'82 xs400. Once warm, will not go above 4k rmp.

So it starts fine, will idle forever without a single sign of trouble. Once it is warm, it sputters and loses all power above 4000 rpm, as if the fuel was being shut off. I rev down and it backfires before running smooth at idle.

Checked the air filter, cleaned it, problem persists.
Connections are tight everywhere, plenty of fuel, fuel filter is clean.
No smoking or other visible signs of stress anywhere.

It sat out in the rain all day yesterday for the first time since I have had it, and I rode it 6miles in the rain just fine without any problems last night. It was not particularly cold out today, just wet from the rain. Problem persisted once it was completely dried out.

Engine has only 7000 miles on it, so I can't imagine it is the valves or anything like that. The only thing I can figure is that there is something up with the carbs, but they were rebuilt only 1000 miles ago.

I don't have a lot of tools, so I can't do much with it. Any suggestions of what this might be? I'll probably be taking it to the shop on thursday.

 

Check the fuel cap vent. There was another thread about that around here somewhere................................

Look here:
http://xjbikes.com/Forums/viewtopic/t=40495.html

 

Carbs... clean them, sounds like the pistons aren't rising, or your mains are plugged (Emulsion tubes as well)

 

I's susspect some moisture worked its way into the coils and HASN'T dried all the way out yet.

dave

 

When the Engine won't attain High-revving when the Throttles are Wide Open you immediately suspect the Main Jet Fuel Delivery components.

Tank to Bowl Supply
Petc0ck Tower
Petc0ck function
Kinked Fuel Lines
Clogged Beenie Screens
• Clogged Main Nozzle (Emulsion Tube) Air Holes
• Diaphragm Pistons -- Sticking*

*Search "Clunk Test"

 

Also going with the carbs,
Following Rick's carb cleaning guides made a world of difference in mine

 

Start imagining. They should have been checked at 3000 miles and probably weren't. If ignored, at 7K you could very well have a tight valve, which would cause your symptoms. ("Starts fine cold, loses power once warm" is a classic sign of tight valves.)

You won't be able to properly set up the carbs until the valves are in spec anyway. Have at it. At least check them, so if by some wild chance they're in spec, you can cross it off the list.

Yes, the carbs will need attention too. But valves gotta happen first.

 

I mis-read his post............nevermind.

dave

 

It was dirty carbs. clay sized rust particles from the gas tank had formed into small balls about 1-3mm in diameter at most of the openings. The left cylinder was misfiring as a result, resulting in the loss of power.

Tank was acid washed, carbs were cleaned, runs better now than when I got it. Valves were fine.

Thanks all,

 

I had a 1980-ish Yamaha 400 special II. It was a cool bike and it commuted me to college from the suburbs. It was a little light for the freeway but was very nice in-town and out on the twisties.

I also rode the bike from a water park (now a 6 Flags) home late at night after counting cash all evening. I worked in the cash vault.

I was puttering at 55 mph at midnight on the way home from counting cash one night on a country 4-lane road when a car went by me at 100+ mph. A minute later a cop went by me at 100+ mph with his lights on but no siren. I always wondered if the cop caught the car he was chasing.