The heart of the problem on your HO is the cylinder head. The ports are to big for the RPM's and valve size. The big ports kill velocity. Port work is not really the answer either .

 

The one ray of hope remains the Mikuni Flatslide. My son's EXP is dead even with my H.O.
in a drag race,(but he weighs 85 lbs.) with an H.O. cam (which did nothing before the Mikuni) a ported out muffler, K&N, and a 38mm Flatslide Mikuni. This is not the cool pumper, but listed for 4-strokes anyway. But it took the Mikuni to wake this EXP from its' nap. Now the limiter is holding it back, and we haven't figured out how to defeat it. The orange and grey wire Scrambler guys are disconnecting controls the reverse solenoid on the EXP.
I have heard many complaints from people who know about the design of this cylinder head.
Ports too big in relation to valves, not enough physical space for a high lift cam. Weisco high compression piston sounds like a good thing. But as Rick says, head work could be a waste of time and/or money, and I'm already an expert at wasting time and money!! Fitting this head with larger valves and a high lift cam would be serious. The guys at Four Stroke Tech are doing just that, along with bore and stroke increases to 630cc. Maybe...
Silly Later,,,Fourlix

 

Hey DieselPro, Fourlix here. I was running 152.5 mainjet here in New Mexico, Southwest Mountains near Silver City. I live at 4300 ft. elevation but ride mostly up to 7,000 ft. With the new airbox horn, it immediately ran like it had no lid, so it must work. Biggest Jet I had was a 165 and it's still lean, but running as well as stock, not better. Up at 6,000 ft it may be just right. If I get it running just right here, it's too rich and doggy up there. The new airhorn is really in anticipation of the new Mikuni Flatslide, which I hope comes this week.
If it doesn't work, I think that will end the bolt-on experimental stage. I'll just have to live with it, go the full Monty with a FST Kit, or leave it as a trail quad and get a Cannondale, or a Raptor and try to make one of them into a Desert Racer. I used to go 90mph down these same sandwashes in my old hot-rod full-size '71 Blazer.
It pisses me off I can only do 50 on my H.O. We're going full throttle with these things and it feels like riding a mini-bike sometimes. Of course in the rough stuff the quads blow the big rigs away. A 350 Chevy with all the goods is "a horsepower of a different color". But like 3TV, when I hit the gas, I expect something. The H.O. does not deliver.
Silly Later,,,,Fourlix

 

3TV,

your idea of a performance quad will never be built because of all the lawyers. there too many idots that would kill themselvs on a quad like that. and the OEM's dont want to get sued like kawie, suzki,and honda did in the 80'

later
jon

 

Ledebuhr may be happy with the status quo, but 3TV and I are not. Three wheelers were inherently dangerous amd marketed to beginners and entry level buyers. Liability City, Downtown.
Expert Motocross bikes are marketed to Expert riders. Todays Sport Quads, like Raptors, are likewise marketed to experienced riders knowing and willing to take the risks.
There is no increased liability in making a better machine. One that handles better, is lighter, has better suspension, more user friendly, like ergonomics, EFI, electronic ignition etc. It's even okay if it's faster, just as long as it can handle its' own power.
I think the ATV industry was nuts going with three wheelers in the first place,their unexpected and illogical success is right up there with pet rocks. 4-wheelers have suffered greatly because of their predecessor.
But there are no more excuses. That era is over. It is time for this industry to get off the pot and start catching up, okay it has begun to catch up, but it has a long way to go. If we don't bitch, we won't get anything better. I look forward to some great new models in the next few years. There is a lot of available technology
that is just starting to get looked at.
In 5 years we should be able to look back at the crap we're riding now and laugh.
Silly Later, Fourlix

 

Youd better go and buy a 500Scrambler quick.If your talking raptor or d.s then for sure you need a Scrambler.
Matt
2000 500 Scrambler and very happy with it

 

Hey 3TV
I had a similar situation were by my buddies rather beaten and older 500 Scrambler smoked mine off the start but were just about the same top end.
After much tinkering and checking I found that my belt alignment was off(from the factory) and I had to space the driven clutch.No laggs any more . You'll need the special line up tool .Either your dealer will lend it you or if there cheap like mine they'll sell you one.
Hope this speeds thing's up for you.
Matt
2000 500 Scrambler.

 

Hi,
Have you tried the Xplorer 400 2 stroke? I just added a Aaen trail pipe, clutchkit, KN airfilter and of course had proper jetting done. It really woke the 2 stroke engine up. I hear its alot harder to get that 500 4 stroke going like a 2 stroke. I have owned both the Sportsman and the Xplorer. The Sportsman is a great machine but I wanted more speed and my buddy zipped by me on his Xplorer so I bought the Xlporer made the mods now I zip by him. It will bring the front tires off the ground in high range now. If you raced 2 stroke motorcycles then you might consider a 2 stroke ATV. Im sure you know all this. With minor mods the 2 stroke goes pretty good! I can feel the power band in mine now.

Thanks,
Paul

 

Fourlix , Do you mean in five years we are going to be riding 1989 250R's ?

Oh lighten up It was just a joke .. Or was it ??

 

There are good features out there on all kinds of different ATV's. The 400 Polaris water-cooled 2-stroke in the Explorer and Scrambler is Polaris'
best performance engine. The 500 4-stroke doesn't measure up despite the extra 120 cc's. Polaris really needs a hot new motor.
But the Sportsman has that great Independent Rear Suspension, giving it great ground clearance and the best ride. Everything else is okay to good, but that is it's best feature. It's biggest negative is it's weight.
On the other end of the scale, the Cannondale has Electronic Fuel Injection, common everywhere else, but a first for this throw-back industry. It's aluminum frame and super light weight are also impressive. And the Raptor has to be given credit for the first Big bore sport bike with a modern engine weighing in super light with reverse. The new Kawasaki V-twin is looking very good, though I think their wet braking system is un-impressive in a standard swing arm design.
Old Standards like Honda 250R's and 400 EX's leave me cold. That's just plain old news. The new entry level 300EX is great for kids, and I think the Honda marketing boys really scored here.

But these are all just shreads. Why not have all of these good features in one quad? or in a lineup of quads? The factory guys all know this. It's like the Tucker automobile back in the 50's. The big auto makers killed it because it was too avant garde and would have cost their industry millions to modernize. The ATV industry needs to MODERNIZE!!!
Silly Later,,,Fourlix