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Carb Jetting

2.4K views 8 replies 5 participants last post by  Xfire  
#1 ·
I've seen lots of recommendations for rejetting stock carbs for simple stage 1 upgrades, but not for 95" builds with headwork and cams.

I've got an 04 FXST that I'll be building shortly, with headwork and Andrews 37G cams. My exhaust is Samson Big Guns II Long Slashers (I know, I'd do better with a good 2-1) and I have a big sucker installed.

When I installed the pipes, I also rejetted the carb at that time, using the Dynojet kit. I installed it as per their instructions, replacing the needle, shimming as directed and replacing the main jet with the supplied 180 jet. I've also replaced the slow jet with a 48.

What, if anything will I need to do with the carb after the build? I do plan on getting it dyno tuned, but would like to set the carb up as close as possible to correct initially, so that I can run through the post build break-in period before the dyno run, without worrying if I am running too lean.

The Dynojet instructions indicate a 185 main valve with cams and a 195 if there are "internal motor modifications". Can I simply replace my main jet with the 195 and be good?

Also, if I do replace my exhaust with, say, a V&H Pro Pipe, after the build, would the jetting change again, due to the different pipe?

Gracias.
 
#2 ·
Seriously doubt if you'd have to go from a 185 to a 195 on the main jet. If you're doing breakin on the new 95" setup (mostly rings anyway), you're not going to be WOT that much, are you?

Personally, I'd wait for the dyno tune, assuming it's not going to be 6 months later.
 
#3 ·
When I went from 88 Stage I to 95 Stage II, I had to adjust the idle speed and mix a smidg. Did some trial jet switching but ended up back where I started before the 95 build. That was with switching from Short Shots to a Python 2:1 pipe as well.
Depending on what headwork and what cams you go with, you might have to richen up a bit, but all other things bein' equal, more cubic inches don't have a major effect on the mixture.
 
#4 · (Edited)
So, it sounds like you guys are recommending to leave it where it is and adjust only if needed then?

While it may not be be 6 months before the dynotune, it could be close to that. The riding season is pretty much winding up here in the PNW. While I do expect to get out whenever the weather permits during the winter, I don't expect to put on very many miles.

Thanks.
 
#5 · (Edited)
I just had mine dyno'd today. Our builds are very close, but had my carb redone by Dakota Kid Custom Carbs. Believe it or not, in addition to the added flow (now 42 mm) he installed a 52 pilot and 204 main. I provided the data to the tech and his jaw almost hit the ground when he saw the combo...thinking it was too fat. After about a dozen runs on the dyno, the fuel/air mixture is right where it needs to be. The tech was very surprised and smiled with the end results. The numbers at the rear wheel were as follows:

98 ft/lbs of torque
93 hp

As for the pipe, I would have a 4-way exhaust test done to see where you're at with the new pipe.
 
#7 ·
According to most of the dyno places around here, the 4 way is the most accurate measure of your fuel/air mixture. Basically, it measures your exhaust gases in the same manner as most emission stations. It measures HC, CO, CO2, and NOx.
 
#8 ·
Backfiring gone?

Xfire said:
I just had mine dyno'd today. Our builds are very close, but had my carb redone by Dakota Kid Custom Carbs. Believe it or not, in addition to the added flow (now 42 mm) he installed a 52 pilot and 204 main. I provided the data to the tech and his jaw almost hit the ground when he saw the combo...thinking it was too fat. After about a dozen runs on the dyno, the fuel/air mixture is right where it needs to be. The tech was very surprised and smiled with the end results. The numbers at the rear wheel were as follows:

98 ft/lbs of torque
93 hp

As for the pipe, I would have a 4-way exhaust test done to see where you're at with the new pipe.
XFire, just curious, what did you do to gt rid of the pops aon your DK carb?

thx