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Followup To
Question -
Hi Chris,
I have a 1978 Honda XR75. I owned it since 1980 and it never did run right. It would not idle and always missed and backfired when revved up.I took the carburetor apart and installed a new Keyster carb kit. It idles perfectly now but will not accelerate. I removed the air intake and I notice that when I try to accelerate it, a lot of fuel is spraying up from the main jet. To me it's getting too much fuel. I tried blowing out all the ports with an air compressor but they appear to be fine. I also re-checked the float level; it's 21mm as recommended by the service manual. If you can figure this one out, I would appreciate hearing from you. Ken
Answer -
Hi Ken.
Well, as long as the carb is working correctly, it will not be flooding the cylinder. The way to be sure if it is, in fact, flooding the engine is to look at the spark plug after a ride. if it is blackened and/or wet with fuel, then it is running rich and needs to be leaned out. If the spark plug comes out clean, then you should be looking at valves, ignition timing and compression.
If your compression is dropping off, then it could be causing power problems as you described, but most likely, the problem is in the valves and/or ignition timing (as long as the carb and mixture is right).
Start with that and let me know what you find.
Good luck.
FALCON
Hi Chris,
I checked the spark plug and it's dry but has a lot of black soot in it. I also checked the ignition timing and the points begin to open exactly at the "F" mark on the flywheel as specified. I took the valve cover off and checked the valve clearances and they are correct. I also checked the timing on the cam gear and it's correct. I then placed the cam gear in a position where both valves are fully closed and carefully clamped it in this position. I then removed the spark plug and did a "poor man's" compression test by holding a tubeless tire stem tightly over the spark plug hole and applied 100 psi of air pressure to the cylinder. No air escaped from either valve.
Also, the air screw adjustment has no effect on engine performance. Is it possible for someone to damage the carburetor by turning the air screw in too tight? Someone definitely was playing around with this carburetor as both the idle and air screws looked like they were adjusted mant times as evidenced by the damage done to the slots where a flat screwdriver is used to adjust them. I bought this motorcycle as a 2 year old machine from a dealer who did not seem to consider customer satisfaction a top priority.
Ken
AnswerHi Ken.
The mixture is slightly off as indicated by the blackened (fouled) plug. It is slightly rich. Start with a fresh spark plug and turn the mixture screw in 1/4 turn and run the bike for a while. Check the plug. If it is still blackened, then clean it, turn the mixture in another 1/4 turn and try again. Repeat this till the plug looks normal. If it starts to look whitened, then turn the screw out 1/8 turn and see what happens. From there you can fine tune it as needed.
Good luck.
FALCON
