Cuts out on hard deceleration

kommando said:
If these new Amal's with 5000 miles on them are Premiers then they probably have 17 pilots fitted, these tune lean and are now supplied with 19's on the 850's and the 750's respond well to 19's too.
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I got them new, about ten years ago. They are 930’s, on my 1973 750, which of course originally came with 932’s, but they were all the shop had on the shelf at the time.
 
Triton Thrasher said:
I have found that a rich idle screw setting fouls the plug on over-run (decelerating or going down a steep hill with throttle closed).

Then it is inclined to miss or stall.

You could test to see if weakening the idle mixture makes it better, or worse.

You could also test to see if enriching the idle mixture makes it better, or worse.
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I think that is a good place to start, thank you.
 
You could induce an engine stall and pull the plugs to inspect them. If it's stalling because of a rich condition it will be pretty obvious because they plugs will show that.... Bear in mind that a weak spark will also show up as a rich condition on your plugs, so if your stalled engine's plugs look rich, you should investigate the causes of weak spark too...

... But since you get the stall from shutting off the fuel delivery, I would bet your stall is caused by a lean condition, not rich....
 
1973x75

Put a single Mikuni on it (if you can still get a new manifold and carburetor kit now that SUDCO is out of the game) and work on the Amals in your spare time. Or box the Amals up and forget about them. ;)

You should be able to get more trouble free milage out of a Mikuni.

If the bike has been sitting for a long time (years), you should remove and clean the carburetors.

As o0norton0o mentioned, you sort of have an auto chop plug test if it dies every time you do the hard deceleration thing. That test would tell you nearly all you need to know.

Chris also mentioned sticky slides which is common with the old 930's.
 
I’ve had my share of mysteries with vehicles, but this one is pretty weird. I can sit and gun the engine in neutral, or with the clutch in, and it idles back down just fine. Going down the road though, if I gun it then decelerate in gear, then pull the clutch in it cuts out. What the hell??!! Making adjustments with pilot screw and idle screw does nothing. I’m just going to adapt to the condition for now, and plan my stops so I’m not “coming in hot” as they say, and give it some more thought.
 
Sorry, my long winded advice didn't help much. As Schwany said, a lot of people switch to mikuni's and that solves their problem. From your replies here, I sense that you're one of those people who would rather solve than switch... I totally get that. I have a bit of that in me as well.

So the only other Amal thing I can say is that when you look at the amal idle circuit, there's actually 2 tiny holes that exit the mixing chamber (you may already know this) One hole is inside of the slide body perimeter and the other hole is beyond the slide body on the engine side of the slide. Both of those tiny holes must be clear of debris for the idle circuit to work correctly... I'll see if I can add a picture or two. Needless to say, poking them with a high E guitar string with a ninety degree bend might clear it, but the swarf might not be gone because it might just relodge in the port again under vacuum... The RPM range which your issue is occurring is pointing toward the idle air circuit. The only other thing I would recommend is to pull the plugs (what are they btw?) and post a picture of them...

Cuts out on hard deceleration



Cuts out on hard deceleration
 
1973x75 said:
750 commando with Boyer ignition and Tympanium regulator. If I snap the throttle shut after a hard acceleration, the engine dies. If I pull in on the clutch, then engage it again, it will bump start no problem. On the other hand, when I gradually ease up on the throttle while coming to a stop, all is fine. The battery is fully charged, and the the alternator is charging the battery as it should. Should I be looking at carburetor issues? It must be starving for gas at those times. They are Amals that I bought new, with probably about 5000 miles on them. Runs great otherwise, idles smooth at 1,100 rpm.
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Have you had the carbs apart?

The jet holder can work loose.
 
I’ve done the guitar string before, after the bike sat for a year with ethanol gas, and it really cleared it up. I’m getting a #78 bit from McMaster-Carr and I’ll try it again. Yesterday I removed a set of NGK BP7ES, which were working fine, and replaced them with a set of Champion N7YC, with a .028 gap, and it didn’t change anything. The weather’s great, and the riding is fun, so I’ll put off any more effort for now unless something occurs to me. Thank you everybody for your suggestions. I’ll get this figured out eventually, and report back.
 
I’ll check the jet holder, and I’ll have to figure out a way to check the fuel level in the float bowl. By now you would think that somebody would have invented a transparent float bowl.
 
1973x75 said:
It has gradually started.
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It does sound like one of the symptoms of wear
But that would be a depressingly low mileage if that were the case
Plus the revs would also be climbing and messing around at idle
Don't forget it's not quite the same revving and closing the throttle whilst stationary
As when you are using engine braking (throttle closed) coming to a stop
Was the engine at full working temperature when you tuned the carbs?
 
It may be that your fuel height is too low. On hard deceleration, the fuel is thrown to the front of the carbs and the pilot circuit is being starved or receiving reduced fuel.

A quick check, if you have the black floats, is to the take the bowls off, hold the pivot pin in place with your thumbnail, and turn the bowl upside down. The edge of the float away from the pin should be in line with the edge of the bowl and even more importantly, both should be the same. This probably works with the old type floats, but I never tried it with them.
 
One thing you didn't mention is if you use both front and rear brake. Too much rear brake could stall engine.
 
Mike40M said:
One thing you didn't mention is if you use both front and rear brake. Too much rear brake could stall engine.
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So can too much front brake if the clutch isn't pulled in
Don't forget he said this has gradually started to happen
 
Last edited:
Reporting back with good news. I took the air filter off (K&N), went out for a test ride, blasted it up to 5,000 rpm, snapped the throttle shut, pulled the clutch in and it idled back down to 1,100 rpm without stalling. It was starving for air, not gas. I guess that explains why it got worse gradually. Time to clean the air filter.
 
1973x75 said:
Reporting back with good news. I took the air filter off (K&N), went out for a test ride, blasted it up to 5,000 rpm, snapped the throttle shut, pulled the clutch in and it idled back down to 1,100 rpm without stalling. It was starving for air, not gas. I guess that explains why it got worse gradually. Time to clean the air filter.
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That would work lol.
 
1973x75 said:
I’ll check the jet holder, and I’ll have to figure out a way to check the fuel level in the float bowl. By now you would think that somebody would have invented a transparent float bowl.
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Its a little off topic but I have a set of Lectron carbs used on a tz750 during the days that the AMA restricted carb choke sizes to try to slow down top speeds. The Lectrons have clear plastic float bowls. I question whether they would hold up to ethanol fuel but interesting bit of history.
 
ilikevelos said:
Its a little off topic but I have a set of Lectron carbs used on a tz750 during the days that the AMA restricted carb choke sizes to try to slow down top speeds. The Lectrons have clear plastic float bowls. I question whether they would hold up to ethanol fuel but interesting bit of history.
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I've seen clear Mikuni float bows but not since the 60's. I would really like clear amal float bowls.
 

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