Unrestricting Zephyr 750 C1 - Page 2 - Kawasaki Motorcycle Forums
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post #11 of 33 (permalink) Old 08-15-2019, 01:21 PM Thread Starter
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Originally Posted by davidr1963 View Post
You could try raising the needles a couple of notches, it sounds like it’s being starved of fuel in the mid range.
Sorry, its vacuum carbs so theres no needle adjustment (if im not being outright stupid, as i noticed no notches on the needle). Please check my latest post #10, for picture of spark plugs. Thank you

Last edited by TimOfSweden; 08-15-2019 at 01:25 PM.
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post #12 of 33 (permalink) Old 08-15-2019, 02:41 PM
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Those plugs look very similar to my GPz750 (same engine). My bike uses cv carbs too but the needles are adjustable.

Iíve just looked on partzilla and your needle, as you say, doesnít appear to have adjustment so it looks like youíll need larger needle jets.
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post #13 of 33 (permalink) Old 08-15-2019, 04:13 PM Thread Starter
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Thank you! Did you leave your GPz750 unadjusted as it is with the plug colors?

I try read up on jets and from this picture



It seems I should try going one size larger needle jet and main jet, or is it sufficient with just needle jet? I've never tinkered with jets before.
Just for clarification, larger jet mean more fuel right?
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post #14 of 33 (permalink) Old 08-15-2019, 04:26 PM Thread Starter
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WHOA! When I consulted Haynes book it says..



There are no needle jets? Can anyone confirm? Is there no needle jet? Then its just only main jet I can change?
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post #15 of 33 (permalink) Old 08-15-2019, 04:29 PM
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Iíve cone filters on the one Iím using and the main jets are 145 (110 standard). Mine are Mikuni carbs and I think yours are Keihin. Theyíre the same in principle but slightly different in practice. Your issue sounds like mid range fuel starvation which means you need to raise the needle or fit a larger needle jet. I must say that Iím no expert though and itís all fairly theoretical but makes sense to my engineers brain.

As you donít have adjustment in your needles, can you buy different size needles in the same way you can buy different size mains? Iím pretty sure that you can get different size needle jets.
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post #16 of 33 (permalink) Old 08-15-2019, 04:31 PM
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Originally Posted by TimOfSweden View Post
WHOA! When I consulted Haynes book it says..







There are no needle jets? Can anyone confirm? Is there no needle jet? Then its just only main jet I can change?


No, there are needle jets; itís just the information re size thatís not available.
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post #17 of 33 (permalink) Old 08-15-2019, 04:43 PM Thread Starter
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No, there are needle jets; itís just the information re size thatís not available.
I thought also so, however when I look for parts for Keihin CVK 32mm carburetors there are no needle jets to be found anywhere (at least in the swedish parts sellers). So it could be that there are actually no needle jets

I'll pull out the carbs and open the bowl to access the jets and see whats going on there. Also, I will contact a carburetor guru in Sweden and ask him a few questions. Will definitely be back for updates
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post #18 of 33 (permalink) Old 08-15-2019, 05:07 PM Thread Starter
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I was thinking... the problem shows when WOT. So... if WOT regardless of rpm range, and it stutters during mid-rpm range then its WOT related problem. Therefore main jet and main air jet that controls the flow during WOT. Concluding, I should try going bigger main jet and smaller air jet... (or am I mad?)
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post #19 of 33 (permalink) Old 08-15-2019, 06:28 PM
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The Zephyr's are not "oil and air cooled"..the engine is only air cooled. The oil cooler only helps with the extra heat picked up by the wet clutch, which is cooled by the oil, hence the term 'wet'.

...I presume the bike has original air box, and stock exhaust,. if it does not, then it is extremmely hard making it run smoothly...very, very hard......obviously; it does not have the stock needles, but could you verify the pilot jet, and main Jet sizes??

In the US the outboard cylinders 1, and 4, main jets have #88, and the two center ones 2 and 3, have #90 jets. And these are very conservative figures to satisfy extremelly stringent pollution regulations with everything stock.

The pilot jets are #35 in all fours, I personally run #38.. The needles are also different; N52A on #1 and #4, and N52N in #2 and #3. The theory is the the carb runners being straight shorter in the two center cylinders provide more air than the two outboard ones, with longer runners.

The Kehin CVK 32 where used in a variety of engines, including off road 4-wheeler, thus, the jets and needles are very different depending on every applications. Even the last KZ750 in the US, the '90s ZR-7 in North America had same CVK 32s, but very different jets and needles from the Zephyr 750, only because it had a 4 into 1 exhaust.

Last edited by Hugojose; 08-15-2019 at 06:38 PM.
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post #20 of 33 (permalink) Old 08-16-2019, 01:00 AM Thread Starter
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Originally Posted by Hugojose View Post
The Zephyr's are not "oil and air cooled"..the engine is only air cooled. The oil cooler only helps with the extra heat picked up by the wet clutch, which is cooled by the oil, hence the term 'wet'.

...I presume the bike has original air box, and stock exhaust,. if it does not, then it is extremmely hard making it run smoothly...very, very hard......obviously; it does not have the stock needles, but could you verify the pilot jet, and main Jet sizes??

In the US the outboard cylinders 1, and 4, main jets have #88, and the two center ones 2 and 3, have #90 jets. And these are very conservative figures to satisfy extremelly stringent pollution regulations with everything stock.

The pilot jets are #35 in all fours, I personally run #38.. The needles are also different; N52A on #1 and #4, and N52N in #2 and #3. The theory is the the carb runners being straight shorter in the two center cylinders provide more air than the two outboard ones, with longer runners.

The Kehin CVK 32 where used in a variety of engines, including off road 4-wheeler, thus, the jets and needles are very different depending on every applications. Even the last KZ750 in the US, the '90s ZR-7 in North America had same CVK 32s, but very different jets and needles from the Zephyr 750, only because it had a 4 into 1 exhaust.
Thank you! Yes it's stock airbox and filter, also stock 4-2 exhaust. I will verify the sizes soon as possible.
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