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

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I still set float level the old fashioned way - hold the float bowl upside down (holding the float hinge rod in place) and measure the distance from the top of the bowl to the top of the float. My personal measurement spec that has worked all this time is 'a little below the top of the float bowl' ;)
 
MexicoMike said:
I still set float level the old fashioned way - hold the float bowl upside down (holding the float hinge rod in place) and measure the distance from the top of the bowl to the top of the float. My personal measurement spec that has worked all this time is 'a little below the top of the float bowl' ;)
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Just to clarify, are you saying the float should hang just lower than the edge of the float bowl ("a little below"), or that it would be a little below if the bowl were right side up?
 
SORRY! I should have clearly stated that the float would be a little below the top of the bowl IF the bowl was right side up! SO, being held upside down, the edge of the float is a bit ABOVE the lip of the bowl.
 
Fast Eddie said:
My understanding is that to ascertain float height you press on the tang of the float, not the top of the needle...
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Readjusted the float tangs to your suggestion, found the plastic non adjustable floats I had were within spec when pushing down on the float tang not the needle . So the fancy pancy stay up floats really were not needed.

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?
 
Right. And still no explanation why the engine stalls after hard braking. Dunno what else to tell ya. I've had carburetted cars do that too and always thought it was float bowl related.
 
Plastic floats are only an issue when you have to adjust the levels. Much easier to bend sheet metal tangs than beating the needle seat up and down in the bowl with a hammer and punch.
 
jimbo said:
so I really didn't have a float level problem
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I think it's more of a flow problem. Did you try the slots in the inlets? I still think the high-speed run is dropping the fuel level and it's not catching up in time for a steady idle.
 
If this was a carbed car of that era I'd first say "check the dashpot" but since we don't have dashpots, that can't be it! :)

But the next two checks would be for:
1. low idle speed and,
2. Non-optimum idle air fuel mixture.

Either one or both can cause an engine to die while pulling up to a stop but idle just fine from a start or slow reduction in the throttle.
 
Danno said:
I still think the high-speed run is dropping the fuel level and it's not catching up in time for a steady idle.
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I still think this as well, the sudden transition to a high vacuum state could also play a confounding role

The slope of the carb would make the issue more pronounced on a Commando, with the idle circuit pickups located at the rear bottom of the bowl. If you take a fuel level measurement with the carbs installed you'll see how close the fuel level is in this area of the bowl.
 
Just wondering. This all seems focused on carburetion. Which makes me wonder if it isn't the ignition or some wiring issue!
 
Because it is a known carburetor thing - it's why dashpots were installed on car engines back in the day, to slowly return the throttle the last few hundred RPM to idle, regardless how quickly you let off the pedal. The other option was to set a higher idle speed which also solves the problem but the dashpot eliminated the issue AND allowed the lower idle speed.

Obviously, some carbs/engine setups are more sensitive than others.
 
Ya, OK. The best trick of the devil was to convince people he didn't exist at all.

(Relax its a movie quote from The Usual Suspects)

How clapped out are the slides on these carbs?
 
MexicoMike said:
If the bike can sustain full throttle on the road in say 3rd/4th gear for more than 10-15 seconds then fuel flow isn't the problem.
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My theory is, the main jet draws down the fuel level in the bowls and the level doesn't quite recover quickly enough to sustain idle. If it idles normally in other circumstances, what else could it be? Also, with the carbs at an angle and the braking forces sloshing the fuel in the bowls forward, the float could be shutting off flow by keep the needle seated even though actual fuel quantity is less than full bowls. This would take even more flow to fill the bowls after a stop.
 
Jimbo

My road bike does that. Full bore run traffic lights go to red. Brakes on down through the box. Engine stalls! If I blip I can save it. Normally with hanging on & doing everything else I'd miss it.
Slides knackered, well & truly. Swapped carbs for another old not so knackered set. This happens as you pop it into first & the revs drop clutch pulled in.
 
I cured this behaviour on my B44 by fitting a micro digital Boyer with idle stabilisation curve, the advance curve has a kick up back to advance just below tickover speeds to increase the revs again, at kicking speeds it's still fully retarded so no kickbacks.
 
No insulating washers ? Big no-no.
Are you running chokes ? If not I hope the cable holes are plugged up .
 
I have an old Bultaco Sherpa T, this bike has an Amal 627 carburetter, it has the bad habit to heat when the bike is on the side stand and hot after long run though the forest, if I touch the carb is very hot, i can even see burbles traveling back from the carb to the tank, I fitted the insulators, and the problem was over.

The metal to metal linkage requires the insulator as this will stop the heat to the carb sufficiently .

My Commando Amal 's have the insulators fitted i would not use it without them.

Regards
 
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