Topic: Why would a 1979 KZ750 Twin surge and sputter when it gets hot?

[BikerBrains]

 My uncle is getting rid his bikes due to early onset dementia. 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 tiime 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.

Should I pass on this bike?

[BenelliSEI]

Does it have points ignition? Change the condenser.

[Alan F.]

Are valve clearances in spec? Terry has one I think, and there have been several around over the years. Someone will have the advice you need.

[HondaMan]

Does it have points ignition? Change the condenser.

They do!

[Tracksnblades1]

Try it again with the gas cap loose..or open it occasionally at the stop signs before it usually acts up..

Or Pull the line off the carbs and make sure gas flows unrestricted with the gas cap tight..

😜 if it’s not the condensers….

[BikerBrains]

Try it again with the gas cap loose..
Or open it occasionally at the stop sign so you don’t get any on the jewels..
😜

yep tried that. its the first thing my uncle's friend told me to try. He thought it might be a venting issue. I wish it had been.

[Tracksnblades1]

Try it again with the gas cap loose..
Or open it occasionally at the stop sign so you don’t get any on the jewels..
😜

yep tried that. its the first thing my uncle's friend told me to try. He thought it might be a venting issue. I wish it had been.

I’d pull the carb lines and see if the sediment bowl is restricting the carbs.. Honda’s have a screen on top of the threads in the tank, not sure what a 2cylinder kawaknocky has..

[Tracksnblades1]

Also, I think the Honda uses 22 microfarad condensers..
Many old cars used the same microfarad rating..
Old new stock Delco/Remey, Autolite, motorcraft, borgwarner,
Chrysler, and Bosch were the ones OEM manufacturers used..

The Honda OEM TEC are well liked here..

I’d stay away from anything made in China…

[Tracksnblades1]

Accel, Vertex, and Mallory all makes good condenser too.
Never used them in a Honda CB though…

[BenelliSEI]

Tried changing the condenser? Those symptoms are typical.

[BikerBrains]

Also, I think the Honda uses 22 microfarad condensers..
Many old cars used the same microfarad rating..
Old new stock Delco/Remey, Autolite, motorcraft, borgwarner,
Chrysler, and Bosch were the ones OEM manufacturers used..

The Honda OEM TEC are well liked here..

I’d stay away from anything made in China…

The bike in question is a Kawasaki. KZ750, mentioned in the title, not a Honda.

[scottly]

When a condenser fails, they usually stay bad, rather than come and go with heat. I have seen a very small point gap that acts up when hot before. What happens if the choke is applied?

[BikerBrains]

When a condenser fails, they usually stay bad, rather than come and go with heat. I have seen a very small point gap that acts up when hot before. What happens if the choke is applied?

I didnt try it with the choke but when I roll the throttle it seems to get worse.
I ordered OEM points and condenser and will see how the bike acts after the valve clearances are checked and the point/condenser are installed.

I dont think its the carbs. They were rebuilt not long ago, BUT there is a tear in the rubber from the carbs to the airbox.

I have been running a Shorai battery in the bike without changing any of the other components. Maybe that fried something, over time.

[scottly]

I have been running a Shorai battery in the bike without changing any of the other components. Maybe that fried something, over time.

I doubt the Shorai battery fried anything. You might check the battery voltage with the bike running to make sure the charging system is working properly.

[BikerBrains]

I have been running a Shorai battery in the bike without changing any of the other components. Maybe that fried something, over time.

I doubt the Shorai battery fried anything. You might check the battery voltage with the bike running to make sure the charging system is working properly.

Yep, I did that yesterday. Charging system is working properly.

[HondaMan]

Since the carbs have been off lately, I'll ask:
how are the fuel lines routed, between the fuel tap and the carbs? is it downhill all the way, or does the fule line go down and then up, and then down again to the carbs?

If there is a trapped air bubble in a down-up-down fuel line path, they can stop fuel flow when hot because the warm air in the carb float bowls then pushes back against the bubble. If it gets hot enough to "burp" it, some fuel arrives and it runs again until the process repeats.

This sort of thing happens on certain Honda twins, like the CB350, when someone tries installing an inline fuel filter. Once in a while it happens on the CB500/550, too, which has a similar shallow fuel delivery path.