Ok, some back ground history, I've just fitted a re-lined cylinder and a standard size pattern piston to my M21, set the ignition timing while the head was off and bolted everything back together. It's now done a couple of heat cycles to torque the head bolts but not ridden yet.

Gradually rolling on the throttle produces an even increase in RPM but snapping the throttle open produces a stumble and spitting form the carb. Adding choke improves the situation, indicating a weak mixture.

Now a couple of months ago there was a thread on the Australian made All Parts 276 brass carbs. I have been using one of these on my M21 for the last 10 years or so, purchased new about 20 years ago but only done around 7-8000 miles. The stock Amal setting for this carb can only be used as a guide and prior to fitting this new cylinder I had the jet needle set for the previous cylinder as high as it would go (bottom groove), anything higher and the engine would over heat.

I have re-checked the ignition timing through the timing hole and it is not at 7/16" BTDC any more, it's more like 3/8" and I will correct this first, but retarding the ignition at the lever makes the stumble worse. screwing in the pilot air screw makes no difference.

If correcting the ignition timing doesn't fix the problem (and I'm not sure it will) I'll go to a #2 cut away throttle slide.

Any thoughts appreciated.

Hi Matt

I would re-check the timing and go for a run (the head off timing check is more accurate than the head on check - that may account for the thou difference). Then I would check the plug to see if it is running weak or rich.

Lots of fiddling around with timing and carb settings on a re-linered engine without going for a run could glaze it (I am in danger of opening up the glaze it or seize it debate).

It does sound a bit like a fuelling issue. If the carb has been sitting around a while with some fuel in (particularly the float bowl), then a good clean and blow out with compressed air might help.

Regards

Pat

email (option): sacombsashtrees@hotmail.com

Cheers Pat, I'm very hesitant on riding with a weak mixture as I have been down that rout on an earlier bore and had the piston nip up. I'll go over the ignition again and clean out the carb. Not too concerned about a glazed cylinder just yet.

Hi Matt

I must admit that I am of the opinion that for most engines I would rather risk glazing them than seizing them. After all a hone and new rings is cheaper than a re-bore, new rings and piston.

Tractors and chainsaws and different, they get used straight after a rebuild. I have no experience of race engines, but my understanding is that they get thrashed straight off.

Regards

Pat

email (option): sacombsashtrees@hotmail.com

How hard you run an engine after a rebuild depends on the materials, clearances and finishes..There is no 'one rule fits all'...

If you have a standard engine set up but don't have specific information relating to the above then you are better off noting and following the advice of the manufacturer than anyone else...They designed it after all...

I used to help build Triumph race engines than ran flat out right off the bench...However, they were built with so much bore clearance that they smoked lightly...As the chap who taught me how to do it explained 'Build them so that they are nearly worn out...but not quite!'....Ian

email (option): ian@wright52.plus.com

Timing has been reset and the carb cleaned but the stumble still persists.

Have you removed the jet block and thoroughly cleaned all the drillings ? Is the float operating freely and filling to the correct level ?

Hi Rik,

Yes I've removed and cleaned the jet block and all galleries and drillings are clear.
I have been talking with John Parker here in Melbourne and he has given me some measurements to check.

snaping the throttle on a m20 with a great big flywheel they dont do snap.If it idles ok nice and slow then the idle air circuit is ok.The next step depends on the slide opening the needle height at this stage doesnt matter.So if it doesnt accelerate or starves when you open the throttle at a reasonable rate it could be the main jet size.
Advancing the timing does make them more responsive but not if there starving.

email (option): mhadfield35@yahoo.com.au

My M21 has a carb off an Ariel Huntmaster
The slide isn't quite right
Whilst running in it would spit back [and sometimes stall]
It doesn't do it now it's run in, in fact I'm hard put to pick up the weakness
Perhaps the additional fiction of a new bore made it more susceptable
On Amals the slide cutaway controls just off idle mix. On Mikunis the slide has far less influence

email (option): j@clogmaker.co.uk

Remember these things are older than my grandparents..they need gentle encouragement not "snapping" open of throttles 😂

While these engines may not like having the throttle snapped open, it should not show indications of a weak mixture when doing so and it wasn't a problem with the previous cylinder.
The mixture is so weak off idle that if I snap the throttle open and hold it open the engine chokes and dies.

I've been in conversation with John Parker and I believe that we have found the root of my problem (I'll find out this weekend). This All Parts carb body has had the main air intake for the pilot jet (the hole opposite the pilot screw) blanked off. Also it appears that the jet block has been machined out to the 1 and 1/16 bore and in doing so has removed the small cylindrical up stand where fuel coming from the needle jet enters the air flow. I've unplugged the orifice and I'm fitting another jet block.

email (option): spacemonkeym@gmail.com

Hi Matt

If you have spent time and money on a liner, piston and rings, go the extra mile and buy a new Amal carb.

It will not only transform the starting of the bike, but the performance too. It will also run much cooler.

I persevered with an old tired carb for far too long and it did not do the bike any good (i.e. liners, rings, piston and exhaust guide suffered).

It will save you time and money in the long run.

It is a bit of an expense, but a must have; much like having a good mag.

Regards

Pat

email (option): sacombsashtrees@hotmail.com

G'day Patrick,
I hear you on the new carb but really this one hasn't done a great deal of miles. It appears that the carb body was set up for the later 276 that had the four holes around the base deleted and the pilot intake plugged. I'll be re-fitting the carb tomorrow morning, we'll see what's up then.

email (option): spacemonkeym@gmail.com