Sxr 849 tuning

M

Mbien34e5

Location
Massachusetts
Trying to get my new 849 motor from group k tuned. Stock exhaust and ignition, carbs have flame arrestors and choke plates removed. Jets are 77.5 and 137.5 pop off around 24psi. It stumbles hard off the line cleans up about 40-70% throttle and falls on its face full throttle only about 5800-5900 rpm. Any advice on getting this thing tuned would be appreciated
 
kevbo

kevbo

Location
dfw
Im sure this is not what you want to hear but......... You need to learn how to tune carbs or else pay someone else to do it. Asking here is useless because nobody can know exactly what your engine needs. You can learn a lot from tuning a cheap leaf blower first.
 
M

Mbien34e5

Location
Massachusetts
kevbo said:
Im sure this is not what you want to hear but......... You need to learn how to tune carbs or else pay someone else to do it. Asking here is useless because nobody can know exactly what your engine needs. You can learn a lot from tuning a cheap leaf blower first.
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More or less just trying to see if my jets are in the right ballpark. I appreciate your help though.
 
Baja 252

Baja 252

Location
LOTO
I ran the Group K 849 kit for several years, but unfortunately I don't know the jet sizes. When I bought it they set the carbs up for me. I would call them, I would think they would help get you in the ballpark.
 
S

smd

Location
Northern CA
I have the same setup and are going through the carb tuning thing to. Group K sends them out with stock jetting L=72.5 H=132.5. They use the stock 1.5 NS and drop from the stock 80gm spring to the 65gm spring. They say 3/4 turn both low and high after break in. I started at 1 1/2 low and high for break in and intial tuning.
I got to 3 turns out on the H at around 6870 max rpms, but stopped wanting to rejet larger. I probably over did the re-jetting and went to L=80 and H=155, but the plugs still look pretty good. I ended up at 1/2 turn on both low and high, but stopped because the water got too rough. At least I didn't have to worry about seizing it going rich. I'm going to rejet to L=77.5 and H=147.5 (+) to try to get the screws out a bit.
Mine has a rough idle, but it pulls good low then really hard from mid to top with very linear power.
Yours is different, so you might have something else going on.
 
S

sturat

Location
WA state
on full wide open throttle stab
if: bogs but doesnt die
then: rich on lows

if: chokes/dies
then: lean on lows

at top speed / WOT, if you let off throttle a little bit and it speeds up, youre rich on the highs. Lean them out til you are happy with the performance and then richen them back up just a hair. I would run the front cylinder slightly richer on the high, just a smidge, if you are running single pulse / fuel pump. Otherwise even settings should work.
 
S

sturat

Location
WA state
kevbo said:
That test will work for pilot jets but assumes the main circuit is perfect.
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Plugs don't lie, and there is plenty of reference material available at this point to get it nearly spot on first try. Pretty easy to break down the tuning philosophy, the jetting itself was always the trick of the trade. Now that we have countless fb groups and over 15 years of posts on several forums to go back and look through, a lot of the thinking has already been done for you.
 
kevbo

kevbo

Location
dfw
The biggest problem with plug color is being able to convert what you see into what the combustion temps are. You can stick a lot of pistons waiting for plugs to color up. It is very difficult to hurt an engine that is jetted rich of peak rpm. I suggest people start there. Using data from the internet is reckless, but a lot easier than learning how to tune.
 
S

smd

Location
Northern CA
Turns out my low speed screws had the wrong o-rings which screwed up my tuning. I went back to the Group K jets and setting recommendations stock jetting L=72.5 H=132.5 3/4 turns out on both screws and it's pretty close now for a stock SXR with the 849 big bore and Tau Ceti air filters. Plug readings in CA are a bit harder with E-10 gas and then in the summer they oxygenate it too. It takes a while for the plugs to color up.
 
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