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Discussion Starter · #1 ·
Bought an ‘07 EX500 that was not running. Cleaned carburetors, replaced float needles, floats, mixture screw and associated gaskets, new air filter, and spark plugs

Bike starts easily, idles well at 1200RPM, and has very good acceleration. The problem is, it will not pass 6000RPM, not matter which gear it’s in. It will just bog and not accelerate past that engine speed. Any ideas, could it be the carburetor or something in the ignition system?

Any would be greatly appreciated. Thanks.
 

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Moderating: Fair & Just
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Possible defective or poorly seated carb diaphragm.
 

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Are the throttles opening completely? Lotsa' cables, hoses and other stuff can get in the way. Hold the throttle open then check at the carbs if the butterflies are fully open. New air filter - did you pull the sides off the airbox to check for mice nests etc?

As BPE said, the diaphragms are a pain to get seated correctly. With carbs in place, can your finger lift both slides fully and do they return properly?
 

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Discussion Starter · #4 ·
Are the throttles opening completely? Lotsa' cables, hoses and other stuff can get in the way. Hold the throttle open then check at the carbs if the butterflies are fully open. New air filter - did you pull the sides off the airbox to check for mice nests etc?

As BPE said, the diaphragms are a pain to get seated correctly. With carbs in place, can your finger lift both slides fully and do they return properly?
Hi,

Yes,the throttle plates are fully opening. I lubricated both throttle cables and installed new filter in a “clean” air box.

I think BPE is onto something as well, the diaphragm on the slides not fully opening would restrict airflow. I may try replacing both slide diaphragms to see what that does. When I disassembled the carbs, I was careful to get the diaphragms seated into the recessed grove and tightened down both plastic top plates snuggly.

Maybe I should spray some aerosol carb cleaner around the carb tops, while the bike is idling, to see what happens. I know this technique works to find vacuum leaks on a car.
 

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Moderating: Fair & Just
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If the diaphragms you have are good and seated properly, new ones won't make a difference.
I mentioned the diaphragms because 6000 rpm's would be about the stopping point if one of the diaphragms was tweaked.
 

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Will it rev to redline in neutral? Is it only under a load that it bogs?
 

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Chain and rear wheel bearing OK? Rear brake not dragging? Exhaust not stuffed up? If nothing else appears take the mufflers off and try again. Just a process of elimination.
 

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Discussion Starter · #9 ·
Chain and rear wheel bearing OK? Rear brake not dragging? Exhaust not stuffed up? If nothing else appears take the mufflers off and try again. Just a process of elimination.
chain is good, wheel bearings roll well(just installed a new rear tire and it spins freely.). New front and rear brake pads, that appear not to be dragging. Muffler is aftermarket, pretty loud, doesn’t seem to be restricted. I’ve got to try a few of the suggestions made so far to eliminate some possibilities.
 

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Discussion Starter · #10 ·
If the diaphragms you have are good and seated properly, new ones won't make a difference.
I mentioned the diaphragms because 6000 rpm's would be about the stopping point if one of the diaphragms was tweaked.
is there a way to bench test the diaphragms prior to reinstalling the carburetors? The diaphragms seem like a pretty good potential candidate for this sort of problem
 

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Loud pipe with zero (or little) back pressure can cause a lean running condition. Both the intake and exhaust systems of the EX are designed to have some restriction. As well, someone may have installed a poorly regulated "jet kit" in a search for more power, finding only less.
 

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is there a way to bench test the diaphragms prior to reinstalling the carburetors? The diaphragms seem like a pretty good potential candidate for this sort of problem
With the carbs fully assembled, lift up on the slides with your fingers. Let them drop. They should take a good second to drop, more importantly the two should drop at the exact same rate.
 

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Discussion Starter · #13 ·
With the carbs fully assembled, lift up on the slides with your fingers. Let them drop. They should take a good second to drop, more importantly the two should drop at the exact same rate.
Found a Youtube video that explains installation and bench testing the diaphragm. Going to pull carbs off and try this

 

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Fast Old Guy
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Check the always overlooked carb vent tube. If it’s still in place junk it

Fog
 

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Use caution with the compressed air. Too much and you could blow out the diaphragms.
 

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Discussion Starter · #16 ·
Use caution with the compressed air. Too much and you could blow out the diaphragms.
Okay removed carbs again, checked the float level was at 17 mm, installed new vacuum diaphragms, used to low pressure air to check operation, verified butterflies would open fully, and adjusted choke to assure it was in the open position. Only thing I didn’t do was sync the carbs, my mercury carb sync was empty, so I ordered a set of mechanical sync gauges.

Road testing, Still stops at 6000 RPM at WOT. It will slowly accelerate with less than WOT, but only to about 7000RPM. If I go to WOT at 7000, engine will begin to slow down.

Both brakes are not dragging, all carb vents and fuel tank vents are open. This has me baffled, I really thought it was the vacuum diaphragms. Could it be an ignition coil cutting out at 6000RPM? As I said, there are new OEM sparkplugs installed. I did not check the main jet and pilot jet sizes when I was in the carburetor today, but I believe they are stock. The idle mixture screws are adjusted to about 1.5 turns out and it idles well at that setting.

I removed the fuel tank petcock and verified by screen filters were clean as well. The guy O bought the bike from had sent the tank to the shop to be cleaned internally. Shining a light onto the tank it looks clean.
 

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could a partially clogged ON circuit be limiting/impeding flow at high speed? Suggest an internal inspection is called for as apparently, all your other bases are covered.
Never know what you'll find. Also time to eliminate jetting as a factor, suggest to inspect mains.

Automotive tire Gas Skull Automotive wheel system Rim
 

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Fast Old Guy
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Did you remove the yellow carb vent/overflow tube?

Fog
 

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Discussion Starter · #20 ·
could a partially clogged ON circuit be limiting/impeding flow at high speed? Suggest an internal inspection is called for as apparently, all your other bases are covered.
Never know what you'll find. Also time to eliminate jetting as a factor, suggest to inspect mains.

View attachment 56529
I will remove the carbs and check main jet size. How do I gain access to the “On circuit”? I have a cyclepedia shop manual, which shows carb disassembly, but I haven’t read about the on circuit. Maybe I should remove the choke plungers and inspect them for damage.
 
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