Vintage Dirt Bike Q & A

As many of you know, I recently got my 74 CR250M out of storage to trail ride with my son. The bike is 100% stock, including the original front tire, down to the worn out grips... I've been having carburetor issues since I got her out of storage. I'm considering on going with a new Mikuni carburetor from Vintage Elsinore. The bike came stock with a Kehin 34mm. Vintage Ellsinore is recommending a 36mm, coupled with a new pro performance pipe. Yes, the original exhaust pipe is dented/cracked, (go figure). Now I really only plan on trail riding, and going to a track to "play" from time to time.

My question is, will the 36mm with the new pipe make that a big of a difference, or should I just go with a 34mm. I need bottom end more than top end, for trail riding, so I've been told, that if I go with the 36mm I need to replace the exhaust pipe as well. Not to mention, I really don't want to drop a lot of money into the project. (yes, I'm trying to be cheap)

thanks for the guidance

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

I bought a Mikuni and a Pro-Form pipe and silencer from Vintage Elsinores and have been real happy. However if you are on a budget and not specially looking for a performance gain, perhaps try and make the stock parts work first? Did you re-build the carb and freshen up the float valve, needle, and needle jet? Old carb parts play hell with jetting. A crack in the pipe will affect performance, but a dent or two can be lived with. These can get fixed up by a welder or exhaust repair shop. In general terms a larger carb will increase top-end power over low-end torque. And an after-market pipe will boost top-end too, although VE does "rev" and "torque" pipes for the same bike sometimes.

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF

I have replaced the seat and needle valve, and it keeps acting up. I hope to take the bike to a friend this weekend who built a 73-74 CR250 to race vintage from a basket case. He's currently running evo on a Husky.

Yes, my original pipe has taken beating... imagine that... Yes it has its fair shair of dents, it does need welded. I don't think I need a large increase in power looking to trail ride mostly... mainly I need something that is as maintence free as a 30 year old bike can be...

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

One thing I have learned sorting out the jetting problem on my 74-MT after mixing different stock carb part is that there were 3 different needles used in the same Keihin carb. One must check the number / letters stamped on the carb to identify the correct needle for that carb. The real trick here is that the needle must match the needle jet seat. Honda sells them as a matched set. My problem started years ago when I bought a new needle jet set and only installed the needle. Why tear down the carb to put this other part in? That's probably the reason I parked it years ago. If you have all your original jets I would try putting them in. Before doing so make sure the internal passageways are clear including the one that connects the airfilter side throat to the needle jet's seat. The lower jet seat should be removed to clean the passageway assuming it is similar to the 28 mm keihin in my bike. You may be able to purchase a new needle jet with seat from Honda.

If you are just trail riding I would stay with a 34mm carb. It will handle fuel delivery at lower speeds better than a larger carb. With a larger carb, everything else being equal, you need more revs to get the airflow speed through the venturi to create the lower pressure, "vacuum", needed to get gas up from the fuel bowl.

If you're looking for top end performance then the 36mm is a good investment but.... I sort of doubt it in the kind of riding you'll be doing. The stock bike is pipey enough and you'll probably make your usable power band a bit narrower. Adding a pipe designed for more low end response would probably be a good thing. Or, if you can beat out the dings in your stock pipe and extend the header pipe about an inch you may pick up a little low end. (Cheap mans way. Not as good as a new pipe but should work.)

HTH,

Loren

After speaking to Don @ vintage Elinores, I've deceide to go with the 36mm, and a new Pro-form pipe. According to Don, the addition of the pipe will allow me to keep the low end grunt, while smoothing out the power band. He can jet the carb to match the pipe. I have dropped a tooth on the front spocket, so hopefully, I'll have the low end I'm looking for, and enough top end to keep spraying those behind me with dirt...

I'll post an update after I get the carb and pipe installed. Thanks for everyone's help.

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

I received the new 36mm carb and the Pro-Form exhaust last night from Vintage Elsinore. I was amazed as to how much larger the carb was then the stock unit. I installed the carb, and the pipe. I then thought before I completely reassemble the bike lets see if she will fire. Up until this point, the bike has not ran well due to carb issues. But we are talking about an Elsinore, and of course, it started on the 2nd kick. The bike just sat there and purred like a loud kitten.... The real test will come this weekend. My son and I are going riding with a bunch of his friends. Its time to show these kids some old school MX fun. I'll let everyone know how it goes.

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

Recently we sold my son's TTR125, and purchased a used '00 YZ125. We went to his friends house over the weekend, who has a small track near his home. He runs a 99 Husky 125. The track is a natural terrain track through a cow field (yes, there were many cow pies lying around) with several long up-hill and down hill runs. Now this would be my son's first time on the YZ, so I decided to make sure he was OK before I fired up the Elsie. He's 14, and it took about 1/16 of a lap before he was comfortable... kids, go figure... So I fired up the Elsie, and putted around for about 2 laps to make sure all was in working order. The bike was running just like it should, and all was well. I gave my son and his friend the thumbs up, and the race was on. I was very happy with the performance of the bike. I was able to pull away for the boys, until we hit the ruff stuff, and their suspension would take over. I was able to keep the boys behind me, until my forearms were so pumped up, I had to take a break... (yes I'm getting older, and out of shape.) We had been racing hard for about 30-40min. My son and his friend each took turns on the Elsie, just to see what it was like. Each said boy if that thing had better suspension, it would be amazing.

We decided to race some more. I let the boys go so 1st so I could chase them this time. It didn't take long for me to pass my son's friend. I was on my son's rear, until he went wide on the up-hill climb, and I blow right by him. I was smiling from ear to ear... until 20 seconds later, the engine just stopped. I reached down and kicked the starter, and oooooo nooooo, no compression. I think I burned a hole in the piston. Now you need to remember something. When I say this bike is stock, it is stock, down to the original 30 year old piston... (I did change the rings once). So I guess its time to do a top end rebuild... I think I'll pull the head over the holiday weekend, after I eat my turkey.

At least one good thing happened while riding. I have the bragging rights over my son and his friend... You know I'm not going to let them forget it all winter, while I get the old girl ready for the spring... It was great while it lasted.

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

Did you take a plug reading before you cranked her up? Changing a carb is a big deal and you need to make sure your fuel/air ratio is good before getting into a race. Also, did you check your timing? I know you where having lots of problems with the bike before these changes and incorrect timing may have been hidden in with your carb problems.

Loren

No did not check the timing. Yes I should have... I didn't check the timing as after I installed the carb, the bike fired on the 2nd kick.

I do not have a dial indicator which is needed to correctly set the timing. Any suggestions putting one together "cheap".

I was running a little too much oil, as my mix ratio was off. I thought I was at 32:1, it was more like 20:1.

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

Your oil mixture ratio and your fuel mixture are two different animals... for the most part. (Increasing your oil to gas ratio will make the bike running an itsy bit leaner but not enough to hole a piston unless you were right on the edge already.)

If you were running that puppy to lean on the carb then it pretty much doesn't matter what ratio you're running your oil. You'll hole a piston but have a nice oily cylinder wall.

I bought my dial indicator from Goodson at http://www.goodson.com . I've seen them pretty cheap at Harbor Freight but I don't believe those come with a gauge holder that screws into the spark plug hole.

I think I was due for a top end rebuild anyway. You know what do I expect, the piston was only 30 years old... You know nothing lasts anymore ... LOL. My mix ratio was off, I was a little heavy on the oil, but it was closer to 18:1. (I mixed the gas/oil, and my son not knowing I mixed the gas/oil, followed my written instructions, and added oil too...) I didn't catch it until after my bike stopped, and we were talking about what could have caused that to happen. But I still think it was time for the piston to say I've has enough. It did not seize up, so at this point, at least until I pull the head and jug, I'm going on the assumption of just replacing the piston/rings, and all should be good.

Thanks for the tip regarding the dial indicator. I'll pick one of over the winter months. Working on this bike is a whole lot easier than my son's YZ. At least I can see my motor, his is buried behind plastics, and of course that liquid cooled stuff... LOL

Firepig, I once read where you were asked why you spent so much on your bike, and you stated "I could be sitting at a bar drinking, or I could spend it on something that is fun, and good for my health. " I can't agree with you more. What a way to spend time with my son, teaching him how to maintain his bike, and I get to watch him ride, all the while I get to ride too, although I wished we could have picked a hobby that is a little less expensive...

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

I asked a friend who used to race a vintage 73 CR250M to stop over last night. We checked the plug, its a perfect tan color. We then decided to pull the head to get a look at the piston. The front of the piston at the exhaust port is where the problem is. The piston is deformed, and yes it messed up the cylinder walls. So a once over bore will need to be preformed, and a fresh piston and rings. My friend was first concerned as to the cause as if something caused it to go lean, but after looking at the cylinder, and the piston, we've concluded, that it was just past due, (way past due)and it should have been replaced prior to me racing my son... LOL.

So the plans are to get the cylinder bored, and put her back together. If all goes well, I be out sometime this winter tearing it up again.

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

I caution you against re-building the top-end without confirming you do not have an air leak past the crankcase oil seal on the ignition side. I would argue it is improbable that the jetting you got with new carb was so off-base lean that it would melt the piston down. I would have also expected an "old" top-end to rattle more and more until some part of it simply broke.

I spent several months trying to jet my CR250R 1979 correctly and it always seemed to run better when it was hotter or I was at higher tracks. When I practiced at a cooler sea level Carlsbad it would pinch sieze and then melted down (just like yours). Once the motor - including oil seals - was rebuilt by Racers Toy Store it ran like a dream and a wide range of tracks and conditions can be accomodated by only 2 main jet sizes and 2 clip positions on the needle. So I believe I had an air leak that got gradually worse until running more laps back to back in practice was just too much. BTW the damaged top-end was a relatively new Wiseco not the original.

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF

Over the thanksgiving weekend I plan to test for an air leak. That's why I just pulled the head, not the cylinder. I can reassemble as it sits pretty quickly, and test for an air leak... I'm hoping for the best, but assuming the worst... The more I think about it, there still has to be something that caused the bike to go lean, and fry the piston. So I get a few months during the winter, to disassemble, repair, and reassemble...

I may need to find a cheaper hobby...

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