Over Revving

[Lucky_Lou]

A question on behalf of my brother,he works away allot so cannot get on the forum much.I was helping him with his rebuild now up and running i rebuilt the carbs for him including ultra sonic cleaning and full jet kit with Bing diaphragms(its a 81 with spring assisted diaphragms).The bike starts and runs fine but is over revving on the throttle shut down,i recall a thread a while ago that concluded that this could be a timing or valve clearance issue.I had this problem with mine but the Bing Diaphragms cured it.
Any thoughts on a remedy.
Lou

[Bengt_Phorqs]

That's a fairly common issue (and topic) with the airheads.  Start with setting the valves to spec on a cold engine, make sure that timing is correct, then go for a 15 miles ride so that the engine is good and warm.  Then it's a matter of balancing the carbs so that they are synchronized.  I won't go into that here but just run a search on the topic.  Usually it's no more than just adjusting the idle screws on the carbs.

[azcycle]

I was having this exact problem on mine last week after I adjusted the valves. I haven't checked timing (yet) but there was two things I did.

1) When I put the left carb back on (took it off so I could see the timing marks better) I unintentionally rotated it so that the throttle cable was stretched a bit. Rotated it back more "level" and that helped 50% of the over-revving.

2) As Bengt mentioned... just adjusted the carb idle screws by ear when out on a ride.  It's still not perfect (balanced) but the revs now drop to idle as they should.

[montmil]

Hum... Is the problem the carburetors or ignition? How to tell?

As the sports above mentioned, do a valve adjustment with the engine stone cold. Then, go for a ride of 15-20 klicks and then check the ignition timing. Adjust, if necessary, to get the timing spot on.

Now, if the high idle revs are still occurring, there's a simple test to isolate the area of concern. Try this during a casual ride; whip in the clutch, bump the transmission into neutral and come to a stop. If the engine's idling way too fast, toggle the ignition kill switch -the one on the right side bar-mounted unit- and let the engine die down almost to a stop. The trick is to catch the engine just at idle rpm.

If the engine returns to its former fast idle, the problem is carburetor tune. Should the engine revs remain at a happy place, the bean can needs work in one of two areas: sticking advance weights or quite likely -given the age of these airheads- worn out advance return springs. Got my new springs from Motobins.

BTW, with a pair of small forceps, it's possible to remove the old springs and replace with new ones working through the access ports on the side of the bean-o can. Pop off the metal plug and have at it. Patience, a cool spot to work and a cold brew helped me.

Luck to you, Lou, and your bro' [smiley=beer.gif]

Monte

[trolle]

My favorit topic  

If the valve clearances are ok, it's either the carbs or the advance springs as stated above. On mine the fast revving in idle was periodic and went away when the throttle was blipped. The cause for the revving was a loose butterfly disc.

If the valve clearances were ok and you fixed the advance springs and the carbs,  the idle rpm then begins to rise after a few hundred miles you should check the valve clearances: If the exhaust valve clearance is diminished it's time for a head job because the valveseat has receeded.

greetings from a rainy north

[steve hawkins]

As Trolle hinted above, make sure both your butterflies are closed when your throttle is closed.

Do this when you are doing your balancing at idle.

You can check your advance and return with a strobe quite adequately.

Cheers

Steve H

[Lucky_Lou]

Thanks for your responses...i do recall one of the butterfly screws was stripped i rethreaded it and peaned it on rebuild but on reflection a new shaft would have been the way to go.
Hope to have a look this weekend if it ever stops raining.Will check the valves and timing as i dont know if LB checked them
Lou
Greetings from the land of the Ark builders...flamin June your havin a laff