Topic: Jetting for 850cc big bore, stock airbox and exhaust

[45-70 fella]

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.

[PeWe]

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.

[Old Scrambler]

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.

[PeWe]

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.