Mk 2 Amal hanging up

R

Randito

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Dec 28, 2024
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Just finished refreshing my ‘72 750 Roadster which languished for too many years. Has less than 15K miles on a single Mk 2 Amal that I fitted in the 80s. Thoroughly cleaned the carb and installed all new jets, float, and needle. Everything works as advertised statically - cable moves smooth and freely and the slide snaps right back to the idle stop. 2 kick starter with a nice 900-1000 RPM idle. However…when revving it, it will NOT return to idle, almost wants to run away. It’s like the slide is hung up, but after shutting it off, the slide is sitting right on the idle stop. Side note: has a Tri Spark ignition.

Been through the remove/clean/install/test fire/fail scenario a couple of times - same result.

Any ideas/suggestions? Thanks -
 
Drop the idle stop by unscrewing, if the revs stay high you have an air leak. To find the airleak spray wd40 or similar over the manifold joints, when the spray finds the leak the revs will change.
 
Randito said:
Just finished refreshing my ‘72 750 Roadster which languished for too many years. Has less than 15K miles on a single Mk 2 Amal that I fitted in the 80s. Thoroughly cleaned the carb and installed all new jets, float, and needle. Everything works as advertised statically - cable moves smooth and freely and the slide snaps right back to the idle stop. 2 kick starter with a nice 900-1000 RPM idle. However…when revving it, it will NOT return to idle, almost wants to run away. It’s like the slide is hung up, but after shutting it off, the slide is sitting right on the idle stop. Side note: has a Tri Spark ignition.

Been through the remove/clean/install/test fire/fail scenario a couple of times - same result.

Any ideas/suggestions? Thanks -
Click to expand...
Are you absolutely certain the slide is on the idle stop when the revs are climbing?
My single carb mikuni used to do exactly that until I fitted an extra return spring
If not that then you are pulling air somewhere
 
With a single carb the cable is very close to the frame spine (unless the carb is angled slightly) so it could also be the cable causing it to hang up slightly.
 
kommando said:
Drop the idle stop by unscrewing, if the revs stay high you have an air leak. To find the airleak spray wd40 or similar over the manifold joints, when the spray finds the leak the revs will change.
Click to expand...
Thought about the air leak - replaced the mankey spigot mount. Will idle smoothly about 900 or so until ref’s over 1600/2000, then not come down. Still a mystery
 
If after checking all of the above and making sure the slide is clean...
It could be a vacuum issue with the round slide.
Mikuni overcame this issue with the 'Heavy' brass, chrome plated slide.
They and Keihin also went to flat slides.
A heavier or newer spring may be in order.
 
Randito said:
Had not thought of that…and…not sure how to identify and cure that?
Click to expand...
If the carb has age to it - change it. I've fought both MK2 and Mikuni due to them. Of course, you can remove it and look at it - pretty easy. The seal can be bad or it can be hanging up. It makes the carb too rich but then people compensate so it becomes very confusing.

One check is to get it started and warmed up, then open and close it - should make a BIG difference if it is working correctly. If it's leaking, then little to no change.
 
If your slide is sitting on the idle stop and you are not touching the throttle and the revs climb then you have an air leak
Any other scenario points to the throttle slide or cable hanging up
If you had a mechanical advance I'd say check that too
But you have an electronic advance
 
baz said:
If your slide is sitting on the idle stop and you are not touching the throttle and the revs climb then you have an air leak
Any other scenario points to the throttle slide or cable hanging up
If you had a mechanical advance I'd say check that too
But you have an electronic advance
Click to expand...
Have a fresh spigot, which I believe is well-sealed to the head. The cable moves smoothly throughout the range, but I might try a new one.

Thanks to all who’ve responded - it’s good having an experienced community available to bounce off.
 
kommando said:
Drop the idle stop by unscrewing, if the revs stay high you have an air leak. To find the airleak spray wd40 or similar over the manifold joints, when the spray finds the leak the revs will change.
Click to expand...
Make sure you don't have too much outer cable clearance, because sometimes the Ferrell can sit a little cocked if it comes out of it hole too much. Cheers
 

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