Jetting changes for increased displacement, compression, and head-flow

[MarsN]

Searched for this specific question no luck....

Looking for some feedback/experience with jetting changes after a similar engine rebuild to mine. I am getting close to assembling my 1216 rebuild and I want to have the jetting as close as possible to 'acceptable' for the initial start-up and break-in, especially for that first half-hour of run-time.

I've done some reading on the affects of increased displacement, increased head-flow, and increased compression ratio and all three indicate a decrease in richness, which I admit, was opposite to what I would have guessed. So smaller main jet and leaner needle position are likely needed.

Rather than specific jet sizes, or needle position, which is going to be individual based on exact engine specifics and altitude, and other variables, etc.  I'm hoping to hear some feedback on just how much of a change I should be looking for. ie 1-2-3-4 jet sizes smaller? I realize I will have to go through the usual steps of dialing in the jetting as I normally do. IE full throttle runs to find best main-jet, followed by needle-jet clip position, float height etc. But just wondering if anyone has any first hand examples of how many main jet sizes they had to change, or how many clip positions etc, with a similar engine rebuild as mine.

I have BST38SS carbs nicely set up before the rebuild. I have the Factory Pro 38 mikuni jet kit installed. Think I have a 152.5 main, and needle at top(leanest), or possibly second from top clip position. I live at 1100 meter altitude which leans things out too. The following are the engine changes:

-1216 JE kit

-11:1

-Head ported by experienced oil cooled head porter

-30/26 valves

-stock cams

 

Any experiences or feedback is much appreciated!

 

[Gixer1460]

The jets you've got presently sound fairly big for an 1100 (assuming that was what you had before) at normal sea level - 150/200ft elevation. At altitude of 3400ft there is less air / oxygen so those jets would give a massive rich condition ! ! ! With altitude you use less fuel to match the lesser air, thats why you lose power if keeping AFR approx the same or you lose power due to overly rich mixture! Very few peeps on here live at those sorts of elevation so any jetting advice would be dubious at best - honestly I'd try it as it is but I suspect richer is NOT what the engine wants!

I don't remember the DJ / Mikuni jet correlation but my 1186 kitted GSXR with 36 CV's and 12:1 CR and unported, at maybe 50ft ASL used DJ136's and made 144hp.

[MarsN]

I definitely was making the mistake of too rich with the main jet when I started with the Factory pro kit. This was on the original 1127 with K & N filters and Kindle 4-2-1.

The Factory Pro recommendation was too rich for my environment/set-up. I think I went two jet sizes smaller than their recommendation. I was pretty diligent however in my test runs to try to find the right size main jet first. Tried over and under on main jet size. However, trying to feel subtle differences at full throttle when getting close to the right jet is definitely tricky. So I could still be too big based on the fact that I'm at the leanest needle position as well.