KZ750 High Speed Problem

Hey Guys, I restored this 78 kz750 twin. Everything was taken apart and cleaned. I'm a mechanic by trade. I rebuilt the carbs,new floats,new petcock, I have stock air box,k&n stk filter.
I have 135 main jets in now and stk idle jets. The bike came with a alphabet header. The bike runs great!all day long at 60 mph or less. Once i get on the highway. and get up past 65 mph. she starts to bog down like she needs more fuel or bowls are empty. does the same with stk 125 jets.
I have the needle jet clip second from the bottom. no leaks, air or fuel.
I checked timing and its advancing. new points & condesor. B7ES plugs. Float bowls are full.
I'm going to check coil and wires next. As per shop manual. I think it is some how fuel related.
I pulled plugs after test, seem kinda lean. Idle mixture is out 1 1/2 turns. Did anyone have this problem? and how did you fix it?
Thanks for any help.
Lawrence

Does it have stock air box? Air filter OK?

Make sure the air vent in the gas cap isn't restricted.

Is there an inline fuel filter? May need replacing.

Make sure to clean the fuel system top down. Rinse the tank, clean the petcock assembly, install new inline filter. Make sure the fuel filters allow enough flow.

Not sure about an alphabet header. If it's a straight pipe, you are probably running way lean.

BTW: the twins use the B6ES plugs, not B7ES. Probably not the source of the problem, but just FYI.

I had a similar problem on my 83 GPZ 550. I traced it down to the vacuum petcock. At highway speed (WOT or near WOT) there wasn't enough vacuum in the intake system to hold the petcock open so it closed down the fuel flow and acted like it was loosing power. I modified the petcock so it was a manual type, no more vacuum control. No more problems at high speeds.

The 78 twin didn't come with a vacuum petcock, but the petcock certainly might need cleaning. Sounds like something is restricting fuel flow.

I had a similar problem w my GPz750. Everything was fine until i hit the highway and couldnt go past 65 without the loss of power. After close inspection their were hairline cracks in my carb holders and would only leak at high speed w the increased vacuum. Couldnt simulate the problem any other way. The slightest of cracks in the middle two boots (in between head and carb) caused quite a headache. Ended up rejetting before i found the problem. Replaced them and hadnt had the problem since. Remember to start simple. Hope this helps

Mutz wrote: ... Once i get on the highway. and get up past 65 mph. she starts to bog down like she needs more fuel or bowls are empty....

May test ride with the fuel cap ajar, to assure adequate tank venting.

If issue persists with cap ajar, it's not being caused by an obstructed cap vent.

If issue disappears with cap ajar, it's likely caused by an obstructed cap vent.

Good Fortune!

Thanks for the info. I checked the holders.
Ran propane tank around boots while running.
Didn't notice any differance. I looked at the holders didn't see
Signs of cracking. I will check again. As they are old.
Thanks again.
Larry

Temporarily set your idle as low as possible (<900rpm). Spray carb cleaner (not propane) around the carb holder/cylinder head joints and the carb holder / carb joints. It the engine quits or the idle drops you need new carb holders. Propane can evaporate too quickly to be drawn in through the cracks. Ed

Make sure the battery is good (12.6+ VDC after charging) & the electrolyte level is correct if applicable.

Check the coil, especially if it's stock. A simple test is to ride the bike at highway speeds, then stop & check to see if it's getting hot - press firmly w your fingers. Mine tested fine w a multimeter but they were getting real hot, so I installed Dyna coils.

Will do thanks

Thanks.
I ordered a coil and wires yesterday. 35 years old.
piece of mind. and one thing to eliminate.

I had a similar problem that i solved removing the fuel filter