Likely cause of idle loss when stopping hard after a high speed run?

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baldy said:
I have the same issue. I have always thought it was carb sloshing. The commando carbs are at a slight incline. Bike runs perfect otherwise. I would check your float bowl heights.
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made up some fuel level checkers, preliminary observations are my float levels are low according to Amal service notes. more to come.

Likely cause of idle loss when stopping hard after a high speed run?
Likely cause of idle loss when stopping hard after a high speed run?
 
Pretty interesting question. Bike is in gear (perhaps even down-shifting). Revs are high, throttle is closed. Pistons are pulling air through with high vacuum for the throttle opening. The higher vacuum could pull more fuel than is usual for the pilot and the air screw setting. I'm sure the design and air screw setting is for a certain velocity of air. It works well at those velocities and those slide openings. But at higher air velocities things might change. For such a small opening and the opening shape you might even generate non-laminar flow! As an experiment you might recreate the conditions and then hit the clutch and chop the spark just at the point, or just before the point it stalls. The condition of the plugs might tell a whole lot. My apologies if I'm repeating what has already been said.
 
I did want to add that if LESS fuel than was needed for running was sucked into the engine, either due to the air velocity being outside the design range for pilot/air screw setting/and slide opening, or due to a non-laminar flow condition, the the engine was way too lean and was barely firing, just being dragged along (so to speak) by the momentum of the bike still in gear. The time between downshifts might be too short for the engine to stop, but once at the end of the ramp the too lean condition does not correct fast enough and the motor dies. Either way I would do the previously described experiment. And, again, apologies if I'm unnecessarily repeating another post.
 
And a question. Say your out on your high speed run and you just cut the throttle while pulling in the clutch. Not cutting the throttle and letting your speed spin the engine with the throttle closed. Does it stall then?
 
Joe Schlaberdowski said:
And a question. Say your out on your high speed run and you just cut the throttle while pulling in the clutch.It coasts and dies Not cutting the throttle and letting your speed spin the engine with the throttle closed. not sure what you mean Does it stall then?
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the road in front of house is getting repaved and its hard to get out, so no testing for a while
 
jimbo said:
the road in front of house is getting repaved and its hard to get out, so no testing for a while
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I think my last post was not quite clear. Very sorry. Usually when coming down from a high speed run you let the engine do a little of the braking. The throttle is closed and the momentum of the bike, if the bike is left in gear, speeds the engine. The pistons, then, are dragging in a lot of air, quite an increase in airflow velocity for the pilot valve. So the setting for the air screw we used to get a proper idle may no longer give the correct mixture. Systems like this are designed to work well in a semi-linear way for only a limited range of input values (air velocity causing a venturi effect). I was speculating that it might cause a lean condition due to the air velocity being above the proper operating range. Then the engine is actually running very poorly and stops as soon as you quit spinning it, or helping it spin, with the bike in gear. All that said, if, as you say, you are at speed and you cut the throttle to idle and simultaneously pull in the clutch, AND IT STILL DIES RIGHT OFF, then all I've said does not apply. In other words, just blather to which I freely admit.
 
might get to monkey with it tomorrow, changing out the floats to metal tab adjustable ones and aluminum needles, adjusting the float height to amal specs and see what happens.
 
after float bowl wrenching today here are my observations;

the stay up floats are pretty nice and makes level changes easy

as found float heights with old plastic and new unaltered stay ups were the same

very small bending causes big adjustments!

I had to lower the right side float below Amal recommended level because of fuel flooding

my needle seats to bowl edges were within tolerance

I had more flooding in the right side after assembly caused by a defect in the Viton seat on a brand new aluminum needle

nylon needles are just little different than the brass and aluminum ones, the brass and aluminum ones are made exactly the same

my before and after fuel levels taken by my level checkers were about the same and that is puzzling,(hard to get real precise measurements looking through a plastic fuel line).

the bike seems to run better ,could be more fuel availability or cooler weather

the tendency of the bike dying after coming off the highway and stopping is gone, but it is a ambient 20* difference

first photo is a composite of the "as found" float heights with different combinations of needles and floats

Likely cause of idle loss when stopping hard after a high speed run?
Likely cause of idle loss when stopping hard after a high speed run?
Likely cause of idle loss when stopping hard after a high speed run?
Likely cause of idle loss when stopping hard after a high speed run?
Likely cause of idle loss when stopping hard after a high speed run?


Likely cause of idle loss when stopping hard after a high speed run?
Likely cause of idle loss when stopping hard after a high speed run?
 
jimbo said:
after float bowl wrenching today here are my observations;

the stay up floats are pretty nice and makes level changes easy

as found float heights with old plastic and new unaltered stay ups were the same

very small bending causes big adjustments!

I had to lower the right side float below Amal recommended level because of fuel flooding

my needle seats to bowl edges were within tolerance

I had more flooding in the right side after assembly caused by a defect in the Viton seat on a brand new aluminum needle

nylon needles are just little different than the brass and aluminum ones, the brass and aluminum ones are made exactly the same

my before and after fuel levels taken by my level checkers were about the same and that is puzzling,(hard to get real precise measurements looking through a plastic fuel line).

the bike seems to run better ,could be more fuel availability or cooler weather

the tendency of the bike dying after coming off the highway and stopping is gone, but it is a ambient 20* difference

first photo is a composite of the "as found" float heights with different combinations of needles and floats

View attachment 12829 View attachment 12830 View attachment 12831 View attachment 12832 View attachment 12833

View attachment 12835View attachment 12834
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Tuning 101, runs better in cold weather, mixture is too rich.
 
concours said:
Tuning 101, runs better in cold weather, mixture is too rich.
Click to expand...

Runs better in cold weather because the cold weather helps with the fact that there are no insulators on the intake manifold... as was said above... I would put the insulators on and see if it makes a difference.
 
o0norton0o said:
Runs better in cold weather because the cold weather helps with the fact that there are no insulators on the intake manifold... as was said above... I would put the insulators on and see if it makes a difference.
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might do that , but I wouldn't be able to test it until next summer ,here in Iowao_O
 
Fast Eddie said:
My understanding is that to ascertain float hight you press on the tang of the float, not the top of the needle...
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I did not think of that, there is slop in the needle slot that would cause misajustment, maybe that's why I had readjust the right side
 
jimbo said:
I did not think of that, there is slop in the needle slot that would cause misajustment, maybe that's why I had readjust the right side
Click to expand...

Yes, and multiplied by the length of the float that slop can make quite a difference.
 
here is a photo of the defective new aluminum float needle Viton seat

Likely cause of idle loss when stopping hard after a high speed run?
 
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