I have a Yamaha 200dx 89 ATV and there is a spot in the throttle where the machine kills. It idles just fine, and goes great once you get moving, but there is the dead spot where the engine kills if you don't get past it fast enough. If you have the choke on it idles fast enough past the spot. I have cleaned and re-adjusted the carburator 20 times and this is the best I can get it. I also adjusted the throttle cable to move past the spot, but that just makes it idle faster. Any ideas would be appreciated.
Thanks for your help.
JWN

 

A couple of places to start:
1) Is there an air leak in the filter box sytem? (Cracks/holes in the Box/Hoses, or Drain plug out, etc.)
2) What is your jet needle clip position? (default is 3rd groove from top)
3) Where is your idle mixture screw set? (default is 2 turns out)

 

I followed the clymer repair manual when I put it back together, I haven't had it apart since late fall, but I seem to remember clipping onto the middle groove, how ever many there are I am not sure. Also the screw is two turns out. Where is the plug on the air filter box? There is a little thing (fogive me I am quite an amatuer mechanic) with a spring clamp on the bottom of the air box. I think I am missing a screw out of the back of the air box also. I thought about that but I think it seems to run without the cover on, as good as with it on, so I thought that that wouldn't affect it. I will try to take it apart this weekend and check the carburator settings again.
Thanks for your help, any other advice is appreciated.
JWN

 

You mention a likely problem when you say it runs fine with the choke closed. That's telling you it is a lean air/fuel mix. Closing the choke simply closes off the air to provide a richer mix when starting a cold engine. If your carb is the original one, it could be a worn jet needle or the needle jet itself. You failed to say where in the band the flat spot occured. If it is in the 1/4 to 3/4 throttle range, that would indicate the jet needle circuit. You might try raising the needle to the 4th position to see if it helps, but unless you have altered the air flow in any way the carb should not become lean all by itself unless something is worn out.

 

Answer to your response, the spring clamp thing on the bottom of your airbox is the drain. It is used to remove dust/water from the airbox.
I have found in the past 17+ years that the smallest thing relating to the airbox can affect the operation of the engine ("home made" filters, breather hoses leakin/not complete connections, lid screws missing, cracks in the airbox, drain plugs out, etc.)
Hot_Shoe is correct in the point that you are running too lean (getting too much air) - to clarify what he is saying (I had to read his post twice) - in order to richen the needle jet circuit, the needle has to move up (clip moves down), to lean the needle jet circuit, the needle has to move down (clip moves up).
A new Jet needle (Mikuni # 4DI1) may correct your problem, but I have rarely seen the Needle jet on these type carbs go bad as they use a Keihin type "inlet valve" (the hole the needle goes through) before the Needle jet. You may require a combination of Jet needle and "inlet valve". Years of operation and vibration wear the Jet needle and "inlet valve" as the Jet needle bounces around in the opening, most often increasing the oriface size, causing a richening situation that fouls out spark plugs.
You can check the condition of operation by the color of the spark plug electrode - black is too rich, white is too lean, tan is about just right. If you read an earlier post "can someone explain in details how a carb works?" last dated 2-25-01, you can get more details on what is going on with the "reading" of your spark plug.

Let us know what you find over the weekend.

 

PaulJ,
Apparently there is a question of semantics here. In my 30 years of rejetting bike & quad carbs, I have found that the accepted method of designating jet needle positions is to number them from #1 to #5 from the top of the needle. In this way one is focused on what he does with the needle rather than where to put the circlip. Hence my suggestion that he raise the needle. I hope this clears up any difficulty in your interpretation of my previous post.

 

Hot_shoe,
I was not trying to stir up anything, but when someone states amature, you have to speak in layman's terms. What you and I would take for granted, they may not know.
When it comes to Jet needle clip positions, some of the many manuals on my shelves count from bottom to top, others top to bottom. What I've come up with over the years is that if the clip default position is below the 3rd groove from the top, the counting reverses and is counted bottom to top. All of the manuals state from which direction they are counting.
Who's count is right? Does it really matter as long as the fact remains - to raise the needle, lower the clip - to lower the needle, raise the clip?

 

Carb parts are cheap (relatively). Rather than trying to clean the carb and assuming everything is OK, simply replace the main jet, pilot jet and needle with new stock ones (assuming the quad is stock). Considering the aggrivation a carb problem can cause, it is well worth the money to go this route.

 

Those are all great places for me start. Unfortunatley I have not been able to work on it much. As soon as I get to take it apart again I will post my results. The dead spot is right in the throttle after idle. You can sit with an idle all day, then as soon as yout touch the throttle it dies. If you can get going fast enough to get past that 1/4 inch spot it goes fine. I have tried to find a carb kit to replace everything in it but have not had much luck. It is a 89 moto 4 200 4 stroke. I beleive the motor is a yfm.
Thanks again for all your help everyone.
JWN

 

I seriously doubt you are going to find a complete kit for your Mikuni, for some reason they don't make many of them like they do for the Keihin. You will have to buy individual parts needed (usually available from Tucker Rocky or Parts Unlimited), or you can go to your local independant ATV shop and see if they save their old carb parts and try to match up what you have (some Keihin parts will interchange, especially the Jet needle and "inlet valve") to find which Keihin kit will fit your carb.