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Author Topic: slow jet number vs. hole size  (Read 4962 times)

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Offline Flying J

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slow jet number vs. hole size
« on: August 12, 2009, 10:50:26 PM »
Im working on tuning my 750f carbs to my mac exhaust and foam filters. I have always had a problem with my idle screws being to far out to idle good, they would rattle loose and fall out. So to fix the problem i was going to go to a larger jet and see how that works. I have a few jet sizes laying around. and got to looking and the jet size and the hole size dont make sense to me. The 35 has a bigger hole then a 42? I checked on a few different carbs and that seems to be how it is. Can anybody verify, explain, disprove this so i can move on to the next part of figuring out what jet i need.

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Offline scunny

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Re: slow jet number vs. hole size
« Reply #1 on: August 12, 2009, 10:55:41 PM »
the 35 will have a smaller diameter hole down the bore so to speak. I bet the other one is aftermarket
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Offline TwoTired

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Re: slow jet number vs. hole size
« Reply #2 on: August 12, 2009, 11:08:33 PM »
The end of the jet tube is NOT the jet orifice.

The orifice is just this side of the emulsion tube at the other end.

Keihin Jets are numbered in mm, ie. 1.00mm is a #100 jet.  A #42 has a 0.42mm diameter orifice.
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Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

Offline Flying J

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Re: slow jet number vs. hole size
« Reply #3 on: August 13, 2009, 08:43:14 AM »
Well loyd, help me do it right. My 78 F carbs are stock 35 slow jet. Would a jump to 42 be to much? Or should i try to get my hands on some 38s or 40s. Im running foam fillters and a mac exhaust. I finally put on a muffler. I was running straight pipes but......it was too loud my girlfriend could not here her 550 at the lights if it died or not. So now i run a muffler. Any way what do you think 40s 42s?
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Offline TwoTired

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Re: slow jet number vs. hole size
« Reply #4 on: August 13, 2009, 11:52:21 AM »
Quote from: ffjmoore on August 13, 2009, 08:43:14 AM
Well loyd, help me do it right. My 78 F carbs are stock 35 slow jet. Would a jump to 42 be to much? Or should i try to get my hands on some 38s or 40s. Im running foam fillters and a mac exhaust. I finally put on a muffler. I was running straight pipes but......it was too loud

What?  Waz that you say?  It was too proud?

 ;D ;D

You got 42s, use 'em.  The IMS can hold back extra with a bit of closure, if needed.  Freer flowing exhaust with more cylinder scavenge, less throat vacuum, make bigger jet orifices sensible.

Cheers,
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Lloyd... (SOHC4 #11 Original Mail List)
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Offline Flying J

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Re: slow jet number vs. hole size
« Reply #5 on: August 13, 2009, 01:52:39 PM »
thanks Twotired. I hated to say it but those straight pipes were to loud. I love the sound, but nobody like to ride by me and im sure my neighbors were starting to hate me. So i gave in.
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