Vintage Dirt Bike Q & A

1974 MT-250, finally fired it up after a complete rebuild. Bike last ran sometime in the 90s.

It will start and idle nicely. It seems to pull strong until 5,000 RPM where it just falls flat on its face, Doesn't matter if it is part throttle or full throttle.

Does this sound like a carb. issue or timing or something else?

Carb. was freshly rebuilt before it went back on, the rebuild kit came with a 120 main jet but I was able to clean up the 115 Main jet that was originally in the carb. before rebuilding it and re-installed it.

Ignition timing was set with calibrated eyeballs and a feeler gauge as I do not have a timing light here to use. It was initially too advanced as it was kicking back a lot on the starter and backfiring and I backed it off a little bit and it starts right away seems to run pretty well until it hits that 5,000 RPM wall.

Where should I look to correct this? I am a 4-stroke, fuel injected electronic ignition kind of guy so I am used to setting air fuel ratios and timing on a laptop, not with a screwdriver. Any help or direction is much appreciated.

Thanks!

And a video in case anyone is interested: http://youtu.be/_PJMQhTaGdU

more than likely the pipe and silencer is plugged up.if equipped with the baffle,remove it temporarily and take it for a quick ride and see if it is any better.

if it runs better the pipe should be removed and cleaned and the silencer needs to be cleaned.

another possibility is the snorkel that goes from under the seat to the air cleaner case can fold itself shut at a higher rpm and this causes it to blubber from the rich mixture as it was shutting off the air flow to the air cleaner case,this happened on my MT250 and i simply removed it and it solved the problem.

i watched it pinch shut while observing it and revving the engine.

this is the item i am talking about on the snorkel,it is parts 7 and 8 in the diagram.

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(2) 1968 DT1 1970 RT1M,1970 DT1C 1971 RT1B Enduro 1974 MT250 1974 DT250A (2) 1975 DT400B 1974 DT100,1978 DT175
1975 CR250M1 1979 CR250R 1979 Kawasaki KZ400H1 Jesus is Lord!!

Thanks a lot for the reply!

After lunch today, before reading the reply I thought about the exhaust being plugged up so I removed the silencer and although gunked up, it was not plugged. I rode it around the block with that removed and there was no change.

After reading this I fired it up with the seat lifted and revved it in neutral while watching the rubber snorkel that feeds the airbox. I did not see it collapse. I did take off the lid to the airbox and ride it around and it did seem to improve a bit. I will examine that a bit closer when I get some time.

I was also able to borrow a timing light and it it appears to be slightly advanced as the light triggers about 1-1.5mm before the "F" mark passes the arrow on the cover. I will dial the timing in a bit closer and then explore the intake snorkel a bit closer.

Thanks again!

In my friend's MT, it wasn't the removable baffle that got plugged up (although that's definitely possible); it was the spark arrestor 'turbine core' thing ahead of the baffle, which was packed with oily carbon. The pipe might have been passing enough exhaust for a 50cc motor, and blew smoke all over the motor from the manifold joint.

We did the 'ring of fire' thing (soak pipe in gas, oxy/acetylene torch in pipe inlet, then switch to pure oxygen) which pushed a dull orange ring through the pipe metal to just before the tailpipe. We finished off with oxy/acetylene into the turbine core from the tail end.

I haven't ridden the bike yet and we're still working out some 30+ year old carb issues, but the manifold leak is gone and the pipe has a slightly tinnier exhaust note, both very good signs. I'm trying to get my friend to drill out the center of the turbine core as this really woke up the top end on my MR175, we'll see.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

keep us posted,i use what is called a buzz box to time my bikes with breaker points.this is a small video checking the timing on a 1968 DT1 engine,with this bike a dial indicator is required but on a MT250 the timing marks on the flywheel and the magneto cover with the small cover held on with 3 screws removed is all that is needed.if you need it i can make a video with a MT250 with the buzz box if you want.

it beeps as long as the points are closed and it stops beeping when the points crack open.click on the image to play the video.

the tool i am using is a Okuda TC-2030.


HVC cycles sells a similar tool.just scroll down this page until you find "ignition Timing Tester "Buzzbox"
http://hvccycle.com/ignition.html

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(2) 1968 DT1 1970 RT1M,1970 DT1C 1971 RT1B Enduro 1974 MT250 1974 DT250A (2) 1975 DT400B 1974 DT100,1978 DT175
1975 CR250M1 1979 CR250R 1979 Kawasaki KZ400H1 Jesus is Lord!!

Thanks for all of the help so far.

I have cleaned the heck out the exhaust pipe, burned all of the oil and carbon out of it and made sure all of the spark arrestor passages were clear. I also nailed down the timing per the service manual. I put it all back together and it started and idled fine, rides well in the lower RPM range but still falls on its face at 5,000 RPM. I have made no adjustments to the carb. at this time.

The snorkel feeding the airbox is not collapsing either. The sides will bow in very slightly when it is revved high, but its still pretty much clear.

Here is the interesting part: I took out the air cleaner element completely and rode it around the block and it seems to be vastly improved. When I removed it, the rubber gasket that goes around the bottom said "FRONT" and was in back. Thinking it may have been on backwards i removed the filter and flipped it around. It kinda fits, but the front is smashed tightly against the airbox and the rear has a lot of open space. . After flipping it around I still had problems. I removed it again and it revs all the way to redline. I wonder if I have the wrong air filter on there? The air filter that I have was new (many years ago) and after it was installed the bike was never ridden and stored inside in a climate control building. It's gray foam that goes over the metal basket on all sides including the top, and has a rubber seal at the bottom. It's perfectly clean, the foam is still strong and intact, and well oiled.

Does this sound like an OEM filter or should it be something else? can I even get new filters? I can provide pictures of what I have on there if needed.

Maybe the foam element is installed backwards on the metal filter frame. Also, the inner plastic lip of the element is designed to snap onto the bottom frame edge, so that tightening the frame drives the filter edge down firmly into the airbox sealing ring.

It's also possible to oil the filter too well. I got my friend a new filter and saw he'd put it in bone dry, and called him on it. So, he squirted about 12 oz. of gear oil on it - looked like a fudge cake. Afterwards oil ran out the airbox drain, and I'm sure the air boot to the carb's got a bit in it too. Now the bike is blubbering, smoking, and won't pull redline, and somehow it seems like my fault.

After I oil my filter - I use Bel-Ray filter oil, it's nasty stuff but works great - I always sop up the excess with paper towels, otherwise the bike runs like the choke's on.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

What do your plugs look like after doing a plug chop?

Here's some great info from the 2-Stroke World forum. Great forum by the way:

http://www.2strokeworld.com/forum/index ... 2#msg41552

Terry