I’ve had a broken ring on my old K&B40(model 4011 / produced until about 10 years ago). I’ve sent it out for replacement and cleaning and now when I started to run it won’t work at all as I was expecting. It won’t idle at low speed and it seems that it reaches full rpm at half throttle. If I open the carb fully it slows down and eventually dies. I’ve tried setting the needle valve differently but I get the same result.
What am I doing wrong(I even took the entire carb assembly out - thinking that it leaks air somewhere – and tighten it back in)? Should I replace the O rings on the carb and where can I get them from? (see my old post HELP ! Old K&B .40 on 1/4/2004)

There's a air leak somewhere? What about the back plate? I got my O rings at the local hardware store the last time but I have gotten them from the bearing store.

Enjoy,

Jim

I don't know. It doesn't seem like. How can I tell?

Your particular engine is a little newer than 10 years ago unless someone changed the carb on it.

While looking at your parts pieces in the picture, there are both a front piece and a back plate to your engine. But most important is the presence of only one gasket. Where's the other gasket?

Enjoy,

Jim

There're two gaskets front and back (see attached picture/you might not see the front one as is attached to the crankshaft). They're not cracked and seem to be all right when I attached them to the engine. Also, while holding carb lever the carb barrel is not moving (up & down) but I thought if the O-rings are old maybe the air still can pass through.

How much break in time have you given the new ring? Is the engine too tight again to turn up to higher RPM or idle properly? When you install a new ring the engine needs to be broken in again to seat the new ring. Even Dykes rings need some break in.

Yes you're right. I've done it and it runs good now. Thanks for your help. I've also changed the plug (it was too worn out and a little cold it seem).