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[1985 GPz550] Modified airbox doesn't run properly 27 Mar 2017 15:24 #757860

  • MaxBell
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HELLO.

I've built a GPz550 into a little street fighter thing. When I cut the tail off and welded in the supports, I also ended up with a smaller area for an airbox.

To solve this, I made an airbox, based on the dimensions of the original.



I don't have a photo of it handy, but it uses the same inlet size, same filter and fits with the intake plenum perfectly.

However, the bike doesn't run right. Does changing the volume of an air box fuck with it's ability to work properly?

If so, do I need to rejet the bike? I also modified the exhaust. I've never done anything with rejetting a bike, would I just buy the dynojet kit?

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[1985 GPz550] Modified airbox doesn't run properly 27 Mar 2017 15:44 #757864

  • Nessism
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Airbox hacks are sure to mess up the carb jetting. Exhaust changes will as well but not to the same extent as intake side changes.

The Dynojet kit is designed around using pod filters, real K&N's or maybe Uni's, along with a 4 : 1 header system. I'm not sure how well it would work with a modified airbox and exhaust.

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[1985 GPz550] Modified airbox doesn't run properly 27 Mar 2017 17:07 #757885

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Do you think it's worth just ditching my airbox and going with some K&Ns and the Dynajet kit?

I've got TK26 CV carbs, but that shouldn't be a huge problem right?

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[1985 GPz550] Modified airbox doesn't run properly 27 Mar 2017 18:57 #757901

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MaxBell wrote: Do you think it's worth just ditching my airbox and going with some K&Ns and the Dynajet kit?

I've got TK26 CV carbs, but that shouldn't be a huge problem right?




Do whatever you wish, but I'm convinced the air box is worth keeping...in one form or another. :)





1982 KZ1100-A2

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[1985 GPz550] Modified airbox doesn't run properly 28 Mar 2017 10:27 #757953

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Ok, so I'll work on getting it to run with my modified airbox then. (Just to be clear, I should leave the plenum alone, but modifying my airbox should have any MAJOR implications for engine tuning?



If I do need to tune, can I do it with just adjusting the air/fuel? Or do I need to rejet?

You posted a plenum with velocity stacks? That's pretty sweet.
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[1985 GPz550] Modified airbox doesn't run properly 29 Mar 2017 13:15 #758046

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You will likely need to jet and sync the carbs to the completed intake system you choose to use...
It's nice when the result winds up better than expected. :)
1982 KZ1100-A2

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[1985 GPz550] Modified airbox doesn't run properly 30 Mar 2017 09:44 #758133

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Alright, awesome. Dynojet kit? Which stage do you guys think?

Is there a guide or something out there I could start with?

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[1985 GPz550] Modified airbox doesn't run properly 30 Mar 2017 10:19 #758136

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Perhaps you have already considered this, but why not take small steps and know where you are going, rather than one big one that might leave you completely in the dark?
If you have the original carbs, you may be able to arrive in a good place by experimenting with different jet sizes before going after something that could complicate your effort. You may be surprised that one size or two change of the mains, and pilot circuit could take care of it, and get away without permanently altering the carbs.
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[1985 GPz550] Modified airbox doesn't run properly 30 Mar 2017 12:01 #758146

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That's smart.

I do have the original carbs on the bike right now, I can get to to start, but it runs weak and tries to die unless I rev it up.

So that means that I should work with my mains, since it's the idle I'm trying to fix? Would I just start with adjusting the fuel mixture? Or should I order in some aftermarket mains? If so, which company?

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[1985 GPz550] Modified airbox doesn't run properly 30 Mar 2017 13:05 #758157

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All that and...

Make sure all passageways for air and fuel are spiffy-clean, and that everything moves freely that is supposed to, and nothing does that isn't supposed to. When it comes to the o-rings, there are members far more knowledgeable than I am. :kiss:
I speak from experience with one Kawasaki motorcycle that has owned me for seventeen years, and how it has come back to life on several more occasions than any cat I ever heard of...

My best advice from experience is buy the best part you can afford, and make sure you use genuine Mikuni for the carbs, and Genuine Kawasaki for most everything else. I tend to plan ahead and make advance purchases off ebay for what I'd consider reasonable prices.
1982 KZ1100-A2

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[1985 GPz550] Modified airbox doesn't run properly 01 Apr 2017 06:02 #758284

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This: but why not take small steps and know where you are going, rather than one big one that might leave you completely in the dark?
If you have the original carbs, you may be able to arrive in a good place by experimenting with different jet sizes before going after something that could complicate your effort. You may be surprised that one size or two change of the mains, and pilot circuit could take care of it, and get away without permanently altering the carbs.


The airbox acts as a plenum chamber, as well as affecting the carb mixture - so two unknowns here.

If the intake is less restrictve now then lifting the needle, upjetting the main jet and experimenting with the pilot airscrews will be the solution. This carb tuning can be time consuming, do things one at a time and feel the difference - even if it makes things worse.
Lifting the needles would be a good start - if non adjustable they can be shimmed with tiny washers.

These run great on pod filters too, but takes a bit of experimentation to get it right. I didn't use dynojet.
1980 Gpz550 D1, 1981 GPz550 D1. 1982 GPz750R1. 1983 z1000R R2. all four aces

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