Cylinder number 4 running cooler than the rest.

Hello, I am a new member and had a question about a bike I recently purchased. It is an '82 KZ750 LTD with low miles. I had to clean the carbs because it would only run with the choke fully engaged.(the jets were all varnished, etc.)

I also needed to replace the wires and caps (they were leaking and gave a shock to the touch)

Everything else checks out fine, except now the #4 cylinder runs cooler, which may have to do with a rough idle. The bike runs great when riding. #4 spark plug was black and the others were a nice tan color.

Is the problem with the #4 carb or is it something else? Should I spend the money on a carb sync? Should I replace the idle jet?

PS The #4 cyl has good compression. No smoke billowing out of the exhaust. Can't see a thing!

Thanks!

Check the valve clearence and sync the carbs. Maybe run a bottle of sea foam through the tank-couldn't hurt.

Since you just replaced the wires and caps get a good look at them at night in the dark and move them around to see if they are arcing to the engine. You say comp is good so I would think the valves are ok. Sometimes when the carbs get cleaned just a small piece of crap can do it. Did you put new lines and fuel filter in?

Thanks, I did install an in-line filter, but didn't replace the fuel line. I will look for arcing. I was hoping that I wouldn't have to tear down the carbs again, The rubber boots leading to the air box are a SOB to put on.

To soften the boots, use a hair dryer at medium setting.If replacing the boots with new ones, soak them in fairly hot water with a little fabric softener. It makes this chore easier.

Great looking bike you got there Koda1.

When I got my 750 a few years back I had carb problems and it took me 3 - 4 shots at cleaning them before I got it right. It was'nt enough to spray carb cleaner in all holes. It was only when I removed all jets. sparyed carb cleaner - wear safety glasses because that stuff will shoot back and get your eyes !!! - probe every hole I could find with a stiff bristle or gently with fine wire and made totally sure all passageways are clean. Only when I did all this did I get it right.

Are the carbs Keihins and did you remove the air mix screws at the top of the carbs towards the front or are they covered with a metal plug ?

I am a believer in using a synch tool on the carbs after I do anything to them. A good envestment in my humble opinion.

Thanks, I was afraid I might have to tear the carbs down again. I was thinking about a sync tool that sells out of England. I think it costs around $100.00. Do you have any suggestions?

If you think it might be a little trash in the carb, sometimes a good spirited ride will clear it up and sometimes not. I wouldn't recommend this if it may be a valve adjustment though. If it is in a certain rpm range I would run it in the range for a few times and see if it clears up. I do this before I tear them down for the upteenth time. Hitting reserve usually means a teardown for me no matter how clean my tank is. I figure if I pull the petcock every so often I will get most of the trash before it gets me.

Click here > Just Do It!

Good Luck!

this problem sounds all too familiar, ay patton

Koda1 wrote:

...#4 cylinder runs cooler....

Would install brand new spark plug in #4 and test for combustion by allowing engine to run a few moments, then water spraying exhaust headers. Water should sizzle and instantly vanish.

If #4 is combusting, would take a test ride to determine whether the new #4 plug soots and fails.

As known, even perfect carbs can't compensate for defects in compression or ignition or fuel supply.

Good Luck!

Koda1 wrote:

................. I was thinking about a sync tool that sells out of England. I think it costs around $100.00. Do you have any suggestions?

Sounds like the Carbtune from Belfast. I have one and love it.