GPZ400 stuttering in midrange

[grantman]

Hi guys
I need some advice for my 1984 GPZ
(i have another 84 for parts)

It seems to rev fine out of gear
(note rev counter isnt working so i cant really tell RPM)

When i take it for a ride it opens up ok until about 7000 RPM then stutters for a about 1-2 sec and then powers thru it and revs out nicely

I brought the bikes from trademe so i dont know if its has been working ok with the 4>1 thats on it.
It didnt have a airbox in it when i got it so I have swapped it over from the parts bike


How do i tell if is a fuel/ carb fault ?

or should i swap over CDI and coils from the parts bike to test?

The bike is quite rideable in the other rev range
Starts easily with the choke on when cold .....starts and idles well when warm



when i got it it had no airbox and wouldnt rev.....but after fitting the airbox it revs alot better and has top end but has the loss over that short midrange
....does this mean it is running to lean?

any help would be great as i am getting it ready for the Cold kiwi rally

Cheers
Grantman

[F5 Dave]

Man I hate the let’s remove the airbox crowd. Try to find out what jets were std with it. Maybe he put large ones in to compensate for when it didn’t run properly with the box removed.

Try here , maybe it will list sizes if you can find your bike. Might not have the list for a 400 as US site though?


Start with the obvious first. Take the carbs apart, clean out the jets (never poke anything down them, only blow). Check if needles are in middle position which is probably std if they are adjustable. Check the rubbers are not perished.

New plugs. Check the coils & plug caps are ok, try swap from donor bike. Is charging system ok? This can cause a breakdown.

Best guess is the carbs though. Maybe the donor bike carbs might be a good starting point once cleaned. The 4-1 will make an effect but shouldn’t be as bad as you describe & likely curable with a slight jetting change.