Vintage Dirt Bike Q & A

Hi,
I have a 'new to me' 1975 MT 250 that I think is running hot on me and I am a little clueless. It cranks great and will run strong for 30-45 minutes then as soon as I come off of the gas it sounds like it chokes down. It wont crank for 10-15 minutes then first kick it fires right up and runs great and idles fine like nothing ever happened. The engine doesnt seem expecially hot. I have reviewed my Clymers on Elsinores and the first thing recommended to trouble shoot is too lean fuel and fuel/oil mix. I am running at 36:1 good Honda synthetic premix, plug seems a tad oily not exactly cardboard brown. Is this typical of running hot? Here is the wildcard, I bought it a 1000 ft elevation and riding it at 5500 ft elevation. All thoughts and comments are greatly appreciated. Thanks

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Awesome Elsinores!

elsinorerick

I think at 5,500 feet it's more likely the motor is running rich, especially with an oily plug.

Try starting the motor and warming it up, then turn off the fuel tap and let it idle. Have your hand near the kill switch in case the revs shoot to the moon after a few minutes, indicating a super-rich low-speed condition.

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

You might want to invest in either a cylinder head temperature gauge or an exhaust gas temp gauge.

I bought one of these cylinder head temp gauges. http://trailtech.net/tto.html
Just remove your spark plug & replace the washer, then mount up the gauge itself with one of their handlebar mount kits. This will provide you with a tool that will provide much useful info especially when changing your jetting, etc.

I run 32:1 with premium fuel & Honda CR type pre-mix.

PS. Check your fuel cap vent to make sure that it isn't plugged. I had one that was plugged & it would cause the bike to just die after 20 minutes or so of run time. After 10-15 minutes it would finally restart. I believe that a vacuum condition was occurring in the tank as fuel was burned.

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WWW.CADDY500.COM
The High Performance
Big Block Cadillac Forum

Two other things to check:

1) Make sure your gas cap vent is unblocked

2) Check the compression - if it's really low you could get the symptoms you describe.

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

Thanks a bunch guys!
I will try all of the recommendations and get back with you with the results. It is currently in my garage with a new tire being ordered. As soon as I can get it out in the fresh air I will be able to do some serious testing and get back with you. After making the post I read one solution in Clymers is changing the jetting of the carb for a change in elevation. It was purchased at an elevation of 1000 ft and now I have it running at 5500 ft. No one mentioned this as an potential fix so I am little curious if this is not a common problem. If the other fixes dont get it I will try that and let you know what happens. Thanks again!

_________________
Awesome Elsinores!

elsinorerick

You will probably have to re-jet for your higher elevation.

I don't think that is the cause of your over-heating problem though if that is actually the case. Air density is at it's highest at sea level. There is less air at your elevation so you should be running rich if anything.

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WWW.CADDY500.COM
The High Performance
Big Block Cadillac Forum

UPDATE... OK I have taken everyones recommendations to heart and done some research of my own. Just got my tire back and got it on asap. According to Clymers the first suggestion was to set the carb for this high altitude for running rich conditions. I dropped the needle one click and took it out for a spin. Now it was only 50 and not 70 degrees but after 45 minutes I had no running hot symptoms. Jury is still out but for now it may have run hot due to too rich fuel due to too high altitude. Did running hot with too rich fuel damage the engine? I will keep you posted on future rides. One thing also my compression may be a little low adding to this problem. Dont own a compression tester yet so we'll see about that. Thanks again for all the tips!

_________________
Awesome Elsinores!

elsinorerick

Oh yeah, I forgot to mention the vent for the gas cap was clear and breathing fine. I have tried all the easiest things first. Thanks for mentioning that one. That is one of those simple things I would have never thought of and isnt mentioned in Clymers. Thanks again for the advice.

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Awesome Elsinores!

elsinorerick