one very hot cylinder, not 2 :(....help!!!

hi, im rather new to the site but I am a proud owner of a 1978 kz750b1 and the right exhaust gets super hot compared to the left. I have checked the plugs and they look the same and good, the rubber boots for cracks leaks etc. they are good, the air/fuel mixture screw and and they are matched I even closed them all the way...the bike kept running. I pulled one plug wire at a time as the bike ran and it shut off whnever one was pulled. I removed the exhaust and replaced copper washer and the metal things and its good. I even beat on the engine hile it ran to see if mabey something was stuck and its still hot on the right. can anybody help me please!!!!

it sounds to me like you may have a plugged pilot cicuit in your carbs. its been a long since i have worked on or even seen a 750 twin but its seems to me that they have a dual fire ignition system [both plugs fire at the same time] if thats the case when you pull a plug wire off all it will do is kill the engine. if i were you i would pull offthe carbs and give them a good cleaning use good spray carb cleaner and lots of air blown thru the passages make sure you remove all the rubber pieces and put them to the side because the carb cleaner can swell them up.

thanx, but it still runs hot, the carbs were rebuilt about 33 miles ago and still look good inside. I guess I will just look for a new set of carbs

even though the carbs had been rebuilt, there is something wrong if you can close the screw all the way, the bike should die.
my first thought is to reclean the carbs, check the float levels, and reset everything, it may take a second time to get it right.

also check to make sure you have a good fire to the cyls, ie coils etc

yeah, I agree but I only ran it for a couple of seconds with them closed and then shut it off myself, fuigured it was burning off the fuel left in the bowls or cylinders or something. the carbs on the inside are in mint condition as is the inside of the intake chamber leading to the cylinders. the spark is great, bike always starts right up too. the fuel valve on the tank wont shut off the fuel though so it might be putting to much work on the floats so too much fuel might be going to the engine regardless. not sure

If the fuel level in the float bowls is correct, and you completely close the idle screw, it will die immediately, at least it's been that way on all the 750 twins I have had.

I suggest a complete tuneup as a starting point, check/adjust the valve clearances, points and timing advancer and timing, and the fuel level in the float bowls. Then sync the carbs.

750 twin has wasted spark ignition, but if the motor is in good shape you can ground one spark plug wire, start it up, then ride away on one cylinder. I've done it.

thanx! I will start with the tune up, and going to re-do the floats, but Im not sure how to check the valve clearences without taking the head off. so I will do the others first and see where I stand.

ok check this out..... sure enough I didnt notice before but the floats had diffrent settings on each. so I syncd them. where they are at now they function properly the fuel stops after the bowls fill. but the problem persits, the right side exhaust gets very hot super fast ...the bike doesnt have to be running for more than 2 seconds and that right exhaust is too hot to touch. does anybody know what the setting is for the floats like a degree of angle for the tab or something cuz I just made sure they were the same and not so much that no fuel got in.

My manual is out in the shop, and I'm not, so I "think" it is 2-3mm below the float bowl mounting face.

I have flipped them upside down (76-80 750 twin carbs) and set the top of the carb float (actual bottom when the carb is right side up) at 1" from the bowl mating surface. When checked with the hose method the fuel level was perfect.

Synching the carbs with vacuum gauges made a huge difference on my 750 twin.



You don't have to remove the head to check/adjust the valve clearances, just the valve/cam cover. A Factory Service Manual is "highly" recommended

cafe750 wrote:

... right side exhaust gets very hot super fast ...the bike doesnt have to be running for more than 2 seconds and that right exhaust is too hot to touch....

With engine running, both exhaust header pipes are supposed to very quickly (a few seconds) get too hot to touch.

With engine running, would spray water mist onto both exhaust header pipes and observe what happens. The water spray should instantly sizzle and vaporize if combustion is happening inside the cylinder. Where the water spray does not instantly sizzle and vaporize, and just runs down the pipe, it indicates that combustion is not happening inside the cylinder (for whatever reason).

Should perhaps consider performing the water spray test, and posting the results, which would aid in diagnosis of the problem.

May just need a brand new spark plug of the correct type and size -- and no guessing allowed -- be absolutely positive about using the exact sparkplug specified by the factory service manual.

Would remove leftside plug and hold its base against the cylinder head while spinning the engine over with ignition switch ON (as in normal cranking procedure), and watch for good fat spark. If there's no spark or an extremely weak spark, indicates a problem in the ignition system.

Good Luck!

thanx for all the help, I will go attempt Synching the carbs with vacuum gauges, and check the floats again. then will do the water method. I will post back with results!!