Hello all.
I've recently acquired a 376 for a friends WM20. The original 289 is so
worn out as to be useless and replacements appear to be prohibitively
expensive.
Anyway, this 376 looks to be almost unused; all the metallic grey/green
paint on the carb body is intact and the slide is pristine.
Now...it is jetted as follows;
Bore; 1"
Cutaway; 3.5
Needle jet; .107
Pilot jet; 25
Main jet; 210
I appreciate that these settings do not conform exactly but I'd appreciate
an appraisal as to what the effects would be. The low cutaway will, I understand, give a rich mixture at 0-1/4 throttle but I'm hoping this might
be ameliorated by the smaller pilot jet (should be 30). The needle jet at
.107 is only slightly smaller than the recommended .11. There's no getting over the main jet being too small so I'm expecting poor top end power.

So my question is, will it work at all or am I being over-optimistic?

email (option): chasgbrook@hotmail.com

Of course it will work. I'ts just an old slogger. It would run alright with a Bantam carby.

Thanks Douglas.

I didn't think it would be quite that forgiving but I'll certainly give it a whirl!
The only other thing I meant to ask about is the extra heat insulation needed for monoblock carbs. Does this just take the form of longer studs and a second one of the black insulators or is there a better way?

Brooky.

email (option): chasgbrook@hotmail.com

The 289 is not the correct carb for an M20...A 276 was the one on WD models and up till about 1954...

Regarding the 376 the 1" choke size is correct...Running weak isn't a good strategy so I would obtain the correct jets as required...The slide cutaway can be filed by hand to increase the airflow...

Personally, I would just buy the right parts for it, they are all available..376 carbs were standard fitment anyway on later M20s..Ian

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

I've got a 289 here, just out of interest, what sort of thing would it have been fitted to?

Thanks for all that gents.

I'll get a 240 main jet and make up a manifold so that the float bowl clears
the dynamo. Would you recommend a phenolic resin gasket either end of the
manifold or a pair of the thicker, black insulators?

Brooky.

email (option): chasgbrook@hotmail.com

As an example...The 298 was the standard carb for my 1951 BSA B33...Ian

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

I fluked a long manifold to clear the dyno at a swapmeet didn't know what it was off. About 1 1/2 inches long.

Thanks Douglas, that's just the thing. Shame its origin isn't known but it
should be easy to make up a suitable alternative. It's knowing the length that really matters. Mine will probably just be two steel flanges welded to each end of a tube. I could even add further flanges along the tube for cooling purposes.

Brooky.

email (option): chasgbrook@hotmail.com

Douglas, just as an aside, the photo seems to show your bike just using one fuel outlet from the tank to the carb. Do civilian M20s only have one tank outlet? This WM has two, presumably to overcome the huge central hump in the tank where it straddles the frame. If yours has two outlets and you're only using one, it presumably works well enough? I'd love to simplify the fuel supply down to one fuel tap if it works.

Brooky.

email (option): chasgbrook@hotmail.com

The tank has a link pipe at the front joining the two sides. My M33 has the same. At a quess the manifold is off of an M21, a later one fitted with a Monobloc carb, I have one of those kicking around in my garage.

The joining pipe under the tank is a pain in the a** when the tank has to be removed..All the petrol must be drained before the pipe is removed but even after that there is a residual amount in the pipe itself which spills out when you undo the fittings....

It's a no brainer to work out why the military (and the AA) specified twin taps without the joining pipe...

On my last civvy M20 rebuild I cut off the joining pipe fittings, welded a small plate over each of them and added a second fuel tap boss at the rear of the tank...Well worth doing if you have to repaint the tank anyway as part of a rebuild..(and it doesn't have tank liner in it)..Ian

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

I agree its a stupid idea. If you need to remove the tank with a full load of petrol to replace a cable, forget it leave the cable hanging in the breeze.

Oh yes, on more careful examination of the photo, I can see the link pipe at
the front of the tank. I can also see why it would be a pain!
@Ian Clare; would that M21 manifold you have be available by any chance?

Brooky.

email (option): chasgbrook@hotmail.com

Re M21 manifold. I'm not at home at present, when I get back next week I'll look in my tip of a garage for it.

Yes the long inlet and welded up head made it pull like a steam train. Cams didn't do much though.

I'd be concerned with any single-feed system, even if it has a reserve-type tap. The two-tap system with one side used as a reserve gives me peace of mind.

email (option): jonny.rudge@verizon.net

The spacer between head and carb is available from Burlen
http://amalcarb.co.uk/pre-monobloc/76-276-series/flange-adaptors/aluminium-spacer-block-1-bore-1-thickness.html

Don’t know if this is the one that the M21 used with a monobloc and I’m not near my bike to measure it. Not cheap either.

Ian, I’ve got a monobloc on my 720cc bike with a 250 main jet and I’ve only now realsied that this is the jetting when an air filter is used. I’ve done around 4K miles on it like this so it can’t be too far out but then again not much full throttle riding either. According to Haynes the difference in jet size for filter/no filter is around 50 (eg B33 210/260) but I can’t find any main jet recommendations for M21 sans air filter. What are you using on yours or do you have any recommendations?

Cheers
Pete

email (option): petercomley@web.de

I'm convinced. I'll retain the two-tap system but would appreciate your advice on a reliable, simple, filter-tipped and not necessarily authentic, taps. I'd also rather they didn't cost the earth as well. Don't want much do I? I'm reminded of the dictum; 'fast, cheap, good. Pick any two and you won't get the third'.
The difference in main jet size between filter and no filter is encouraging as the main jet on this 376 is 210 and the carb came with a rather lovely circular thread-on metal air filter with oil-soaked metal mesh innards.

Brooky.

email (option): chasgbrook@hotmail.com