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TOPIC: Jet needle improves acceleration.

Jet needle improves acceleration. 10 Jul 2011 11:50 #462029

  • calvin17d
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Driving in first and observing engine performance I noted surging most noticeably at 2K rpm's. It was however present at 1-3 & 4K as well. Approaching 5K the surge went away. I assume this was where a transition was occurring. Prior to heading out, I did in fact tune the mix screws as suggested. After Finding the optimum rpm at idle how would one begin tuning after a driving test that revealed a rich condition?

Lean all screws an 1/8 a turn and try the riding test again?

If I understand this, we are saying that a good idle tune of the mix screws may not directly translate to a good tune under load?
1976 Kz900 A4

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Jet needle improves acceleration. 10 Jul 2011 12:14 #462039

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calvin17d wrote: Driving in first and observing engine performance I noted surging most noticeably at 2K rpm's. It was however present at 1-3 & 4K as well. Approaching 5K the surge went away. I assume this was where a transition was occurring. Prior to heading out, I did in fact tune the mix screws as suggested. After Finding the optimum rpm at idle how would one begin tuning after a driving test that revealed a rich condition?

Lean all screws an 1/8 a turn and try the riding test again?

If I understand this, we are saying that a good idle tune of the mix screws may not directly translate to a good tune under load?


i feel that way.

as u richened the pilots during the 1st gear test, was it obvious that the bike was running smoother/better at some rpms? how many turns out were the srews before and after ther 1st gear test ?
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. 10 Jul 2011 12:31 #462044

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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?
1976 Kz900 A4

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Jet needle improves acceleration. 10 Jul 2011 12:34 #462045

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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?
1976 Kz900 A4

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Jet needle improves acceleration. 10 Jul 2011 13:01 #462049

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calvin17d wrote: Driving in first and observing engine performance I noted surging most noticeably at 2K rpm's. It was however present at 1-3 & 4K as well. Approaching 5K the surge went away. I assume this was where a transition was occurring. Prior to heading out, I did in fact tune the mix screws as suggested. After Finding the optimum rpm at idle how would one begin tuning after a driving test that revealed a rich condition?

Lean all screws an 1/8 a turn and try the riding test again?

YES , but i would encourage u to try it booth ways to be sure u are going the right way the first time. this is all about u learning what rich and lean feels like . don’t be afraid to go too far for experimental purposes so u can REALY FEEL the dif.

If I understand this, we are saying that a good idle tune of the mix screws may not directly translate to a good tune under load?

it usually does but obviously, not always. Also a carb sync helps to smooth things out, after jetting is done.

Not all Carbs have AIR SCREWS. Some have FUEL SCREWS, it is important to differentiate as to which one you have. This depends on the location of the screw on the carb. (looking at the center of the carb) If the screw is in between the cylinder and the carb body , it's a FUEL SCREW, turn it OUT to get more fuel (richer) if the screw is in between the carb body and the air box, it's an AIR SCREW, turn IN to richen it. As far as I know this rule of thumb is correct.

dont forget to take notes as u go. right down the adjustment (1/8th turn out)ask yourself did it get better or worse and a what rpms.
put tape on the bar grip. mark off closed ,full then 1/2, 1/4 1/8th throttle . when marking the bottom openings dont get fooled by the throttle play. take up the slack in the throttle cable by twisting the grip till snug, then mark off the lower sections on the tape. u will be surprised how fast u can go in 5th gear at 1/4 throttle.
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. 10 Jul 2011 14:44 #462070

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Just got back from the 1st gear test.

I started out making qtr turns at my first adjustment as my adjustment made for idle was rich and the bike surged through all rpm ranges. I then reached a point where I was about 2-1/4 turns out and the it started to backfire on deceleration. I began turning them back in over the course of 3 stops making adjustments of 1/8 a turn. I finally reached a spot where it is no longer popping and surging has faded. This is located at 1-5/8 to 1-3/4 on all screws. Unfortunately my acceleration is suffering.
1976 Kz900 A4

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Jet needle improves acceleration. 10 Jul 2011 16:57 #462088

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calvin17d wrote: Just got back from the 1st gear test.

I started out making qtr turns at my first adjustment as my adjustment made for idle was rich and the bike surged through all rpm ranges. I then reached a point where I was about 2-1/4 turns out and the it started to backfire on deceleration. I began turning them back in over the course of 3 stops making adjustments of 1/8 a turn. I finally reached a spot where it is no longer popping and surging has faded. This is located at 1-5/8 to 1-3/4 on all screws. Unfortunately my acceleration is suffering.


so as u turned them out it got leaner. are your pilot screws on the airbox side of the carbs?
ok. so, are u happy with the cruise . is it smooth there now? if so we will leave that and move on to the accell. they are 2 dif things.
describe the accell problem.
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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~ (k) / (z)

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Jet needle improves acceleration. 10 Jul 2011 17:14 #462093

  • 78kzturbo
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i can help you with your carb problems.

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Jet needle improves acceleration. 10 Jul 2011 17:42 #462097

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Being unsatisfied with previous results. I brought it back to the shop to get a better tune on the pilot screws (located on the air cleaner side of carbs).
Watching the tach and listening I brought it back to optimum without load. Went for a test drive and after an acceleration that sounds of fluttering it took off smoothly and accel'd nicely up to 5 k with a silky transition. I hit the side streets to Start the 1st gear test again. All was smooth with the exception of a light yet noticeable surge at 3k. I made adjustments of 1/8 a turn on the screws in both directions and It got worse. After this I couldn't get it back to the silky smooth operation. My effort on the side of the road led to a rough running condition and again I think my plugs began to foul and the exhaust very smokey.

I am happy with the cruise when those screws are adjusted at idle for idle, but it seems to be fouling when left at idle. Am wonderin if the increased height in the needle is allowing premature entry of fuel. Sadly I attribute some gains in midrange to the new clip position. 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.

The accel problem lasts about 2 seconds and occurs at take off and also during aggressive throttling at cruising speeds. I feel like this got better after I raised the needle.
1976 Kz900 A4

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Jet needle improves acceleration. 10 Jul 2011 17:51 #462101

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this is not rocket science it is a matter of air fuel mixture screws set properly and correct jetting.dont worry about needle position right now.after your bike is jetted properly and you have the right fuel mixture fine tuning will be easy.my computer skills are not good but i've been building these bikes for 20 years now.if you like i can help you get it jetted properly.

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Jet needle improves acceleration. 10 Jul 2011 17:53 #462102

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78kzturbo wrote: i can help you with your carb problems.


welcome 78kzturbo


please feel free to jump right in.
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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~ ~ _- \,
~ (k) / (z)

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Jet needle improves acceleration. 10 Jul 2011 17:58 #462103

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what year is the bike and what size are the carbs?

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