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Offline thelowmax

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Jetting
« on: July 05, 2015, 03:23:15 PM »
Everything I know about my bikes, I've figured out myself. Maybe thats why Im having problems. :)

There must be a better way to figure out the proper jetting than trial and error for Jardine crossover 4 into 2 pipes (or any other configuration) on a cb750k8 . Is there? What is it? And where can I get a selection of k7/8 jets where I don't have to buy the whole rebuild kit? When rejetting, do I need to replace all jets or just the mains? Does the thingamabob need to be cajiggered?
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What am I doing and why am I doing it? Those are excellent questions.

'72 Amen Savior/'77 CB750K Chrome/Da Bhudda(project)
'73 CB750K Green/El Verde (beat)
'76 CB750K Red/The Cinnabomb (sweet)
'77 CB750K Black (frame and parts) CANNIBALIZED
'77 CB750K Dark Purpley/Scooty Puff, Jr. (la beast)
'78 CB750K Black (struggling) SOLD
'78 CB750K Blue Flake/CiocioSan (minty)
'81 CB750C Poiple/Barbie'sDreamMotorcycle SOLD (darnit!)
'89 Trek 21" 21 speed Green/YaBiatch (the wife)
Converse One Stars size 8.5 Black/Sneaks (suede)

Offline links56

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Re: Jetting
« Reply #1 on: July 05, 2015, 08:37:32 PM »
It depends on the issues you are having? What size jets do you currently have?
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Offline martin99

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Re: Jetting
« Reply #2 on: July 06, 2015, 03:20:35 AM »
IIWY I would read up on plug chops, lots of good info to be had here.
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Build threads:
77 750F2 Refresh Project http://forums.sohc4.net/index.php?topic=144075.0
TRIBSA http://forums.sohc4.net/index.php/topic,160296.0.html

1977 CB750 F2
1958 Norton Model 99
2011 Triumph Street Triple 675

Offline flybox1

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Re: Jetting
« Reply #3 on: July 06, 2015, 06:43:25 AM »
Quote from: thelowmax on July 05, 2015, 03:23:15 PM
Everything I know about my bikes, I've figured out myself. Maybe thats why Im having problems. :)

There must be a better way to figure out the proper jetting than trial and error for Jardine crossover 4 into 2 pipes (or any other configuration) on a cb750k8 . Is there? What is it? And where can I get a selection of k7/8 jets where I don't have to buy the whole rebuild kit? When rejetting, do I need to replace all jets or just the mains? Does the thingamabob need to be cajiggered?
buy OEM jets from siriusconinc or jetsrus. 
I've had the best luck with jetsrus.com.  Customer service is great.  Parts are easy to locate on their site.
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline thelowmax

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Re: Jetting
« Reply #4 on: July 06, 2015, 07:27:10 AM »
Quote from: flybox1 on July 06, 2015, 06:43:25 AM
Quote from: thelowmax on July 05, 2015, 03:23:15 PM
Everything I know about my bikes, I've figured out myself. Maybe thats why Im having problems. :)

There must be a better way to figure out the proper jetting than trial and error for Jardine crossover 4 into 2 pipes (or any other configuration) on a cb750k8 . Is there? What is it? And where can I get a selection of k7/8 jets where I don't have to buy the whole rebuild kit? When rejetting, do I need to replace all jets or just the mains? Does the thingamabob need to be cajiggered?
buy OEM jets from siriusconinc or jetsrus. 
I've had the best luck with jetsrus.com.  Customer service is great.  Parts are easy to locate on their site.
Sweet. Thanks.
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What am I doing and why am I doing it? Those are excellent questions.

'72 Amen Savior/'77 CB750K Chrome/Da Bhudda(project)
'73 CB750K Green/El Verde (beat)
'76 CB750K Red/The Cinnabomb (sweet)
'77 CB750K Black (frame and parts) CANNIBALIZED
'77 CB750K Dark Purpley/Scooty Puff, Jr. (la beast)
'78 CB750K Black (struggling) SOLD
'78 CB750K Blue Flake/CiocioSan (minty)
'81 CB750C Poiple/Barbie'sDreamMotorcycle SOLD (darnit!)
'89 Trek 21" 21 speed Green/YaBiatch (the wife)
Converse One Stars size 8.5 Black/Sneaks (suede)

Offline thelowmax

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Re: Jetting
« Reply #5 on: July 07, 2015, 11:18:17 AM »
Quote from: links56 on July 05, 2015, 08:37:32 PM
It depends on the issues you are having? What size jets do you currently have?

I'm pretty sure there are 115s in there right now. When I got it, it had a 4 into 1 Kerker (no baffle) and seemed fine. I replaced that with a baffled Mac 4 into 1 and it noticeably kicked up the power a notch.

Now, with the Jardine 4 into 2 crossovers, it jumps off the line but after that I can't get it past 6000 rpms. It starts to cough at about 5500 and I really have to finesse it to go higher. It's not a good feeling on the highway.






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What am I doing and why am I doing it? Those are excellent questions.

'72 Amen Savior/'77 CB750K Chrome/Da Bhudda(project)
'73 CB750K Green/El Verde (beat)
'76 CB750K Red/The Cinnabomb (sweet)
'77 CB750K Black (frame and parts) CANNIBALIZED
'77 CB750K Dark Purpley/Scooty Puff, Jr. (la beast)
'78 CB750K Black (struggling) SOLD
'78 CB750K Blue Flake/CiocioSan (minty)
'81 CB750C Poiple/Barbie'sDreamMotorcycle SOLD (darnit!)
'89 Trek 21" 21 speed Green/YaBiatch (the wife)
Converse One Stars size 8.5 Black/Sneaks (suede)

Offline flybox1

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Re: Jetting
« Reply #6 on: July 07, 2015, 11:28:50 AM »
that's in the transition between clip position and main jet.
once you get through this 'coughing', how does it perform?

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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline thelowmax

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Re: Jetting
« Reply #7 on: July 08, 2015, 03:31:59 AM »
Quote from: flybox1 on July 07, 2015, 11:28:50 AM
that's in the transition between clip position and main jet.
once you get through this 'coughing', how does it perform?



Seems to run great, just limited to below 5500 rpms.
Logged
What am I doing and why am I doing it? Those are excellent questions.

'72 Amen Savior/'77 CB750K Chrome/Da Bhudda(project)
'73 CB750K Green/El Verde (beat)
'76 CB750K Red/The Cinnabomb (sweet)
'77 CB750K Black (frame and parts) CANNIBALIZED
'77 CB750K Dark Purpley/Scooty Puff, Jr. (la beast)
'78 CB750K Black (struggling) SOLD
'78 CB750K Blue Flake/CiocioSan (minty)
'81 CB750C Poiple/Barbie'sDreamMotorcycle SOLD (darnit!)
'89 Trek 21" 21 speed Green/YaBiatch (the wife)
Converse One Stars size 8.5 Black/Sneaks (suede)

Offline Cqyqte

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Re: Jetting
« Reply #8 on: July 08, 2015, 04:40:20 AM »
Quote from: calj737 on July 06, 2015, 04:30:11 AM
The easiest way is to take the bike to a dyno with an exhaust analyzer. The dyno will tell you the Air/Fuel mixture exactly from idle to WOT. You can then jet based upon known, accurate circumstances. You may need to alter the carbs two or more times while on the dyno to dial them in, but it is the best and most accurate means.

Else, it's buy 3/4 sets of plugs, and 2/3 sets of jets and chop away!

One very important piece of the tuning puzzle is dynamic air flow!  On a dyno you don't get true air flow around the bike as you would while riding at speed down the highway.  So techincally you are correct you will find out exactly what your air/fuel mixture is from 0 to WOT in a static setting but not in the dynamic.  This is why in many cases the properly tuned bike on the dyno falls flat on it's face when taken on the first run down the highway. 

Air travelling past the air filters or air intakes of a stock or modified bike are ever changing and can create significant differences in the way air is drawn into the carburators throat.  These differences are minimized by the design of the air intakes and their position on the bike.  OEM designs attempt to position air intake in the quietest location possible to reduce the dynamic airflows affect.
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Offline flybox1

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Re: Jetting
« Reply #9 on: July 08, 2015, 07:18:40 AM »
Quote from: thelowmax on July 08, 2015, 03:31:59 AM
Quote from: flybox1 on July 07, 2015, 11:28:50 AM
that's in the transition between clip position and main jet.
once you get through this 'coughing', how does it perform?



Seems to run great, just limited to below 5500 rpms.
You'll need to perform a series of plug chops to ensure you have the correct jets and needle clip position.
Start with them WOT (main jet) chop. 
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"
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