Amal Mk2 Settings and Issue

Fast Eddie said:

Number 3 slides should be fine on a 750cc 4 stroke twin. 3.5 may suit it better, may not, but that’s into the realms of fine tuning and isn‘t your issue.

You haven’t mentioned the needle. What needle type do you have and which clip position are they in? Again, T140Es had odd ball needles that you don’t want.

And another possible needle issue is that the washer (#5 in the diagram above) must be correctly located under the spring and on top of the needle clip to hold it in place properly.

But the float tang thing... it is the new floats that have the issue. So fitting new floats isn’t the solution! A bit of dressing may be in order. Mine had a bur that had cut away a tiny slither of the float needle. A few minutes with a file and some emery is all we’re talking about here.

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Agree, cutaway is fine tuning. I also bought new needles and needle jets as I have come across issues from the past where they have worn. The needles I believe are 2a1, with 5 positions for the circlip. As a start I put them in the middle position.
Washers are correctly located.
Right, understand now on the floats. I had some new ones laying around so decided to fit them. They work fine when I bench tested them. And yes i have dealt with the new floats requiring some trimming before.

MichaelB said:

34mm Mk2's on a 750 Combat.
They are NOS, 15+ years old.
#3, 106, 25
Had wierd tuning issues, unstable idle.
Went down to a 20. Better, but.........
Completely depotted them, again.
Had a piece of grit on one choke holder, knackered O ring at pilot on other.
After clean up, new orings all around, back to 25, runs great


Edited.. No balance tube.

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Thanks for the reply Michael. Interesting, the Atlas I'm building has the same cam but is a little lower in compression so should be similar settings to what you have. I will do a run around of all the o rings and double check them.

2A1 is correct. Middle position should be fine, again fine tuning might be different but it ain’t your problem.

Everything you have said seems fine. So something must be wrong!

I can’t remember all of the nuances off of the top of my head, but a problem with Amals is that many jets from different carbs or different applications will still fit, but be wrong.

Needle jets and jet holders are different between mk1 and mk2. Some jets, and I think the spray tubes, are different for 4 stroke vs 2 stroke.

So, although you must be very tired of stripping these carbs by now, I’d suggest doing so again to confirm, one by one, all jets, jet holders, spray tubes are correct for a 4 stroke application in a mk2 carb.

Other than that, I’d start wondering if the carb problem was electronic or something else. I chased my tail for ages on one bike with mk2 carbs before I realised fouled plugs were sending me down rabbit holes !