rough running under 3000 rpm

Hi there,

Wonder if any one can help. I recently bought a KZ750 LTD, 1980, done 17,800 miles, not been used much. It has been running well for about a month until last week. I rode out to a friends house and it was fine. Came to come home and noticed bike was running rough, rough and spluttery until you get to 3000 revs then she is fine. have changed all 4 plugs which were all the same colour and looked fine. One HT plug lead gave me a shock when engine was running so have cleaned all plug leads and the caps and sprayed with water dispersant. Got a gut feeling it might be a fuel problem. As I said the bike hadn't been used much by previous owner, but I have used it every week and filled up twice with 2 clean tanks of fuel which have gone through. This happened literally over course of one day. Do you reckon crud has come loose in tank or carbs? Many thanks for any help

Hi Redwitch,

You may have run the fuel low enough to were it picked up debris which ended up in the carbs. You may possibly have to clean out the carbs (sounds like your pilot circut) to get it right again. Or you may be able to get it right by simply running some seafoam mixed in with the fuel for a couple of tanks. But what ever you do, make sure there is no debris in the tank and at the least have an inline gravity type fuel filter in place. Otherwise, solely cleaning the carburetors will only mean they will get clogged again if you don't take care of the source of the problem.

Redwitch:

Have you checked the ignition mechanical advancer? It sounds to me that it is frozen or stuck in the full advance position (weights open). Lube the pivots and apply grease to the camshaft axle end.

Thanks for the replies. I will take a look on Friday when I am off. Give the tank a good flush out and take the tap out and clean it. Fingers crossed!

Fer sure it's the advancer.Take off the right side cover. It should twist and snap back super easy and the fly weights should move out and drop back super easy.If you do need to take it appart,make sure you note which way the pointer is pointing!!!Big P.I.T.A. if you reassemble it 180* off.Also,take a good look at the pick up wires.They should not be hanging loose or exposed in any way.Pointer>If you do need to replace the pick-ups at some time,they are all the same.The only difference was the lead length to the igniter box.

Hi all,

Update on the rough running. I took the tank off Friday and it had a nail in the bottom! It also had a lot of very fine paint flakes, the idiot who resprayed the bike obviously didn't seal off the fuel cap properly. Anyway I drained the tank as best I could and sluiced it out with some fresh petrol. I also dismanatled the fuel tap and the filter looked okay and clean, I washed it out with clean fuel and reassembled. I tried to get the carbs off but failed miserably, so i poured a load of Redex fuel cleaner in the tank and sprayed the carbs with carb cleaner through the air filter rubbers. As it was late in the afternoon and the light was going I reassembled everything and went for a ride. The bike was even worse and had to be revved really hard to keep running over 3000 revs. I headed for the local bike shop and she died on me 3 times. I had difficulty restarting her and had to switch the fuel tap to Prime and put the choke on. I made it to the bike shop and booked her in for the week after next. Today I decided to have another go at removing the carbs and succeeded. Light was going again so I have only taken one float bowl off so far but everything looks very clean, including the pilot jet. I did try to look at the timing rotor and it does move okay, although not really snappy. It's really bugging me as the bike was running fine a couple of weeks ago. I will continue to clean the carbs but if anyone has any other ideas, they are most welcome, thank you