Vintage Dirt Bike Q & A

I have a 76 MT 250 that I completly rebuilt the engine on. My problem is that the motor when driving and trying to accellerate, it bogs out like it is being choked, and it seems to billow out excessive smoke. Sometimes it trys to snap out of it but not really. I took it for a 4 mile ride last night and was worried that I would have enough to get home. The carb was just rebuilt for the third time, and the engine had all bearings and seals replaced and the cylinder was bored for the new piston. I have adjusted the oil injection system to the lowest setting and still the same. Has anyone ever experienced this problem? Might I have some type of internal leak? Is there any method of checking pressure in the crankcase or transmission area?
I am anxious to ride and appreciate any help!

Hey curcuit1 I just got my mt250 bottom end back together. You say its smoking but dont mention what color the smoke is. White smoke is generally a leaky seal was it white or blue? I belive the test you are looking for is a leak down test there are kits made for this and probably arnt to expensive.

I just had a similar problem with my 79 cr250. It ended up being an air filter problem. Check to make sure you are getting enough air through the air filter and nothing is blocking the air on that side of the carb.

I checked the air filter and even tried running without it with the same results. I got the opinion from a couple of friends as to the color of the smoke, they both thought it looked to be a light blue or green but very close to white. I looked at another engine that I have and noticed the only way for oil to leak in from the gear case was for the gasket to leak. Is that correct? I hate to tear it down but will have to if I need to. Thanks for the information

did you paint your engine and or frame if so you could have a bad ground engine needs to be grounded to the frame. your other elect. that need to be grounded need to have a clean ground coil, cdi box or what ever

Yes I did strip and paint the frame, and painted the engine as well. Thanks for the post, I will check the electrical system today.

I think you are jetted too rich. The easiest way to check this is to lower your needle. Regardless of what the factory setting is, I would lower the needle a couple of steps (by raising the clip). If this helps through the mid-range but still blubbers at the top, then you need to put in a couple step smaller main jet. I have a 1975 CR250 and to get it running clean I had to lower my needle two steps and go down 3 main jet sizes. This was after pressure checking the crankcase, etc. The factory numbers are sometimes way too rich. Trying the needle first is your easiest choice - you don't even have to remove the carb. If you tear into the motor and later discover that it was just jetted rich you'll really kick yourself, so I would try that first. Good luck.

a bad ground will act like its jetted to rich also depends on where you are located from sea level as you go up you need to jet down two strokes are some what sensitive to how hi up in the air you are jd

Well I checked and ground clean all the electrical connections, with the same results. I also lowered the needle as far as it would go with no results, same problem. I also tried running it on mixed gas without the oil injection hooked up, again same. I did not however change the jet size, I live about a 1/2 mile from the east coast at about 220ft above sea level and figured I was ok. I have not been able to locate the needed stuff to do a leak test. I called both Honda shops in my area and they refused to work on it! I did however get the name of a guy who works on elsinores from the service manager, so I will be bringing the bike to him and keep my fingers crossed. Thanks for the help, I will post the results when I get the bike back.

Check your ignition timing.

Neither of my MTs smoke much even with the choke/enrichment on. Only time one did was a bad center case gasket. Also had a '76 CR that had a cracked case from someone feeding it a small bolt but that is a different story.

When I had the bad center case gasket I had to rev the engine up to ~5000 RPM just to pull away from a stop, lots of smoke, and the plug was wet with oil. Running rich kind of produces a black soot on carb and unburned fuel fumes/smell will make it obvious.

Sounds like a seal of some kind to me anyway. Could be center case gasket, cylinder base, oil seals, just about anything.

Rick

when i got my 79 cr250 it it smoked like a son of a bitch. tried replacing the crank seal that has oil up against it 3 times and finally figured that it had to be the crank center gasket. i rode it once and my friend probably stayed back 50 yards just so he could see. i finally took off the jug and drained all the oil lightly blew air in the transmission and could obviously see is was leaking from the tranny into the crankcase. i'm just waiting on my gasket kit to come in the mail so i can get my bike ready for the next long weekend. When i install the gasket i'm probably gonna put a super thin skim coat of three bond 1104 on both sides of the gasket when i install it so i feel better that i won't have any more problems

When I put the gasket on my mt bottom end the center one I applied a coat of spray on copper gasket stuff to both sides of the gasket just for peace of mind.

Well, I got my bike back yesterday from a home mechanic and it runs 90% better. It turns out the needle tube was severly worn. He replaced it with one from a junk carb I had and it now has plenty of power and accelleration, it does however bog out on the very low end, like when I am taking off from a stop. He told me that I need a jet sized about 54-55, so I am now in search of one. If anyone has any ideas as to where I could find carb parts, I am all ears. I thank everyone who offered their assistance and ideas towards my problems, what a great site this is. I wish I had discovered it sooner than I did.