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

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jimbo

jimbo

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What's a likely cause of idle loss when stopping hard after a high speed run? I am guessing float levels are too high (Never changed the height, needles, floats are OK). Heat? MK3 primary chain tension getting too tight? If I do the Harley throttle blip thing I can catch it then it seems to keep running barely .
 
jimbo said:
What's a likely cause of idle loss when stopping hard after a high speed run? I am guessing float levels are too high (Never changed the height, needles, floats are OK). Heat? MK3 primary chain tension getting too tight? If I do the Harley throttle blip thing I can catch it then it seems to keep running barely .
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When you tune your carburettors do you do it with the engine fully warmed up?
That's what an engine can do if set up whilst just warm
 
Then what happens when you drive as you mostly do and you adjust it thusly?
 
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More likely empty float bowls. Maybe try the flycutting the inlet above the needle seat trick to keep fuel delivery up.
 
Danno said:
More likely empty float bowls. Maybe try the flycutting the inlet above the needle seat trick to keep fuel delivery up.
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If your float bowls run dry you’d feel it whilst at WOT. When this happens the bike loses power, starts missing etc and generally sounds like something serious is wrong.

For this to be the OPs issue, the float bowls would have to become empty at the exact moment the throttle is shut, so probably not likely.
 
cool temp? high humidity?
You may have frost forming where the air expands at the spray tube/fuel evaporation point. The fuel draws heat to evaporate and I have suffered eventual shut down/very poor running until the engine heat soaks in to melt it all down.
 
Fast Eddie said:
If your float bowls run dry you’d feel it whilst at WOT. When this happens the bike loses power, starts missing etc and generally sounds like something serious is wrong.

For this to be the OPs issue, the float bowls would have to become empty at the exact moment the throttle is shut, so probably not likely.
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Empty was probably the wrong way to express it, but float levels drop during full-throttle runs and the main jet can still pull fuel, but the sudden stop and reduced fuel level could keep the idle circuit from picking up fuel at that fuel level. JMO, no empirical evidence.
 
The idle pickup in the float bowl is toward the back of the carb. If the float level was low, would it be possible for fuel to "slosh" forward and starve the idle circuit? I know in automotive carbs they sometimes have a baffle in the float chamber to prevent that.
 
maylar said:
The idle pickup in the float bowl is toward the back of the carb. If the float level was low, would it be possible for fuel to "slosh" forward and starve the idle circuit? I know in automotive carbs they sometimes have a baffle in the float chamber to prevent that.
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Wow , things I didn't think of, but if the fuel was high and the fuel sloshed forward wouldn't it flood the engine,kill the idle and feel the same way?
 
Simple acid test.
Lean the idle circuit, so it’s leaner than you’d normally set it. Blaze it down the high speed run. Idles better? Then it’s the idle mixture was originally too rich. I said that to say this: the Amal is a big compromise. That is, the idle circuit is doing several things. Tip-in throttle mixture, under VARIOUS loads, on the way to the needle. With no accelerator pump. So, often the ideal “idle mixture setting” is not the best IDLE mixture, rather the setting that covers all the duties acceptably well, which means slightly rich(er than needed for HOT idle).

Mikuni has a more complex idle and pilot
circuit, helps this small throttle opening area of operation.
 
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From your discription, I would go with Danno's guess. You sucked the fuel level height down on the bowl so it's not going to idle until the fuel height returns to normal, where you have previously adjusted the idle mixture screws to. Blipping the throttle may help because it gives a shot of fuel which gives the fuel level a few extra seconds to refill.

I suppose you could argue the fuel level after a WOT run returns to it's resting height pretty quickly, but maybe the mixing chamber empties at WOT and it takes a few seconds to refill...

I'm very careful to adjust my idle mixture screw to try and get an A/F ratio that smoothly transitions from idle to 2000 rpms. I feel like the A/F ratio changes slightly as the needle/needle jet/cutaway begin to influence the mixture. If I feel lifting the slides leans out the mixture, then I try to be on the richer side of the working idle screw adjustment. This way when I accellerate the A/F ratio will waver from the throttle influence, but not outside the range of working A/F ratio.

I'm going to try to run a WOT and come to an idle next time I am out on a ride. Now you have me wondering if I will have the same result.
 
concours said:
Simple acid test.
Lean the idle circuit, so it’s leaner than you’d normally set it. Blaze it down the high speed run. Idles better? Then it’s the idle mixture was originally too rich. I said that to say this: the Amal is a big compromise. That is, the idle circuit is doing several things. Tip-in throttle mixture, under VARIOUS loads, on the way to the needle. With no accelerator pump. So, often the ideal “idle mixture setting” is not the best IDLE mixture, rather the setting that covers all the duties acceptably well, which means slightly rich(er than needed for HOT idle).

Mikuni has a more complex idle and pilot
circuit, helps this small throttle opening area of operation.
Click to expand...

Some good points there. And it reminds me that I’m sure I recall reading somewhere that in order to ensure good snap acceleration, the idle needs to be set rich, which would concur with your points.

Now I just gotta try and remember where I read that...
 
Triton Thrasher said:
A properly adjusted idle mixture will not allow the engine to idle well when cold.

The engine doesn’t get cold during a ride.
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True, but many, ridden at parade like speeds, never get HOT.
I have observed the phenom mentioned by the OP after holding it at or near triple digit speed.
 
Fast Eddie said:
Some good points there. And it reminds me that I’m sure I recall reading somewhere that in order to ensure good snap acceleration, the idle needs to be set rich, which would concur with your points.

Now I just gotta try and remember where I read that...
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Is the mixture screw is backed out more to richen?
 
Just remember that it’s called an AIR screw, slacken it and allow more air. Tighten it and allow less air. Just makes it easier to remember.
 
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