Polaris RMK 800 snowbike

[Hawkster]

Looks like you've been given a pretty good list to go threw
The seat height thing your addressing is a good one also , your building a machine with handling characteristics to the power it has to offer .
There's a magic number that all manufactured high performance motorcycles set them at and if raising or lowering it would give them a competitive edge they'd do it .

Didn't one of the manufacture's drop the whole machine 3 inches for this year ?

By the way if you build a new tunnel make sure the rear upper idler wheel brackets don't allow rubbing , polaris is scrubbing ponies by allowing that

 

[Brett Kobernik]

My suspicion is that the airbox is
part of your culprit for low rpm.

do some searching on “gutted airbox” on pro rmk sleds. (Most guys lost low end power and peak rpm when the air horns / shelf was removed)

The pipe also changes the “powerband” when modified. You need to account for the material lost in the cutting/ fitting process. I added a 7/16” ring into the fattest part of my skidoo pipe i modded to fit my home built snowbike.

having a different powerband can also be partially compensated for with clutching and gearing. Pro rmk sleds are high geared to begin with stock and i had great luck gearing them down 20% even though i ride close to sea level.

drop your primary clutch weight 4 grams and gear it down a bit should transform your performance.

(all this is assuming your engine internally is performing correctly)

a bad exhaust valve solenoid can be diagnosed by plugging the line going to it and seeing if the peak rpms raise.

Thanks for the thoughts. I'm definitely suspicious of my intake. Currently, I've completely removed the box. Only the horns are on the throttle bodies. Air sensor is zip tied about 6 inches above them.

I did some reading about gutting air boxes. Seems like mixed opinions about whether they affect performance or are mainly for noise reduction. We'll get to the bottom of this.

Currently I'm more suspicious of the expansion chamber/can set up.

 

[Brett Kobernik]

Wow did it run right before the project? losing over 1000 rpms on a cvt would mean you lost well over 20hp! that's more than half the power a 450 even makes at 10000 feet!
I Doubt the pipe or intake could make that big a change by them selves unless you drastically screwed up the pipe angles. Check the power valves for sure. If you cant tell that they are getting 100 percent open see if there is a way to jam them manually open for a test.
Also check the clutch aligment and all the rollers and bushings in the clutches.

Yeah, engine was running in the donor sled. And you're on it with HP loss. Serious issue.

There's a couple of tight angles in the pipe. That's one of my concerns. I'll double check all of the components but I suspect the issue is physics.

Side note, I realized we have a mutual acquaintance Jeremy in Orem.

 

[Brett Kobernik]

A pipe could make all the difference. I tried a set of twins on a 2007 ski doo rev that were made for an 800ho not an 800r and I lost huge amounts of rpm at elevation. But without a dyno to adjust things its anyone's guess what's going on.

You might have to try a fuel controller to get it dialed.

Exhaust is highly suspect.

 

[Brett Kobernik]

Thanks for all the thoughts boys. We'll figure this out. Bottom line is that this thing is a TOTAL success and is only a few minor tweaks away from being exactly what I'd hoped for.

Got a number of tests in mind including hanging on another stock expansion chamber and stock can and running it across a field to confirm if exhaust is the main culprit here.

I'm close and this thing is sick. For that matter, the Unicorn with its 580 is completely on the next level. Unbelievable......

 

[Brett Kobernik]

The seat height thing your addressing is a good one also , your building a machine with handling characteristics to the power it has to offer .
There's a magic number that all manufactured high performance motorcycles set them at and if raising or lowering it would give them a competitive edge they'd do it .

The funny thing is that my two buddies both agreed that the sitting stance was better on the Unicorn than the 800. And I have to agree. The odd thing is that the Unicorn has just been tweaked and modified over the years with no real serious direction. One mod happens then another then another. But the ****er rips!! I'm just trying to duplicate that with the 800 and a full width 3" paddle track.

 

[couch]

Thanks for all the thoughts boys. We'll figure this out. Bottom line is that this thing is a TOTAL success and is only a few minor tweaks away from being exactly what I'd hoped for.

Got a number of tests in mind including hanging on another stock expansion chamber and stock can and running it across a field to confirm if exhaust is the main culprit here.

I'm close and this thing is sick. For that matter, the Unicorn with its 580 is completely on the next level. Unbelievable......

Yep - likely one of the first simple tests to establish the problem

Sent from my motorola edge plus using Tapatalk

 

[NEWDUCK]

Thanks for all the thoughts boys. We'll figure this out. Bottom line is that this thing is a TOTAL success and is only a few minor tweaks away from being exactly what I'd hoped for.

Got a number of tests in mind including hanging on another stock expansion chamber and stock can and running it across a field to confirm if exhaust is the main culprit here.

I'm close and this thing is sick. For that matter, the Unicorn with its 580 is completely on the next level. Unbelievable......

I was just going to suggest that. If you can rig up the stock pipe quick and take it for a rip that will eliminate the issue hopefully.

If it is indeed the pipe there is a very talented guy in malad, ID that could duplicate the stock pipe but fit it to your chassis.
Hes "retired" but might enjoy an odd project like yours.

He built these. Wish I had a better picture. Works of art.

 

[Brett Kobernik]

Had a setback last week. My alternative spindle mounts blew apart when I clipped a rock. I didn’t clip it very hard. My design was weak. Had to find another set of forks. Fabricated some standard style clamps. This is a bit lighter than my original setup. Back on the snow tomorrow for some more fine tuning.

 

[Answ3r]

Had a setback last week. My alternative spindle mounts blew apart when I clipped a rock. I didn’t clip it very hard. My design was weak. Had to find another set of forks. Fabricated some standard style clamps. This is a bit lighter than my original setup. Back on the snow tomorrow for some more fine tuning.
View attachment 365541

Put a cross tube to act like a nut for both front bolts. Otherwise if you hit it wants to rotate and split your fork appart. That's the weak spot of all timbersled mounts.

 

[Brett Kobernik]

Wow. This thing is fun. “Smooth” was my buddy’s take today when he tried it for the first time. I’m still chasing down lack of maximum rpm with no solid conclusion yet. I’m not worried. I’ll get it.


****ty winter so far. Dealing with lots of riding on roads with firmer conditions. I decided to swap tracks with another project I have that is on hold. Same Camso Conquer track but cut down to 13.5” wide. It has the outside corners trimmed off to give it a convex profile. No news here but the convex profile is superior on firm surfaces. I’ll most likely trim the corners on the full width track intended for this machine.

 

[Hawkster]

I wont do the straight lugs , I'll accept what ever the lug height loss is to have a rounded tire response compared to a self leveling straight lug . I want the handling characteristics of a motorcycle not a sled .

 

[Pjjansson]

If you want the most "wheel like feelig" then go whit a monorail setup. Sliding on roads or hard pack trails it is just so much better!

 

[Brett Kobernik]

I wont do the straight lugs , I'll accept what ever the lug height loss is to have a rounded tire response compared to a self leveling straight lug . I want the handling characteristics of a motorcycle not a sled .

I think I'm in the same boat. While I don't do much trail riding, I find that it's nice to have the bike roll over when I'm up on exposed ridges where the snow is firm from wind. I'm finding only a slight taper is all that's needed for me.

If you want the most "wheel like feelig" then go whit a monorail setup. Sliding on roads or hard pack trails it is just so much better!

View attachment 365626

I really do like the single rail theory. If I were in more firm terrain, I think I'd go that route. Most of my riding is in soft snow. That looks like a recipe for a wet rider with no tunnel over the track. That definitely wouldn't cut it in my zone. We'd be soaked within an hour.

 

[Brett Kobernik]

I think I found my issue with not getting full rpms. One exhaust valve diaphragm had a big crack which means only one exhaust valve was opening. Easy fix. Testing tomorrow.

 

[Hawkster]

Those are prone to that , be careful that you don't stress them on cleaning also . They did some really stupid things on some models , like the air plenum drain hole directly above the mag side . What bulletin , that's normal wear and tear

 

[Brett Kobernik]

Whoa.

 

[Sheetmetalfab]

I think I found my issue with not getting full rpms. One exhaust valve diaphragm had a big crack which means only one exhaust valve was opening. Easy fix. Testing tomorrow.
View attachment 365684

Rpm up?

 

[Brett Kobernik]

Rpm up?

Yes, but not all the way yet. 7500 to 7600. The exhaust valve fix was huge. My coach says we should look into clutching. Got another sprocket coming to gear down as well. I'm working on intake also. This thing is so sick. Pulled a small hill on it the other day. Freaking thing felt great. Handles soooooo nice and will only get better with more fine tuning.

 

[Brett Kobernik]