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Question -
Hi, and thank you for assistance.
I have a 1980 CX500 custom. Starts and runs decent, but at speeds over 50mph will lose power fast when trying to overtake a car. Cleaned out the carbs, didn't see much problems. Balanced them. I know they run lean per Honda, maybe a rubber part rotted? No obvious once found. Runs best at lower speeds. Usual CV carbs.24000 mls.
Answer -
Martin,
http://www.powersportspro.com/partsfish/login.asp to check some on-line illustrations of your machine.
The fuel system must be very clean, so check the insides of the tank and the little bowl on the bottom of the petcock. Speaking of the petcock, 1980 was when Honda started using the infamous vacuum operated petcock that has a diaphragm that may be the source of your fuel feed/starvation symptoms. Honda replacement parts are very expensive, but the K&L brand aftermarket parts has remade a lot of those little parts. Check with your local m/c shop for their catalog.
There is also an air-cut (anti-backfire) diaphragm on the side of the carburetor that will cause a lean condition, usually most noticible at idle speeds, however. Looks like it is mounted on the outside of the left carburetor. Worth checking.
I would drain the float bowls into a little cup and see if you have water/rust lying in the bottom next to the main jets. If you do drain the bowls, the petcock may not be able to refill them quickly, so expect a prolonged restart sequence. Be sure your battery is in good shape for that.
Pull the plugs and check them for signs of fouling or misfire. They should look pretty clean and dry on both sides. Check the resistor values on the plug caps. Should be around 5K ohm.
If the bike has had a change in the mufflers or air filter, the carbs may need rejetting. If you do have signs of rust/dirt/water in the fuel system you may need to clean the carbs and check for buildup behind the float valves, which allow fuel flow into the bowls.
Bill Silver
Thanks, very fast response. I dissasembled and cleaned every part I could find with carb cleaner. No sediment found. Old style non-vacuum fuel tap on tank. Air cut off with some light areas, may be leaking, but idles ok, all issues at high passing throttle settings. No changes after all this work in performance, still great at low power, dies at passing power. Could the air cut off on the carb side still be the cause depite its function as a low speed device?
Thanks again.
AnswerAir cut valve should not have any holes, but shouldn't really affect high speed fuel metering that much....
Other possibilities could be fuel tank venting issues and beyond fuel, get a timing light on the flywheel... messy because it runs in oil... they used to make a screw in window to keep the oil in and allow you to check the timing.
Spark advancer may be sticking... or CDI may be failing at pulser coils or have bad wire connections. Check coils, plug caps and be sure that you have a fresh set of correct plugs. Could be dropping one side as you accelerate to higher rpms.
Valve adjustment okay? Compression checking out correctly?
Bill Silver
