198A carb info

[DickyM]

Hi All

Getting on with restoring my Sherpa 198A and currently trying to sort out the carburettor. Stripped it and generally wasn't too bad except for worn throttle slide and a bashed about float chamber.

Anyway jets and needle were as specified in the Owners Manual but as general detail is a bit thin I have referred to an Amal Mk2 Technical Manual available off the internet. Totally confused now so I'm hoping forum members can put me right.

Attached pdf shows the needle jet, Carb Bits.pdfmain jet holder and main jet taken from my carb. Needle jet is the 2 stroke variety without the bleed hole but according to the Amal Technical Manual (extract on pdf) the main jet holder is a 4 stroke type. Picture in the Owners Manual seems to show this 4 stroke holder but it is not very clear. Is the main jet holder currently fitted the correct one or not??

Owners Manual and the Amal Mk2 Technical Manual both show a main jet 'screen' on their respective exploded views which seems to be a mesh of some sort that fits over the main jet. Is this necessary? I've never seen one before in my life.

Spray tube. The one on my carb looks a bit like the one described in the Technical Manual extract as coming from a late Norton 850 with the cutaway facing the engine. Is this correct?

Throttle slide was badly worn so bought a new one which appears to be teflon coated. Relationship of cutaway to needle height is identical to the old one but internals where the spring fits is different. See picture on pdf. The needle retaining disc doesn't fit properly into the new slide because of the lug. All shown in the picture. Is there a different type of needle retaining disc available or do I just bash the lug flat and reuse that?

Finally I have a problem of my own making. As the float bowl on the original was badly bashed I shopped around and bought a new one at the best price I could find. Problem is in my ignorance I didn't realise there are 2 stroke and 4 stroke versions. I appear to have ended up with a 4 stroke version which comes with a fat float needle and a brass seat with a 3.1mm (.125') bore. The original float bowl has a slimmer needle , a nylon or similar seat and a bore of approximately 2.2mm as near as I can measure. Does anyone know what will happenif the bike was run with this 4 stroke set up? Would it work ok or do I need to faff about sourcing and fitting a new seat, needle etc??

I would be extremely grateful for your advice.

Thanks

[bultobill]

The Jets I cant help with but the little mesh (gauze) thing I can.

Its role in life is to hold a little bead of fuel under the hole in the main jet in negative gravity situations with large throttle openings  (like jumps etc ) when the fuel in the float bowl moves to the top of the float chamber. Without it on a momentary loss of power can take place as the jet will draw air up instead of fuel, similar role to the large brass washer with the 4 holes in you see on Mikuni,and similar, main jets.

I know it sounds a bit made up but its true.

 

[feetupfun]

Not sure what you are calling the spray tube cutaway. If you've got a semicircular shield that sticks up into the airstream where the needle comes out of the needle jet, the open side of the shield always faces the engine.

No idea about that new type of slide.

Your main jet holder looks like what was in the MK2 AMAL concentric on my M198.

Having a bigger float needle and orifice doesn't matter but make sure the fuel height setting is not affected by the change.

I figure you must be pretty keen on originality to be asking all these questions and you may well know already that this carby has a reputation for rapid wear of the moving bits. You didn't mention wear of the slide bore but they are well known for that. The fastest wearing bits are the needle and needle jet.

There are many excellent alternative carburetors available.

[DickyM]

Thanks for the replies. Apart from the situation with the new throttle slide that basically answers all my questions. The guys that sold the slide don't know the answer to the question about that either so I will make a new needle retaining disc to my own design. Sorted.

Very little wear on the slide bore actually but the slide itself was badly worn so not that keen on originality feetup just trying to keep costs down a bit for the time being. Not sure what I'll find when I take the engine apart  but I dare say there will be lots of expenditure.

I was originally after a british two stroke for a project but they are very expensive and not many about that I was interested in. Always admired the Sherpa and when I found out the gear change can be arranged on either side (rhs for me) and there were plenty available at reasonable cost I went for it. Cost me £900 but I can see by the time I've finished it would have been far cheaper to have bought one already done up. But where's the fun in that? I have though been quite shocked at the cost of spares, particularly second hand junk on ebay. 

Cheers.