Vintage Dirt Bike Q & A

Hi everyone,
So I am making progress with the MT250. I got a replacement mixture screw for the one that fell out and I made the rock-hard airbox boot fit. (boiled it)

NEXT PROBLEM PLEASE...

I started the bike up and it sounded great. I let it sit while I got my helmet (3 or 4 mins). Then the bike just dies. I start it again and it runs for 10 secs or so then just dies. It does not matter if i am reving or idling, it just die out. I then kick it a couple of times, it runs for 10 secs then dies again. Any ideas? I seem to remember someone mentioning that this sort of thing could be a coil problem!?!? but i could not find the posting.

Any help welcomed.
Check out the bike pics below.
Thanks,
Newfie

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Newfie
1974 MT250
1974 MR50
1975 CB750F
http://spaces.msn.com/members/andrewmer ... photoalbum

1st thing is check fuel flow from tank. Just disconnect the hose and see how it runs. Another trick is: If you took the time to loosen/fix the brass fitting on the bottom of the float bowl, shut off gas as soon as it dies and remove the fitting. That should give you an idea of how much gas is in the carb when it dies.

The MT carbs have part of the choke circuit in the bowl. Actually has a little brass jet down inside one of the passages around the lip. Those seem to clog up frequently. What I do is fray a used throttle or brake cable to get the single strand of steel wire and use that for cleaning jets. You may want to poke it through the slow jet while you are at it.

Could be coil but that is a bit rare of a fault. If they are going south they either fail outright, have problems when they heat up, or at high RPMs.

Thanks the tip. Sure enough it was the fuel line. I had put some silicone in the bowl of the petcock to fix a leak there. The silicone had broken down and clogged the fuel line. Once I cleared it out the bike ran fine. I just hope none of the silicone got down into the carb. With the hardened boots it would be nearly impossible to get the carb off again.

The bike seems to be pulling along great. The power dies off at around 5000 rpms. i am figuring that this is ok. from 3000 to 5000 it pulls like a train. let me know if that sounds about right?

Thanks again.
Newfie

Now what will be the next thing to work on

_________________
Newfie
1974 MT250
1974 MR50
1975 CB750F
http://spaces.msn.com/members/andrewmer ... photoalbum

The my '74 doesn't have a tach and isn't quite stock anyway, pipe and piston. I got my '76 cheap after the oil pump/tach drive went south and previous owner melted the motor. I am just running it on mix since I redid the top end while I am hunting down parts like fenders, lights, and tach drive.

That being said, good power up to 5,000 sounds OK. I think mine rev higher but I can't be sure. I can't recall what the top speed of an MT is but something over 70MPH seems to ring a bell. If it looks like it does 14 MPH/1,000 RPMs in 5th you are there with what they did NEW. I think the tach on them must of been off a overhead cam multi cylinder 4 stroke: Damn 9,500 redline on mine. That just isn't realistic IMO. I think you have the carb right so I wouldn't fiddle with anything other then timing/points gap maybe.

My slow jets are 58 and 60 and they are too rich in the low end. Main are 115. Seems like it is deliberately set too rich low and main chosen to lean it out at WOT.

Main on a CR is 148 and slow a 55. This seems to make more sense to me.

Thanks for the info. I noticed a pool of fuel under the bike. So I guess my needle valve must be sticking. I turned off the fuel and left it for the night. I was hoping I would not have to take that carb off again anytime soon...oh well. Is there a trick to getting the carb off with rock-hard boots on it? Can I remove the airbox??

Thanks
newfie

_________________
Newfie
1974 MT250
1974 MR50
1975 CB750F
http://spaces.msn.com/members/andrewmer ... photoalbum

I'm a "If it ain't broke don't fix it" kind of guy. I would leave the floats alone until I had a couple of hours on it. Sometimes the vibrations are enough to wear the seat for a good fit. I've even used rubbing compound to lap the seat when I've been in a hurry and didn't want to wait for parts.

If you do decide to replace it, try to find a Keyster kit. They are a bit more expensive but come with both jets and new float seat/needle. Maybe do the rubbing compound trick if you want it working now.

Got a hair dryer or a hot air gun? I spray the boots with WD-40 and heat them up a bit. Softens them and they get slicker then snot on a doorknob.

Thanks again for the info.

I took apart the carb to see if there was any silicone plugging it up, but I did not find any. after checking all the passageways I put it all back together. I am sure I am not getting the power I should from this bike. After 3500 RPM it almost sounds like the bike is missing. I am probably not right in saying this but if the bike is firing properly down low as I pass 3500 she only fires every second time she should be. There is no way I could reach the indicated redline it just could not make it up to that - very sluggish.

I did notice at idle that there is an exaust leak between the exaust pipe and the cylinder. Could this be part of the problem?

Does anyone have any ideas because I am running out of things to check.

Thanks again,
Newfie

_________________
Newfie
1974 MT250
1974 MR50
1975 CB750F
http://spaces.msn.com/members/andrewmer ... photoalbum

Exhaust leak wouldn't cause that big of a problem. They'll even run with no pipe on them better then that.

I've had bikes do that when the main jet needle messed up: The clip and cover on the needle was letting it float up and down depending on RPM.

You should have a tiny C-clip on the needle jet, a large washer the same diameter as the slide with couple of cut outs , and a Ss looking spring clip to hold it all in place. Anyway when it is put together the needle shouldn't move more then a fraction of an inch up or down. The C-clip prevents it from moving down and the big washer prevents it from moving up.

~the way works 0-1/4 throttle is all slow jet, 1/4-3/4 is shared between the slow jet and needle taper/main, WOT is just 10% slow jet + 90% main jet. Can't tell for sure without running it long enough to check the plug of course, but if you are getting black sooty deposits on it, carburetion. Clean plug => electrical.

Thanks once again.

After running for a while the plug is black and wet. Oilly I guess.

I took off the exaust to see if it may be plugged up. There was alot of carbon in there. I don't think there was enough to be causing the problem though. I just scrapped out what I could see. My shop manual has not arrived yet. So can anyone give me the directions to decarbonizing the exaust, if that sort of this is necessary ?!?

With the exaust off I checked to top of the piston to see if there was much carbon in there and there was none.

Perhaps I am expecting too much... Does this bike have much power? I am not seeing much at all... I have hear that it should give you a good kick in the pants... am I expecting too much???

Thanks Again,
Newfie

_________________
Newfie
1974 MT250
1974 MR50
1975 CB750F
http://spaces.msn.com/members/andrewmer ... photoalbum

To get the carbon out of your pipe, remove the pipe and take it to a radiator shop. Have them put it in their caustic tank (cost about $10-15) and repaint it.

You aren't expecting too much. I'm not much of a rider, that being said a well tuned MT scares me. WOT in low and the front gets light or come up.

I've done front brake on/smoking tire burn outs with mine. This was in response to someone who said they thought the clutch was slipping when I could clearly see it was rear tire spin on pavement.

If you want to try a known good carb and will return it after let me know. I don't have a spare but the MT I am currently restoring is apart for paint. You could borrow the carb for a week or two. If it works, maybe send the one that doesn't to me and I'll check it out.

The oily plug could be a problem, bad case seal or clutch side crank seal but IMO it would NOT cause the problems at 3,500 RPM you are describing. If it smokes using a good two stroke specific oil.

It's like I said, often it is multiple problems that take vintage bikes out of action. It could be your oil injector isn't adjusted right or someone filled it with whatever motor oil Kmart had on sale that week. The shop manual has instructions for adjusting the oil injector.

Thanks once again. i will be back in the garage this evening. i will try the bike once again and see where we are. Thanks for the offers. i may take you up on that. I will check it out and get back to you.
Thanks
Newfie

_________________
Newfie
1974 MT250
1974 MR50
1975 CB750F
http://spaces.msn.com/members/andrewmer ... photoalbum

grrrr. I thought that maybe the float needed adjustment. It was out 2 mills but there was no difference.

I scraped the carbon from the exaust and no difference.

I am still not getting the power I should. It seems like as the main jet opens the bike looses power.

What else can I try?

Newfie

_________________
Newfie
1974 MT250
1974 MR50
1975 CB750F
http://spaces.msn.com/members/andrewmer ... photoalbum

I find that most the carbon collects in the in the narrow section just upstream of the baffle. If you can't soak the pipe as Nathan suggested I would get a long thin flat head screwdriver and carefully work it through the carbon from the rear of the pipe with the baffle removed. There should be 3 passageways as I recall. Did you try blowing out all the passageways in the carb with a compressor while you had it apart?

The last thing I'll say is my top end is a little rough until it's warmed up.

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74 MT-250
06 Husky TE-450

Thanks Jake,
I did not blow out the passageways with a compressor. I will try that today. I did use a long screwdriver on the pipe for the carbon and I got quite a bit out of there.
I will pull the carb off again and let you guys know.
Thanks again
Newfie

_________________
Newfie
1974 MT250
1974 MR50
1975 CB750F
http://spaces.msn.com/members/andrewmer ... photoalbum