Setup and Jetting

[jTrue]

Sorry for the novel, new to carbed 2 stroke motors. So I just purchased an 04 Mountain Cat 900 a couple weeks ago. When I would get into deep snow it would bog very badly. I riveted another stock snow deflector onto the original deflector. I also plumbed a snorkel to the speedo hole and gutted the airbox. Also put a ring of rtv around the exhaust outlet to seal it, so exhaust didn't come back in. Just took it out this weekend and it seemed to run a bit better, but still not getting the power I think I should and it's still bogging a bit.

I ride in Utah about 8k to 10k feet 0-20 F. Usually run a mix of 110 and 91 for a final octane of around 94-95. I was told a squish measurement was taken and it was on the edge between pump/race gas.

I checked the plugs and it looked like I was running pretty rich even after the snorkel/airbox gutting. I pulled the carbs and checked the main jets, 480 PTO / 470 MAG side. This seems really high compared to what others run with a similar setup at the same altitudes/temps. I know every sled is different, but this seems excessive. I have a KOSO Dual EGT gauge on its way, but should I be ok dropping down to 460 PTO / 450 MAG to see if the bog clears up? Appreciate any help.

Mods are as follows:

Black Magic Heads Pump Gas Insert(anyone know listed CR for these?)
SLP Twins Pipes and Silencer
Bored Carbs
VForce 3 Reeds
Gutted airbox
Snorkel to speedo hole

 

[jTrue]

Any recommendations?

 

[Idcatman3]

I would guess that with the bogging you describe you would be ok dropping your jets that much. We'd know better if you took some good pictures of your plugs and posted them. It makes me a little nervous telling someone what changes to make unless I know all the details. I don't want to recommend something that burns someones engine down.

Also, if you could find out what the squish measurement was, or take another one (I can give instructions if needed) that would be helpful.

With the bored carbs and twin pipes, you're going to be running larger jets than someone with stock carbs and either the stock pipe or an aftermarket single.

 

[jTrue]

I would guess that with the bogging you describe you would be ok dropping your jets that much. We'd know better if you took some good pictures of your plugs and posted them. It makes me a little nervous telling someone what changes to make unless I know all the details. I don't want to recommend something that burns someones engine down.

Also, if you could find out what the squish measurement was, or take another one (I can give instructions if needed) that would be helpful.

With the bored carbs and twin pipes, you're going to be running larger jets than someone with stock carbs and either the stock pipe or an aftermarket single.

Interested into taking another squish measurement, thought the previous owner said it was around 65?? Did he mean .065?

 

[Idcatman1]

.065" is what he probably meant for the squish clearance. With that you should be good to run pump premium depending on what your compression is.
Every 800 and 900 Cat I have been around has been way rich at low speed and I would bet even with the mods you have you could benefit by using smaller pilot jets. They come with #50 and I have used as low as 27.5, usually 30s is what I end up with. I would drop one size at a time and see if it improves. It has always seemed to me that the low end jetting has more to do with the engine bogging in deep snow than the main jets do, but some will disagree with that.

 

[jTrue]

.065" is what he probably meant for the squish clearance. With that you should be good to run pump premium depending on what your compression is.
Every 800 and 900 Cat I have been around has been way rich at low speed and I would bet even with the mods you have you could benefit by using smaller pilot jets. They come with #50 and I have used as low as 27.5, usually 30s is what I end up with. I would drop one size at a time and see if it improves. It has always seemed to me that the low end jetting has more to do with the engine bogging in deep snow than the main jets do, but some will disagree with that.

Thanks, appreciate the help. Probably just gonna drop one jet size for now to 470 PTO 460 MAG. See if that helps, then hook up the EGT gauge for further tuning and playing with pilot jet sizes. I'll take pics of the plugs when I get out to do a couple 3/4 throttle and WOT runs this weekend.

 

[Meatman]

if you are running a smaller jet in your mag side than your pto side something maybe wrong. keep a close eye on your plugs...if running staggerd jetting you should be larger on your mag side.

with that being said, try dropping the clip on your needle.

 

[high time]

Yeah, I shouldn't tell you to rejet but,

You're richer than me and I'm at 1.300'. I am running 13 to 1 heads , 2 deg advance and Jaws twins. 1/4 110 octane. I sure don't need any race fuel at high alt.
I run 360s at 8 to 10,000 and no race fuel. It never bogs. Learn how to read plugs and piston wash.

Owen

 

[mp_2009]

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the mag side should have the bigger main jet like mentioned above