I have a 99 fatboy, the carb has a thunderjet kit on it (previous owner). My slow jet is a 45 and main is a 170. Off idle and part throttle are fine but WOT just doesnt pull. It seems like the 170 main might be a little lean.
I also tried to adjust the idle mixture and I can back the screw almost all the way out without messing up the idle. And, to top it off I get conflitcting stories on whether the screw in is richer/leaner. So I don't knw if I need a smaller slow jet or a bigger one.
The air mixture leans out when turned in (assuming you have a stock CV carb not a Mikuni) and richens when turned out. Your low speed jet is too rich if turning the mixture out has no effect. Put a 42 in it. Normal mixture screw would be from about 2 1/2 to 3 1/2.
If you're pulling at WOT and let off the throttle a little, does the bike speed up? This usually means lean. Sluggish usually means too rich. Just my opinion.
my evo heritage had 165/42 jetting. when i put my thunderslide and s&s intake i rejetted to 185/45 and it made a whale of a difference in performance. ive got vance and hines bigshot longs exhuast also. i might be a tad rich on top, but it is real close. i might cut back to a 180 but that wont be till april seems how the bike is up for the year but i am gonna put an ev27 cam in er this winter! cant wait to see the results of that.
Well, the bike feels sluggish at WOT, but I just can't imagine that it needs a jet smaller than a 170. With an AC kit, drag pipes and thunderjet kit the bike only made 58 hp and 71 lbs of tq. The plugs are sooty. I was originally thinking that the slow jet needs to be leaned out while the main needs to be fattened up. But then someone told me that the idle mixture screw out meant rich, so I need a bigger slow jet. That threw me way off. I guess I'll go with my original plan, 42 slow and 175 or 180 main and see what happens.
Or, maybe trying to get 65+hp with such few mods is unrealistic.
I would only change one jet at time. install a new set of plugs too. Put in the 42, set the mixture screw to 3 turns out, get the engine good and warmed, then re-adjust the mixture screw. See what that does for you. I'm not that familiar with the thunderjet stuff. What kind of needle setup does it have? Did a needle come with the kit? Is it adjustable?
Your torque figures are OK. Don't worry about hp. It's torque you want unless you plan on running at wot all the time. If you still the stock cam in it, that's a big part of real good performance problems. FWIW, drag pipes also make it hard to tune.
The thunder jet kit has a new needle and a few jets, it's just a recalibration kit.
As for the pipes, I hear yah, I am thinking of Thunderheaders.
I just got off the phone with my HD service dept. and they said that the idle mixture screw is not for the idle, it for while you are cruising. And that the screw should just be set 3.5 turns out. I asked which way is lean and which is rich and he told me it does not work like that, just set it 3.5 turns out. So I asked, "If that's the case then how do I know if my slow jet is the right size?" and he said it's fine. (he knows the bike)
I can't get over the fact that he couldnt tell me about the mixture adjustment, 3.5 turns out?!, I always thhought that was just a starting point for fine tuning.
So he says the drag pipes are killing it, I tell him about the Thinderheader and he tells me that their quality is going to ****, I should buy the V&H pro-pipe. HMMM I wonder if that's cause they sell they V&H and not the Thunderheader?
I think the guy at the dealer is full of sh**. He needs to do his homework about tuning a CV carb. If your plugs are sooty, the bike is runnning too rich. Now you have to find out where. The drag pipes are not causing all this problem. Nothing wrong with Thunderheaders. They are LOUD. A lot of guys report a sorta flat spot about mid range but nothing that bad. Nothing wrong with Vance and Hines either, they are just too high dollar for me. Cycle Shack used to make some good slip on mufflers. The White Bros E exhaust (2 into 1) along with Super Trapp 2 into 1 are also good. I would not buy any more exhaust systems or mufflers until you hear and see something you like. Then ask the owner about them. If you're not comfortable with tuning that carb right, see if you can find a good independent shop. I, personally, would not trust that service dude you talked to.
Here is a good link about tuning CV carbs -