I have a 1974 850 Commando fitted with a single 36mm Mikuni. It is has the stronger return spring fitted. Earlier this summer, I was at the Dijon circuit for the classic bike track sessions. On the last session of the weekend, I reached the end of the long main straight at over 6000rpm in fourth and, whenIclosed the throttle,thebike continued at full whack.After aride across the run off gravel, I clattered into the tyre wall.The bikeisvery bent and is nowwaiting to be delivered to Norton guruNorman White for treatment.
Ihad fitted a new throttle cable after Norman had experienced the throttle sticking on a recent test run prior to the trip to France .I thought this had fixed the problem as the slide was returning firmly to the closed position when the throttle was closed. By coincidence, my mate has a similar 850 to mine, fitted witha 34 mm Mikuni conversion earlierthis year.This was also being test ridden by Norman and had the sameproblem . Another Commando owner hasconfirmed a similar experience.
Now Mikuni conversions for Nortons,as we are all aware, have been around for yearsand have had excellent reports from riders. My bike has a fibreglass tank which might be a factor in this. Perhaps, as has been suggested, the carb could be contaminated with the fibreglass being affected by the additives in modern petrol. However, my mates Commando has a steel tank. My carb has been sent back to the dealer and nothing, apart from average wear, has been found wrong with it.
Has anybody on this forum got anytheories? Be careful out there.Iam only just off the crutches!
And me. Off the ferry from…
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I have had similar problem…
I have had similar problems with a single Mk2 concentric with two springs fitted. It was more frequent when I removed the K&N filter. It was bad enough to become unacceptable and I took it off. I put it down to freezing due to the large volume of air going through one carb and the resultant pressure drop and hence temperature drop. (worse in wet or damp conditions) As the carbs are rubber mounted there is little heating from the engine. I have a 34mm mikuni fitted now and so far haven't had a problem. There was a thread on the american forum about Mk2 Amals nipping up when heated, one chap heated his in a vice with a hot air gun and managed to replicate the problem. The mikunis are machined to quite close tolerances but I would expect a carb body to increase in dia when warmed up and I wouldn't expect a brass slide to expand much.
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Hi Peter, This seems to co…
Hi Peter,
This seems to come up quite a bit these days.
Ive had a 36mm Mikuni on my 750 for quite a few years now & I started to get the problem of sticking throttle after a rebuild 2 years ago. I put in new rings which improved the compression but this also increases thepressure of air being pulled in through the carb.
Itried Stripping the carb, Changing pilot & main jet, new throttle cable &re routing the cable & had no luck.
As soon as I fitted a 2nd strong spring in the carb the problem stopped.
I do think its somthing to do with lots of fuel passing through &frezzing the carbthen making the slide stick causing the problem.
After chatting to other owners it seems to happen less on the 34mm carbs.
My bike now runs without any problems & I can use the engine to slow down.
The throttle is a hard pull now but I managed the 145 mile trip to Hunstanton& 145 back withoutany troublewith my wrist.
Try experimenting with stronger springs & different lenghts of spring until you cure it without having too hard a pull to cope with. Try not too have too long a spring as this stops getting maximum opening if the 2nd spring gets fully compressed.
It makes the bike much nicer to ride & a lot safer, it has also cut the extra washing I was doing with my smalls after a few hairy moments.
Good Luck, Don
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Hi Peter I've had similar…
Hi Peter
I've had similar problems with my 750 which uses a 34mm Mikuni. Even though the manifold (see pic) seems to angle the carb into a updraught position, the cable still sometimes interferes with the bottom of the tank. The engine will race due to a sticking throttle slide which only seems to clear when you grasp the throttle slide cable and jiggle it a bit. This can be tricky with a thick gloves/ going mental engine interface situation...
I'm also in the middle of a rebuild and will try Don's idea of a stronger return spring..!
Peter W
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And me. Off the ferry from the International Rally in Colombres last July at 01.00 in pouringrain, I thought it couldn't get any worse until I started to approach a roundabout faster than planned. A few miles later and some hard tweaks on the twistgrip to try and clear whatever it was, it settled back to normal. Mine's a steel tank and 36mm Mikuni with a standard spring.I have never seen this discussed before, but there are the beginnings of a pattern emerging here.