Modded SL900 Hesitation. Jetting? Prop?

[RobbyP]

Hello all. Last year I built an SL900 with the following modifications:

Chopper city high compression domes (150 psi)
Nujet 6 degree progressive pitch impeller
CarbonTech reeds
Virage 6 vane ss stator
Hot Seat waterbox
Bored .5mm over with NOS polaris pistons
converted to premix (40:1 ratio)

Carbs where rebuilt with kits from Watcon. Pop off in the mid 20s between all carbs. As of now it has a low-mid range hesitation where it will lose power until you pump the throttle a few times then it will take off. This dosent happen all the time but enough for me to know its an issue. Also it will only top out at around 6100-6200 RPM. I think I should be getting alittle more RPM out of the top end. Ive talked to a few people about this and Im getting two answers. Some people are saying the prop I have in it is too tall for the 900 and thats my hesitation and why Im not getting full RPM at the top end. Most people are saying its the carb jetting. Yes the jetting is stock right now. I know with my mods it should be changed, Im just not experienced enough with tuning these to know what jets to use. Is it possible someone has a build close to mine and could let me know what jets theyre running?

Thanks

[xlint89]

Welcome to the Hulk.

According to the impeller chart, the Nujet PL-6.0 is recommended for the 900.

https://www.greenhulk.net/forum/personal-watercraft-performance-skis/polaris-pwc-performance/polaris-tech-tips-and-info/12914-driveline-propulsion

So I'm going to guess it's carb/jet related.

1. Have you verified the accelerator pump is working?

2. Are the carbs in sync?

3.When you did the carb rebuilds, did you verify all the circuits and jets were clear?

4. Did you write down the jets you have for each cyl? What are they?

5. Did you replace the fuel hoses also? The gray Tempo brand fuel hoses are known for forming some green goo that clogs things up.

6. Did you set your carb screws according to the manual for your model and year?

It's very tough to recommend jetting because of so many variables. Plus the fact you made a few changes at the same time makes it harder to nail down. The easier way is to make a change and adjust the jets. Then make another change and adjust the jets, etc... because you have only made 1 small change at a time. When you make multiple, you have a harder time trying to figure out how everything is effected.

Are you familiar with spark plug reading and chops? Are you familiar with carb adjustments?

Are you familiar with piston wash?

These are the tools that will help you identify what changes will need to be made to your fuel requirements.

[RobbyP]

Welcome to the Hulk.

According to the impeller chart, the Nujet PL-6.0 is recommended for the 900.

https://www.greenhulk.net/forum/personal-watercraft-performance-skis/polaris-pwc-performance/polaris-tech-tips-and-info/12914-driveline-propulsion

So I'm going to guess it's carb/jet related.

1. Have you verified the accelerator pump is working?

2. Are the carbs in sync?

3.When you did the carb rebuilds, did you verify all the circuits and jets were clear?

4. Did you write down the jets you have for each cyl? What are they?

5. Did you replace the fuel hoses also? The gray Tempo brand fuel hoses are known for forming some green goo that clogs things up.

6. Did you set your carb screws according to the manual for your model and year?

It's very tough to recommend jetting because of so many variables. Plus the fact you made a few changes at the same time makes it harder to nail down. The easier way is to make a change and adjust the jets. Then make another change and adjust the jets, etc... because you have only made 1 small change at a time. When you make multiple, you have a harder time trying to figure out how everything is effected.

Are you familiar with spark plug reading and chops? Are you familiar with carb adjustments?

Are you familiar with piston wash?

These are the tools that will help you identify what changes will need to be made to your fuel requirements.

accelerator pump is working

carbs are in sync

cleaned carbs in an ultrasonic cleaner, never had one not come out 100% clean

not sure of jets, id assume stock?

all fuel lines replaced

carb screws are set 1 turn out. i believe factory spec is 3/4 turn out.

somewhat familiar with plug reading and tuning. ive had the ski out twice and the plugs dont look dangerously lean. as for tuning I know how to make minor adjustments and get a ski dialed in that is almost there but when it comes to jetting I usually can ask and someone can recommend jets for my yamahas and kawis.

I know about piston wash but honestly I havent checked this ski. I have maybe 6 hours on it since the rebuild. Is that enough to tell?

Also, could my oil/fuel mix have something to do with it? maybe the mixture is too heavy on oil at 40:1? maybe try 50:1? I wouldnt go higher than that.

[xlint89]

OK, what year is the SL 900? That will matter when it comes to stock jetting and screw adjustments.

6 hours should be good to start looking at the wash pattern.

Premix will have a small role in jetting. I'd stay at 40:1 with a quality TWC3 rated oil.

[RobbyP]

It is a 1996

[xlint89]

Manual shows 96 as having Keihin CDK II 38mm carbs.

Main jet MAG 138
Main jet CEN 140
Main jet PTO 135

Mid range jet 65

Slow jet 60

Low speed screw at 5/8 turn

Pop off pressure of 20-24 psi

Idle speed in water 1250-1300 RPM

Without knowing what jets you actually have installed, we're just guessing/hoping you have stock.

Hopefully the mid and low jets aren't switched.

And if you're 1 full turn out on the slow speed screw, you're prob too rich.

It's much safer to be too rich and take fuel away, than to be lean. Less chance of piston damage.

[RobbyP]

Manual shows 96 as having Keihin CDK II 38mm carbs.

Main jet MAG 138
Main jet CEN 140
Main jet PTO 135

Mid range jet 65

Slow jet 60

Low speed screw at 5/8 turn

Pop off pressure of 20-24 psi

Idle speed in water 1250-1300 RPM

Without knowing what jets you actually have installed, we're just guessing/hoping you have stock.

Hopefully the mid and low jets aren't switched.

And if you're 1 full turn out on the slow speed screw, you're prob too rich.

It's much safer to be too rich and take fuel away, than to be lean. Less chance of piston damage.

Ill take a look at piston wash and get back. If its too rich I might tweak the screws down slightly and just live with the slight hesistation. Idle speed is right in that spec and the ski always starts up very quickly and has never stalled out

[RobbyP]

Got these with a borescope. Mag is the one you can see the arrow, center is the next one and the PTO is totally covered. The white spot in the centet is the reflection of the camera light

[xlint89]

I agree ^^^^^

Pic on the left is way rich

Pic it the middle is rich

Pic on the right is slightly blurred, but I would say lean. I don't see much wash.

It's not mine and I don't want to be blamed for ruining your engine. So I'm going to play it safe and suggest dropping 1 jet on the left, leave the middle alone, and add 2 jet sizes on the right. Run it like that for a little while longer and re-check the wash. We can adjust again after that. It's the safe way of doing it.

I find it interesting that the first 2 are rich and the PTO is lean. I'm guessing 1 of 2 things here. PTO jets are different from stock, or you have a rear main seal leak you are compensating for.

What was the reason you ended up doing the rebuild? Did you ever do a leak down test after the engine was reassembled?

It's always a good idea to do after a rebuild. Normally there's something that fails and leads to needing to be repaired. If that problem is not found and you just replace the pistons, it will most likely happen again if the original cause was not resolved. A lot of times it's gummed up carbs, but it's also common for the crank seals to suck air and cause the MAG or PTO to run lean.

[RobbyP]

This ski was interesting.

Cen piston had damage on top of it like the prong of the spark plug broke off and smacked the piston and head. Cylinders where fine though. Was only getting 80psi from that cylinder and the mag and pto where at 120psi.

When I pulled the top end apart I found that the mag and cen where bored to .5mm and the pto was stock size. So I got the pto cylinder bored out and the other two honed so every cylinder is now .5mm over.

I know its not much but maybe the jets are different in the pto since it used to be smaller. id assume the previous owner had to know about it and maybe jetted up the other two cylinders to compensate?

[xlint89]

That is a possibility.

If/When you open up the carbs please write down what jets you have for each cyl. Also make note of the Slow and mid range jet location. That could also play a role in hesitation.

[RobbyP]

I will do that for sure. Might be a few weeks before I get to that but I will pull jets from all carbs and update you. Best case hopefully ill match the PTO jets to the CEN jets if they are if fact different.

[casey67]

Manual shows 96 as having Keihin CDK II 38mm carbs.

Main jet MAG 138
Main jet CEN 140
Main jet PTO 135

Mid range jet 65

Slow jet 60

Low speed screw at 5/8 turn

Pop off pressure of 20-24 psi

Idle speed in water 1250-1300 RPM

Without knowing what jets you actually have installed, we're just guessing/hoping you have stock.

Hopefully the mid and low jets aren't switched.

And if you're 1 full turn out on the slow speed screw, you're prob too rich.

It's much safer to be too rich and take fuel away, than to be lean. Less chance of piston damage.

This was/is a good guide and starting point.

[RobbyP]

UPDATE: took in out yesterday for alittle on the water. turned out the PTO screw 1/2 a turn to add more fuel. First two hours the ski was pulling hard and running good then I was at mid throttle cruising up to a buddies boat and RPMs wouldnt go over 3000 and had no power. Limped it back to the boat launch and got it home. PTO piston got burned through. Cylinder and head suffered no damage so I will just be putting a piston in it and running it. I did pull the carbs and checked the jets on the CEN (has the best fuel mixture) and PTO. Here are the jet sizes

CEN:
both smaller jets at the top size 58 (I dont know CDK carbs that well)
larger main jet 142

PTO:
both smaller jets at the top size 58
larger main jet 138

Seems like I want to atleast run a size 142 in the PTO. Maybe alittle bigger. also seems like the slow and mid range jets are smaller than what they should have been from the factory? My idle is fine so not sure if I should mess with the slow speed jet. Like I said before I dont have alot of knowledge about rejetting these carbs so hopefully someone here can point me in the right direction. (I wouldnt mind being alittle on the rich side just to be safe)

Also, is it possible for me just to put new rings on a used piston (its in good shape) and run it or just try and find another NOS polaris piston and run that?