Let me preface this post by saying I've read every word and letter relating to predator tuning, and I've tried dozens (literally) of configurations before I posted this.

I picked up a 2004 Preddy 500, the suspension was shot and it had two vent valves and a destroyed piston. I replaced all of the bushings, tie rod ends, etc, and put on new tires, new chain, etc. I built a new top end with kibblewhite valves and hotcams stage 1 exhaust and intake cams (the stock cams were damaged). While I had it apart I ported and polished the head pretty well, not a mirror shine but I took out the rough spots. I ran a standard Namura flat head piston because I live in California and it's tough to get anything over 91 octane. I also replaced the stock muffler with a pro circuit T4 (I also have a polaris pure but it's incredibly loud - may need to be re-packed). I know the pure may be better but I like the headers that came wit the pro circuit.

I put it back together with a new carb, mikuni 150 jet, stock idle and starter jets, and a square K&N air filter in the OEM box, it runs as expected with no hesitation. I wasn't too impressed with the low end and mid-range so I started making changes to fuel/air. I ran 2" schedule 40 PVC from the carb to a K&N filter (Honda Civic type via reducer). When I run a 150 or 155 mikuni jet, it dies with anything over half throttle OR anything over 5000 rpm. Complete loss of power, like the engine kill switch was activated. I went to a 165 and 170 mikuni, same exact behavior. The pilot and starter jets are unchanged. The pilot screw is 2 3/4 turns out. I moved the needle all over (2nd, 3rd, and 4th from top) with the 165 and 170 jet and started getting some serious stuttering and hesitation (as opposed to total power loss), but overall it was the same behavior.

I've jetted a dozen bikes, but this is the only bike I own that has a diaphragm carb, so it seems the air volume makes a huge difference due to the vacuum behavior with the throttle body and needle, but I'm speculating.

After maybe 45 mins of time on the new top end, I adjusted the valves to spec. I also tried a third carb and rebuilding the original carb, but they all result in the same behavior, so I have it pretty well narrowed down to a jet/needle issue.

Thoughts? Should I try a 180 jet? It's back to my first configuration now and runs, well, a little better than stock.

First time poster, long time reader. Thanks for any help. If anyone is running a similar configuration, I would be interested what jet and needle position you're using.

 

I ended up getting this running really well if anyone is interested. I built a new air intake using a 2" --> 3" reducer to 3" flex tube and a K&N air cleaner. In the picture I don't have a filter on the breather but I have a mini K&N type air filter installed normally. I kept the box there to keep water and mud from coming up from the suspension and getting in the filter but I'm considering drilling 1" holes on each side to improve airflow.

Mods:
180 Mikuni main jet
OEM pilot and starter jets
3" pipe to K&N air filter
Needle 4th from top (2nd from bottom)
3 turns out on the air/fuel screw
Pro Circuit T4 exhaust w/ headers
Procom CDI
HotCams Stage 1 Ex/In
Ported and polished head, exhaust and intake ports (done by self using Dremel tools, etc)

It runs harder than almost any quad I've driven and runs smooth all the way to the limiter without hesitation. The low end is still lacking a bit, but over 4000 RPM it reminds me of a banshee. I took it out to test the 180 jet and the front end came up on me unexpectedly, which has never happened before. I'm a big dude and I ride up in front as a habit, so this was a major increase in power.

 

The problems you are having are definitely related to not running the air box. Try around factory jets with the air box in.

 

Got it working really well. It didn't post before yours because the image had to be approved by mods over the weekend. With the factory jets and airbox, yes it ran good but that's boring.

 

I understand,the old way i remember for the airbox mod was use pvc to line the inside of the air box tube with enough stickinng put to clamp the filter on,then cut a few holes in the lower section of the airbox for flow. I always worry about not running a lid(manual says definitely no lol) because of the imminent streams i cross. I have an 03 that only has a pipe on it and am considering doing the 560 bb kit and maybe cams(although i keep seeing that they are pointless along with the header) and porting with bigger valves. Hope yours works good. Was wondering how you like that cdi,i heard only two are actually worth it and the others dont retard the timing

 

I like the CDI, I haven't heard much about it except for a few bad things lol, but I had already bought it so I stuck with it. It changes the max RPM in both forward and reverse (mine doesn't have reverse) and also changes the timing curve, but I don't know any more details. I don't have much water around since I'm in California, so no worries about that but I am going to chop up the airbox a bit now that I'm convinced I'm sticking with this configuration. I can't believe I got it to work.

I forgot to mention I also ported and polished the head when I was changing out the camshafts. I drove a preddy with the 560 kit on it and it was actually what got me to get mine. His was faster than mine in the low end but I wanna say the top end is about the same. I considered doing it but I hear its dangerous to run it on 91 fuel... racing fuel is hard to get in California and we can't even buy 93 at the gas station. I love this state and it's only getting better every day!

 

have you ever considered running e85? Ever since i had my 04 i got ride of,i always toyed with the idea of a turbo predator. I may have to get another bike lol.

 

No but that's something I'll look into. My buddies run their RZRs on e85 religiously.