Dear Friends I'd value any helpful advice people can offer. I was out the other day and after running smoothly the engine became rough, some banging and irregularity not really idling. When I got home the spark plug was sooty and the bike continued to run like this when i tried to get it going again a few times, although it would keep running but not really idling. So i wonder if the pilot jet is the cause of the problem with some kind of blockage.

I have dismantled the carburettor having been advise to blow it through with an air line to clean out the pilot jet and use carb cleaner also.

I now have the body of the carb in which the jets are positioned separate.

I'm not really sure which the pilot jet is and how to make sure it is clean. i don't want to damage it by prodding it with anything.

Any additional advice at this stage would be very helpful. with sincere thanks. peter

Whenever these things happen to me, I find out its the magneto or ht leads or something related. I usually dismantle the carburetor first though because I am an idiot.

As Cas said, it could be something else, but you might as well start with the carb as it's simple. There is this tiny drilling in the throat that can get blocked with verdigris. It needs poking out with a piece of thin wire, But you need to remove the jet block first and also poke out the corresponding drilling in the jet block. Blow everything else through with an air line before re-assembly. Ron

email (option): ronpier@talk21.com

Dear Ron, sincere thanks for your advice which i have been following. I have tried to clear the holes with fine wire, use as good as I can get to an air line to blow things clear. It does now i have reassembled it, seem to be running better. One thing though I was wary of trying to unscrew the main jet assembly from the body that holds needle. It didn't seem to want to come loose and I was worried about breaking something. Should it come unscrewed?
sorry if this sounds dim. any further possible advice more than welcome. all v best peter with thanks

Yes the small main jet is screwed into the bottom of the needle jet. Both should come undone quite easily. They should only be nipped up. Maybe someone has over-tightened them which can end up with a snapped off thread in the jet block.

The Jet block must also be removed to clear the drillings properly......By undoing the big union nut on the base of the carb body and gently tap out the block with a wooden or nylon drift (as big as you can fit in the carb to spread the load) I usually use a hammer shaft by hitting the hammer head with another hammer, all the while cupping the carb in my hand. It often helps to heat the carb up in the oven/boiling water/hot air gun/blow lamp to expand the body. Ron

email (option): ronpier@talk21.com

thanks again Ron, I got the jet block out ok using a piece of shaped hard balsa wood and taps from a hammer. it is the bit at the bottom which has seemed reluctant to come unscrewed. I am a bit nervous about snapping off the thread in the jet block. would doing things like spraying with Wd40 or putting in hot water help do you think? all best again and thanks for any further guidance. peter

The upper half of the jetblock is very easily damaged/distorted and lots of them are.....especially if the carb has a larger bore, like the M21, here the top only stands on 2 very thin brass legs.

To tap out the jetblock without putting force on the top half I made this very simple tool from a spoke and a piece of aluminum.






BR Michiel

Well it's up to you if you want to risk undoing it, as you might need to undo it in the future. But as long as you've blown it through and can see no debris. Here is an exploded view. # 27 and 28 Ron
https://www.hitchcocksmotorcycles.com/partsbooks/pages/4769

email (option): ronpier@talk21.com

Thank you both for further advice and clarification. Now at last i think i understand how to remove the jets - pushing them out rather than unscrewing them. I haven't tried to remove them as I think the jets are clear and the engine now seems to be running much more regularly at low speed, so my next plan is to go for a run and see if how it runs in the shed is how it will stay running out over a distance. I am crossing my fingers. Sincere thanks for your really helpful feedback. peter

No No No! The jets are screwed in. Don't try and drift them out. Michiel's advice and home made tool are for removing the jet block. Ron

email (option): ronpier@talk21.com

Got it, thanks Ron, that at least is one mistake i won't be making now!! I feel a lot more confident about this now and if things don't go well with riding then i will try taking out the jets. I found getting the block out, with care and balsa wood and hammer - and then putting it back, straightforward. thankfully.

it is great to have this website and all the knowledge that goes with it. with thanks again and all best peter