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Discussion Starter #1
Hello,

I am from Sweden and the bike is also bought there. I found out when reading the registration papers of the motorcycle, that it is restricted from 53 kW (71bhp) to 37 kW (49bhp)by restriction added to carbs. I also found out that ALL Zephyrs sold in Europe were restricted due to emission standards to be met.

I tried to look for cable, shims, draughts etc restriction and found none. Until I checked the diafragm slide and I found out there is 4 holes drilled in there, two at each side. When consulting Haynes book and/or looking at spare parts in US I do not see any hole in the slide. They're also very expensive to afford buying new spares from US. So, my plan, is to add chemical metal and plug those holes. However, should I plug all those 4 holes or is it just one of them? I become unsure which holes I should go for.

I will add two photos, I hope you can help my Zephyr to breathe real wind again!




A bit dark on the right side, but its a hole there too. This side faces the valves


This side face airbox


There is also a hole at the bottom however it's as it should be, from what I gather.

Thanks in advance!
 

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Discussion Starter #4
I bought chemical metal and goddamnit the fumes was noxious and probably not good for health. However added it and it dried metal hard almost at once.




I'll let them dry over the night, tomorrow, I will mount it all back and go for a test ride!
 

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Discussion Starter #6
Sounds good. Hopefully that "chemical metal" is fuel resistant and doesn't dissolve.
You may have to make some carburetor adjustments to account for the plugged holes.
It is fuel resistant and doesn't melt until temperature is over 160 degree Celsius (320 degree Fahrenheit).
I might, will have to ride first and check plugs afterwards.
 

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No, in the US they do not have the 4 holes. The one in the bottom is only an indentation, not a hole, I presume to indicate the orientation of the slide. The indention should face the intake side.
 

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Discussion Starter #8 (Edited)
I put everything back in, went out for test ride. Let it warm properly first. And then tried WOT, results:

positive

# It revs much faster when using clutch or no gear in.

# Very aggressive rev/acceleration when in high range rpm.

negative

# Much more apparent stuttering (hestitation) in midrange, its like its starved for fuel when WOT around 4-5k rpm. With that I mean it is not smooth power transistion as when above 8k rpm.


What can I learn from this? Is jetting bad for such fast power surge when wot?
 

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Discussion Starter #10
I put everything back in, went out for test ride. Let it warm properly first. And then tried WOT, results:

positive

# It revs much faster when using clutch or no gear in.

# Very aggressive rev/acceleration when in high range rpm.

negative

# Much more apparent stuttering (hestitation) in midrange, its like its starved for fuel when WOT around 4-5k rpm. With that I mean it is not smooth power transistion as when above 8k rpm.


What can I learn from this? Is jetting bad for such fast power surge when wot?
Countinuation of latest reply!

Ok, went out for another ride. Still same symptoms, motorcycle do not bog. It's just like when riding against heavy winds. Acceleration do not have linear power output all the way, there is weakness there and there. As if it would be starved and not getting full fuel power as needed. Mid-range is where problem lies. No hestitation or bogging when idling and wot.

Pulled out the spark plugs and they look like this;



I know Zephyr 750's are oil and air cooled, so they generally runs hotter than nowadays fluidcooled machines. However if you ask me, I would say it runs too lean/hot. I think the carb setup is original/untouched.

However it's just a laymans give and take.

So before going to clean carbs and s*** which is just bothersome, I would like to add S1 injectioncleaner and rejet to make it go richer.

My questions, what jetting should I go? You who have no holes in slide in US, which jetting? Help me!
 

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Discussion Starter #11 (Edited)
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
 

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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.

I’ve just looked on partzilla and your needle, as you say, doesn’t appear to have adjustment so it looks like you’ll need larger needle jets.
 

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Discussion Starter #13
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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Discussion Starter #14
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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I’ve cone filters on the one I’m using and the main jets are 145 (110 standard). Mine are Mikuni carbs and I think yours are Keihin. They’re 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 I’m no expert though and it’s all fairly theoretical but makes sense to my engineers brain.

As you don’t have adjustment in your needles, can you buy different size needles in the same way you can buy different size mains? I’m pretty sure that you can get different size needle jets.
 

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Discussion Starter #17
No, there are needle jets; it’s just the information re size that’s 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 :confused:

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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Discussion Starter #18
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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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.
 

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Discussion Starter #20
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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