Still rough running

Hi there,

Thanks for all your suggestions, this is what I have done so far. Cleaned out fuel tank as best I could. Rebuilt petcock with new diagphragm. Dismantelled carbs and cleaned them, there was some varnish at the bottom of each float bowl but the jets seemed to be okay. Took the timing advancer off and greased it. It is working fine, springs and wires all look good. She has new plugs, oil and filter, air filter and battery. All hoses are in good condition, no splits or kinks. Fitted an in-line fuel filter as well to be on the safe side. Put everything back together, started up and was still running rough, bike ran for a couple of minutes on the choke and then just died. Fired up again but could not keep her running, very rough sounding and then just dies. Opened fuel cap to see if breather was blocked, have also run on prime to check if there is a vacuum leak, zilch. Used a plug tester, all 4 plugs are sparking and checked the float bowls and there is fuel there. I am out of ideas. Bike suddenly started running rough under 3000rpm overnight and has steadily got worse. Any ideas greatly appreciated. Bike is 1980 Kz750Ltd H1 4 cylinder US spec. Thank you

I know you don't want to hear this but carbs are still dirty, pay particular attention the idle circuit passages in the carb bodies. Insure all of them are clean and clear. If the float bowls had varnish then the small passage ways in the carb bodies are clogged.

If the carbs at hand have the tiny passageways as shown in this diagram, the passageways may be cleared using a can of pressurized carb cleaner spray.

Where so equipped, the plug shown in the diagram must be present.

Fuel must pass through the pilot screw hole into the carb bore (as shown in the diagram).

Wear eye protection while applying carb spray, because the spray always backfires. Always.





Good Fortune!

yep, what they stated, sounds like lowspeed in carbs. I'd also replace plugs anyways. Sometimes you can see a spark but it wont fire correctly under compression.
Might want to install a clear fuel filter to make sure you aren't still getting crap in from the tank.

+1 with what they said. Just to demonstrate how hard it can be to clean these passages. My bike had sat 10+ outside. When I bought it, there were wasp mud packs in some of the carb passages. I completely disassembled them and boiled them for 20 min in a solution of lemon juice and water (1 pt lemon, 6 pt water). But you don't need to do that. That is for extreme gunk. Afterwards, I used carb spray followed by compressed air on all the passages. Bike ran so and so. Then I saw Patton's diagram and decided to take the rack off and spray the passages again using the method he describes. Used carb spray and compressed air again. While doing one passage, some kind of bug embryo shot out, LOL. So put everything back together and the bike ran better. However, the bike finally ran perfect after adjusting the valve clearances and balancing the carbs. But the point being is, you really need to make sure those passages are clear. Patton's diagram is very effective.

Possible that the Condenser(s) have gone out. Is it a nice blue spark or just a orange spark? they need to work as designed to make proper spark.

idle mixture screws, and intake leaks?

Hi there,

Update on the bike. I took her down to a local garage yesterday and have just got her back. Pilot jets were blocked (I obviously didn't do a very good job cleaning them, fuel was contaminated with water (bad local petrol station) and two of the shims needed replacing as they were too tight. All back together, carbs balanced and mechanic took her for a test ride and said what a lovely bike it is to ride. Have just picked her up in the rain with a huge grin on my face. Thanks to everyone for their help and suggestions, this is a brilliant forum! Just need it to stop bloody raining so I can go riding now
:woohoo: