Carb Jet sizes

[Mickirig1]

My problem child Ski 13 needs new Jets I think. Too rich at the low end! Been round and round with this ski for the last Year! My question? What size jets, main and pilot? It has a BN401Mikuni. The shop manual gives me no sizes I can find. It may have another problem. Not pumping fuel real well. I tested all the carb and line leak downs. It all holds 5 pounds for 10 + min. All the pulse line and carb check valve tests are right too. Is there a way to test the pulse line ? Vacuum / pressure measurements? I am about to buy a new carb and be done with it!

 

[Sportster-2001-951C-Stock]

75/167.5/115gram, according to 2001 GTI shop manual, 717 motor. According to Mikuni, that spring should pop at 55psi while in a 1.5 seat.

 

[Mickirig1]

Thanks man. I could not find them in the shop manual. Book says pop at 36# . It pops all day at 36. I rebuilt carb last summer. But,I had it apart twice. So I think something is amiss with the rebuild! Maybe one of the expensive carb gaskets? The Ski's are 15 years old. Maybe old age gasket wear out?

 

[mikidymac]

Any time the carb is off you should replace the base gasket.
You rebuilt the carb with all new parts?
Make sure the 3 small holes below the throttle butterfly flow carb cleaner good when you spray it through the pilot jet.

If you have covered everything carb related check the compression and rotary valve clearance.

If you have done everything a new carb is an expensive guess.

On the other hand I had a fresh 720 in a 2003 GTI that always had a bog just off idle that drove me crazy, Rebuilt the carb and had it apart many times and I have done a lot of these carbs. Finally I threw in the towel and bought a new carb from OSD and it has never ran so good. Best money I spent on this ski.

 

[Mickirig1]

I am starting to think the past owner had the same problem. The filter was full of dirt, not green gunk. I think something got in the gas tank. It's sister Ski's filter was spotless. I got the carb clean. All the holes flowed well. It is bogging because it loads up. Fails the choke test. Kills it instantly. Base gasket got replaced both times.

 

[Sportster-2001-951C-Stock]

Thanks man. I could not find them in the shop manual. Book says pop at 36# . It pops all day at 36. I rebuilt carb last summer. But,I had it apart twice. So I think something is amiss with the rebuild! Maybe one of the expensive carb gaskets? The Ski's are 15 years old. Maybe old age gasket wear out?

But, but, but, you didn't say if or whatever, not much detail to go on here.... So do you have the gold 115gram spring in your carb?

I'm thinking maybe your manual is wrong and this is the problem causing the too much fuel issue, are you sure you've got the correct spring?

2001 manual says 115gr spring and 1.5 seat, mikuni says that combo pops at 55psi

I'm not just making stuff up for entertainment, trying to guess what's going on on the other end of this horn I'm speaking into.,.

EDIT: Oops sorry, I just realized maybe your carb should have the 2.0 seat. In that case, the pop would be 32psi, so that is fine.

So let's examine the problem more closely..... Where exactly is the mixture too rich, just off-idle or part throttle? b/c there will be some overlap as the high speed circuit begins flowing at part throttle. Or, maybe your idle is too rich?

Need to isolate which part of the range is overly rich, and concentrate on the possible reasons that might cause rich mixture.

 

[Mickirig1]

It has a 2.0 seat. Spring and arm are OEM / original with carb. Pops at 36# all day. Just like it's sister Ski. Got a used carb from Minnetonka4me. It pops at 36#. I have read almost everything here on carbs and tuning. I think the jets are just worn. It loads up at 1/2 throttle, pull the choke a little it dies instantly. Out of a long no wake zone it will big time stumble at 1/2 throttle. Will see if the used carb solves the problem. It starts right up for the last 3 days with used carb. No one to go with me to lake test yet.

 

[Sportster-2001-951C-Stock]

Yes, you have some kind of stumble at the transition from LS to HS fuel circuit. That's helpful But is it too rich or too lean during the transition? You're thinking it's a rich stumble but let's keep an open mind so bear with me....

What do we know and what don't we know for sure...... A stumble of some kind at the LS->HS transition.

We can blame either the LS or HS fuel circuit at the transition. But we need to isolate which one is the culprit. If the motor revs out WOT just fine then I'd suspect the LS circuit. Especially if LS operation seems rich. A tattle tale is plenty of 4-stroking burble (rich missing) during low speed operation.

But I'm still wondering if you're experiencing a lean spot at the transition from LS to HS circuit. There have been instances of the mylar check valve on the kidney shaped metering plate causing the HS circuit to begin flowing late, b/c the check valve is opening too late. If this piece of mylar film is too stiff the HS circuit operation is delayed enough to cause a lean stumble just before the HS circuit begins flowing. Aftermarket HS check vales are suspect, might be too stiff and cause HS circuit delay.

I wanna know what you discover, looking forward to next update.

 

[Mickirig1]

It does have after market check valves. A year old with the last rebuild. ODS kit. The kit did not have the rectangular check valve come to think of it. You may be right and that is the problem! I get the stumble near 1/2 throttle. Just pull choke 2 hairs and it dies. I towed a 18 foot boat 2 miles back to the ramp.
(They didn't think a Jet Ski could do it) As long as it was past the 1/2 throttle spot it pulled like a tractor. You may be right, a long low speed cruise it does seem to be loading up / missing. I want to rebuild the carb and put it back on next season. So I have a spare for problem child or it's sister. Since parts are always days or a week away.

 

[Sportster-2001-951C-Stock]

Yes, unfortunately some of the aftermarket parts are "iffy" and don't work as well as the Mikuni stuff. It doesn't take much to throw off the timing and calibration. The gaskets don't seem to be a problem and the n/s's Ive used aftermarket worked fine but I suspect the metering diaphragms and those HS check valves might cause problems.

If you have the original mikuni parts might consider throwing them back in. All except the n/s, I think those need to be new and n/s's are really the only thing that wears aside from the metering diaphragms which might stiffen after a decade of fuel exposure. If they're still soft almost like rose petals the originals should be fine..

 

[Mickirig1]

My used Carb work perfect! A little low speed adjustment and idle adjustment and it runs like a champ! Even My Wife noticed the difference! And She has no clue! Both Ski's run the same. They don't stay away from Ski 13 now. I towed an old GTI back to the beach 2 miles and it didn't even notice it was attached. Another new member of the forum will be checking in soon. His battery line was not tight.

 

[Sportster-2001-951C-Stock]

Nice! Interested in knowing what you discover on the problem carb.

 

[Mickirig1]

Going to take it apart this weekend. Working 12 on 12 off for 3 days straight. Hard to get much done.

 

[Coastiejoe]

[MENTION=69623]Mickirig1[/MENTION], few will know the fuel system as well as you with all you went through about a year or so,,,,


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[Mickirig1]

This was another problem after the air leak was found. Not a huge stumble but annoying enough!
Driving old trucks my whole life you learn "work arounds" to keep going.
When he mentioned the back flow check valve: I was like DUH ! the only thing I did not replace with the rebuild! All the reading on throttle curve points and tuning pointed at the jets as being the problem. Till, he mentioned the check valve. I pulled the new to me carb apart and cleaned the fuel filter and pump section with warm water and Dawn Dish Soap. At least I have a spare carb now, after I fix the old one.

 

[IDoSeaDoo]

One way to be sure it's the check valve. Take the valve out of the good carb and put it in the problem carb. See if that fixes it. I know I've chased this gremlin for years on my spx. I've replaced check valves, and everything else in there. No dice! Ski falls on it's face during takeoff. Rode my buddy's 787 XP and his throttle response is crisp as can be. I'm insanely jealous. I will definitely be working on this over the winter...

 

[Sportster-2001-951C-Stock]

Ski falls on it's face during takeoff.

So have you thought about which portion of the carb is at fault? Sounds like low speed circuit to me (not idle, not high speed).

 

[IDoSeaDoo]

Yea, I feel like I have tried tweaking every portion, modifying pop-off pressure, tweaking the LSA. The only thing I haven't tried is to go up or down in jet sizes, and that's mainly because I feel that I would be putting a bandaid on the problem. Technically, it should run according to spec as everything is bone stock on it. Maybe next season I will mess with the jetting... I'm starting to not care about bandaids and the such, I just want it to work right.

 

[Sportster-2001-951C-Stock]

The only thing I haven't tried is to go up or down in jet sizes, and that's mainly because I feel that I would be putting a bandaid on the problem.

For off idle and part throttle stumble/hesitation/stalling...... Suspect low speed pilot transition circuit.

If the three precision-drilled pilot transition ports in the bore are even slightly restricted, by natural corrosion oxidation or otherwise, by just a small imperceptible amount, you will experience part throttle stumble/ hesitation.

This is why a new carb always works great, those holes haven't corroded yet.

Even larger jets may not work for you depending on how much the holes are closed, so you might consider a caustic cleanser carb dip (not just a carb solvent), some guys will use liquid toilet bowl cleaner, or muriatic acid (diluted), or CLR lime remover to etch out those three holes (remove the light corrosion).

A nose or medicine dropper can get the acid right where you need it, and a flush with water(like from a water pick) and compressed air through the passages will rinse the acid out of the passage. How long and how much to etch depends on how much corrosion has taken place.
I avoided the acid etch for my carbs and just jetted larger, I didn't want to go that route but would've if the jets hadn't done the trick.

It's just a matter of getting enough fuel to do the job but not so much that 4-stroking becomes the issue.

 

[Mickirig1]

It's a rich condition on transition. Pull choke and it dies. Loads up off idle, would 4 stroke. Son on sister ski told me he can smell gas fumes when it does it.

 

[Sportster-2001-951C-Stock]

It's a rich condition on transition. Pull choke and it dies. Loads up off idle, would 4 stroke. Son on sister ski told me he can smell gas fumes when it does it.

This is why it's important to understand what's happening. You said before, it loads up at 1/2 throttle. That's not an off idle description. 1/2 throttle gets you well on plane and turning about 5,000RPM, no?

Pulling the choke at any time will immediately flood even a lean engine by making it flip to rich. So a lean stall can get the stick save from a quick blip of the choke but hold choke too long it will flood with fuel quickly. A quick blip of choke well timed, are the operative words to help isolate a lean stall.

A four stroking (slightly rich) engine won't normally stall unless it's grossly rich and the plugs become wet-fouled with fuel.

Also, a lean engine doesn't burn the weak mixture b/c lean mixture doesn't ignite, so that gets pushed out the exhaust unburned and smells like raw fuel just as an overly rich mixture would. Going by smell, all you can say is the cylinder didn't fire the charge (for whatever reason).

So if your carb was rich off idle and the needle/seat were popping correctly and not leaking then I'd suspect the metering diaphragm if it's not Mikuni OEM. If it's an aftermarket diaphragm, the metal nub that pushes on the metering arm might be too long, which would actuate the meterng arm too soon, often and early, holding the needle off it's seat an extraordinarily long time and flowing too much fuel.

Stuff happens fast in these carbs, that's why it's so important to describe the symptom exactly. More than a quick blip of choke will flood an engine.

Jets, passages and orifices tend to close up over time, and thus restrict fuel flow due to corrosion and fuel gum deposits. Diaphragm carbs never go rich on their own unless there's some problem with the metering, such as wrong or inferior parts. They can also be hard to start if the pop is too great, this keeps fuel fomr entering the fuel chamber.

There's an arms long list of things that could go wrong, that's why a precise description of the symptom is important.

So bottom line it sounds like maybe the metering diaphragm in your carb is probably aftermarket? I guess the metal nub that pushes the metering arm is too long, or possibly the rubber is thicker and not well suited.

 

[IDoSeaDoo]

Hmm, I cleaned those transition ports several times wroth carb cleaner and once wroth 1600 psi pressure washer. Still the problem persists. I will try the acid, drop it right in there and quickly rinse the carb out after a minute or two. I wish there was a way to poke them out with a wire or something. .. every time I sprayed carb cleaner, all three seemed to flow it well...

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[Sportster-2001-951C-Stock]

Yep, I also cleaned my transition ports VERY WELL several times only using solvent, air and even poked them out with a thin stainless wire with a 90 bent onto the end. I've never had to resort to using a caustic cleaner in any carb. Still had the lean stalling.

So in my case I didn't try acid route, first I tried a larger jet and that worked for me.

Hypothetically speaking, what else can be said, except if there's not enough fuel from low speed circuit the result will be a lean hesitation and lean stalling. Yeah, there could be other causes such as an air leak somewhere like bad crank seals for instance so not all lean hesitation is caused by the carburetors. We checked all that out, right? Oh wait, we tossed a new carb on there and the hesitation went away, so that rules out an air leak, lol...

Kinda unusual I think to have too much fuel but if the metering is goofed up somehow like the diaphragm is the wrong one (nub too long) for instance or the metering needle arm geometry is goofed up then yeah, that could lead to too much fuel in the fuel chamber, even to the point the large venturi drips and dribbles fuel at low speed.

The one single trait of being too lean is the lean stall, this doesn't happen from being too rich unless it's WAY, WAY rich practically gushing fuel. I blame such gross richness on the metering system, like a float bowl that's overfilling.

 

[IDoSeaDoo]

This has to be carbs, I've had these on different engines with new cranks, and that passed the pressure tests. Would the acid eat the aluminum or just the buildup? Could I just toss the whole carb in there? What is the longest time I can leave it there for?

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[Sportster-2001-951C-Stock]

Would the acid eat the aluminum or just the buildup? Could I just toss the whole carb in there? What is the longest time I can leave it there for?

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Since my Seadoo carbs are made of aluminum, I had planned on using an acid for etching aluminum oxide. Just to clean those three holes. No need to etch an area that doesn't need etching, that's asking for trouble.

Thus, I would suggest trying phosphoric acid metal prep, for etching back the aluminum oxide (nitric acid is for etching the aluminum itself).

No need to dip the whole carb in acid!!!!!

This product, I'd use 100% concentration for a few minutes applied directly into the three holes (or just flood the low speed passage after removing brass jet and mix adjust screw):

http://www.homedepot.com/catalog/pdfImages/3f/3fad4c2b-34e1-4f45-8410-f64dd7feea48.pdf

And, here's why I'd choose phosphoric acid, along with a brief technical explanation of what's happening in a nitric/phosphoric aluminum etch and expected ALUMINUM etch rate. Remember, no nitric just phosphoric, should remove just the AlO2:

I can't say for sure this will work for you b/c I haven't tested it myself but I'd like to know how well it does and maybe I'll try it myself so I can return to factory jetting. BTW, This corrosion issue is unusual for carbs b/c most aren't made of aluminum, most carbs are made from zinc alloy and even then all of the critical orifices are made of brass.