1979 KZ400 H1 - Tuning VB32 Carbs

[keeno]

Hi Team! Long time listener, first time caller. I picked up this bike a couple years ago for $600. It has about 70k kms, runs well (but not first kick kinda bike, more like 8 kicks to start). Not long after buying it I notice the exhaust couplers were rotting out. The stock megaphones also looked pretty gnarly so I put on some shorty mufflers. I cleaned the carbs out at the same time.

So I'm debating what my next steps should be. The bike starts and runs okay but there is noticeable popping, especially when I let off the throttle while riding. I expect the mixture is too lean so I should jet up. I've been reading lots about tuning KZ400s and it seams most people just prefer to go with a vm30 or 32 instead of messing with the stock diaphragm carbs.

Option 1 - Re Jet
Can anyone make sense of the carb specs? I'd like to know what the stock jets are but the table isn't very clear. It looks like there are lots of options on JetsRUs.

Option 2 - VM30 or 32 Carbs (possibly with 2 into 1 intake?)
I'm leaning towards this option since my longterm goal is to remove the airbox and tidy up the electronics under the seat. I also figure pod filters might make tuning the stock VB32s even more frustrating.

I know this topic has been beaten to death but I'd really appreciate some advice, or similar experiences you guys have had.

Thanks!

 

[barnett468]

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the stock jet size is clearly posted in your link . . exactly what do you want to know?

Jet your bike so it performs well under acceleration . . Never jet it just to try and make it quit popping when decellerating because it doesnt matter how it runs when you are slowing down, besides, the popping can be from leaking exhaust valves or the exhaust pipe design.

post photos of your plugs.

how does it perform when you accelerate?

pay attention to how it runs the first few feet when you take off . . if it hesitates slightly, i would go up on the pilot 2 sizes . . this may also help it to start easier.

 

[keeno]

I guess my confusion with the carb jet sizes is the "Air Jet" column. What are these? Also, the Clymer manual I've been using refers to a needle jet. Is this the same as the pilot jet in the table above?

Acceleration is good, no signs of hesitation. Really the only difference I've noticed since swapping the megaphone exhaust for shorties is the popping when letting off the throttle.

I've never inspected spark plugs before but it looks like normal wear based on the image chart I found online. The plugs have 60ish kms.

 

[barnett468]

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don't mess with the air jets.

the needle is the long needle in the slide . . the needle goes into the jet for the needle which is called....the needle jet . . the needle is called the jet needle.

your jetting looks a little rich on the bottom . . if it starts easy without the choke, the pilot jet is a little big, hoewever, if it runs perfect;you could leave it.

 

[keeno]

I actually haven't tried to start it without the choke yet. The bike actually stalls without the choke being about 50% engaged. So I should have mentioned that before as a difference between stock exhaust and now. Could this also be related to the pilot jet.

Can you explain to me where the pilot jet is on the diagram in this manual? Is it the pilot screw in this case (53)?

Thanks for the help on this btw

 

[keeno]

Diagram in the Clymer manual

 

[barnett468]

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ok, the pilot jet is number 61.

if you go up on it 1 size, it will not die as easily when cold, plus it may reduce the popping sound. its common to need to go up at least one size on it when headers are instslled . . its also common to need to go up on the secondary main jet 2 sizes also when a header is insatalled.

if your needles are adjustable, you could raise them up i position but i doubt that would have much affect on the popping.

 

[DohcBikes]

This bike is obviously lean. Make the change. Considering the info given thus far I would go up 2 pilot sizes but 1 is minumum.

It is not an H1. Lol.

To make something clear, the needle jet is the piece that the jet needle resides in. Also at times referred to as an emulsion tube. These terms are easily confused as they have been in this thread. In many carbs the needle jet must be installed in a particular orientation so take note in your manuals.

Air jets are the passages at the intake side of the carb that help create the pressure difference that causes fuel to be pushed/pulled through the jets. Some carbs have removable air jets, some don't. These are also known as air bleeds.

Jet up.

 

[keeno]

The KZ400 H1 (aka LTD) is actually the correct model name, but obviously this is not the 2 stroke H1 which you are referring to.

Thanks for the advice and clarification. I think I'll purchase several sizes as you've suggested and start experimenting.

 

[DohcBikes]

The KZ400 H1 (aka LTD) is actually the correct model name, but obviously this is not the 2 stroke H1 which you are referring to.

It was a sarcastic comment. I will try to add a description next time.

Keep us posted, always nice to know how it works out.