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Jet needle improves acceleration. 09 Jul 2011 00:20 #461796

  • calvin17d
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120 mains
Stock pilots
Pods
4 into 1
Timed
Clearances set.
Strong spark
Carbs cleaned twice
Float levels dialed

Have the bike running very smooth off idle and performing quite well at the top. Not shabby, but acceleration was a bit sluggish from take off and aggressive mid range throttle openings. I decided to raise the needle to the 4th notch.

Result:
Bike still running great with a very noticeable improvement in acceleration and smoother operation through higher rpm's. Unfortunately it seems that the high speed accelerations have tapered off?

Would you think that the top just seems more flat now that midrange has more snap, or is the new height of the needle allowing too much fuel through the 120s at the high end of midrange.

I am considering trying the 115s with this new needle position .......

Thoughts?
1976 Kz900 A4

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Jet needle improves acceleration. 09 Jul 2011 02:50 #461815

  • testarossa
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Don't use the 115's IMO. That's two sizes down. Try 117.5's if you like, but the reality is that you need to get the main jet size correct first, and then set the needle clip position.

Are your needle jet and jet needle new? Needle jets get worn by the jet needle "filing" on them due to constant up and down slide movements. The constant in-rush of air pulls the needle against the front of the needle jet. It results in an oblong orifice in the needle jet which leads to all sorts of tuning issues.

It's common to need to increase pilot jet size by one for pods and a 4-1 exhaust. Raising the needle may be masking a pilot jet that is too lean. Don't forget that all of the jets affect each other.
1978 KZ1000 A2 Click--->Build Thread
2004 ZX-10R
2007 Harley Sportster 1200
2020 Harley Street Glide Special
Angola, IN

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Jet needle improves acceleration. 09 Jul 2011 04:03 #461819

  • calvin17d
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I am feeling good about the 120s in the main. They are on the cold side of optimum but that is where I like them for safety sake. I agree 117.5 are probably the ticket. The needle and there jets are not new but have no significant indications of wear.

I tried poking around the forum to get a feel for thoughts on raising the jet needle. If in fact the needle jet were worn I would assume a rich condition. As stated my issue was with a lean quarter to 3/4 throttle.

Where to starr first in Attempt to reduce excess spending ? Pilots?
1976 Kz900 A4

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Jet needle improves acceleration. 09 Jul 2011 05:37 #461822

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There is no sure fire way to reduce spending on a tuning issue. You have to have the different jets to play with, but if you're happy with the way it's running and the plugs read okay leave it alone.
'78 KZ1075 LTD
stage 1 head by Larry Cavanaugh
race built crank by John Pearson
Mikuni rs34's
k410 cams
back cut tranny
8" over D&G swinger
proving once again that age and treachery is better than youth and enthusiasm

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Jet needle improves acceleration. 09 Jul 2011 08:38 #461832

  • mark1122
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in order to test the mains for correct size. run it at full throttle, then back out to 3/4's . if u feel or hear a stumble /stutter , or it gets better, your main is not correct. if it gets better at 3/4,your main is too big
it helps to put some tape on the grip, and mark off the 3/4 throttle position.
to fine tune the main u can ride at say, 50mph., then time how long it takes to accelerate to 70mph. at full throttle. do this test with dif. mains until u find the shortest time.
do all tests on a flat section of rd (or all on the same rd.), so u dont distort the test.
Then u can fine tune the needle.
Have fun.
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.
01 CBR600F4i Track bike.
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Last edit: by mark1122.

Jet needle improves acceleration. 09 Jul 2011 09:02 #461840

  • Patton
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Another option to help determine best main jet size is the throttle chop test.

This allows spark plug reading immediately after a run at wide open throttle where ignition is cut and clutch simultaneously disengaged (then brake/coast to a stop).

Air/fuel mixture at wot is governed by the main jet.
At wot, jet needle function within the needle jet is immaterial.

The plug readings show results of the air/fuel mixture being produced by the particular main jets inside the combustion chambers at full throttle.

Spark plug readings are reportedly more difficult when burning modern fuel, at least a longer test run is needed for adequate coloration.

Good Fortune! :)
1973 Z1
KZ900 LTD

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Jet needle improves acceleration. 09 Jul 2011 09:07 #461842

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Patton wrote: Another option to help determine best main jet size is the throttle chop test.

This allows spark plug reading immediately after a run at wide open throttle where ignition is cut and clutch simultaneously disengaged (then brake/coast to a stop).

Air/fuel mixture at wot is governed by the main jet.

The plug readings show results of the air/fuel mixture being produced by the particular main jets inside the combustion chambers at full throttle.

Spark plug readings are reportedly more difficult when burning modern fuel, at least a longer test run is needed for adequate coloration.

Good Fortune! :)


i have found this to be the case myself. the fuel today does not color the plugs the same, so u end up running longer to get color. then the question becomes, HOW LONG.
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.
01 CBR600F4i Track bike.
Cobourg,Ont.Can.

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Jet needle improves acceleration. 09 Jul 2011 15:34 #461896

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Surprising results from this mornings adrenaline overdose and plug chop!!!!

Taking into account the diameter of the bore (26mm) I found a socket with diameter 19.5 mm = distance from bottom of bore to a 3/4 throttle opening (19.5/26) = .75 or 3/4. I then taped and marked a line on the switch block and matched lines on the grip for respective throttle openings.

When I was marking the grip I didn't realize I'd be running upwards of 110 mph to achieve greater than 3/4 throttle. Thanks for the treat!

Killing the engine at 110 I coasted to safety and after a few deep breaths I checked the plugs which were all extremely lean. I don't ride like this very often but performance did not seem much of an
issue. No backfiring, missing , etc.

Even still in the 1/4 to 1/2 throttle range I was lean as before. Not a spec of carbon or soot. My last chop at 1/8-1/4 openings were starting to show
signs of color. It was not until I drove around town accelerating, stoplights, etc that the plugs went rich.

Was surprised to see the mains 120's running
lean?

Chops were taken at 2-3 me intervals
1976 Kz900 A4

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Last edit: by calvin17d.

Jet needle improves acceleration. 09 Jul 2011 15:57 #461898

  • mark1122
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how do u know how long to run before doing the plug chop? the longer u run the more they will show color, and u are only interested in color at the base of the plug. it will be a dark ring right at the base. a white plug tip after 1 pass means nothing.
how does it feel? do u have any stutters or misses? if not, then u can fine tune.
if i were u i would go by feel not plug color. use the timed test i posted.
did u try backing out of full throttle to 3/4? any isuse 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.
01 CBR600F4i Track bike.
Cobourg,Ont.Can.

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Last edit: by mark1122.

Jet needle improves acceleration. 09 Jul 2011 16:15 #461899

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For carb tuning considerations, if not already aware ---

Main jet size has no effect on air/fuel mixture until after 3/4 throttle, and has most effect at wot where the jet needle is high enough in the needle jet to allow all mixture available from the main jet to pass through the needle jet.

Jet needle taper and position in the needle jet (plus slide cutaway) governs air/fuel mixture from 1/4~3/4 throttle.

The pilot jet governs air fuel mixture from idle~1/4 throttle.
Up to 1/4 throttle position on large Kaw fours produces a surprisingly brisk road speed.

There is a progressive transition as the jet needle takes over from the pilot jet, and another progressive transition when the jet needle thereafter surrenders to the main jet (as the main jet supplies the needle jet absent limitation by the tapered jet needle).

Pods are typically more difficult to tune (than the stock air box) over the entire range of carb operation.

Good Fortune! :)
1973 Z1
KZ900 LTD
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Jet needle improves acceleration. 09 Jul 2011 17:58 #461906

  • bountyhunter
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When I got new headers and was screwing with main jets, I would go WOT and could feel the engine "bog" a little when the mains were too rich as the engine went to about 5k and above (mine redlines at 7500). As said, if it's too rich, you will feel it bog as you go to WOT and it will "perk up" a little if you close the throttle a touch.

I selected pilots as follows: good response at 1/8 throttle. On mine, I would put it in first gear and try to hold the engine exactly at 2000 RPM. If the pilot was too big, the engine would surge. Too lean, and it would feel weak and pop a little. A lean pilot also makes it pop when you let off the throttle while cruising. If you let off and hear bang bang, the pilot amy be too lean.
1979 KZ-750 Twin

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Last edit: by bountyhunter.

Jet needle improves acceleration. 09 Jul 2011 18:13 #461910

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bountyhunter wrote: I selected pilots as follows: good response at 1/8 throttle. On mine, I would put it in first gear and try to hold the engine exactly at 2000 RPM. If the pilot was too big, the engine would surge. Too lean, and it would feel weak and pop a little. A lean pilot also makes it pop when you let off the throttle while cruising. If you let off and hear bang bang, the pilot amy be too lean.


i use this method as well, but i add a little. i will cruise at 2000,3000,4000. as i tune i can feel it get better at a particular rpm, and i keep going untill all rpms are smooth. take a note pad with u and write everything down.
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.
01 CBR600F4i Track bike.
Cobourg,Ont.Can.

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