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TOPIC: Jet needle improves acceleration.

Jet needle improves acceleration. 10 Jul 2011 22:51 #462156

  • Patton
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calvin17d wrote: ...Am feeling like a smaller than stock pilot could help reduce the excess fuel at idle that has come at the hand of the raised needle....


Jet needle clip position has no effect at idle.
Raising the jet needle does not contribute additional fuel at idle.

The pilot circuit governs air/fuel mixture at idle.
Application of choke adds more air/fuel at idle.

Btw, who's on first? :lol:

Good Fortune! :)
1973 Z1
KZ900 LTD

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Jet needle improves acceleration. 11 Jul 2011 01:55 #462187

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I must have been fouling the plugs while making the screw adjustments. Or something else was making it smoke.

I put the needle back to stock and put in an order for a larger pilot. I had heard the pilot increase recommendation in the past but before knowing how critical they can be to operation I decided to shine it. Perhaps this will provide the the correct amount of fuel when needed.

Thanks for all the help. I am almost there. It runs so well and smooth from mids up. Just need to smooth out that low end transition?
1976 Kz900 A4

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Last edit: by calvin17d.

Jet needle improves acceleration. 11 Jul 2011 03:24 #462191

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calvin17d wrote: I did not get a chance to make any adjustments to the first run on the first gear test. I was unclear on how this should be done? If I had a rich result (surging) during my first gear test, would I simply turn the screws out uniformly and test again?

The screws only adjust the idle mixture. My carbs have a "pilot bypass" which is fed by the pilot jet but unadjustable. That is what runs mainly in the region just above idle at slight throttle opening. hence, the 1500 - 2k running at lightest throttle tests the pilot jet regardless of screw setting.

BTW: make sure the fuel levels are correct as that really affects low end response and pilot jet operation. Also, do a mechanical sync on the throttle plates to make sure they open the same amount. That can really screw up low end. I just use the smooth end of very small drill bits to set the throttle plates open a touch to sync the idle positions.
1979 KZ-750 Twin

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Last edit: by bountyhunter.

Jet needle improves acceleration. 12 Jul 2011 11:19 #462441

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bountyhunter wrote:

calvin17d wrote: BTW: make sure the fuel levels are correct as that really affects low end response and pilot jet operation. Also, do a mechanical sync on the throttle plates to make sure they open the same amount. That can really screw up low end. I just use the smooth end of very small drill bits to set the throttle plates open a touch to sync the idle positions.


I have spent quite a bit of time working on both of these. I used a sewing needle to sync the carbs to the clymer spec'd height.

1976 Kz900 A4

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Jet needle improves acceleration. 12 Jul 2011 11:42 #462446

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calvin17d wrote:

bountyhunter wrote:

calvin17d wrote: BTW: make sure the fuel levels are correct as that really affects low end response and pilot jet operation. Also, do a mechanical sync on the throttle plates to make sure they open the same amount. That can really screw up low end. I just use the smooth end of very small drill bits to set the throttle plates open a touch to sync the idle positions.


I have spent quite a bit of time working on both of these. I used a sewing needle to sync the carbs to the clymer spec'd height.


As known, with manual slide carbs, the "bench sync" matches opening heights of the throttle slides at their lowest positions, which achieves a basic beginning position for the later "engine-running sync" using a manometer or vacuum gauges.

The engine-running sync equalizes performance among the cylinders, which usually results in slight differences in throttle slide heights of the carbs.

Good Fortune! :)

1973 Z1
KZ900 LTD

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Last edit: by Patton.

Jet needle improves acceleration. 12 Jul 2011 14:36 #462469

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Most differences in engine vacuum arent as slight as one would think.
The vacuum requirements of each cylinder are off by more than one can imagine.

The bench sync is almost uselesss except to just get the old engine to run. A proper vacuum corrected manometer sync is considered mandatory basic maintenence for proper operation, and not performing one leaves you guessing what other issues are at hand.

It doesnt matter how close you make those throttles/slides off the bike, either with micrometers, by eye, whatever - they are required to be at slightly different heights once installed, and the engines demands are then seen with the manometer.

No more bench syncing! Its useless and ametuer.

Get a good set of vacuum guages even...

B)
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Jet needle improves acceleration. 12 Jul 2011 14:38 #462470

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Patton wrote: As known, with manual slide carbs, the "bench sync" matches opening heights of the throttle slides at their lowest positions, which achieves a basic beginning position for the later "engine-running sync" using a manometer or vacuum gauges.

The engine-running sync equalizes performance among the cylinders, which usually results in slight differences in throttle slide heights of the carbs.

Good Fortune! :)


I assume this would be best performed once all the proper jetting and mixtures have been achieved, and as the last touch on the carburetor tuning?
1976 Kz900 A4

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Jet needle improves acceleration. 12 Jul 2011 18:03 #462496

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calvin17d wrote:

Patton wrote: As known, with manual slide carbs, the "bench sync" matches opening heights of the throttle slides at their lowest positions, which achieves a basic beginning position for the later "engine-running sync" using a manometer or vacuum gauges.

The engine-running sync equalizes performance among the cylinders, which usually results in slight differences in throttle slide heights of the carbs.

Good Fortune! :)


I assume this would be best performed once all the proper jetting and mixtures have been achieved, and as the last touch on the carburetor tuning?


Thats the way i do it.

my 1135cc motor had vm33mm smoothbores. those carbs dont even have a spot to hook up a sync guage, and she ran sweet.
i'm NOT saying dont do a sync, just that i get everything else close first.
u may still be way off on jetting. but i think u are getting there.
76 kZ, frame gusset work,1200CC.Ported by Larry Cavanaugh,This email address is being protected from spambots. You need JavaScript enabled to view it., 1.5mm.over intakes,.ZRX12 rear end,and seat,96zx9 front end.
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Jet needle improves acceleration. 16 Jul 2011 21:34 #463224

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Well my special order on the pilot jets still has not arrived from the local shop. Ordering from Z1e hAs been more efficient.

Meanwhile I pulled the needle and put them back to stock position, after which I also mechanically syncd them. A test drive quickly revealed how the added fuel from raising the needle had helped so much. I was unable to achieve smooth operation but made the needle change in preparation of the larger pilots. I had previously used a home
Made sync kit to sync the carbs before all this needle swapping. It must have worked well as the bike has not ran this poorly in over a month. It may be time for vacuum gauges. I am quickly pissing my summer away. Any recommendations on a decent set that won't require a half months pay?

One other thing to note. My # 1 carb continues to run rich and is not influenced by seating the air screw. I have the pod filters off. Covering the intake for the pilot on all other carbs nearly kills the bike but doing so on #1 does nothing. Could this be sync related. I have verified clear pilot circuit by covering the air intake, the mix screw and pilot jet holes while blasting carb cleaner through. I alternated different hole plugging and spraying combinations at each hole and all were clear. I have also interchanged jets, needle valves and the choke plunger with no improvement in mixture response???? could this be a poor sync?
1976 Kz900 A4

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Jet needle improves acceleration. 16 Jul 2011 22:53 #463239

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calvin17d wrote: ... # 1 carb continues to run rich and is not influenced by seating the air screw....


Seating the screw is the richest position.

Good Fortune! :)
1973 Z1
KZ900 LTD

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Last edit: by Patton.

Jet needle improves acceleration. 16 Jul 2011 23:04 #463243

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I understand that. Allow me to clarify.
After cleaning with carb fluid thru the pilot jet hole, the idle air intake hole, and also thru the mixture screw hole and achieving carb cleaner spray thru the pilot circuit into the bore, I am still rich at idle on that one cylinder with the screw at 2-1.5 turns out. If I slowly turn the screw to seat and back out there is no change in idle. The other cylinder will nearly kill the engine when I do this.

I Am aware that this indicates an obstruction in the pilot circuit. Are there any other legs Of the circuit I did not mention above? I have cleaned 3 times the items listed Above.
1976 Kz900 A4

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Jet needle improves acceleration. 16 Jul 2011 23:35 #463247

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Sounds like the pilot circuit passages are clean.

And let's presume that the pilot jet is the correct size and in good condition and that the holes in the pilot jet are clear and have not been enlarged by over zealous cleaning.

What the heck, let's presume that the pilot circuit is perfect.
But still producing a too rich fuel mixture.

One reason might be a too high fuel level.

Another reason might be an imperfect enrichener circuit function which could be due to some malfunction of the choke plunger.

Yet another reason might be a dirty or obstructed pod filter (if pods are fitted), or possibly the pod rubber is partially blocking the pilot air entrance hole (as some less expensive pods are reportedly prone to do).

For the benefit of other readers, please see below for the pilot circuit cleaning method to which I believe you're referring.

Good Fortune! :)


1973 Z1
KZ900 LTD
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