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second chimp in space
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3,344 Posts
Discussion Starter #1
I have an '87 ZX-750 that wasn't tuned properly when it had the pods and muzzy exhaust fitted. Now I get to do that. It has a larger jet (keihin 155, stock is 118), but I got it with the needles shimmed with washers (raised 1.6mm) so I think they're stock. I can get idle and full throttle going great, but everything inbetween is too lean (lean stumble).

Questions:

1. Roughly how much (if at all) do I have to raise the fuel level for the pods? 1mm, 3mm, 10mm?

2. How long do I have to run at a particular throttle for the plugs to reflect that? If I were to be testing the main jet, how long do I have to run w.o.t. before killing it and looking at the plugs? 10 seconds, a mile?

3. What do you guys like for jet kits? Dynojet, K&N, other? (I think I need a stage 3)

4. How is a jet kit needle different from a stock one? Is the adjusting clip the only difference, or is it narrower, or a different profile?

5. What is the main effect of raising/lowering the needle? Does raising the needle only make 1/4-3/4 throttle richer, or does it just make the main jet come on sooner?

6. Does anyone know of a good book about carb tuning theory? I'd like to have a reference on how to do it properly and how things interrelate. A semi-comprehensive troubleshooting guide would be cool too. I have a semi-descent idea now, but I'd like to know more.

Thanks in advance.
 

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No Whammy No Whammy STOP!
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1,752 Posts
Your stock main is 118, and you have a 155 in there now??? And both are keihin mains? If thats the case you bike must be spilling gas right out the tailpipe. Thats a drastic jump in main jet size. Something in the range of Stage 17. However, if you have a dynojet jet kit in there, they have a different way of numbering their main jets. Typically it is a much higher number than what Keihin or Mikuni uses. So find out if you have a Keihin jet of a dyno jet main in there. There should be some sort of stamp on the jet that signifies who's jet it is.

Anyways, it sounds like someone did a discount jet kit on your bike. Some say they work well, others (like me) dont.

As for your questions:

1) It is hard to say. All bikes run different. If you raise the float level (measuring from the float bowl base) it leans out the mixture. I dont know what the stock float height is for your bike, but a service manual should tell you. With my carbs, I raised my floats from 18mm to 24mm and it is just fine, but this is what Factory Pro recommends. Maybe you can give them a call and ask what they recommend. I am no carb expert, I know a few things about carbs and tuning, but I dont do the research that they do.

2) Dont bother with checking out the plugs. What a pain in the ass that is. Plus you would have to do it on the side of the road. I recommend you go with a main jet that is 3 sizes larger than your stock jet. For you, that would be a 125 Keihin main jet (118 (stock), 120 (stage 1), 122 (stage 2), 125 (stage 3). With your full system and pods, it should be a good size to start off with. Go WOT with that main jet and see how it feels. If it has great power than gets a very quick stumble, then good, then quick stumble, it is too lean. If it goes fine then it stubles a lot and feels like power is dying off, it is too rich.

3) I like Factory Pro for ket kits. Their needles have a nice taper to them and you dont have to drill out the holes in the slides. I think it is more user friendly than the Dyno Jet kits.

4) Typically stock needles have very little taper at all to them. There is no adjustment to them at all. A Dyno Jet needle has a strange taper to it. It is kind of stepped and not smooth. They also have 6 different settings on the needle. The Factory Pro needle has a nice smooth taper and has 5 settings.



5) Raising and lowering the needle clip placement leans and richens the mixture, respectivly. Raising the needle clip (clip in the #1 position as far away from the needle tip) means that you are keeping the needle pushed in farther and blocking off fuel from getting in the the mixture. If you lower the clip (clip in the #5 or #6 position closest to the needle tip) you pull the needle out of the emulsion tube and it lets more fuel into the mixture because of the taper of the needle. This is why I dont think discount jet kits (shimming needles) work well. You can shim the needle, but because it doesnt taper like a jet kit needle, the extra fuel doesnt get into the mix, so you have to shim it like a mofo, and it F's up all your other carb settings.

The needle effects throttle from 1/4 to 3/4 throttle, but can carry along into the main jet a little bit (when looking on a A/F graph). It effects that part where the first big rush of power comes on. Also with pods you will hear them start to growl. That is because the slides in the carbs are opening up allowing in more air into the engine. You can test out if your needle is in a good place by setting it at the #2 setting and going for a ride. Go WOT and see how the bike feels from 5000 rpms to 10000 rpms. The same stumbles and bike characteristics still apply here. Quick stumble is lean, bogging is rich.

6) There are some good sites on the internet that you can find out what all does what. They were helpful for me when I was first tuning my carbs. But remember that carb theory is just theory. Every bike is different, and no one setting will work for everyone. And with those pods it will be very sensitive to temperature, humidity, barometric pressure, and everything else. So if your bike runs great at 70 degrees, at 90 degrees your bike will try and flood itself.

Good luck, and I hope that I helped out a little bit.
 

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second chimp in space
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3,344 Posts
Discussion Starter #4
Wow, thanks for the info.

It is a Keihin 155 main jet (it has that Keihin star/K). I saw them selling them online for watercraft for like $5 each. I bet the needles are stock because they were shimmed and there are no clips. I don't have a jet kit on it yet, though I will be getting one. I didn't see any Factory Pro kits for my bike, though maybe the one for the '89 ZX 750 will work. I'll have to ask them to make sure. If not, maybe I'll try the K&N. I'd rather not have to drill any holes in the slides...

Thanks for the reply, it was really helpful. That link is pretty good too.
 
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