It seems operational issues with carburetors come up about one a week so I pulled together some stuff that I've learned the hard way about common operational problems with the Keihin carburetor. Almost all of the parts from the Keihin PD27 (XR185 and 81-83 XR200R) and PD 97 carburetors are the same except for air passages so the pilot and main jets have different sizes. Almost every Keihin carburetor operational problem I have had during the past 25 years has been related to fuel deposits and dirt, so here goes:
Poor idle, hard cold starting, sometimes hot starting, throttle hesitation, popping on decal, and spitting. These symptoms are all related to lean low speed circuit that is caused by misadjusted of the mixture screw or dirt and fuel residue blocking passages in the low speed discharge ports. The low speed discharge ports are so small that they easily become obstructed and cause a lean condition that affects throttle setting from closed (idle) to about 1/3 throttle. Low speed meaning low air flow because of small throttle positions, not engine speed.
Usually the symptom appears suddenly or after the bike has sat for a long time and is probably caused by fuel residue or dirt in the fuel. The first thing to do is unscrew the large plug on the bottom of the fuel bowl and inspect the fuel in the bottom of the plug for debris. Next I turn on the fuel for a moment to see if it flows, this also flushes the fuel needle/seat. If you find debris it is probably time for a carburetor cleaning. At this point I strongly recommend that you install one of these to block debris coming from the tank.
Carburetor Cleaning Procedure
Pull the carburetor: On the 200Rs I find it easiest to pull the carburetor from the left side with the manifold attached. I start on the right by first loosening the carburetor top, the air boot clamp, the carburetor manifold nut, and then remove the right side bolt holding the manifold to the head. Then from the left I pull the fuel hose, loosen the left manifold nut, and then remove the left side manifold/head bolt. I then pull the carburetor to the left and down, unscrew the carburetor top and pull the slide, the carburetor is now free. I then unscrew the left manifold nut and remove the manifold.
In general you will need to sight all jets and opening against a light source to look for dirt and crud. And blow carburetor cleaner thru all passages and check for flow. I use a small bread baking pan to catch falling parts and hold the them during cleaning, an aerosol can of automotive carburetor cleaner, and compressed air with a blow nozzle. Use some gas and clean the outside of the carburetor with a stiff brush. Spray with carburetor cleaner and clean again. Clean out the pan.
Undo the float bowl, the white plastic thing is the jet holder, often called a main jet shield, we'll cover this later on re-assembly. Use carburetor cleaner to clean all of the debris and varnish deposits from inside the float bowl. Push the float pin out and the float and needle will drop out. Unscrew the main, the needle jet, and pilot jets. Unscrew the mixture screw, watch carefully the order of parts on the screw; first is a spring, then a small washer, and finally a small O-ring that often sticks inside the carburetor; don't lose the O-ring it is important and expensive to replace because it only comes in a $16 kit. Hold your hand over the mixture screw port and use a little blast of carburetor cleaner or compressed air from the venturi to loosen the O-ring.
Blow carburetor cleaner thru the two brass ports at the air horn, carburetor cleaner should flow out of both jets holders and the mixture screw hole. Spray carburetor cleaner over the outside and inside of the carburetor, set it aside in the pan and go have a cup of coffee.
The low speed jet feeds two tiny ports in the floor of the carburetor, one right under the rear edge of the slide and another right above the slow speed mixture screw. I've had stubborn ones that could only be cleaned by blowing air thru from the venturi side. The mixture screw passage leads up to a small discharge port in the venturi, sight this against a light. Add some carburetor cleaner, sight again. Blow compressed air from the venturi back thru, sight again. Next clean the forward port in the floor of the venture just behind the needle jet discharge, this is the other low speed discharge port. Also back blow with compressed air.
Re-assemble in reverse order and check the float level: Hold the carburetor upright from the left side (the choke lever side) and slowly rotate it counterclockwise. As you rotate the carburetor towards 90 degrees the float will swing down and just touches the float needle, this is the point at which you measure the float height. Tipping the carburetor further will compress the little spring on the float needle and provide a false float level measurement. Measure the distance from the float bowl flange to the bottom edge of the float near the end opposite the float needle, it should be 12.5mm (1/2"). I set my dimension a little higher but not over 13.5mm (.530") to lower the float level which helps reduce fuel surge on rough terrain.
Installing Jet Holder and Float Bowl
The next problem is properly installing the jet holder and the float bowl. The jet holder has a notch in the side that fits around the overflow tube in the float bowl, when looking at the bottom of the carburetor this will be near the float needle. The main jet casting has two ridges on one side, place the jet holder so its matching notches fit over these with the large side notch towards the needle seat. Now you should be able to set the float bowl onto the carburetor. Reinstalling the jet holder with the carburetor on the bike is very difficult, it doesn't stay up on the casting while you put on the float bow, I never pull the float bowl with the carburetor on the bike.
Mixture Screw Settings
Initial adjustment of the mixture screw (courtesy of Clymer) in turns out from lightly seated is:
- '79 XL185S: 2 turns out
- '80+ XL185S: 2 turns out
- All XR185: 2-1/4 turns out
- All XR200: 1-3/4 turns out
- '81-'83 XR200R: 2-1/2 turns out
- '86+ XR200R: 1-1/8 turns out
- '79 XL185S: N/A
- '80+ XL185S: N/A
- All XR185: 4th
- All XR200: 3rd
- '81-'83 XR200R: 4th
- '86 XR200R: 2nd (I think this is too lean, go with '87 setting)
- '87+ XR200R: 3rd
Credits: Article void of copyright submitted by a member of SoCalXRs. Edited and published by 4Strokes.com.