Jetting - 91 Indy 500 with PSI pipe - Snowmobile Forum: Your #1 Snowmobile Forum

skidooosl · 01-02-2008, 01:45 AM

Could anyone tell me the stock jet sizes for a 91 Indy 500 just above sea level (Michigan)? (for a stock sled)

Also…

The sled currently has a PSI pipe on it… could anyone please tell me the jetting and other changes recommended for that pipe?

I googled PSI for a bit but could not find a web page for them or any support numbers to call and find this info…. The previous owner said it was jetted to run very rich because he was worried about meltdown… It shows… the sled BBLLOOWWWSSS through spark plugs and they always come out black and fouled… I am not sure the current jet size in the sled.

Thanks

skidooosl · 01-03-2008, 11:01 AM

anyone?

IRONDOG · 01-03-2008, 11:13 AM

Quote:

Originally Posted by skidooosl

anyone?

Stock pilot is 40, main jet is 280.
Jerry

ridgerunner · 09-07-2008, 01:02 PM

I'm going to bump this thread cause I just bought a PSI pipe for my '90 500 and was wondering what I will need to do for jetting. Also wondering if I swap to foam filters and ditch my airbox will that need jet changes also?

BC_Dan · 09-07-2008, 03:03 PM

When I put PSI twins on my 700, PSI told me to go 3 sizes up on the mains as an initial start. Jets aren't terribly expensive, start rich, work your way back, checking plug color and piston wash as you go. You should have cardboard brown insulators on your plugs, if you have lighter than that, you are too lean and take a chance of burning holes in the pistons. Until you can locate a proper jetting chart for temp and elevation, you will have to check plugs often to develop safe jetting for the engine. Cold temps require more fuel, as does going down in elevation.

If you swap the airbox for filters, you may not see any increase in performance. The airbox is designed to take cold air from outside the engine compartment. If you remove the airbox, you will be breathing warm air from inside the engine. Cold air is denser than warm air, which means there is more of it, which has the potential for more power. Factory jetting charts are also based on the airbox in place, so again, you have to come up with your own jetting chart.

The airbox on the early Indys was also designed to reverberate with the engine, which actually slightly "supercharged" the air charge going into the carbs, giving slightly more hp. If you go to pods, you will lose that as well.

ridgerunner · 09-07-2008, 04:08 PM

wow... thanks for that. so the airbox stays and i should just buy some jets, start rich and go from there. i am new to the jetting thing so i want to make sure, larger jet is richer (more fuel) i assume. also, you talk about piston wash, what is that. i know the plug thing, but "wash" is a new term to me. thanks again.

BC_Dan · 09-08-2008, 12:50 AM

You are correct, larger jets flow more fuel and make the fuel/air ratio richer.

In a two-stroke engine, the fuel/air mix comes up from the crankcase and into the cylinders via porting in the cylinders. The area on the piston where the fuel enters will get "washed" by the fresh fuel/air mix and will not develop carbon like the rest of the piston will. You can easily see the area washed by the fuel/air when the piston is at bottom dead center and if you have a good light (bend-a-lights work great for that). If you have a thumbnail sized area of wash at each intake port, you are jetted safely. If you have a large area of wash, all around the piston, you are jetted rich. If you have carbon completely side to side, you are lean (no wash). Lean is bad, lean will burn hot and you can get hot enough to burn holes in the pistons.

Here's a thread to a pic of acceptable piston wash...

http://www.snowmobileforum.com/engin...ck-out-my.html

**smallengineguy** · 09-08-2008, 07:13 AM

Awesome thread BC Dan.

ridgerunner · 09-09-2008, 08:31 PM

i agree with smallengineguy (i just now got a chance to read the reply). i am always amazed at how much knowledge that there is on this site, and how much people are willing to help. thanks to all

white lightning · 09-16-2008, 06:24 PM

Ridgerunner - I used to have a '89 Indy 500 with PSI single pipes. I bought the pipe from PSI new and they gave me the instructions/recommend sheets with their pipes. I dont remember where I put them sheets in my garage but Id highly recommend you to contact PSI directly via email - good luck. WL
Bruce@PSIPOWERInc.com

ridgerunner · 09-21-2008, 01:50 PM

thanks for the contact white lightning.... i sent him an e-mail and this is what i got "Dave, you will need to jet up 2 sizes and add 2 grams of weight on ea. arm." i assume he meens 2 grams on the clutch weights. do i have to do this now, or can i just set the sled and do a full clutch kit later (i want a trail performance kit for primary and secondary clutches)?

BC_Dan · 09-21-2008, 10:05 PM

When you put aftermarket pipes on an engine, it changes the RPM where the engine makes max. HP and torque. By going bigger on the flyweights as PSI is suggesting, you are putting the RPM where they think the power should be (and they did the testing, so they should know!). You will notice improvement if you jet it and clutch it to what PSI said. If you don't add the weights, you may actually see a decrease in performance, which isn't what you are looking for after you do all this work! PSI probably has a recommendation for the secondary as well, for spring, ramp angle, which hole to put the spring in. To get the most out of their stuff, follow their recommendations.

ridgerunner · 10-07-2008, 10:54 AM

Quote:

Originally Posted by BC_Dan

When you put aftermarket pipes on an engine, it changes the RPM where the engine makes max. HP and torque. By going bigger on the flyweights as PSI is suggesting, you are putting the RPM where they think the power should be (and they did the testing, so they should know!). You will notice improvement if you jet it and clutch it to what PSI said. If you don't add the weights, you may actually see a decrease in performance, which isn't what you are looking for after you do all this work! PSI probably has a recommendation for the secondary as well, for spring, ramp angle, which hole to put the spring in. To get the most out of their stuff, follow their recommendations.

alright, i was going to order the jets and weights this week-end and realized, i don't know what the stock weights are in my sled. I don't have it here to check and see if they are marked. i plan on doing as psi suggests because, well, you are right, they did do the testing, but i want to know if i can change the clutch engagment alittle bit to be higher in the rpm range. if i remember right from last year, it felt alittle bit low, but i don't remember the color of the spring either. will changing the weights to a heavier bring the rpm down at engagement, higher, or no change? this is my first time trying to mess with a clutch, and i want to learn what i can before i tear it apart.. thanks for the help guys.

ps. farmers almanac is calling for 2-3ft more snow in northern maine (where i sled) than last year. thats sick considering last year was at like 220-230in. of snow. I would recomend a trip to anyone that can get there, amazing riding when they get 8-12in of fresh snow a week.

white lightning · 10-07-2008, 04:40 PM

When I had my PSI single piped, I dont remember even having to monkering with the weights or any clutching components - Only thing I changed was the gearing and change the jettings per PSI's spec at the time. With single PSI piped, it uses same stock revs at 7.8k Also I see you have 192 studs - you have way too many of them and it will rob u on top end speed. Id go with 96 instead or at the most, 144. WL

ridgerunner · 10-07-2008, 09:15 PM

Quote:

Originally Posted by white lightning

When I had my PSI single piped, I dont remember even having to monkering with the weights or any clutching components - Only thing I changed was the gearing and change the jettings per PSI's spec at the time. With single PSI piped, it uses same stock revs at 7.8k Also I see you have 192 studs - you have way too many of them and it will rob u on top end speed. Id go with 96 instead or at the most, 144. WL

when i emailed psi directly about the jetting, they told me to go 2 sizes up and to change the weights. i have never had to tune a sled for pipes, so i figured i should probably do what they suggested. and i know i have way to many pics, but i needed a track and some other stuff and a friend sold me a wrecked ultra w/ a brand new track for almost nothing, so figured i may as well. why did you change the gearing of the sled? did they recomend it? I like low end, and mid-range... top end to me is whatever, i meen i ride a 18yo indy 500. i was just wondering if the clutching would change my engagement.

white lightning · 10-08-2008, 11:02 AM

I changed the gearing because they stated two things, if I keep the gearing the same, only thing Id noticed, more snappier off the line and maintance the same top end, or change the gearing to increase top end speed for the lake racing - thats what I did. I was racing against guys with WILDCATS at back in that time. If you like the low-end and mid-range, then leave the gearing the way it is - just change the clutch weights per PSI's spec. WL

PS - I may had to changed weights with mine, I just dont remember as its been back in the early 90s when I did that. I have the PSI spec sheet but its somewhere in the box, in my garage.