What would make a 1979 KZ750 Twin surge and sputter when it gets hot?

I joined this forum only a week ago, and I dont know if this goes in Carbs, Electrical or something else, so I put it in "engine."

My uncle is getting rid his bikes due to early onset dementia. Its sad to watch this happen but hes taking control. We have always been close and he has about 13 bikes, 7 of them are old Kawas. He gave me a KZ650 which hadnt run for a while, but needs some carb work, and he has a KZ750 twin, 1979, that he was going to give to my brother, but my brother doesnt want it.

The problem with the KZ750 is, the last time he was on it was last summer. Then he drained all the fluids and oil before storage. According to his maintenance log, he stop liking the bike because it would be fine until it got hot and then it would surge really badly, backfire and eventually shut off. He rebuilt the carbs, changed the coil and everything he could think of, and it still happened. I told him I might want the bike but wanted to ride it first.

So, we put all the fluids back in today and she started right up. I went about 9 miles at 60mph and sure enough, though it ran fine and strong at first, it then started surging and backfiring and then shut off. I pulled over and let it cool down for 30 min and it ran OK for a minute, and then the same thing happened but I was able to get it back to my Uncle's.

Is it worth taking on this bike? This is it, a photo from when he first bought it a long time ago, from a private seller. :


 

Has the valve lash been checked? Is the fuel cap vent plugged?

Only you can decide if it's worth taking on, it's a 4 stroke twin from 40 years ago (not the most complex of machines) - there is enough knowledge here to get you sorted if you're committed.

TexasKZ wrote:

Has the valve lash been checked? Is the fuel cap vent plugged?

 

Hi. Its hard to get an answer of my uncle as he deals with his health. He gave me his notes and said thats all he can offer. He did tell me that he rode the bike at lot in 2021- 2023, from 10K on the speedo up to 14k and never tuned it up, never checked valves, nothing. His disease made him forgetful since 2022 so sometimes he cant remember.. but he was only recently diagnosed.

I just got in from testing the bike and man oh man, what an experience! It sounds starved for fuel, but what do I know?! It starts up with a kick (he took the starter out so it takes a strong kick on that thing) but once it gets going while Im on it, its jerks forward, sometimes really hard, and makes loud popping sounds from the exhaust. If you give it more throttle, its gets worse.

Checking his manual, the carbs were rebuilt just before he put it away. He put about 75 miles on the new carbs. I tested the battery, its lithium, and its working properly and it and the regulator are doing their job. Petcock was changed in 2023 to stock. I dont know what it could be thats wrong, and neither does he. From what little I know, it either needs a tuneup or there is a fuel delivery problem.

I want to thank all of you smart guys for also being kind and helping out with this. Id love to have the bike because it looks so good, but I dont know. Hmmmm

Like Wookie58 says, these are pretty simple machines and there is a lot of help around here. 

One of the most common problems with motorcycles that have been sitting for an extended time is clogged carburetors. When the gasoline evaporates, it leaves behind gummy deposits that eventually dry out. The only solution to them is to disassemble the carbs and soak them in carb cleaner. I know they can seem intimidating if you are not experienced, but there are experienced folks here who can walk you through the process. There a a couple of members who might be talked into doing them for you.

Another common problem is the valve lash. Too many owners have been afraid to try and too cheap to pay somebody else to do it. As the miles pile up, the gap gets smaller and smaller. In extreme cases, a valve can actually be held open a tiny bit, causing really poor running, and maybe a burned valve. The problem can get worse as the engine heats up. 
Checking the lash is not difficult.

The 750 twin is a fun ride, and the one you have looks to be in good shape. It likely just needs a little love.

Thanks. Im going to go through everything: changing points and condenser, checking valve clearance, charging system, carbs, all of it.

As "TexasKZ" wrote:
It's always very important that the valves are checked and, if necessary, adjusted correctly! - This can't be emphasized enough! It's essential for smooth and healthy engine operation in all temperature ranges!

( I also checked the valves on my KZ1000A2 the day before yesterday and adjusted two of them!)
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:And as other members have already written before me:carburetors need to be disassembled and meticulously cleaned. Today's gasoline, with its relatively high bioethanol content, dissolves all plasticizers from hoses and seals, and this filth accumulates in narrow passages and idle systems, causing problems!
 

As stated check the tank is venting properly, just pop the cap open to see if it changes anything.

Check the size fo the float valves. I bought a carb kit once that had the wrong size and it was starving the bike for fuel on the highway.

DoctoRot wrote:

As stated check the tank is venting properly, just pop the cap open to see if it changes anything.

Check the size fo the float valves. I bought a carb kit once that had the wrong size and it was starving the bike for fuel on the highway.
 

My uncle's friend thought of the tank-venting issue and yesterday I did ride it with the cap open and it had the same problem. Going to do a thorough run through and let you know what happens. It could be anything so we are going to go through everything, and I do mean everything!

MFP-Joe wrote:

As "TexasKZ" wrote:
It's always very important that the valves are checked and, if necessary, adjusted correctly! - This can't be emphasized enough! It's essential for smooth and healthy engine operation in all temperature ranges!

( I also checked the valves on my KZ1000A2 the day before yesterday and adjusted two of them!)
-
:And as other members have already written before me:carburetors need to be disassembled and meticulously cleaned. Today's gasoline, with its relatively high bioethanol content, dissolves all plasticizers from hoses and seals, and this filth accumulates in narrow passages and idle systems, causing problems!

 

the carbs were rebuilt one month before he drained the fluids and put the bike away. Im learning that it could be anything as people are pointing out. so, the bike will be checked at every point, fuel, spark, carbs, electronics, all of it. and I will let you know what the problem was.

Im so confident that I ordered a new exhaust for it, knowing that things will work out fine. Delkevic has two nice 2 into 1 options and I got one today.

Seems you've become the provisionary of your uncle's estate of motorbikes? Be keen to the fact that Everyone one of those machines has a different discipline and effective cause/resolve.     

Delkevic makes nice systems and their prices are surprisingly reasonable.