Dirtdame

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Apr 10, 2010
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I own a 1986 KDX200 C1. I bought it new in September of 1985 and have had it almost all this time, except for a couple of years when I sold it somebody else but then bought it back. I have rebuilt that engine from top to bottom a couple of times and have always kept it tuned and jetted properly.

A month ago, I came across a basketcase bike just like mine for cheap, so I bought it to restore. I got it to run after a carb clean, but it ran pretty bad, loading up and smoking awful, so I decided to rebuild the engine. New rod kit, main bearings and seals, flex honed the cylinder, put in a set of reeds, fresh piston kit. All nice....go to start the engine and the bike still was running terribly rich. It is strange because the carb was jetted very lean compared to my original bike's carb. It had a pilot jet that was two sizes smaller than the one in my other bike, the needle was dropped all the way and the main jet was smaller too. I dipped that carb, checked every jet and piece that went in it, made sure that the floats were good and levels adjusted right, checked the fuel valve needle o-ring and fuel valve needle (physically on the bike with the gas tap on), made sure that the choke assembly was operating properly and wasn't leaking when closed, but the bike is awful with that carb on it.

Then I took the carb off my good running bike and put it on the freshly rebuilt one. Guess what? The bike ran perfectly. I took both carbs apart and double checked all the parts. Both had the same needle jets and needles. I put each carb back on the machine it came off of. But I don't get it. The carb that is jetted lean runs so rich that the bike won't even idle. The other carb runs perfectly. Grrrrrr. I think that there must be something wrong with the carb body on my rebuilt basketcase, because theoretically I should be able to jet it identically to my other carburetor and have it run identically to that carburetor.

Any insight into this?
 
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sr5bidder

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Oct 27, 2008
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you did not mention the air/fuel mixture screw, though from reading your not a dummy and probably just forgot to mention it.

also when you have the 2 carbs out compare where the slide in on the good one sets and set the slide on the bad one the same and tune from there.
 

Dirtdame

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Apr 10, 2010
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I did not mention the air adjustment screw because on this model of Mikuni, there is no air adjustment screw. You have to get the idle circuit set by replacing pilot jets.

The slide was set way high on on the bad carb (both are 3.0), so high that there was no more adjustment left in the idle screw, but it still would not idle.
 

sr5bidder

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Oct 27, 2008
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Dirtdame said:
I did not mention the air adjustment screw because on this model of Mikuni, there is no air adjustment screw. You have to get the idle circuit set by replacing pilot jets.

The slide was set way high on on the bad carb (both are 3.0), so high that there was no more adjustment left in the idle screw, but it still would not idle.


did you lower the slide setting then?
 

sr5bidder

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Oct 27, 2008
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I have had similar problems with rich running right after a rebuild that cleared after break in, but seems with you other carb not to be the case
 

dirt bike dave

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May 3, 2000
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sr5bidder said:
did you lower the slide setting then?

In theory, that would make it even more rich.

FWIW, I also suspect the carb body is bad. Never heard of it on a KDX, but I've hard some years of RM250 had a carb that, over time, would get microscopic cracks in the bottom of the venturi, that would allow fuel to get sucked in from the float bowl. This was a devil to diagnose and only curable with a new carb body.

I guess you could transfer all the parts (slide, everything) from your good carb to the bad carb, and test. If it still runs badly, then you know it has to be the carb body.
 

sr5bidder

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Oct 27, 2008
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dirt bike dave said:
In theory, that would make it even more rich.

FWIW, I also suspect the carb body is bad. Never heard of it on a KDX, but I've hard some years of RM250 had a carb that, over time, would get microscopic cracks in the bottom of the venturi, that would allow fuel to get sucked in from the float bowl. This was a devil to diagnose and only curable with a new carb body.

I guess you could transfer all the parts (slide, everything) from your good carb to the bad carb, and test. If it still runs badly, then you know it has to be the carb body.


well I was just thinking that with the idle screw all the way in that it would pull from the main circuit some..

the reason the screw was all the way in too leeds me to believe it was set that way to keep the bike running on a worn out piston/ cylinder with low compression and- or the pilot circuit in pluged up.

good idea though to swapp out all the parts from one to the other to troubleshoot
 

Dirtdame

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Apr 10, 2010
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dirt bike dave said:
FWIW, I also suspect the carb body is bad.
That's what I was afraid of, too. :uh: I guess I'll be looking for another carb on e-bay (unless somebody here has one), because I'm not going to toss the good carb back and forth between bikes.
 

Dirtdame

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Apr 10, 2010
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dirt bike dave said:
I guess you could transfer all the parts (slide, everything) from your good carb to the bad carb, and test. If it still runs badly, then you know it has to be the carb body.
I have another needle jet that is one size leaner than stock (that I got for high altitude applications). If anything could be the culprit that wasn't the carb body itself, that would be the most likely suspect. If I were to put that spare needle jet in and the bike cleaned up, i would just need to get a new R-2 needle jet. I hope that it will be just that easy. Probably check that out today or tomorrow.

I suspect that the original owner had tried to make the problem go away by rejetting the bike leaner and leaner before he stuck the piston, starved the bottom end, burned up the reeds and tossed the bike in a corner for the next ten years. :|
 
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glad2ride

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Jul 4, 2005
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How is the float valve on the old carb? Is it letting too much fuel into the bowl?

Congratulations on getting another '86! Is it called Bentley's Buddy? :-)
 

Dirtdame

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Apr 10, 2010
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glad2ride said:
How is the float valve on the old carb? Is it letting too much fuel into the bowl?

Congratulations on getting another '86! Is it called Bentley's Buddy? :-)
The float valve/needle set was replaced and manually and visually checked for the proper operation with the carburetor installed on the machine with the float bowl removed.

The bike was originally dubbed Fugly because of the condition of abject neglect it was in, but has been renamed Fentley since starting his makeover.
 
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Dirtdame

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Apr 10, 2010
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Update: Jeff Fredette had a good carb body in the carburetor box at his shop. We were discussing the problem and he has occasionally run across this problem on a bike or two. He commented that the carb probably was manufactured with some overdrilled passageways or something. Anyway, a new carb should get here later this week and hopefully the bike will be ready for it's maiden voyage by the weekend. :nod:
 
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