Author Topic: Jetting for 850cc big bore, stock airbox and exhaust  (Read 1514 times)

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Offline 45-70 fella

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Jetting for 850cc big bore, stock airbox and exhaust
« on: March 27, 2016, 10:04:11 PM »
Hello,
I have the stock airbox and exhaust on my K3 750.  I have the 850cc kit installed as well as powerarc ignition.  Will I need to change jet sizes?  Where should I start for jet sizes?  I like the way my 4 into 4 looks and do not want to change it.

Offline PeWe

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Re: Jetting for 850cc big bore, stock airbox and exhaust
« Reply #1 on: April 12, 2016, 03:39:33 AM »
Everything original except for the bore, probably not much difference in jetting.  Filter to carbs is clean....
 If it runned OK before bigger bore, same settings must work. 1 step up to see if you get more torque at lower rpms when twisting the throttle

Different flow such 4-1, ported head + cam etc need more air and fuel

You will see when driving....
Full speed and bike will go faster when reduce throttle just a little..... too lean mains.
« Last Edit: April 12, 2016, 03:47:41 AM by PeWe »
CB750 K6-76  970cc (Earlier 1005cc JMR Billet block on the shelf waiting for a comeback)
CB750 K2-75 Parts assembled to a stock K2

Updates of the CB750 K6 -1976
http://forums.sohc4.net/index.php/topic,180468.msg2092136.html#msg2092136
The billet block build thread
http://forums.sohc4.net/index.php/topic,49438.msg1863571.html#msg1863571
CB750 K2 -1975  build thread
http://forums.sohc4.net/index.php/topic,168243.msg1948381.html#msg1948381
K2 engine build thread. For a complete CB750 -75
http://forums.sohc4.net/index.php/topic,180088.msg2088008.html#msg2088008
Carb jetting, a long story Mikuni TMR32
http://forums.sohc4.net/index.php/topic,179479.msg2104967.html#msg2104967

Offline Old Scrambler

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Re: Jetting for 850cc big bore, stock airbox and exhaust
« Reply #2 on: April 12, 2016, 08:49:34 AM »
My K3 had 105-jets when new. After more than 60,000 miles I installed the +0.25mm pistons and rings. I ran it rich with 120 mains and then 115s..............still too rich with a K&N filter but all else is stock.

Your big-bore will give you added compression as well as more torque. If your pistons are flat-topped and all else OEM I would be ready to go with larger jets..........maybe up to 125s or 130s to be safe. If you jet too small you will get detonation in the upper rpm-range..........and that is to be avoided ;) ;) ;) ;)  If your pistons have dome-tops.........especially begin with big jets and tune down to the correct size to be safe.  My brother-in-law installed a slightly-domed 836 kit with a mild performance cam..............his #2 & #3 pistons were toast from detonation with 115-mains.
Dennis in Wisconsin
'64 Triumph Cub & '74 Honda CB750 Bonneville Salt Flats AMA Record Holder (6)
CB750 Classic Bonneville Racer thread - http://forums.sohc4.net/index.php/topic,135473.0.html
'63 CL72 Project(s)
'66 CL77 Red
'67 Triumph T100C
'73 750K3 Owned since New
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Offline PeWe

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Re: Jetting for 850cc big bore, stock airbox and exhaust
« Reply #3 on: April 12, 2016, 10:22:47 AM »
Interesting...
I burned the 2 3  836 pistons back in 84.  cyl 2 had a strange hole on IN side, 3 had a bump. All small ends of rods bluish.
Did not that fast 100mph in about 5 minutes or less. I thought I had missed to check the 2-3 point, checked 1-4.
CB750 K6-76  970cc (Earlier 1005cc JMR Billet block on the shelf waiting for a comeback)
CB750 K2-75 Parts assembled to a stock K2

Updates of the CB750 K6 -1976
http://forums.sohc4.net/index.php/topic,180468.msg2092136.html#msg2092136
The billet block build thread
http://forums.sohc4.net/index.php/topic,49438.msg1863571.html#msg1863571
CB750 K2 -1975  build thread
http://forums.sohc4.net/index.php/topic,168243.msg1948381.html#msg1948381
K2 engine build thread. For a complete CB750 -75
http://forums.sohc4.net/index.php/topic,180088.msg2088008.html#msg2088008
Carb jetting, a long story Mikuni TMR32
http://forums.sohc4.net/index.php/topic,179479.msg2104967.html#msg2104967