Compression Release Cylinders vs No Compression Release

[izzni]

I have an 02 800 RMK. I had a piston fail and ended up trashing a cylinder. I rebuilt it and managed to put both pistons in wrong.....at least I was consistent. In anycase, 2 pistons and cylinders later I now have the VES cylinders that have compression release on them (the 02 800 RMK cylinders have VES but not compression release).

The first thing I noticed is that I was pig rich. I normally run 530s (equivelent of a 495ish or so), but with the new cylinders I was loading up on a day that had a high of about 0 degrees. I rejetted to 470s and I'm seeing that the EGTs are still pretty low on a 10 degree day, but it isn't loading up anymore. I'm guessing I might have to go down to about 450 to get nominal 1225 degree EGTs I used to aim for on my machine.

The question I have is whether anyone knows that the rejetting I'm having to do is related to cylinder change, or whether I have something else going on.

 

[HammsRMK]

hey thats a good question, we had the same motor we sent it to a shop to get replated and we got differnet jugs back with the decompression ports. i didnt even think of that but we have not rode it much sence the rebuild to even notice if its lean or running fat. sorry i cant help but am interested in your situation.

 

[sled_guy]

No way will the decomp holes cause that sort of jetting change need. At altitude in fact, they don't cause enough to need at most 1 jet size change. From what I have seen the decomp holes just change the throttle response. Dyno guys say they are worth 3 hp or so.

Where did the new cylinders come from? Are you sure they are stock cylinders and not radically ported? Or were your old ones ported and these aren't? I've seen porting make a 3-4 jet size difference.

sled_guy

 

[izzni]

The old ones were not ported, and the new ones are remanufactured ones, so it is unlikely that they are ported.

To add a few more wrinkles:

My dad bought an 2005 800 Touring (which has the cylinders with the compression release) and the jetting across the chart is 30 to 40 smaller than the jetting on the chart for my 2002 800 RMK.

Perhaps in addition to adding the compression release they also changed the porting? Would porting have an effect on the jetting? I always figured that it was more related to the CDI's programming on my dad's sled that it needs smaller jets, but I never looked into it.

 

[HammsRMK]

what jets are you running? for some reason the jets we are supposed to run are the 520n from polaris but they are way to fat. i actually have 460 in mine and have had no problem and if you convert the 520n to normal jets that a 560 so im not sure why mine is still running. i know it rips and have never had a problem but my bro did, and his is the same exact sled and setup. his has gone down twice.

 

[Hardass]

Some of the reason there is a difference between the 02 and 05 is the needle and jet sizes were different i believe.

 

[clutch man]

the release hole does indeed make a difference yes!!
change to the needle that was in the 04 sleds and this will help too
sounds like your on the right track. watch you wash too when you get up to 1200 or higher. my 04 i run at 1340 all day long with Polaris pistons. and a good 1/4 wash too. this is at elevation 6000 ft plus for me.

I have an 02 800 RMK. I had a piston fail and ended up trashing a cylinder. I rebuilt it and managed to put both pistons in wrong.....at least I was consistent. In anycase, 2 pistons and cylinders later I now have the VES cylinders that have compression release on them (the 02 800 RMK cylinders have VES but not compression release).

The first thing I noticed is that I was pig rich. I normally run 530s (equivelent of a 495ish or so), but with the new cylinders I was loading up on a day that had a high of about 0 degrees. I rejetted to 470s and I'm seeing that the EGTs are still pretty low on a 10 degree day, but it isn't loading up anymore. I'm guessing I might have to go down to about 450 to get nominal 1225 degree EGTs I used to aim for on my machine.

The question I have is whether anyone knows that the rejetting I'm having to do is related to cylinder change, or whether I have something else going on.

 

[izzni]

Thanks!

I didn't think about the needle size, I'll change that as well. That would explain a lot of the lower throttle range behavior I've seen as well.

 

[sled_guy]

The 02 has way leaner needles than the 03 and newer.

But again, just the difference between compression release hole and not won't cause that huge of a jet change.

sled_guy

 

[izzni]

Ok, so I was investigating what needles I need, and I found two options.

I currently have the 2002 800 RMK needles, which are J8-9D6J03-57.

I was looking at 2003 and newer 800 motors and I found the following theme:

2003 800s have J8-9DGN05-57

2004 and 2005 800s have J8-9DGN05-57 or J8-9DGN06-57 depending on the particular model.

In the same year range the 700s usually have J8-9DGN05-57.

So...I'm trying to figure out whether I need to acquire a J8-9DGN05-57 or J8-9DGN06-57. Anyone know why some 800s have one versus the other? Which one is leaner? Polaris likes to set things fat to protect their ***, so I assume I want the leaner one, especially since I have EGT gauges and know what are the safe temp ranges.

 

[sled_guy]

You want the 03 needles. You want the richer ones because the 800s are lean in the midrange and those needles richen it up. Then you can step down the mains quite a bit.

sled_guy