1978 KZ400 B1 - Scramber to be

I ran the 180s again and the bike ran really well - plugs look fine, not really showing any signs of being too lean. I changed to the 170s and it ran fine there too - not sure if it was any better or any worse than the 180s by pure feel, but the ground was wet after raining for a couple of days and I did not get a chance to do a proper plug chop. If I do go with the 170s or lower I will probably have to richen up the mid-range as it is not quite as smooth as it used to be when main jets were richer. Or it could be because my latest testing has been done at 70-80 degrees whereas previously I was testing when the air temp was well into the 90s. Time will tell but unfortunately I have been pulled into another project by my wife, re-staining and sealing our deck and then probably sanding and painting the front porch after that. The bike will be on the back burner for a week or so.

In my experience, a 20 degree difference in temps can show up in the jetting. The solution is to just live with it a little rich on the super hot days. It can still run smooth a little rich, but cruising while a little lean on a cold day results in a lot of hesitation.

So far I've only tried the Chinese VM30 carbs with 140 mains and initial indications are it's too rich. I only rode it a block with a plastic auxiliary gas can and no mufflers but if I rolled the throttle slowly, I could get more throttle. A quick snap would kill it. This usually indicates too rich. I have some 120 mains to try next, but we have to get some mufflers happening and we'll see if the tank will actually hold gas.

I suspect the Chinese VM30 differs somehow from the Mikuni VM30. I notice little differences, but the needles are probably different so these things are probably going to end up with different jet numbers than a real Mikuni VM30. At least it can use actual Mikuni jets.

It seams that most of the vm carbs being sold these days are originally setup for 2 strokes.
With the wrong needle jet and jet needle people are running way big jets. Causing the bike
to run good up top or down low but not both. If you look at what the KZ900/1000s are using
for jetting to give you an idea of what is needed. I'm running 29 smoothbore on my 440.
The jetting is as follows, 15 pilot jet, 1/4 turn out, 110 main, P8 jet needle 4th slot down.
and 5J11 needle jet. My engine is not like most others but the jetting should give you a direction.
And if I remember right the RS34s on my 1100 have only 135s.


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