Folks, we've got another one. Previous owner said it ran until one day it just quit. I am able to get it to start and idle well. However, it's got the dreaded common disease where it dies at full throttle. It will rev up, but after a second or two it loses all power. If I choke it with my hand until there is just a sliver of gap for air to enter the carb it will run at full throttle for as long as I am willing to hold my hand there. Here is what I have done so far:

Taken the carb apart three times. It looks brand new inside and I did not find any dirt or gunk of any kind. I have run a small drill down all ports that I can and blown compressed air through the rest.

Checked the reed valves. I could see daylight through the reeds. I took them off and tried flipping them over, but it made the gap bigger. Then I carefully flexed them backwards to get them to seal better. They look a lot better, but not perfect. No change in performance.

Verified good fuel flow through the gas lines to the carb.
Verified the float and needle operate correctly.
Verified the operation of the enrichment valve (choke).
Adjusted the idle.

I thought perhaps the spark arrester is plugged. I cannot locate a removable one on this exhaust, so I unbolted the exhaust pipe from the jug. No change in performance.

Here is where it gets interesting. There is a screw near the idle screw. I presume that is the air-fuel mixture screw. It runs the best with that screw turned all the way down (off). If I back it out a turn the engine rpms lower significantly. Any more than a turn and it will die. With the screw backed out a turn it will not rev up at all, period.

Misc. things I can tell you, I don't have any sort of air cleaner for it. Not sure if running it without an air cleaner will cause it to run lean and die at full throttle? I am willing to get an air cleaner, but only once I am confident it's worth spending the $$.

The E-clip for the main needle was in the middle groove. I moved it down a groove, then down again to the bottom groove (moves the needle uo towards the top of the carb). No change in performance. The carb has very little in terms of markings on it, so I have no idea what it is other than a Mikuni knock off.

At this point, I have to think this carb is just junk and cannot be adjusted properly. And/or the reed valves are causing problems because they aren't sealed 100%.

Anyone have any thoughts beyond what I have tried?

Thanks,
Aaron

 

A few things I forgot. The orange cap visible in the pictures was installed by me as an experiment. Since it is the vent for the float bowl I have since removed it. The black cap, on what seems to be a vacuum port, was there when I got it. This atv had oil injection, but was removed by previous owner and uses mix gas now.

 

Does it have a Chinese spark plug in it? Will it run well at say 3/4 throttle or 7/8 throttle?

 

I have had the plug out to look at the end but I did not note the brand. It has really nice blue spark. It doesn't seem to want to run well anything above idle. It will stall out at half throttle too.

 

All your symptoms lead to a clogged carb.. its definitely a lean condition.. a air filter would help but will not be as restrictive as your hand. If its got a chinese plug.. start with that.. since this is a 2 stroke the small screw is a air screw.. opening it only adds air which you have plenty of.. ill think about this some more..

 

I figured out today that if I plug one of the air ports it runs great. Notice the white wire I used to plug the port. So why does this work?

 

We cracked the case. As it turns out this carburetor does need the air box for it to work properly. I didn't know this but the owner had the air box. However, they were missing the rubber tube that connects to the carb. We roughly fabricated one out of a drain hose from the plumbing section. The quad ran like a champ with the piece of wire removed that I had previously used to plug the port on the carb intake. The air mixture screw works too. I can't tell you the number of engines I have run without the airbreather attached with no problems. Evidently this one is different. So there you have it, case closed.

 

I'm of the opinion the most if not all atv's are designed/jetted to run with the air box on.

 

Aint that something..hmmmm?... Id have almost bet $$$ it was a carb problem..

How restrictive is the stock airbox/filter?

 

MWQ, as an example: my race motor runs a 37.5 pilot and 77.5 main. Drill 4- 1" holes in the air box and the main jumps to a 110.