Reving out of control, then Mikuni set up

[Lloyd Boland]

Racing my '76 250 MC5 a few months ago, a ring caught the exhaust port and broke the ring.  I pulled the motor, got a new piston and bore, and put it back together.  When I pulled the Bing off, I just pulled it off and even left it attached to the throttle cable. No modifications.  It started right up.  If I just "blipped" the throttle, no problem.  If I slowly increased the throttle, when it reached about mid-range, it would just go crazy and rev to the max and would need to use the kill button.  The slide was working fine. Thought an air leak.

Did a leak down test, it kept 5.5 psi for 10 minutes with no change.  Tried it several times with the piston in different positions.  I tested the gauge and is working fine.  

Put the Bing back on and the same reving problem.  Thought it might be at the carb - motor mount so I started it and increased the idle a little and sprayed an entire bottle of WD40 around the manifold - no change.

I need to mention I pulled the Bing apart while doing the leakdown test and everything looked good, but some wear.

Time to try a 36mm Mikuni.  I read the prior posts here and modified the intake manifold to accept a Mikuni (filed down the bumps.  Tried to find the exact size gas tank hose to match the spicket but was unable to find one at local auto parts stores.  I looked in the MC5 Parts book and found that the GS Model has a spicket type manifold, perfect for a Mikuni.  Ordered the new manifold and the rubber mount from Al.  Bolts right on, no need to use the Bing type manifold.  It is a tight fit to the airbox and had to remove as much as possible off the air box side of the carb and a little off the airbox.  The jetting was a little off, but it started on the second kick and NO reving problem.  I have no idea why the Bing started acting up so suddenly, it ran fine during the race.  Anyway, the Mikuni transfer was fairly easy using the GS manifold and rubber mount. I still need to fine tune the jetting but raced it a week ago and it ran fairly well. (Out pulled a 250 CanAM).  I need to remove the motor brace to access the carb, but once I remove the motor brace, I can remove the top off the carb and adjust the needle, and I can loosen the manifold and air box camps and rotate the carb to access the bottom bolt and main jet.  Hopes this helps anyone considering a carb change.

[Dale Sonnenschein]

Thanks for the info Lloyd. I did the mikuni swap with cutting. It was a pain but I got it done.

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