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Popping/banging sounds...

Submitted by david_henshaw on Thu, 23/02/2012
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Commando

Hi guys, I have 1975 Commando mk.3 sidecar outfit...

I posted a thread a while ago about replacing the kick start with an electric one. This isn't such an issue now, brand new NYC7's and some fresh petrol got her turning over on the second kick.

I left her to idle for about 10 minutes with the choke fully on. Then I turned it down to half choke and she sounded a bit weak, so I kept the revs up with the throttle and took her for a few slow laps around the block.

However, after about half an hour of the engine running continuously, I tried to turn the choke off. She started sending some loud pops and bangs out of theexhaustbefore cutting out completely.

I put the choke fully on and she turned over again on the first kick. Turned the choke off, same thing happened. Loud pop and bangs, engine cuts out.

I'm pretty new to the vintage bike thing, should I be concerned?

Here is some more info as well as some nice pictures!

Frame:Wasp RT17AZ 1979, rebuilt and converted by Rob Rhind-Tutt whilst Wasp were still at South Newton.Registered Year:1975.Engine type and Size:1975 Norton Commando Mk3 828cc, rebuilt by Norman White Motorcycle Racing Thruxton, inc:Reconditioned gearbox.Belt drive conversion.Mikuni VM36 carbrettor with K&N filters.Boyer Bransden 3 Phase Power Box.

http://img844.imageshack.us/img844/3212/imag0108x.jpg

http://img62.imageshack.us/img62/6844/imag0126mp.jpg

Attachments 2151471.jpg

Submitted by alan_dawes on Thu, 23/02/2012

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I don't think you need to be concerned but you need to strip down and clean the carburettor. Once you know that the carb is clean and has no blocked jets, you need to make sure that it is set up correctly. If you carry on running the engine with a weak mixture, you might end up with a holed piston.

Good luck.

Alan

Submitted by ashley_cutler on Thu, 23/02/2012

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Before stripping check the simple things, is tight on the manifold. I have this problem, drove me mad as the rubber inlet rubbers make it look ok, so give them a good shake.

Submitted by alan_dawes on Fri, 24/02/2012

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But if there is a manifold leak, using the choke will not compensate.

More generally, if one has acquired a classic machine of unknown provenance and unknown maintenance history, it seems to me over optimistic to assume that all is well, start it up, and give it a good thrashing. It only takes a few hours to make some basic checks that might prevent serious damage to the machine or the rider.

Cheers

Alan

Submitted by david_evans on Fri, 24/02/2012

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Hi Peter,

The VM Mikuni carb doesn't have a choke but an enrichment device. It is normally on or off, If you are having to keep it on, ie. enriched, there is not enough fuel getting through at tickover/low throttle settings and the fuel is being supplied by the enrichening mechanism. The popping and banging is typical of a very weak mixture and in your case so weak as not to maintain life. I suggest removing the carb and stripping it down to ensure that the jets are correct and particularly the pilot jet is clear. The correct settings for the 850 with 36mm VM are 240 main jet, 159 P2 needle jet, D6H4 needle, 45 idle jet, 2.5 slide, 2.0 idle air jet.

My money is on the idle jet being too small or the slide having too big a cutaway in it ie. 3 or 4 not 2.5 You can download a tuning guide for Mikuni VM carbs and searching for parts on line should take you to Motocarb. These carbs are easy to work on and very well made the slides and main jets are however quite expensive when compared with Amal parts. Once they are set up correctly they are almost a fit and forget item and will be well suited to your chariot.

Yours Dave

Submitted by david_henshaw on Sun, 26/02/2012

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If anyone was wondering about an update...

Spent the morning stripping down the carb (with plenty of assistance from my Dad) there was no noticable fouling and all the jets were nice and clear. Gave the parts a wipe down with some ambersil fast dry and put it all back together.

She starts a little easier and the pops and bangs are gone, but the fuel mix is still a bit off, plugs are coming out covered in soot and she dies with the choke off.

However my foot can't take any more of that kick start today, and some of the local curtains are twitching, so I've put her away until tomorrow. Then I'll do some fine tuning...

 


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