High revs on Mk 3 Commando

Previously john_holmes wrote:

Air leak letting too much air in say at carb/manifold joint, or timing stuck too advanced if you are still on points. 72K is a lot on amal's and not surprised as the bodies would be worn and you would have too much air coming in even with new slides.

Appreciate reply, bike has electronic ignition fitted since early nineties. Also when I removed alternate plug leads bike was still over revving on closed throttle, I assume there would need to a air leak at both inlet manifolds to cause that? When new carbs were fitted problem was cured so it's obviously a carb issue.

Previously dominic_gomes wrote:

I have found sometimes worn carb slides may stick when the bike is hot causing these symptoms. Once cold the carbs work perfectly so it is initially difficult to diagnose.

Even starting up from cold still had same problem

Previously julian_wells wrote:

What was John Hudson's objection to twin-pull throttles? I've always thought the standard set-up was absurdly complicated and unhelpful:

1) you have to carry two spare cables instead of one

2) even if you have the spares taped to their working equivalents, you have to take the tank off in order to swap them over (because you need to get at the junction box -- which is a source of potential trouble independently of the cables, as Richard notes).

Previously richard_payne wrote:

Carb slides temporarily sticking or a piece of slide broken off?

It occurs to me that if the problem is on both cylinders as demonstrated by removing the HT lead then the 'hanging' might well be in the cable junction block. I hate the blasted things as they can wear horribly and I use a twin-pull throttle but John Hudson always used to tell me off about it.

I used the same cables when fitting new carbs and they are working fine.

Previously les_howard wrote:

As Richard Payne suggests above....after thinking long and hard about what might have occurred, I can only suggest it was something to do with the throttle cable junction block. Most likely is that the slide stuck momentarily which allowed the cable from the throttle to ride up and out of the tube and catch there. This would hold both slides up....Strangly though, even after you did quite a bit of work on the carbs the trouble remain so what ever was holding open the slides remained. You also said the carbs were returning fully after checking...What would have been interesting is that you could have screwed in the throttle stops with the motor running fast and see if this added any more revs but I reckon it would not have. Bear in mind that it only needs a few millimetres for the revs to race to 4K without any load on the engine. I would definitely check the cable block...perhaps even tape in the cable ends so they can't move out of their sockets...Obviously if you have twin cables all the way to the throttle it's not going to be that...so completely stumped....glad it's now sorted.

Les

I used the same cables and junction block when fitting new carbs and bike ran ok.Maybe it was just a symptom of badly worn carbs, although there had not been any sign of revs increasing prior to problem manifesting itself

Previously dominic_gomes wrote:

I have found sometimes worn carb slides may stick when the bike is hot causing these symptoms. Once cold the carbs work perfectly so it is initially difficult to diagnose.

Even when starting from cold revs still shot up.

Previously david_evans wrote:

Whilst, if the carbs were the originals, they would have been well worn out by now, you have to ask yourself why did this happen all of a sudden and not a gradual increase? something has changed on your journey to make it happen. The prime candidate is the manifold balance pipe or dare I say it the throttle cable outers have become displaced but both of them is strange. What was the last part of the engine you were fiddling with before your journey? Did the old carbs appear to be worn out when you removed them? even completely shagged out carbs would not give you that symptom all of a sudden.

I hadn't been doing any work on bike , I was just on way for annual mot, when problem occurred. I used existing cables on new carbs without any issues.