Keihin PWK advice please

[Testarossa]

When I rebuilt the '70 TR6R I gave it a brand new Keihin PWK 30 carb, and the bike started and ran just fine. Then someone told me that it would have a better midrange with the Amal, so I rebuilt the original carb and put that on. The bike started and ran just fine, but took a mile and a half to warm up and accept throttle beyond 2500 rpm; to get going after a cold start I had to coax it up to 6000 rpm and slip the clutch.

So I wanted to go back to the Keihin, with its choke. Reinstalled that and it starts great and takes throttle find from cold. But after it warms up now it does something it never did before -- it pretends the throttle is stuck open. I ran without the air filter so I could monitor the throttle slide and that wasn't the problem -- it closed just fine while the engine raced. To be on the safe side I installed a heavier spring and went thru the whole cable and throttle system to make sure the slide slammed shut when I release the throttle. Still does it.

This appears to be a known problem with these carbs. In the dirt bike forums it's variously ascribed to lean running due to water in the float bowl, or blocked vents, or bad float level. I dropped the float bowl and everything seems normal. The carb does have a couple extra vents not present on either the Dell' Ortos, Amals, or Mikunis I've worked with, and I don't know their function.  In particular I'm curious about the vent in the bottom of the float bowl, which goes to a vinyl hose routed upward and open to atmosphere, almost as if it's a fuel level monitor for the float bowl. Maybe it's an overflow vent in case the float valve sticks open? In some photos of this carb that vent is blanked off on the outside, so I suspect it's irrelevant to the phantom stuck throttle problem.

I'm stumped. Can anyone clue me in?

[RinkRat II]

      Depending how long it sat between uses, it may have picked up some flotsam in the needle and seat and it's not shutting the fuel off completely. Remove carb, fill with fresh fuel, gently shake and stir the carb, dump the fuel into a funnel with a coffee filter or napkin and see what comes out. My $.02

     Paul B

[mtiberio]

I'd suspect the floats. Damaged by ethanol while sitting, not floating or clogged valve as suggested.

[Don G]

Why not smack it flat with a large hammer and fit a new Amal Premiere carburetor? No point futzing about with a old worn out Amal, the new Amals are also ethanol tolerant as well. DonG

[bodine99]

wiff around the intake spigot with brake clean at idle ck. for an air leak

[larrys]

I ran Mikunis or Mark II Amals on my Triumphs. Amal Concentrics and Monoblocs were junk. I understand the new Amals are pretty good.
Larry

[moto-uno]

  ^^ +1 ,  Peter

[Rough Edge racing]

Is the carb real Keihin or an unbranded knock off? A lean low speed can hang up the idle but it will come down slowly.Besides the obvious air leaks ,do check out the cold start enrichment, the plunger may be leaking air. The hose you mentioned is a float level sight tube.
  On the Amal, if the slide fit is good,then make sure the pilot jet isn't plugged as they often are. It's also easy to wrap the carb flange by over tightening mounting nuts. Carb cleaner won't do it on the pilot jet, you need to use a .017 drill or guitar string. After removing the mixture screw, gently work the wire into the jet. Lots of info on this on Brit Bike
  The new Amals Concentrics are exactly the same as the old ones except the Premier model has a better float and anodized slide for longer life...Generally speaking,for just riding around, the Amal Premier is best because it won't require messing with jetting. But you will need to check the float level..

[Testarossa]

Is the carb real Keihin or an unbranded knock off? A lean low speed can hang up the idle but it will come down slowly.Besides the obvious air leaks ,do check out the cold start enrichment, the plunger may be leaking air. The hose you mentioned is a float level sight tube.

This. Thanks. It's not a real Keihin, which I didn't realize until you asked. No Keihin logo anywhere. I bought it in 2014 from STDFST for about $100 and asked them to jet it for 5600 ft elevation, which is where I live. When it arrived I dropped the float bowl to see what jets were there, but never thought to check the float level.

Once you clued me in to the sight tube it all became clear. It showed way high, and I realized fuel must be flooding into the venturi thru the jets. It took some searching to figure out how to reset the float level but now that's done and the bike starts, idles and runs perfectly.  Many thanks.