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Johnny Blue Lightnin'
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200 miles on a tank is about when I filled mine up. The faired ZR-7S seemed to do a little better on gas as mine was naked. It also depends on your riding style, condirions, and even fuel quality. When I rode south to the mountains away from the overpopulated area with smog induced high alcohol containing gas I always picked up about 2 mpg.
 

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if you measure the two needles you will find that the N4MD used in the inner carburetors are about 2mm longer than the N6NC.
This seems backwards because the inner cylinders should be run a little richer to cool them better.
I believe the mixture is richened up on the inner cylinders by the use of emulsion tubes with 18 holes compare to 14 holes for the outside cylinders.
Just to clarify, the needles in cylinders 2 & 3 are LONGER than 1 & 4

and the reason for them being longer isn't to cool the innermost cyclinders it has to do with the air pressure through the carbs. As the air passes through the airbox into the carbs the path of least resistance is that of carbs 2 & 3, therefore petrol is attomised and drawn up easier than in carbs 1 & 4.

to match the same flow at the lower pressure of carbs 1 & 4 a bigger opening is required for a given throttle position, which is what the different needle lengths and tapers replicate.

It's strange then that all the aftermarket jet kits make them all the same length :( maybe they are just concerned with full throttle max power than drivability.

I've looked into this at great depth as it's always stumped me on why the carbs are different, nice to finally find an answer I'm happy with so thought I'd share :)
 

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Just to clarify, the needles in cylinders 2 & 3 are LONGER than 1 & 4

and the reason for them being longer isn't to cool the innermost cyclinders it has to do with the air pressure through the carbs. As the air passes through the airbox into the carbs the path of least resistance is that of carbs 2 & 3, therefore petrol is attomised and drawn up easier than in carbs 1 & 4.

to match the same flow at the lower pressure of carbs 1 & 4 a bigger opening is required for a given throttle position, which is what the different needle lengths and tapers replicate.

It's strange then that all the aftermarket jet kits make them all the same length :( maybe they are just concerned with full throttle max power than drivability.

I've looked into this at great depth as it's always stumped me on why the carbs are different, nice to finally find an answer I'm happy with so thought I'd share :)
Very interesting observations...
So far I've been lucky with my bike. No carburettion issues etc.-- I have the stock OEM exhaust with a K&N air filter.
I get around 40-41 mpg in NYC stop-n-go traffic and about 50mpg when I'm out of the city. (that's US gallons).
 

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Discussion Starter #84 (Edited)
Just to clarify, the needles in cylinders 2 & 3 are LONGER than 1 & 4

and the reason for them being longer isn't to cool the innermost cyclinders it has to do with the air pressure through the carbs. As the air passes through the airbox into the carbs the path of least resistance is that of carbs 2 & 3, therefore petrol is attomised and drawn up easier than in carbs 1 & 4.

to match the same flow at the lower pressure of carbs 1 & 4 a bigger opening is required for a given throttle position, which is what the different needle lengths and tapers replicate.

I've looked into this at great depth as it's always stumped me on why the carbs are different, nice to finally find an answer I'm happy with so thought I'd share
Hello TWOQUIDKEV,

Interesting idea concerning different flow resistance thru the inner and outer carbs. I'm still thinking this over, but my initial reaction is that this would not explain the different needle lengths, since these are CV carbs. If carbs 2 & 3 have higher air flow rate the diaphragm above the carb throat would feel the lower throat pressure caused by the higher flow rate and pull the needle out further to adjust the air-to-fuel mixture ratio appropriately for the higher air flow rate. That is the nice thing about CV carbs, the mixture is automatically adjusted in each carb for air flow rate and air density by sensing the pressure in the throat of the carb.

Question, in your research on this topic did you ever read anything concerning the different number of holes in the emulsion tubes affected the fuel atomization and mixture? I had noted that the emulsion tubes for carbs 2 & 3 have more holes than the ones for carbs 1 & 4, but have never uncovered any information on their design that might enlighten us as to why they were used in combination with the slightly longer needles.
 

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Hello TWOQUIDKEV,

Interesting idea concerning different flow resistance thru the inner and outer carbs. I'm still thinking this over, but my initial reaction is that this would not explain the different needle lengths, since these are CV carbs. If carbs 2 & 3 have higher air flow rate the diaphragm above the carb throat would feel the lower throat pressure caused by the higher flow rate and pull the needle out further to adjust the air-to-fuel mixture ratio appropriately for the higher air flow rate. That is the nice thing about CV carbs, the mixture is automatically adjusted in each carb for air flow rate and air density by sensing the pressure in the throat of the carb.

Question, in your research on this topic did you ever read anything concerning the different number of holes in the emulsion tubes affected the fuel atomization and mixture? I had noted that the emulsion tubes for carbs 2 & 3 have more holes than the ones for carbs 1 & 4, but have never uncovered any information on their design that might enlighten us as to why they were used in combination with the slightly longer needles.
The part about the flow (speed) and pressure is true, and therefore also for the part BEFORE the carb throat, maybe that needs to be compensated by the needles/different setups?
If you look at the airbox design, this is not a sample of optimised flow... the filter is in the middle, so the air flow tends to go straight ahead... being the path of least resistance.
The two outer carbs have other flow paths, localised pressure differences might occur.
 

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I never read anything specific but just thought the design of the tubes and needles must be to compensate for the different airflow.
 

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Thanks so much for your info bolserst, i too have the issues you had and my question to everyone is has anybody put pods on and removed the airbox? Its truely a pain in the *** and im used to pods from my old 79 KZ1000 LTD. Im sure i would have to go up a bunch in the jets but my other question that goes with this is that im at about 4k ft elevation and i have 108s installed. In anybodys travels have you figured a good jet size for the elevation? My bike is stock but i would love to put on a Jardine and pods to get rid of the airbox crap. Any help would be very appreciated.
 

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Discussion Starter #88
my other question that goes with this is that im at about 4k ft elevation and i have 108s installed. In anybodys travels have you figured a good jet size for the elevation?
Fortunately, the ZR-7 uses CV carbs, so the change in air density is sensed by the diaphragms and adjusts the needle height to keep the fuel/air mixture pretty darn constant even with large altitude changes. So, whatever main jets you are happy with at sea-level, you should be happy with at other altitudes. Now, you may need to adjust your pilot screws in a bit to lean the idle mixture out at higher altitudes since this feeds a fixed fuel quantity at idle no matter what the air density.
 

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I've read the whole thread, and find it very interesting stuff! At this point my ZR is not very economical with the fuel (I get about 200 miles out of a full tank, not sure what that does in MPG, but it mean I need 1 litre to go about 17 km (just over 10 mile per 1 litre).
and i hear and read stories about the super fuel efficient ZR7.. doing up to 23 km per 1 litre) i do know my bike runs rich, but not overly so.

one of the things i do no like is working on a product that has already been over used. In this case the carbs, they have been on this bike for 40.000 miles and i suspect the main needles, tubes that house them and some other parts to be fairly used up (worn). and there for hard to tune correctly. This made me want to go out there and buy a overhaul kit for the carb (needles, tubes, gasket, springs.. the whole nine yards).

BUT... so far no luck. all i could find was a pilotscrew kit with a float-bowl-gasket and float-valve. no needles, no tubes and so on.

is there someone here that has done this before and has like "the complete list" of part numbers or a address where one could obtain these parts?
200 miles out of a tank isn't far off, I hit reserve between 200-230 depending how I ride it, here are my mpg figs
52mpg
55mpg
53mpg
54.34mpg
49.99mpg
58mpg
52.31mpg
58.65mpg
58.20mpg
56mpg
51mpg
its easy to workout with the mpg websites on the net, just google for one for your country
this is the one I use for the uk
MPG Calculator - Miles to the Gallon - Calculate my MPG
 

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anyone have a copy of the airbox mod you get in the ivans jet kit?
I would like to see where the holes have to be drilled
thanks
 

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WOW, great posts!!! Thanx everyone for the good info, because I just bought a bank of 2000' Zr7 carbs for my 93' Kawasaki Zephyr 750.....(predecessor of Zr7), and unfortunately they have been operated on before. I found 1 bent needle that appeared to have been run over by a truck, 3 homemade washers under only 3 of the needles, and emulsion tubes with less holes in 3&4 and the 1's with more holes in 1&2.

Unfortunately, I will not be able to get a new needle anytime soon, so I'm going to use the new carb bodies, diaphragms, springs, float needles, and floats, and I'll install the old "zephyr" main jets, pilot jets, pilot screws, and needles.
Also, I wonder if the bigger needles and jets from the Zr7 would even work ok in the Zephyr? If the engines are the same, then I don't see why not. But if the engines are the same, then why change to bigger diameter and shorter needles on 1&4 carbs and longer on 2&3 carbs, and why are the jets bigger? Is it just temperature/emission control measures and also the stock 4 into 1 exhaust?

I wonder if my zephyr would benefit from bigger jets and different needles? What do y'all think?
 

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The ZR7 does have different parts in carbs 1 & 4 verses 2 & 3 for temperature/emission I believe
carb zr7.JPG
 

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The stock carbs do have different needles and tubes. They are not all the same. I can't speak to modified carbs.

The tubes in carbs 1 & 4 are the same with 14 holes. Needles are marked N6NC.
The tubes in carbs 2 & 3 are the same with 18 holes. Needles are marked N4MD.

My understanding is the outer ones are different to compensate for the fact the air has to travel further to the carbs from the airbox thus affecting vacuum and fuel atomization.

carb_needle_needle_jets_100p.jpg
 
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Correct, and to compensate for higher cylinder temperatures than #1 and #4 cylinders, which are kept cooler because of better air flow for cooling.
 

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I’ll get the apology out of the way first. I owned a 2002 ZR-7 for several years. This bike ran great right out of the box, no hesitation issues. Factory pilot screw settings were 1 1/4 turn out on all 4 carbs. I bumped them up to 1 3/4 turns as per the service manual and noticed the bike being much more eager off the line. Having read all the discussion about jetting on this forum I figured you guys were either nuts, or just needed something to complain about.

My sincere apologies…I now understand.

I picked up a 2001 ZR-7 as my replacement bike. This bike has HUGE amounts of hesitation at anything below 1/4 throttle position and an erratic floating idle. Also I noticed lean surging at cruise with throttle settings below 1/4. I removed the carbs and spent a day cleaning and checking them. Everything was stock and spotless, except one of the pilot jets was partially plugged, but I got it cleared out. The pilot screws were set at 0, 1/2, 0, 1/2 . I also noticed that when completely closed, the #2 pilot screw protruded nearly 1mm further in to the carb throat than the other 3. I set them all to 2 turns out. Idle was now solid, but the hesitation and lean surging persisted. I also noticed that the header pipes are blue where as my 2002 pipes had a nice straw color. I used the carb spray test to check for any airleaks around the airbox, carb boots, etc. None found. Once throttle gets past 1/4 things are pretty much as I remember on my other ZR-7, a nice smooth pull to 7k, and then a slower climb from there.

HOW CAN 2 “identical” unmodified completely stock bikes run SOOOO differently? Has any root cause been identified for this variance in jetting requirements for the ZR-7?

What I have done:
I used tape on the throttle grip to mark off 0 thru 100% throttle positions so I could identify the areas where hesitation occurred. As I mentioned above, all hesitation occurs at less than ¼ throttle. Most prominent at 1/8. As suggested elsewhere, I plugged up 1/3 of the intake and the bike ran great at low throttle settings but bogged down above 1/4 throttle. Based on these observations I ordered #38 pilot jets to richen up the pilot circuit. The bike is running better at idle and pulls a little better off the line(don’t have to rev and slip the clutch to get moving) but still hesitates at 1/8 throttle and some lean surging at cruise persists. The pilot screws are at 2 turns out except #2 is at 2 1/2. I set this based on pulling plug wires at idle and looking for equal RPM drop for each cylinder. With #2 set to 2 turns like the rest, the RPM didn’t drop at all when #2 plug wire was pulled. Smoothed out the idle a bit. Oh, and of course a careful carb sync has been down.

What I will be doing:
1) Tonight I will be trying 2 1/2 turns out on all carbs(maybe 3 on #2). This will probably be too rich at idle as blipping the throttle already results in RPM dropping slightly below idle momentarily and then recovering.
2) I have an dyno appointment for tomorrow to get some fuel/air mixture reading information. I will post my findings.

Questions:
1) Has anybody ever needed to use a pilot jet larger than #38 to cure hesitation?
2) I am beginning to think that the larger pilot jet is just helping out the real problem of a too lean setting for the straight section of the stock jet needle. Is it possible that there is a variance in stock needle diameter? I will measure mine next time I have them out.
3) Has anybody tried to modify the straight section of the stock needle to richen up the mixture for low throttle settings? I have done this before on a jetski carb that had problems with lean running. Of course it only had 1 carb.


Thanks for listening,
Steve Bolser
Saginaw, TX
Hi, Just found your excellent post regarding the ZR7(S) carb modifications. I retired my first 00 ZR7 at over 125K miles and have 92K on my 01 ZR7S. My 3rd one, an 03 with 6K miles is getting revived after years idle by previous owner. The only carb mods were idle mixture screws(2-2.5 turns out) and disconnecting the gas tank evaporator hoses. The rest of the bike is completely stock. From your research and testing I gather the following: keep pilot jet stock, change main jets to 105 or 103, and modify jet needles-remove approx. 1 mm. I use my bike for commuting and want better driveability with little or no fuel economy loss. Do you have a picture or diagrahm of the jet needle modification areas. I have spare carb parts to experiment. Thanks for your lengthy and detailed testing and development.
 

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Hi, Just found your excellent post regarding the ZR7(S) carb modifications. I retired my first 00 ZR7 at over 125K miles and have 92K on my 01 ZR7S. My 3rd one, an 03 with 6K miles is getting revived after years idle by previous owner. The only carb mods were idle mixture screws(2-2.5 turns out) and disconnecting the gas tank evaporator hoses. The rest of the bike is completely stock. From your research and testing I gather the following: keep pilot jet stock, change main jets to 105 or 103, and modify jet needles-remove approx. 1 mm. I use my bike for commuting and want better driveability with little or no fuel economy loss. Do you have a picture or diagrahm of the jet needle modification areas. I have spare carb parts to experiment. Thanks for your lengthy and detailed testing and development.
Hi KawAir4S, and welcome to the forum. You may not get much of a reply as this thread is over 4 years old and Bolserst hasn't been on here according to his profile since June 2018. Sadly there's not very many of us 7 /7s folks active here anymore. There are lurkers about - harmless souls, but they never add anything :)
 
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