Jetting/Carb Tuning Help Please

[FLY6584]

I am currently running stock N/S, 95g spring, 75pilots, and 135 mains on stock 701 with limited bpipe, ADA Head (185psi), stock 38's/stock flame arrestor(2 screens and chokeplates removed), and Carbon Tech LT Reeds.

I am 1.25 turns out on my high speed screws and 1 turn out on my low speed screws.

I am pretty happy with the way it is running right now, but I think I can get more out of it.

Occasionally when I hit it hard off idle I get a slight hesitation about midway up the power band.

Off idle hit is great and top end is a great as can be expected with a 12/17 swirl, but I just dont understand why it has that slight hesitation about midway through the power curve.:dunno:

I've got an idea I'm right on with my jetting specs, but does that hesitation sound like a lean/rich condition?

Like I said it's a very slight hesitation and not a bogg so I'm guessing its a lean condition.

Appreciate the help:biggthumpup:

 

[Big Kahuna]

might try opening the bottom screw a bit and trying that, then go the opposite direction to see what change you get.

 

[Matt_E]

Sounds like a lean bog. You should tune your low speed adjusters properly.
Put the ski in the water and have the engine idle.
Adjust the low speed adjusters slowly for highest/smoothest idle.
See how it feels after that.

might try opening the bottom screw a bit and trying that, then go the opposite direction to see what change you get.

IMHO, the bottom adjusters are too often used for attempts to adjust engine performance above idle.
They adjust idle mixture, nothing else. That said, any well tuned carb has its foundation in a properly tuned idle mix circuit.

The optimum setting for the lo adjusters is extremely easy to attain - as described.

 

[FLY6584]

Sounds like a lean bog. You should tune your low speed adjusters properly.
Put the ski in the water and have the engine idle.
Adjust the low speed adjusters slowly for highest/smoothest idle.
See how it feels after that.




IMHO, the bottom adjusters are too often used for attempts to adjust engine performance above idle.
They adjust idle mixture, nothing else. That said, any well tuned carb has its foundation in a properly tuned idle mix circuit.

The optimum setting for the lo adjusters is extremely easy to attain - as described.

Right now Ive got the idle set low so the engine will die because I hate lanyards.

Would you recommend turning the idle back up and following your procedure then when I have it dialed in go ahead and turn the idle back down?

Appreciate the help. It sounds like you and Harrison both agree it probably has something to do with the low speed circuit.

 

[Matt_E]

Right now Ive got the idle set low so the engine will die because I hate lanyards.

Would you recommend turning the idle back up and following your procedure then when I have it dialed in go ahead and turn the idle back down?

Yes, absolutely. Turn it up a bit so that you can detect both higher rpms and lower rpms when you mess with the adjusters.

A word of warning: Whenever you change anything else on the carbs (popoff, jets) you need to first retune your low adjusters. That's always the first step.

BTW, I hate lanyards, too. However, I think I will wear one in the winter time when I am alone in the water. Last Friday my ski got away from me and was idling upside down for far longer than I am comfortable with.

 

[FLY6584]

Forgive these very ignorant questions. I am new to carb tuning.

A buddy of mine and I were debating the other day on whether going richer on the main jets would affect low end snap or not. He said the main jets had nothing to do with low end snap and suggested I go up to 145 mains to improve my topspeed. I explained to him I didnt care about top speed and I didnt want to hurt my low end snap at all.

If I went to 145 mains would that affect my low end snap?

Also I was wondering are pilots solely for idleing and mains are for anything above idling? I was under the impression that pilots are for idle and when you are above idle you are actually on the mains and pilots.

Once again thanks for the help. I am trying to learn as much about tuning carbs as I can. Luckily for me I have a very common setup that should be very easy to tune.

 

[Big Kahuna]

your jetting is not going to really effect top speed, just how it is going to run at Max RPM's......... your prop is what will get top speed, that is why you always pick a prop for the style of riding you will be doing.

 

[Matt_E]

Forgive these very ignorant questions. I am new to carb tuning.

A buddy of mine and I were debating the other day on whether going richer on the main jets would affect low end snap or not. He said the main jets had nothing to do with low end snap and suggested I go up to 145 mains to improve my topspeed. I explained to him I didnt care about top speed and I didnt want to hurt my low end snap at all.

If I went to 145 mains would that affect my low end snap?

Also I was wondering are pilots solely for idleing and mains are for anything above idling? I was under the impression that pilots are for idle and when you are above idle you are actually on the mains and pilots.

Once again thanks for the help. I am trying to learn as much about tuning carbs as I can. Luckily for me I have a very common setup that should be very easy to tune.

Your mains will be optimal at only one specific jet number. Go below that, and you're lean - you will be below max RPMs. Go above it, and you're rich. You will be below max RPMs. A properly tuned carb will give you the best high speed possible with that setup.
So, you can't just increase main jets indefinetely to increase top speed - it doesn't work that way. If it did, why not go with a 250 main jet to begin with?
The pilots affect the carburetion throughout the entire RPM range. Their main effect will be noted from 1/4 to 1/2 throttle, but they contribute above 1/2.

 

[Jp116]

The tech article on carb tuning on the groupK site are what you are looking for. You can ask for advice all day long and any suggestion someone gives will only you get you a baseline anyway. The only way to properly adjust them is to put the boat in the water and like Big Kahuna said turn the screws and see what happens. If the problem gets worse turn them the other way. just a little at a time thou.

 

[SkiDaddy]

You need to check max sustained RPM w/a tiny tach, & tune the HS adjusters for the highest sustained RPM. There are articles decribing this. If you obtain max RPM w/HS adjuster around 1.0 turns out your main jets are correct.

Have you tried small adjustments on the FPP head pipe screws? BIG gains can sometimes come from there.....

 

[Jr.]

Here is a link to the Mikuni carbs web site
http://www.mikuni.com/fs-manuals.html

On the bottom of the page is the watercraft carb section
Read the SBN manual, you will get a good idea & theory on tuning your SBN..............

Have fun!!!

ski ya, Paul

 

[Watty]

Here's the way I learnt to carb tune...

You have to start somewhere with your jetting, so generally you are going to have some reccommended specs. Start with these.

Tie the boat up to a dock or trailer so the pump can be fully loaded (even at WOT) and start the ski and let it warm up.

From here, my goal is to be able to sustain the maximum smooth rpm when the ski is idling and also having my low adjusters 1 turn out. If the ski won't idle (without using any throttle), chances are you're rich. If you're lean, you're probably really lean if the engine is dying from a lean condition at idle.

Change your jets until you can get the ski to idle indefinately and the low speed adjuster is at 1 turn out. Once you think you have it right, let it idle for a minute or so, and then snap WOT. You're looking for a nice clean snap off idle. If it hesitates, but clears up as you continue to hold WOT, it's rich. If it dies, and stalls, or requirews you to fan the throttle to get the revs back up it's lean. If you can't tell the difference between the two (a lot of people can't) go richer and see if it gets better or worse. This should get your bottom end dialled in, but keep in mind, if your popoff isn't set right, sometimes no amount of tuning will get rid of an annoying little bog, but by all means, play around with it and see what works.

As far as the top end goes, I usually go around 5 or so jet sizes above what is reccommended and start with a 3/4 throttle run for about 20 seconds. Check the plugs, and if they are nice and rich, go ahead and give it a WOT run for 20 seconds (work your way up to 20 seconds so if there is a lean condition, you can detect it early). What you're looking for is a nice coffee bean color, or a little richer on the end of the plugs. I like to run my jets a little fat on top just to be safe.

Once you think you've got it right on the trailer/dock, take it for a ride and do the same thinkgs that you have just done. Sometimes when testing against a dock and riding the ski, adjustments have to be made.

Get on your knees, let the ski idle for a minute or so (no throttle whatsoever) and then grab a handfull. You might find the bottom a little on the lean side because of your weight/drag of water etc...

Adjust until it feels good and away you go.

I don't really understand when people say "I don't care about top end", as even a good freestyle engine needs to have the top end dialled in to get the most out of it. I think the difference is that when you're not holding WOT for prolonged periods, you can run the top end a little leaner to get more out of it.

Anyway, I've crapped on for enough, and my methods might be really left of centre, but thats how the (less) than average Joe does backyard tuning!

 

[FLY6584]

Thanks for all of the help guys especially Wattage for the detailed writeup.

 

[ClayMax]

Bringing up this oldie as it seems pretty related to my problem.

Have an 04 SXR with; Wet pipe, Tau Ceti's, Super Pump Cone besides the non relevant mods.

Recently put the Tau Ceti's back on after switching back to stock box for a bit. So I'm out riding yesterday, third ride of the month (no problems on other identical rides) when during a wot run my ski dies suddenly, just like running out of gas when empty. Coast to almost a stop, turns a couple times and starts up fine.

This is repeatable. Can ride all day long at 3/4 throttle but after about 20 seconds or so of wot it cuts off. Again, it cuts off with no warning just running full tilt then out.

I've changed plugs, didn't look bad, flushed it out well, looked very clean.

The wierd thing is I used to have the Tau Ceti's on and never had this problem, seems like it came out of nowhere.

Am I running lean or is this something else?

And can someone edumacate me on the screws, clockwise = leaner or richer?

Thanks.

 

[Dustin Mustangs]

cw = right tighty = closing off fuel passage = leaner

I'm not the best person to comment on this, but it sounds to me like your problem has nothing to do with carb tuning. I am guessing you didn't put everything back together right after installing the flame arrestors (or whatever the last thing is you did to the ski). Unless of course it's lean on top and your temp switch is shutting you down.

BTW, there is some really good reading in this thread. I will second the value of the link jr. posted.

 

[shepster]

Sounds like your running out of fuel from the wot run, meaning your lean. Get it looked at before you blow your motor and learn the hard way ! If possible pop the head off and check the condition of the bores etc, they mabe scored already.

James

 

[Matt_E]

Sounds like your running out of fuel from the wot run, meaning your lean. Get it looked at before you blow your motor and learn the hard way ! If possible pop the head off and check the condition of the bores etc, they mabe scored already.

James

That's what I'm thinking.

Putting in less restrictive flame arrestors may have leaned out your mix throughout.

Do a compression test.

 

[ClayMax]

Thanks for the advice guys. I'll pull the head and check things before riding again.

I guess I'm a little confused about the instructions and advice though. The steps given for tuning a ski seem like they'd take a fair amount of time to do correctly so I'm wondering what do the repair shops do in these cases? I know a lot don't have water tanks to put a ski in and do the tune up procedures listed by Mikuni.

I can't see them walking through each one of these steps every time they have a tuneup needed. And I know they don't take your ski out to the boat dock and do all of that. So what do they do? Is it a case of just adjusting the high speed adjuster?

If you opened your head up and didn't see any damage but were leaning out at wot, what would you do?

 

[Watty]

One thing you can do, is put your ski against a dock, and do a 3/4 throttle run for 20 seconds, or less if the note of the engine changes enough to notice.

Check your plugs....if they are white annd powdery, it's lean as....

If not, go for a WOT 20 second run, or rather, work your way up to it checking plugs etc for early warning signs.

I have a friend who has had this exact problem with a superjet more than once, and each time the cause was different.

If you've installed a primer tee off one of your fuel lines, make sure it's not leaking or sucking in air, this was one of the causes. Check your fuel filters. I've noticed that the small inline filters you get that look like they have a brass centre section with tiny rough looking little holes clog up way before you can actually notice that they are blocked.

I would suspect that your carb settings are ok if they have worked before, and changing the popoff by changing flame arrestors shouldn't make the ski run out of fuel so violently.

Do you run an aftermarket CDI? I've seen a few SX-R's with aftermarket ignitions that leave the heat sensor plug exposed, and when water gets on there, it shorts the pins out and stops it dead in it's tracks.

Come to think of it....Another friend of mine has been having similar troubles with a wet pipe as well, and I've read on the forums (X/PWCT) that this is a common thing. It must all be related.

To this day my friend with the SX-R hasn't sorted it out, and it still dies randomly as you described from time to time. Hit up FPP and see what they think, as it seems a huge coincidence that installation of the wet pipes usually exibit this problem. I'm not saying that it's the cause, but it might be a good start.

 

[D Slicker]

How many turns out can you go before you need to go up a jet size. Also if you go up should you go up one size on the piloit and main?