Strange Amal problem

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Apr 15, 2004
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In hot weather, after finishing a ride, my right carby seems to overheat. Gas will start coming out the overflow (even with the taps off), and if I press the tickler lightly pressurized fuel squirts out. Sometimes I can hear what sounds like the gas boiling inside.

The left one doesn't do it, only the right. Neither cylinder seems to be running hotter than the other and the carbs feel about the same temperature to the touch. Compression is 140 psi each side, both cylinders are timed the same.

A friend suggested it might be a clogged breather in the gascap, resulting in the tank getting pressurized. But the breather is working fine.

The right cylinder does have a major oil consumption problem, apparently from bad rings (lots of blue smoke on acceleration). I don't see how that would affect it though.

This one has me stumped. Any ideas?

72 750 Roadster
Bad/sticking float needle? Double check the tank vent in the cover anyway, although that usually causes a vacuum.

Even with the gas taps off, there will come some gas out if the tickler is pressed, at least for a moment, the "boiling" noise may be gas overflowing from a bad needle and filling the bowl up so far that it ends up running into the hot intake manifold and makes a hiss. I wouldn't think it has to do with the rings, but I'm not an expert. I go with a bad float needle. Might to some extent reduce smoking on that side too...some of the smoke may be gas, instead of oil....or? Could be washing down the barrel and causing more problems...thin oil? Check it. Check the float level too...see a recent thread. Mine had a constant wet gasket because the float was too high. Good luck, let us know what you find.

Best...and ride safe!
right carb boiling

Hi Debby,

Did you forget the heat insulator on the right intake manifold?
Both insulators are present. Could the insulator lose its heat-insulating properties over time??

I removed both float bowls yesterday for an inspection. Both look fine. No sign of dirt or any other problems. Float level is the same on both carbs - holding the bowl upside down, the top of the float sits slightly beyond the top of the bowl.

We had a lengthy thread here a year ago about float levels and the conclusion seemed to be that's correct due to the downdraft angle of the carbs. The value stated was 3/32 inch. I haven't measured mine but they look about right.

One thought I had was, maybe that cylinder has a lot of carbon buildup and is running hotter because of it. That's the side with the damaged oil rings. The bike burns a lot of oil, that side smokes pretty heavily, and the plug on that side accumulates carbon rapidly.

Was hoping to get through the season before the rebuild but maybe that's not a good idea... :cry:

Maybe not a bad idea anyway.....I will be getting to mine if the parts for the other 3 bikes I have apart at the moment ever come in and I can get the machines out of my garage...4 apart at once, is not so bad, just having the place to keep all the parts sorted and organized and separate and free from the bird poop...(swallows are starting ANOTHER nest in there now), is a problem. I just don't want to get the commando mixed into the situation yet. Order the parts and then when you will be able to get most of it done in one shot...ring job isn't so bad, parts for my ringjob were about 115 bucks and the boys will keep you informed with tips....Best!

PS what about the float needle? I didn't figure you would have forgotten the heat insulator...I would think a bore that is running oilly and maybe rich, would not be running too hot, though. Rather cooler, I'd think. Bet it's the needle.
I don't see anything wrong but who knows. Think I'll swap the float bowls and parts and see if the problem goes to the other cylinder...

good idea......also check that the gasket can't stop the float from coming up far enough to seal the may have to be trimmed in the area of the float hinge pin...soem gaskets are just a bit in the way of the float... :wink:

I can’t imagine what could be causing the fuel in your right-hand carburetor to become pressurized. The only area where pressure could possibly build up is between the petcock and the float needle, assuming the gas cap is vented. Try leaving the petcock open for a while after riding and see if you experience the same pressure build-up.

Regarding the noises coming from the right-hand carburetor, perhaps it’s the sound of raw gas or oil being cooked by “glowing” carbon deposits in that cylinder? In the past, I've seen oil-burning engines not stop running, even after turning off the ignition. This run-on condition was due to hot carbon deposits (from the burned oil) acting as a glow plug and igniting the air/fuel mixture, even in the absence of an electric spark.

Indeed, this is a strange one.

It would seem that some passage way is blocked. If you think about it, the float chamber is not sealed. Fuel is siphoned into the venturi through the main jet or pilot jet passage. In order for pressure to build in the float chamber it would seem that both of these would need to be plugged.

In addition, the tickler doesn't seal even when closed.

Strange indeed.
Well, I swapped the parts around and now both carbs do it! Seems to be an overheating issue - the engine gets very hot and after a ride all that heat radiates off the top end and cooks the carbs. That's when they start drooling and squirting and gurgling. The carbs do get quite hot to the touch after the bike sits for a few minutes.

Maybe the bike just doesn't like hot weather. It came from Michigan where I understand they have snow on the ground 11 months out of the year :wink:

So, still a mystery.


Is the bike running too lean? A main jet that is too small will cause the engine to run super hot. However, at the altitude where you're living the bike will have a tendency to run rich.

What about an air leak at the intake manifolds? This would cause a lean hot engine, too. But an air leak will cause some idling problems that I'm sure you would have noticed and reported to us.

And you do have the insulators between the manifolds and carburetors, right?

I think you've stumped the panel with this one.
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