I have a 1998 Fat Boy with a stock Evo 80" motor and CV carb. I'm the second owner of the bike and recently put around 6000 miles on in since I bought it a year ago. Also, the bike only has 11,000 miles total on in.
I've got a Screaming Eagle Intake kit with a K&N filter and Baloney Cut slip ons.

The problem I'm having is when I roll on the throttle from idle. When I roll on the throttle (smooth or aggressive) I hear a popping and the bike wants to stall. It seems to happen more often when its cold or really humid. Also, I leave right on the Pacific Ocean in San Francisco and temperature changes can be huge around the Bay area.
I've dug around the postings and this seems to be a common problem with intake manifold seals and slow jets.

Here's what I've done so far:
-Checked jets and cleaned carb. I have a 48 slow jet (HD Part # 27165-90) and a Sportster XL1200 California Main Jet (HD Part # 27114-88.) Jets, SE intake kit and slip on mufflers where installed by the dealer when the original owner bought the bike new in 1997.
-Replaced intake manifold seals (all three)
-Set idle mixture in the sweet spot (where it seems to give me good performance given the temperature changes around here.)
-Checked for leaks around intake and exhaust.

One thing I noted after the last time I pulled the carb off was that my rear engine breather bolt is spraying out fluid after I ride the bike more than 35 miles. Could I have an issue with the air cleaner/breather bolts causing a vacuum problem?

I know this is a long posting but I wanted to tell you all as much info to help narrow down the issue.
Thanx

 

Check the slide diaphragm to make sure that it is installed right and has no holes. isn't torn, etc. Did the original owner put a "Jet kit" in it???

 

The HD dealership put the 48 slow jet and the XL1200 main jet in when the bike was sold to him in 1997 per his request. As far as the documents I have from the 1st owner those two jets were all that was changed in the carb. I'll check the carb again but when I cleaned and inspected it recently I didn't notice any punctures or damage.

 

When you took the carb apart did you remove the little silver needle jet that sticks up into the throat of the carb?? If you did you may have it in upside down. One end is square on the inside and the other is radiused. The radiused end goes up into the throat(venturi) of the carb. If it is in upside down the engine will run poorly. Hope this help.
Was the bike running ok before you took the carb apart?? Is the accelerator pump working??

 

Don't forget the float-level either. If you let in continue to pop like that....it will rupture the diaphragm. Lots of good stuff already been posted here, but with the air breather off...you should be able to tell if the slide is lifting when you open the throttle.

I don't think your air breather bolts are causing any problem that relates to this. You may have a bigger-than-normal blow-by problem, but the thing you describe should not be related.

 

When I took the carb off, I only sprayed it with carb cleaner and did a visual inspection. I didn't take the needle out.
You guys make some good points regarding the diaphragm. I'll pull the carb off this weekend to check out what you guys have suggested.
I took the carb off once to replace the manifold seals. This was after the bike started popping and the first step I took to remedy it was the seals. The bike was running fine for about six months, then it started popping and stalling.

 

My 2 cents:

A 48 slow jet is too fat for a stock Evo. I just finished tuning the carb in my Evo FXR with slip-ons, an air cleaner and a mild cam and ended up with a 45 slow jet and the idle mixture screw about 1 1/2 turns out. 46 was too fat for my bike. Runs like a dream now.

Check again for vacuum leaks. I believe your bike has a vacuum operated petcock. Check that the vacuum line for that and the VOES switch is properly seated on the brass tube for it on the backside of the carb. If that wasn't reconnected or if the line is cracked, you'll run like ***** and the bike will pop like you described. Check your intake seals again and look at your exhaust gaskets for leaks there. The diagphramn on your slide may be ripped, but if you didn't open the top to remove it, I doubt that's the problem.

Keep at it. You'll get it figured out.

 

I pulled the carb of this weekend. There weren't any holes in the slide diaphragm. Slide needle looked good too. I also checked to make sure my jets weren't clogged or loose. The main is a 180. I didn't mention that earlier.

Everything looked ok except there was a thin black residue in the throttle body. Correct me if 'm wrong but I think I'm running too rich so I turned in my mixture screw a 1/4 turn. Now I'm at 2 turns out from fully closed.

Also, I noticed the gasket between my carb and the air cleaner back plate seemed dry. I lubed it up and put the whole thing back together. I plan on replacing that gasket ASAP.

I fired her up and took a spin. The carb popped a couple times right after I started the motor. It normally doesn't do that. I rode it all afternoon without it popping once. It sat for a couple hours then I rode her again. No popping or stalling!!!!

When I replaced the intake manifold seals a couple months back, the popping went away for a week or so. I'm hesitant to think simply pulling the carb and putting it back on again solved my problem (it's too easy!!!) I did lube the dry gasket and adjust the mixture screw though.

 

The air cleaner to carb gasket is not a likely suspect. The seal where the carb plugs into the manifold and the manifold to head o-rings/seals are. The carb has three circuits....idle, mid-range (slow) and high-speed. If turning the adjustment screw has any effect on it, it sounds like your slow-jet is problematic. Omgtkk has already pointed-out that jet may be too large. With the correct slow-jet...that mixture screw normally runs around 1.5 to 1.75 turns off the seat. Turn it in another 1/2 turn, or even a bit more with that #48 in there and see what results you get. It sounds to me like you have improved matters when you turned the mixture screw in. I think you could probably turn it in a fuzz further.

Sometimes, simply taking stuff off and putting it back on really does help. Maybe something wasn't sealing correctly. The manifold bolts need to be torqued down on the heads in a criss-cross pattern to insure a good seal. You can check that by gently spraying some carb-cleaner around those joints while the bike is running. If you have a leak...it will show-up. Some folks will even take a propane torch (un-lit) and "spray" a little gas in those areas for fear that carb cleaner will eat their paint off, but you can be careful with it. If you detect a "note" change in the engine, you still have a leak. It can be difficult to obtain/maintain a perfect seal at that point and I suppose many bikes are running around out there with an imperfect seal. On cold mornings, mine may pop a time or two, but I have not been energetic enough to get out there and replace the seals. It is EFI so I don't worry about a diaphragm bursting.

 

Gonna find a 45 slow jet and give it a whirl. Do you guys recommend getting a full jet kit or just individually. Any recommendations on brands or kits? My local parts shop doesn't carry many. I'll have to order them.

Thanx again for the good tips!!!