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I've a 1980 CX500C, I've been working on getting it into running shape. It was previously sitting for about five years in an old barn. I pulled the bowls of of both carbs, and they were definitely full of crud & green dried up (gas?).... The right-side carb was pouring fuel out of the tube running out of the bowl... I found the needle to be stuck open. Cleaned it up and it moves freely now. It was caked in black greasy gunk.

The problem is the bike runs on the verge of OK when it is cold... will rev up to about 5000. It coughs & spits a little bit... once the bike is warmed up it won't rev above about 2500 and it bogs (but doesn't rev down or kill) with the throttle wide open, never getting above about 2500 RPM. If I turn the fuel off at the petcock the bike will come to life as the gas begins to run out... revving freely well above 6000. I haven't the courage to find out if it will hit redline in this condition, seems like a bad idea.

I'm wondering if I need to take the carbs completely apart and clean them, or if I need to completely rebuild them? I've also read about the petcock & filter within the fuel tank. From what I can see inside the tank through the filler opening (which isn't much), the tank appears spotless. I don't think the bike was taken care of very well by its previous owner, as the tubes that have plugs in them coming from under the airbox (crankcase breather?) were completely chock-full of black gunk I had to clean out with a pipe cleaner.

I've rebuilt several snowmobile carburetors, and I have the official Honda shop manual for the bike, so I feel I'm competent enough to take apart the carbs if that is indeed the problem. If they need to be rebuilt I'd like to know the best place to find rebuild kits. The gaskets on the bowls were in fine shape and don't leak after reassembly, so I hope that luck continues throughout the carbs if they need to be taken apart.

Also, according to the shop manual, that fuel/air mix pilot screw on the bottom of each carb should be out exactly 1.75 turns unless the carburetors have been overhauled. Something to worry about if I just clean them up? Also, will I have to resync the carbs if I take them apart, clean them, and reassemble them?

I'm a little of a veteran when it comes to snowmobile & small engine repair, my motorcycle tech skills haven't really been tested yet, so any input or other ideas would be great as well.

Just curious, is the petcock on the gas tank vacuum operated? If so, maybe the diaphragm in the petcock has a hole in it and it's sucking straight gas into the manifold. Maybe even float needle not seating fully? Just some ideas I guess.

Stratus311 wrote:Just curious, is the petcock on the gas tank vacuum operated? If so, maybe the diaphragm in the petcock has a hole in it and it's sucking straight gas into the manifold. Maybe even float needle not seating fully? Just some ideas I guess.

I don't know... it is entirely possible that the float needle isn't seating properly on either side, I cleaned them out as best I could with just taking off the bowls... they move freely but they could definitely be not seating quite right. I'm going to take the carbs completely apart today & clean them up.

I have no idea about the petcock... I'll have to look at the shop manual to see what it says... it looks like a regular L-shaped On-Off-Res petcock to me, exactly like the petcock in on the tank in this eBay auction (sorry I couldn't find a better pic really):

http://cgi.ebay.com/ebaymotors/ws/eBayI ... 4647773280

The tank however, looks completely different. Different year I suppose.

My bike looks exactly like this: http://www.bikepics.com/pictures/566357/

I believe your problem to be in fuel delievery. Remove the petcock clean or rebuild if needed. Clean both carbs thoroughly or rebuild if needed and I really think that your bike will run good.

I had a float needle once that never actually seated too well and what I did was, pulled the needle out and put some Armor-All on the rubber tip and let it sit for a while to soften the rubber a little. Never had a problem since. Also check the float height. If it's set too high it could be letting gas out through the overflow which is getting sucked into the engine causing an overly rich condition.

Here's what I came across:

Most of the jets & such were caked with chalky green (dried up gas?), a fair amount of black greasy crud in there as well... One jet on the left side carb was completely plugged. I imagine if I had kept everything I cleaned out of both carbs I could about fill a Dixie cup 1/2 full.

My carbs have plastic floats, non-adjustable. The manual shows floats with a metal arm and plastic buoys. However I measured them anyways and they came in right at 15.5 mm, dead on.

The air cutoff valves on the sides and the vacuum chamber with air jets on top of both carbs were pretty clean. Looks like besides the gunk buildup most of the problem was that the gas was never drained properly when the bike went into storage.

I didn't purchase a single carb kit, needle, gasket, or O ring. I was extremely careful with all of the existing parts and managed to get both carbs apart and reassembled without a single leak.

The bike now runs great with the air filter removed and my fist creating an about 85% block over the opening where the air filter should be. The exising air filter was so dirty that I couldn't see light through any part of it when help up to a three million candlepower halogen bulb. Without the filter in place the bike just sucks too much air, with it it gets almost no air at all. I'll be picking up a new air filter tomorrow (hopefully), and all problems should be solved.

All in all, for $200 + a free carb cleaning, air filter, fluids, a few bolts to replace missing ones, and 43150 miles (no idea if the odometer has rolled over) I'd say the bike runs beautifully.

Thanks for all the help!

P.S.: this 1980 CX500C has a regular old L-shaped fuel petcock, nothing special, and no problems. At one point an in-line filter was placed about 3" after the tank and it is pefectly clean ;D

Sweet! You fixed it. Glad to hear you got it running good again.

By the way, that's alot of junk in those carbs!!