Bike Losing Power

[Shockwave3724]

I have a 1990 Katana 750 that I am trying to get running right. I have owned it since last spring, and rode it last summer with no problems. At the end of the summer, the bike started to run a little rougher, not too bad, but it was noticeable. I put it away for the winter and just got it back out. It wouldnt start so I pulled the carbs, cleaned everything out really good and put new spark plugs back in. The bike started right up and idles pretty good.

The problem is that it doesnt want to rev beyond 5-6k and if I open the throttle more than a quarter turn, it dies out like its running on three cylinders. As long as I keep it between 1-5k rpms, I can ride the bike fine and it still has power like its running on all four cylinders, as soon as I hit 5-6k rpm, or open the throttle up, it cuts out and loses power. I can baby it up to about 9k rpm, but that was the max I could get it up to. Anyway, it doesnt seem to be getting any better or any worse. I also noticed the right exhaust pipe has some vapor coming from it after I shut it off, probably indicating the bad cylinder is one of the two on the right and that its getting fuel, but not burning it under full throttle. I took the carbs off again to make sure I didnt miss anything, but it all looked good.

So, I am thinking its a weak spark in one of the cylinders, possibly bad plug wires, coils, maybe even a new spark plug that is bad. Hopefully I'm working on it tomorrow and can get it fixed. Anyone have this problem before? Anything else I should check?

[Newbie2it]

pull the carbs and give 'em a good bath, Sounds like the mains either are plugged or varnished a bit to much. You can check spark, but i'd wager on the carbs needing a bit of help.

[elkat]

I agree with newb, most likely a carb issue. Sounds like when you open the throttle, it doesn't get any gas. Have you tried running some seafoam or techron through it yet? Did you take out the old gas and put in new?

[Shockwave3724]

Yeah, I have put new gas in it and put seafoam in it. I had the carbs apart twice, making sure I didnt miss anything, all the main jets are clean, ran a tip cleaner through them, then cleaned them with carb cleaner, I cant find anything wrong with the carbs, unless its an internal passage that I cant see thats blocked. At first, I figured it was a carb issue, not enough gas, but I cant find anything wrong.

[Shockwave3724]

I worked on it today, determined electrical system is fine. The problem is that the bike is losing vacuum pressure whenever it revs up. Its alright up to 5-6k rpm, but above that it loses pressure. My buddy said it sounds like the valves need to be adjusted and they arent closing and providing vacuum at high revs. Part of the problem was that the bike wasnt pulling the fuel in because it was losing vacuum, on prime it runs better, but hesitates at a higher idle around 10k instead. So tomorrow I'm going to tear into it and check the valves. Anyone had this problem?

[Newbie2it]

One thing to check while you got your stuff in peices. Rip the four black caps off the tops of the carbs and inspect the black rubber diaphrams for any cuts or cracks. They should also be soft and flexible. Just make sure one or more isnt an issue there. Also before you put them back in, Oil them, Like take something like 3 in 1 oil or something simular that is a real oil and work it into the rubber. This will help soften the rubber more. The new gas sux and dries out rubber. Also, around 5k your using the main jets. So while your holding the rubber thing in your hand, Look at the needle and see if its shelac'ed and how does the emulsion tube look? 0 or o If its oval like an 0 then its worn.

Just some things to look at while your there.