In the resources section there is a great trouble shooting chart in the carburation section and talks about "engine stops when closing throttle" mentions bad diaphram, metering system set too high, inlet needle and seat binding and nozel check valve.

Kelly

Yes that is a great little chart! I would suspect the diaphragm first...easy to replace too.

Here is the link to the troubleshooting chart: www.modelgasboats.com/Resource...shooting_Chart/

Replaced the 644,with a new 257 carb. Still does the same thing,so I guess I can eliminate the carb as the problem. Could it be the seals,on the crankshaft leaking air? Ron

How odd is that .... Ok, could be an engine torque issue flexing the header at engine manifold, get off the gas hard and engine relaxes allowing water in threw the o-rings killing engine ?

Could be a TANK or line issue with an air bubbles ?

Could be a bad spark plug that is misfiring under the leaner off throttle transition ?


Those are all WAY stretches of ideas ... you have a really tough one.

Yeah,Scott! I noticed the mounts have gotten a little soft,and ordered replacements. I removed the engine today and tore it down. The area between the transfer ports is real rough on the lower part,and the piston ring dosen't look like it has seated in a few places. I can't get the three screws loose from the engine "block" on the front plate. They may have thread locker on them. I'm going to order a new "stock" engine,then go over this one later. Will the new 257 carb work well on a stock engine? I really like it not having a choke plate to mess with. Ill try to get a picture of the rough area up on the post Thanks Ron

marc wrote:Thanks for the offer Mark! I just got a new 257 carb,but I appreciate the offer! Ron

Here's a pic of the cylinder. Ron

Not sure if you posted this and I missed it, but what is your High speed set at? I would run that carb at 1-1/4 on Low and 1-5/8 on High. I am getting the feeling you have to run it fat because of an air leak. I would start with surfacing the isolator block on both sides and checking it for trueness on a piece of glass. Also double check the water pump fitting in the iso block and even cap it off to eliminate that possibility. Make sure the pulse passage is clear. Last check cylinder gasket and crank seals for leaks. Of course do not forget the fuel delivery...check it all for leaks...air bubbles in the gas lines are not good. If you are using a quick fill valve, take it out of the equation for testing. Keep us posted.

I had the same problem last month. In my case it was a loose isolator block.

That would make sense.