Cutting out at idle

Have experienced the same problem from time to timeon both my Commando 850 bikeswhich both have the original twin Amal set up. Probably a combination of engine age and Amal carb wear. Suggest you increase idle a tad by screwing in throttle stop screw - a slightly faster tick-over won't hurt. Also try different pilot air screw positions in combination with the throttle stop setting - some trial and errorfiddling required but you get there in the end.

Wear occurs between slide and carb body on standard Amals at a rapid rate. From new there seems to be alot of clearance between these two items.My bikehas suffered from similar symptoms in the past, but since having thebodies bored to remove wear (I use two rubber mounted carbs) and slides brass sleeved to approx. 0.001" clearance this problem has stopped. I think it is due to air leaking past the sides of the slide on the engine overrun i.e when the carbs are shut resulting in a highvacuum and therefore a weak mixture occurs resulting in the engine stalling. It appears o.k on tickover because the engine revs are less and so the engine vacuum is less and the air takes theroute throughthe slide cutaway thus giving a more correct air/fuel ratio,

Simon.

Fuel starvation would be indicated whilst moving as the system could not replenish the bowls quick enough, usually indicated when you crack open the throttle and the bike stutters as the bowls try to refill. Go with James plan and this should cure it.

High vacuum is good, but it must be felt across the pilot jet as it will lift fuel with the effect of the venturi through the carb. Low speed of air across the venturi = less vacuum and less fuel lift through the pilot system, this explains why your carbs do leak fuel whilst the engine is stopped as there is no air through the carb to lift the fuel. Remember engines stop for 2 reasons, no sparks or no fuel, you know you have sparks, so it must a fuel issue. With the above in mind you should be able toachieve a good setting. If the slides are really worn, this would account for the drop in speed of the air over the pilot jet. If needed, then renew as above or maybe new carbs a better and cheaper solution. My twin AMALS have covered 28K and the slides are still good, just seems a case of dodgy engineering standards in the 70's as to whether you got good ones or bad ones.

Ashley,

the vacuum I refer to is created by the pistons descending on the intake stroke. This vacuum does not 'lift the fuel through the pilot system' as you mention, but creates a negative pressure thus air flows through the carbinto the engine.Venturi is the effect the narrowing of the carb port has on fluid pressure. As air flows through the restrictionits velocity increases and it is this which creates a lower pressure than atmospheric at the restrictionand thus fuel flows from the float bowl through the pilot system and into the engine.

So when the throttle is shut,e.g changing down, the engine vacuum is relatively high due to engine speedand so theair velocitythrough the carb is relatively high which creates greater negative pressure across the restriction due to the Venturi principle so fuel flow through the pilot system also increases. Therefore if the engine idles and starts easily I suggest the carb/slide gap is too big i.e excess air is mixing with the fuel on the overrun, weakening the mixture and stalling the engine.

Your carbs should not leak when the engine isn't running and if they do it hasabsolutely nothing to do with the Venturi principle.

Also there are more than two reasons why an engine wont work.

Every day's a school day as they say, glad you've got it sorted. Personally I enjoy improvingthe bike fromits 'as it left the factory condition'but not having to modify brand new components and spares which is an all too common problem,

Simon.

Mac Macdonald wrote; Wouldn't it have been easier to make some bendable tangs on the float rather than the hit and miss method tapping the valve housing in and out the float bowl?

Amal now make an ethanol resistant float that also has a metal bracket that can be bent to set float height.

"The new adjustable height StayUp float is resistant to modern ethanol based fuels and has a military spec closed cell construction making it puncture proof. The ability to adjust the float arm allows for quick and easy minor alterations to float levels where required."

They retail at £12 inc vat.