not the first to mention the main seal may be the culprit. My question is, How difficult is it to replace the main seal. Is any special tools required. I am somewhat mech. inclined, meaning I know which end a wrench to use, but I want to make sure I don't get in over my head.

thanks for your help.

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74 CR250M, bought new in 74, still stock, except for the new 36mm Carb, and proform exhaust.

I've been tinkering with the bike, and this is where it stands. I starting looking closely at the carb, and the needle jet. I discovered that instead of being smooth, it was ruff, (years on gunk build-up). I took fine sand paper, and smoothed out the rough surface. I also did this on the idle screw, and the air screw. The bike started and was running worse than before. I had to give it full throttle to keep it running. I then noticed fuel coming out the overflow. I shut off the fuel with the engine running, and eventually the engine "cleared" up, it fact, it ran strong with good throttle response. So my 1st problem is the bike was flooding out, and the carb needs rebuilt. I previously replaced the needle valve and the "O" ring on the needle seat assembly, but something about it doesn't seem quite right. So I just ordered a Keyster Carb rebuild kit from mailto:sci@siriusconinc.com , and will start there with getting the beast running again.

Next problem, I believe there is a small air leak, as the rpm would continue to grow. What is the best way for a garage mechanic to track down an air leak. I can turn a wrench, but I also don't want to get in over my head.

Any and all suggestions are appreciated.

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74 CR250M, bought new in 74, still stock, except for the new 36mm Carb, and proform exhaust.

You need to rig up some sort of leak-down tester. Basically you plug up the exhaust port, using an expanding stopper available in most plumbing departments, and then make a plug that will fit in the intake manifold where the carb goes. In this plug mount something like a tire valve and a pressure gauge. Pump it up to about 7-9 PSI and watch for pressure drop. It should be no more than 1psi per minute.

Now if you don't have a pressure gauge you can still put some pressure in there and listen closely for a leak. Last year a buddy of mine and I drove up to the Chehalis national. We stopped in at his folks house just north of Portland the first night. His dads old Husky wasn't running right either. Power surging. I riggen up a real makeshift gludgy leakdown tester with a rubber ball in the exhaust and a bunch of radiator hose, duct tape and an old bicycle innertube. As soone as I'd pump some air in it we could clearly hear it hissing out through the crankcase breathing tube. Big time main seal or case leak. Needless to say that Husky didn't make it to the race the next day.

Bottom line of the paragraph above is that he can *improvise" something.

If you have access to a lathe you can built a really slick tester like I did that I use whenever I go into the top or bottom ends.

Loren

You mention that you turned the fuel valve off because gas came out the over flow and eventually the engine smoothed out and ran faster. When you shut the gas off and the level lowers in the float bowl the mixture leans out raising the RMP just like an air leak. When you install the carb kit make sure all the passage ways are clear. Don't use a drill bit or anything that will scratch surfaces. I have stripped cooper wire to remove a single strand for this purpose. Air compressors with basketball inflation needles are useful here.

Poor compression will prevent a 2-cycle engine from idling. How was the bike running when you parked it?

If the left side crank seal were gone it would likely be running lean with a fast idle. The seal can be changed without splitting the case but it ain't pretty. It involves a thin clean sacrificial chisel or small screwdriver and some good needle nose pliers. You must be real careful to avoid the race and not to let any of the debris get in the bearings behind it. Look at a new seal before you start to understand what you are removing. LorenÌs suggestion for a vacuum test would be very useful here.

It ran fine before I parked it. The compression is very strong. When I first got it out, and kicked it over, I had forgotten about the back pressure... I wasn't wearing the proper shoes... and yes, my foot was bruised.

When the rpm's would rise, It didn't just take off and scream, it would build slowly. So maybe.... just maybe, it's still a carb issue.

I'm going to take the card to a local shop, he's agreed to soak it in some cleaner. I'll then rebuild using the kit when it arrives. I'm building a unit to test for an air leak as Loren suggested. Its just tuff sitting on my hands waiting for replacement parts, knowing (hoping) its so close to running again.

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74 CR250M, bought new in 74, still stock, except for the new 36mm Carb, and proform exhaust.

I preformed the air pressure test per Loren's suggestion. It revealed a left side air leak, as the one Jake described. I also found oil inside the left side engine cover. So I have some more work to do. I have contacted a long time friend who I used to ride/race with 25 years ago. He currently races street oval, and he as agreed to give me a hand.

My wife keeps asking me why not just get rid of the "old" bike and get a newer one that will require less maint. I can't put it into words she can understand, but I wouldn't be able to forgive myself if I didn't bring this old girl back to life. I guess I'm starting my mid life crisis. (At least I'm doing something that will keep me out of the bars, while having fun with my son)

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74 CR250M, bought new in 74, still stock, except for the new 36mm Carb, and proform exhaust.

Glad to hear you isolated the problem.

The way I look at it, anyone can buy a new bike and maybe even save a few $$$ but I enjoy turning a few heads. Every where I go someone will ask, "hey what year is that" and proceed to tell me about 70 something bike they once owned. I get it at red lights, do it yourself carwashes, gas stations and on the trails, They almost seem a little disappointed to be using the past tense. And I've got a measly MT. Just think of the fun you'll have showing off the CR.

The air leak is caused from a small crack in the engine case. Can this be welded by an alum welder?

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74 CR250M, bought new in 74, still stock, except for the new 36mm Carb, and proform exhaust.

Progress has been made, I found the air leak and it was not the engine case as I thought, but , 2 bolts on the head were not tightened correctly, and the exhaust port bolts were loose too. Both have been tightened. I ran the bike for 5-10 min in the garage, at which time the idle did not raise... I still need to fix the crack in the engine case. Oil from the transmission is getting into the left side case. A little JB weld, and that should be taken care of.

Now onto the carb, I'm been having problems with after market parts. The float needle and seat I purchased on e-bay from 2 different suppliers did not fit, so I'm still working on that problem. I just received an e-mail from Western Hills Honda, with the correct part, and yes its in stock. So we are making progress.

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74 CR250M, bought new in 74, still stock, except for the new 36mm Carb, and proform exhaust.

Just an update

The rebuilt kit from Western Honda would not fit my carburetor either. I contacted a friend of a friend who races vintage loacally, and he was trying to find a original stock carb for me. I also was checking everywhere.

I just received last night a carburetor purchased through e-bay, from a 73/74 Honda CR250M. Since I've been going crazy wanting to get the bike going again, I installed it, and the bike fired right up. Since it was 10:30 PM, I only let the bike run for 5 seconds. (I don't want to upset the neighbors or the wife by riding the bike in the yard that late at night.) I will remove it today, and give it a proper cleaning. Then I'll take her out to the trails, and see what happens.

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74 CR250M, bought new in 74, still stock, except for the new 36mm Carb, and proform exhaust.

Ahhhh, the way to do it.
Thanks for keeping us updated. We guess and some are right, some are not. Glad to read you are on top of this.
Rick

Just a final note, I finally found time to pull the carb I purchased off of e-bay, and give it a proper cleaning. I then changed the jets with a rebuild kit I purchased. I also replaced the needle and seat, and also replaced the gasket. Put in fresh fuel (with the proper mixture), a new spark plug, flipped the choke, and kicked her over. It fired on the 2nd kick. I could tell from the moment it started, that it was running like it was new.

I took her on a trail just to work out the bugs, and I'm happy to report, that the Ïbeast is backÓ. I got some looks from some of the "kids" when I unloaded her of the trailer. But when I went flying around them, they all took notice.

It took two days to wipe the smile off my face... The best part was the look on my sonÌs (14 years old) face, when he took it for a little spin. Lets just say he didnÌt expect it to have the power it does÷

When I get a chance, I'll post a picture. I just want to say thanks to all those who gave advice, I really do appreciate it.

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74 CR250M, bought new in 74, still stock, except for the new 36mm Carb, and proform exhaust.