340 LX Poor thottle response when warm

[kazaz123]

2007 340 LX that I bought for beginners who visit. It has only 1300 miles. The guy I bought it from hadn't used it in a couple of years and the fuel had turned to varnish. He couldn't get it started so I bought the thing betting I could get it to run for $800, which I considered a bargain.

Long story short, I removed the fuel tank and carburetors and replaced the fuel lines. Cleaned the tank and rebuilt the carbs.

The sled starts and idles without problem. When cold, I throttle up and can go miles (literally) with no problem. Once I stop, the RPMs won't go past 4000, which isn't enough to engage the clutch. Wait for it to cool down, and it will go miles and miles again as long as I keep the revs up over 5000 RPM. Once I stop, same problem - no throttle response past 4000 RPM until it cools down again.

This sled is simple as hell and yet I am stumped. Any ideas on how to diagnose the problem?

Al

 

[retiredpop]

Shot in the dark here but Is it firing on both cylinders when warm? Sometimes it is hard to hear a misfire when it is operating at higher rpm and it will appear to be running fine until it slows down.

 

[kazaz123]

Shot in the dark here but Is it firing on both cylinders when warm? Sometimes it is hard to hear a misfire when it is operating at higher rpm and it will appear to be running fine until it slows down.

Well I did disconnect one plug wire at a time and connect it to a new spark plug to see if both cylinders had spark - they do. Since the plugs were the first thing I replaced when I noticed the problem I didn't remove them to take a look for fouling. I probably should.

I also poked around the electronics with some freeze spray (canned air turned upside down) once I replicated the problem to see if any particular component made the problem better, I did find that when I hit the coil pack the engine died almost immediately, which was not what I expected, but suspicious.

Disconnected the e-stop switch plug. I assume this sled has the WOT sensor as a safety feature. Made no difference.

I had the sled up on a stand in the garage and the problem is quite reproducible. Sometimes I can get it to rev past 4000 RPM by feathering the throttle a lot around the max rpm point.

 

[retiredpop]

How are the fuel enrichment plungers for adjustment? I recall having a bit of an issue with an Indy Lite a friend had and it turned out that one plunger was not fully seating because of some crud in the bottom of the bore. Float levels good?
Other than that you may be onto something with the coil pack. Very strange it stopped when you cooled it off.

 

[kazaz123]

Tomorrow I think I will remove the coil pack to the bench and see what happens resistance-wise when I heat it and cool it. It's easy to do and avoids the temptation to throw parts at it. I hate not knowing the cause of a problem.

The carbs were thoroughly cleaned when I rebuilt them. I'm not that familiar with the Mikuni's and carburetors have always seemed like Black Magic to me even though they are simple devices. These remind me of the old SU carbs I had in an MGB I owned 40 years ago. I so prefer fuel injection and computer control!