runs better with choke on

[skiracer]

As a follow up to my on going problem on my 76 MC5 250, I changed the neddle jet to one richer.  Now, when I start the bike, it runs almost normal with the choke on, rich, but somewhat normal.  When I take the choke off, the bike begins to rev fairly high on it's own, with touching the trottle!  Any suggestions?  I would really apprciate it.  I have plans to come to the RR, but I need a running bike first!  Thanks, James

[Randy Kirkbride]

Hi James,
Sounds like you're sucking air from someplace. Carb tight??

[tomale]

I think I would start by seeing if the carb was tight... but then I would move to see if the igniton side crank seal is bad.. they are easy to replace. It would also be a good idea to do a leak down test. I built the things I needed to do the test using plumbing products. 10 pounds of air pressure is all that you need. and using some soapy water spray all your gasket areas , for that matter, all of the motor, I had a leak in a barrel once that developed a crack in the aluminum, in the intake manifold area.. I have also seen cracks develop in the transfer port. After all these are old engines... anything is possible.

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)74'
250 hare scrambler (project bike)

[Rain Man]

thats a big leak if its revving high with the throttle off...  Maybe your cylinder head nuts loosened up,(torque wrench).  A blown cylinder or base gasket will provide a big leak also.

Raymond
 Down East Pentons

[firstturn]

This may sound crazy, but be sure you aren't confused as to when the choke is on and when it if off.  I have seen this happen more than once.  If this isn't the case at least we have made sure that this isn't the problem.

Ron Carbaugh

[Steve Minor]

That's the first thing I thought of too Ron...I've done it myself.


Steve Minor
Wilmington, NC

[Ron]

Are you sure the slide is closed. Its not un-common for an engine to run a little faster with the choke on.
RonW

[skiracer]

Thanks for the input guys!  No, I am not confusing the position of the choke lever; push the lever down, the choke is on, pull the lever up, the choke is off.  If the main seal was leaking on the mag side, shouldn't I see gas leaking into the case area?  Or, can I have a leak there without any gas leakage showing?  I am building a pressure tester today thanks to Dave sending me some great photos.  I will check the tightness of the head today also.  On the base gasket, again, wouldn't there be visible signs of exhaust leakage where the leak is?  I will write back later today after I build my pressure tester.  Thanks again for all your input, and help, I really appreciate it, and I owe you guys a beer at the RR!  James

[firstturn]

James,
  As far as gas leaking in the areas you are talking about you may just have an air leak with the gas evaporating.  Also, on the choke I had someone I was working with on this and they said the sme thing until they checked inside the throat of the carburetor to see when the choke on and off.  Sorry I can't be there to help you, but you are learning some good troubleshooting.

Ron Carbaugh

[tomale]

Hi James, with big air leaks, yes there should be some visable sign but with small leaks, no.. and if you have several air leaks then it is possible that you have no signs. ask me how I know. I must admit, that given the discription of the problem, it sound like a bigger leak. I just had a crazy idea. on KTM motors there is a small bolt on the bottom of the crank case. It is a nice feature if you ever flood the bike. If is missing, then that would be more than enough to cause your problem..

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)74'
250 hare scrambler (project bike)

[skiracer]

You guys are great!  The bolt is still in the bottom of the motor.  I had some unexpected company this afternoon, so I didn't have a chance to work on it.  I did build the pressure tester, so, tomorrow morning will be the time of finding out!  I will let all of you know the results.  Thanks again.  James

[Big Mac]

If you have a lever up/down on the carb, you must have a Mikuni replacement for the stock Bing? On the Bing with the plunger-in-slide type of choke, the lever's on the handlebars...cable pulled full is off, cable loosened puts the plunger choke "on", which is contrary to typical logic and often creates the confusion.

Diagnose for air leaks by spraying WD-40 or parts cleaner in behind the flywheel or around intake while the engine is idling. You'll hear the revs go up noticeably when it sucks in the spray and should be able to pinpoint the leak if there is one.

[skiracer]

Yes, it is a Mikuni carb.  I was looking for better performance at the bottom end.  The engine starts to rev higher and higher as soon as I take the choke off.  I am in the process of doing a pressure test on the bike.  So far, I have not found any leaks around the head or base gasket, or the mag side seal.  I am checking the intake manifold later today.  Thanks for your help.  James

quote:Originally posted by Big Mac

If you have a lever up/down on the carb, you must have a Mikuni replacement for the stock Bing? On the Bing with the plunger-in-slide type of choke, the lever's on the handlebars...cable pulled full is off, cable loosened puts the plunger choke "on", which is contrary to typical logic and often creates the confusion.

Diagnose for air leaks by spraying WD-40 or parts cleaner in behind the flywheel or around intake while the engine is idling. You'll hear the revs go up noticeably when it sucks in the spray and should be able to pinpoint the leak if there is one.