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Author Topic: Amal Carb Vapor Lock  (Read 488 times)

High On Octane

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Amal Carb Vapor Lock
« on: November 11, 2013, 03:58:03 AM »
Anyone else ever have trouble with their Amal carb vapor locking on them?  I've noticed it's been a constant issue on my brand new 930 Concentric.  At first I thought it was crappy fuel petcocks, replaced them.  Then I thought the gas cap, so I gave it a thorough cleaning and drilled out the vent hole on top a couple sizes bigger.  Then I went after float bowl adjustment and adjusted that probably a half dozen times and I KNOW it's perfect now.  But even still, with the bike tuned superbly I might add, I'll be cruising along and the bike starts acting like it's starving.  Look down at the fuel filter and it's empty.  Crack open the reserve and both filters fill up immediately and bike starts running fine.  Close the reserve, and sure shit, no more than 15-20 minutes further into the ride and it's doing it again.  Weird part is that it happens more frequent when the tank starts getting below about 2/3 of a tank.  So when the tank is fuel there must be enough weight/pressure behind it that it's not an issue?  All I know is that I'm convinced it has something to due with the fact that there isn't 1 single vent tube on the entire carb.  What's up with that?   ???  Maybe time to invest in a Mikuni flat slide?

Scottie
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Arizoni

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Re: Amal Carb Vapor Lock
« Reply #1 on: November 11, 2013, 04:56:07 AM »
Just a few thoughts.

Vapor lock is usually something that happens when a mechanical fuel pump suddenly has to try to suck vaporized fuel from the fuel tank.  Many mechanical pumps don't pump vapor well because they are designed to suck liquids so, they just sorta gasp at the vapor without really pumping anything.
With a gravity system like your motorcycle has there isn't any pumping going on.

With a gravity system the only way I can imagine that vapor like locking could occur is if some gasoline in the fuel bowl of the carb started to boil and it created enough gas pressure to hold the float valve closed.
Unless your carb float bowl is almost too hot to touch I don't think this could be happening.

Even if it did happen that would not explain why your switching to the reserve position on the fuel cock suddenly makes things start to fill and work normally.  After all, the expanded pressurized gas wouldn't have the foggiest idea that you switched the valve position.

That leaves the fuel cock as the most likely culprit.

Either something is plugging up the "run" fuel feed or the top of the pipe inside the fuel tank is too tall.

I know you cleaned the inside of the tank but it's still possible that some floating debris or rust is getting into the "run" feed tube to plug it.

If the boiling gasoline seems to be the most likely answer I suppose you could drill a 1/16 inch diameter hole in the top of the float chamber cover.  That would let out any pressure that might be building up in there.  It might also allow raw gasoline to escape from the vibration of the engine.  Doesn't sound like too good of an idea to me if that happened.

I think if it were mine I would look for a replacement tank valve.  I would also check the height of the "run" tube to make sure it was not sticking up too high.

If that isn't possible I would try just running the bike with the valve set to the Reserve position every time I rode it and I'd keep a record of the mileage when I last filled the tank, just like I do on my G5.
Jim
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High On Octane

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Re: Amal Carb Vapor Lock
« Reply #2 on: November 11, 2013, 01:09:16 PM »
It's most certainly not the fuel valve.  If I pull the fuel line off and crack the valve open it spews fuel rapidly and freely, instantly.  No junk or debris clogging anything in there.  I'm convinced this carb is a POS.  I have never had this problem with any other carb I've ever had before this.  Maybe the fuel is cooking out.  I know if I park with the carb in direct sunlight it will cook all the fuel out of the bowl.  Whatever the problem it's annoying as hell.

Scottie
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ace.cafe

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Re: Amal Carb Vapor Lock
« Reply #3 on: November 11, 2013, 04:51:18 PM »
I think the Mikuni flat slide is the ticket.
Actually, per our previous conversation on the phone, I think that individual manifolds and a pair of Mikuni flat slide carbs would be the ticker. Individual runner intakes is the ticket for power. And it looks great too.
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barenekd

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Re: Amal Carb Vapor Lock
« Reply #4 on: November 11, 2013, 07:19:03 PM »
Put a thick fiber gasket between the crab and cylinder to reduce the heat transfer from the head to the carb. The carb should not be heating up enough to cause a vapor lock. If an Amal is getting that hot, so will a Mikuni or what ever.
Bare
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ace.cafe

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Re: Amal Carb Vapor Lock
« Reply #5 on: November 11, 2013, 07:24:54 PM »
Put a thick fiber gasket between the crab and cylinder to reduce the heat transfer from the head to the carb. The carb should not be heating up enough to cause a vapor lock. If an Amal is getting that hot, so will a Mikuni or what ever.
Bare

Yes, cork works good for that. One-eighth inch thickness of cork gasket insulates as well as one whole inch of phenolic spacer.
« Last Edit: November 11, 2013, 07:29:11 PM by ace.cafe »
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Re: Amal Carb Vapor Lock
« Reply #6 on: November 11, 2013, 07:33:25 PM »
I should add that about every 6 times I start the bike this problem occurs with the engine ice cold as well.  It's freaking weird.

Scottie
Bulldog Kustoms - Denver
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barenekd

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Re: Amal Carb Vapor Lock
« Reply #7 on: November 11, 2013, 07:42:54 PM »
Then it's definitely not vapor lock. Check the needle and seat closely and make sure nothing is hanging up there. Replace the filter, too. But I'm still on still thinking that the problem is in the tank/petcock. Can you keep it on reserve and it keeps running? That would definitely point to the petcock. I don't really think the carb is the problem. The pickup tube in the tank may just be too long. Saw some of that off.
Bare
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