Bike surging at full throttle and high speed

Hello to my fellow Beamer riders,

I restored a Honda 1981 Honda C70 a while back and I am having problems. I moved the bike to Albuquerque and installed a high altitude jet. The bike starts and idles well but under way at full throttle the bike surges. I tried a smaller jet and rebuilt the carb making sure all passages are clear. Set the points and readjusted the mixture 10 times all to no avail. The plug always is carboned up. I have a hotter plug on order.

I have no problem balancing my Bings but the little Jap carb has me besides myself. Most say carb problems are electrical. Condenser? Coil??

Any help appriciated.

Well it's not a BMW but this is my guess. The idle circuit jet is running rich and adjustments to smooth out the top end have resulted in the richness. If you can shim up the needle the thickness of a small washer, then re-adjust the idle. There probably is a forum for those sweet old Honda's...They may have already seen the problem. The TW200 forum I frequent has a lot of the carb info on the TW.
Good luck. OM

Omega Man,

I was thinking to lower the needle as it might be rich at idle. This type of carb has a spring to push the slide down and has a idle adjust screw to limit the downward travel, the trottle cable lifts the slide i.e. no leaking diaphram prolems. Second thought I had is that the engine might be dieseling intermmitently due to bad rings similar to the old VW diesel engines.

I'll try the other forums but I put more faith in the BWM riders.

Thanks for your thoughts.

Yeah it might be old enough that the E-clip may be adjustable. I just had to go through the carb on the TW- it had been messed with. It seems to be at about 90% which is pretty good- I can drive it without the choke
Also for some obvious, have you checked the whole airbox and connection rubbers? The mice love 'em.
Let us know how you make out. OM

OM,

Get ready to give me a dope slap!!!. Why running rich?? I'll try running it without the air cleaner. Runs like a clock. Dirty filter, no it looks clean. Try it with the air cleaner in. Bike runs like a clock. WTF.

The problem was that I added the air filter cover without installing the fairing. Without the fairing the cap seals off the air inlet completely. I am suprised it ran at all. Glad I did not take the carb off again.

Live and learn, and I am a 72 year old Mechanical Engineer

Regards to All

Airbox problems are real easy to overlook. Not sure on your carb but some of the older ones run on a venturi/airflow to lift the needle. Of course as you can't really see the airflow, it's tough to see anything wrong. Hey- at least it was simple.
Have fun on that sweet little Honda, in general with a valve adjustment and a new spark plug from time to time, it will run forever.
Gary