yet another carb spring/popoff psi question

[PatinMD]

I'm rebuilding the carb on the 02 GTI with a 717 engine. This is a single carb engine. Seadoosource calls for a 2.0 needle with a popoff psi of 36-40. The carb is BN-40I-38-48. Part diagrams call for spring 270500428.

However, all of the charts that I can find indicate that with a 2.0 needle, even the 115 gram gold spring would yield a popoff psi of 32 psi.
There is no spring that will yield the higher popoff psi called for on seadoosource.
What am I missing?

If everything works out as planned and the popoff psi is 32, is that ok to stay with? An option would be to switch to a 1.5 and a black spring to yield the expected 38 psi popoff.

I'm going to put it together and test it, but I was hoping to have a bit more confidence in doing it correctly the first time.

thanks

 

[racerxxx]

OK, first thing first. Has this carb ever been apart before you owning the ski? It's pretty easy to tell if someone was in there before you, look at the screw heads. I always just use the spring that was in the carb, replace the N&S and all the soft parts in the carb. Check pop off and THEN adjust as necessary. But I WOULD NOT just start buying springs and tweaking until you check with you new parts.

 

[PatinMD]

It appeared that the carb had never been off the engine or opened up. The paint was still on all of the screws. Scary actually, since that's a long time.
Ok. Once I have it open to replace the N&S, I'll be able to check and see what size N&S and spring are in there.

thanks

 

[PatinMD]

I've been reading other threads on carbs. One of the threads mentioned using a certain dealer for a replacement carb. I checked the site and it was a BN40I-38-24. The list of skis that it fit included my 02 GTI. But my part list shows the 02 GTI having a BN 40I-38-48.
What does the last number mean? What's the difference between the 24 and the 48?
I'm trying to wrap my head around all of this. Dang, the carbs can be as complicated as the rest of the engine.

 

[racerxxx]

It appeared that the carb had never been off the engine or opened up. The paint was still on all of the screws. Scary actually, since that's a long time.
Ok. Once I have it open to replace the N&S, I'll be able to check and see what size N&S and spring are in there.

thanks

Yes, if you carefully remove the seat you can polish the bottom side up with some steel wool and with your good eyes read the number stamped in there. The carbs are really easy once you get into them. www.osdparts.com is the site you were looking at. If you really want to bow out and just buy one , I would call them and discuss the factory settings and have them set it up before they ship it.

 

[Coastiejoe]

One thing you shouldn't do is replace the spring even from the new kit if yours was good and operates within the correct pop-off. They just never seem to work from the kit even when OEM.


Sent from my iPhone using Tapatalk

 

[Sportster-2001-951C-Stock]

I wouldn't get too worked up over pop-off, you probably have some wrong info. Even Seadoo info isn't always correct in every case If the original seat was 2.0 and you have the original spring then reference the Mikuni chart for pop-off and that should be very close I bet, within a couple psi of the chart. There's some bad info floating around on the internets causing confusion.

If anything, the small pilot ports and the passage delivering fuel to them are corroded internally, so make sure to clean that area (low speed circuit) really, really well.

 

[Strizzo]

go with the info in the factory service manual, not all of the info on seadoosource is perfectly accurate, especially when you get away from the more popular years/models, like 95-98 787/951

 

[PatinMD]

I wasn't so sure the rebuild of the old carb was going to work out, since it had so much corrosion when I broke it apart. I went with a new carb. I changed the jets to match what the seadoo manual and seadoo source agreed they should be, 167.5 and 75.
Both sources also say the carb should have a popoff pressure of 36-40 with a 2.0 needle and a xxx428 needle.
I tested the popoff thru the fuel inlet which I have read may raise the popoff readings due to going thru the various parts of the carb.

The new carb as received had a 1.5 with a shiny silver spring (65g) which gave a popoff of 36 psi.
I changed the N&S to 2.0, retained the same shiny silver spring and the popoff went to 16 psi.
I pulled the spring from the old carb, which was longer and not as shiny silver as the new carb's spring, so I assume it was a 115 g spring. In the old carb it gave a popoff of 32psi.
In the new carb with the 2.0 N&S the 115 g spring gave a popoff of 25 psi.

I've read that this carb can't flow more fuel than the 1.5 N&S allows no matter what size N&S and that the larger N&S are used to regulate the popoff psi.

So I went with the new carb 1.5 N&S and the shiny spring for a popoff psi of 36 psi, which is very close to the spec of 36-40.

Was that the right choice?

 

[howie099]

Does the ski run fine with the settings you have in it with new carb? If it does you are all set to go.

 

[PatinMD]

I just finished installing the new carb. While I was at it I did a compression test that turned out at 140/145. So that was a good thing.

The ski started up and sounded fine idling. It was on water hose so it didn't go for too long. I'll put a tach on it asap. Probably tomorrow. Today weather was in the 70's F, tomorrow in the 50's F.
I asked about the N&S set up hoping that someone wouldn't shout out that running a 1.5 instead of a 2.0 would starve the engine for fuel when at higher rpm's and ruin the engine.
There's a bunch of folks on here that know sooooo much about these engines that since I was veering off spec's I wanted to ask if I was unknowingly doing something stupid. Or that there was a better way of doing it.
Thanks

 

[Sportster-2001-951C-Stock]

I would go back to the original factory calibration. If that was a 2.0 seat then so be it. But if the factory calibration used a 1.5 seat that should go along with the corresponding high speed jet.

The 951 engine is the only one that MUST have a 2.0 seat due to the flow through the carb anything smaller would leave it lean no matter how far open the HS adjuster was.

The 951 isn't the only Rotax that used the 2.0 seat, the factory changed calibrations over the years as data was available, improving durability, performance and EPA emissions.

But just swapping out whatever parts b/c you think you like them isn't usually a success story. I didn't look up your specs but sticking with those, whatever they are and if that's what you did, should get you bang on. Especially considering your carbs are shiny new and not corroded.

 

[Sportster-2001-951C-Stock]

I pulled the spring from the old carb, which was longer and not as shiny silver as the new carb's spring, so I assume it was a 115 g spring. In the old carb it gave a popoff of 32psi.
In the new carb with the 2.0 N&S the 115 g spring gave a popoff of 25 psi.

So I went with the new carb 1.5 N&S and the shiny spring for a popoff psi of 36 psi, which is very close to the spec of 36-40.

Was that the right choice?

The 717 is the best Rotax jetski motor IMO, it just runs and runs well. At least mine did and others that I know of. Early versions of this motor I think used the 1.5 seat and I don't recall the spring or pop pressure off hand but can look it up if need be.

There are 4 springs available for these carbs and a few different seats, full array of jets, etc. If you run an airbox that's not factory you'll likely find the factory calibration doesn't work well so make other changes accordingly and good luck.

Having a good clean carb is key, one that's corroded internally often can't be tuned to run properly no matter which jets and springs are in it. Make sure it's clean.

The 1.5 seat with corresponding spring and jets, etc. is a little better choice IMO, due to the larger seat is more prone to dribbling fuel at low speed due to vibration.

Mikuni's pop charts and spring selector guide are here, OEM part numbers are in the Mikuni SB manual.

 

[racerxxx]

Just pull the seat from the original carb and verify what was in there.

Sent from my SGH-T889 using Tapatalk

 

[PatinMD]

I would go back to the original factory calibration. If that was a 2.0 seat then so be it. But if the factory calibration used a 1.5 seat that should go along with the corresponding high speed jet.

The 951 engine is the only one that MUST have a 2.0 seat due to the flow through the carb anything smaller would leave it lean no matter how far open the HS adjuster was.

The 951 isn't the only Rotax that used the 2.0 seat, the factory changed calibrations over the years as data was available, improving durability, performance and EPA emissions.

But just swapping out whatever parts b/c you think you like them isn't usually a success story. I didn't look up your specs but sticking with those, whatever they are and if that's what you did, should get you bang on. Especially considering your carbs are shiny new and not corroded.

Thanks. The info on the 2.0 being necessary on only the 951 is very good. The specs of the ski I'm working on are 2.0; pop off of 36-40.
I can't make that happen with a 2.0 nozzle in the carb. Using a 115g spring yields a popoff of 25. Using a 1.5 and a 65 g spring yielded a pop off of 36.

My question was basically, since I can't get to the specs, which is more important, the N&S size or the the pop off psi so that I don't kill the engine. Since the carb can't use the flow rate of the 2.0 it seems that going with the 1.5 and the spring that gives a very close to spec popoff is the best way to go.

 

[PatinMD]

The 717 is the best Rotax jetski motor IMO, it just runs and runs well. At least mine did and others that I know of. Early versions of this motor I think used the 1.5 seat and I don't recall the spring or pop pressure off hand but can look it up if need be.

There are 4 springs available for these carbs and a few different seats, full array of jets, etc. If you run an airbox that's not factory you'll likely find the factory calibration doesn't work well so make other changes accordingly and good luck.

Having a good clean carb is key, one that's corroded internally often can't be tuned to run properly no matter which jets and springs are in it. Make sure it's clean.

The 1.5 seat with corresponding spring and jets, etc. is a little better choice IMO, due to the larger seat is more prone to dribbling fuel at low speed due to vibration.

Mikuni's pop charts and spring selector guide are here, OEM part numbers are in the Mikuni SB manual.

Thanks for the information. It's a standard air box. And I've stuck with the spec's jets.
The chart confirms that the spring that yielded the 25 psi popoff was a 115 g spring. And that the 65 g spring yielded 36 psi.

 

[PatinMD]

Just pull the seat from the original carb and verify what was in there.

Sent from my SGH-T889 using Tapatalk

I"m not sure that the old carb was the original carb. Due to circumstances, I can't verify that the ski ran as it should to make the old carb a benchmark. I only ran the ski a couple of times and didn't know enough to test the response to throttle position. I just ran it to make it go. It started, it ran, it ran fast. It didn't sink. I've learned a lot since then.

 

[howie099]

The OEM carb should have a 2.0 needle. I just rebuilt my 2001 GTi carb which is the same as yours using the OEM spring an new 2.0 needle and I am getting a 39-41 Pop-off. You look at the mikuni chart and it shouldn't be but it is. So the question is how does Seadoo get high of Pop-Off when a gold spring gives a 32 psi and goes down from there? I can tell you my spring wasn't gold. The only thing I can think of is a custom spring that just seadoo uses or something in the carb that gives it a higher reading. There must be some difference on why they choose the 2.0 needle instead of the 1.5 since you can get the reading you need with the 1.5 needle very easily. You might just want to ask benjilafouine from here he has a 2003 GTi he just put a new carb on he bought from OSDparts that runs perfect on his ski. I don't think he changed anything in the carb from them.

 

[racerxxx]

I"m not sure that the old carb was the original carb. Due to circumstances, I can't verify that the ski ran as it should to make the old carb a benchmark. I only ran the ski a couple of times and didn't know enough to test the response to throttle position. I just ran it to make it go. It started, it ran, it ran fast. It didn't sink. I've learned a lot since then.

Call me crazy, but whether or not it's the original carb, why would you not open it up and verify what WAS in there. If there's a 2.0 in there you'd be safe to assume the carb is original.

Sent from my SGH-T889 using Tapatalk

 

[Sportster-2001-951C-Stock]

The OEM carb should have a 2.0 needle. I just rebuilt my 2001 GTi carb which is the same as yours using the OEM spring an new 2.0 needle and I am getting a 39-41 Pop-off. You look at the mikuni chart and it shouldn't be but it is. So the question is how does Seadoo get high of Pop-Off when a gold spring gives a 32 psi and goes down from there? I can tell you my spring wasn't gold. The only thing I can think of is a custom spring that just seadoo uses or something in the carb that gives it a higher reading. There must be some difference on why they choose the 2.0 needle instead of the 1.5 since you can get the reading you need with the 1.5 needle very easily. You might just want to ask benjilafouine from here he has a 2003 GTi he just put a new carb on he bought from OSDparts that runs perfect on his ski. I don't think he changed anything in the carb from them.

Yep, you've got the right idea. Maybe there's a 5th spring available. Otherwise I can't explain why you found pop isn't as listed in Mikuni's chart but would like to understand why that is. Some carbs even used a spacer washer under the spring.

In my experience speaking generally, the more restrictive the air box is, the higher pop needs to be, accordingly. Too little pop will result in more fuel down low and you might not have reasonable control over idle mixture. If your idle mixture screw is working, I think you can conclude the pop isn't too low.

If pop is too high, you might find it's hard to start the engine cold, b/c the metering diaphragm doesn't see enough vacuum to pop the needle off the seat allowing fuel into the fuel chamber. I'd expect it might even require choke for a successful warm restart.

If the metering arm is bent down too far (isn't near flush with the chamber as illustrated in Mikuni's manual), pop will be increased but the valve needle might not lift far enough off the seat and WOT might become a lean experience.

Conversely, if the metering arm is too high in the chamber the result could be the metering diaphragm pushes against the metering arm and holds the metering arm off it's seat, causing flooding.

Also I want to say since we're discussing a single carb 717 application, it stands to reason the 2.0 seat is correct considering the single carb feeds two cylinders.

The OP should concentrate on installing the factory spec tune just as you did, 100% chance it works great.

 

[Sportster-2001-951C-Stock]

I"m not sure that the old carb was the original carb. Due to circumstances, I can't verify that the ski ran as it should to make the old carb a benchmark. I only ran the ski a couple of times and didn't know enough to test the response to throttle position. I just ran it to make it go. It started, it ran, it ran fast. It didn't sink. I've learned a lot since then.

Sounds reasonable. I think it's established you should be using the 2.0 seat, the 1.5 seat may not be large enough to feed the amount of fuel necessary and the jets would need to be accordingly larger as well to make that work so it opens another can of worms to avoid.

And hopefully, whatever your original spring was gives you about 40psi pop pressure on the 2.0 seat. There may have been a 5th spring specially made, honestly I wouldn't have that level of detail.

It's been a really long time since I watched my 717 leaving the driveway, not sure what I was thinking at the time, lol.

 

[howie099]

Seadoo sells the spring for $11 for the carb. I know some OEM parts are high but that is high for that but if you need it to you need it. I know there wasn't a washer or anything under my spring. I didn't mess with the arm since I was just cleaning and replacing rubbers. I have the standard OEM Seadoo flame arestor that is on every 587-717 from mid 90's-2000's. Here is the kicker I have 1998 1800 Sportster w/dual 717's the engines are 100% the same. Same carb, instake, and same flame arestor setup but the needle in them is 1.5 and has the same pop-off of 36-40. There is a difference is the boat has and external fuel pump and the spring is a different part number the ski's . That spring by the way cost $14 dollars WTF. So what I see here you must need the 2.0 needle in the ski's since the internal fuel pump can't supply enough gas that maybe the external one can? I am not seadoo engineer here just trying to pass sometime and figure out how they do this stuff.

 

[PatinMD]

Call me crazy, but whether or not it's the original carb, why would you not open it up and verify what WAS in there. If there's a 2.0 in there you'd be safe to assume the carb is original.

Sent from my SGH-T889 using Tapatalk

This is a jet ski, nothing is that simple. lol. I tried but the screwhead of the clamp on the seat stripped. It'll be a project to drill and remove that clamp that I didn't have time for, yet. Plus, I don't know that that carb was set up properly or ran properly. So I'm not sure that it's the gold standard that I want to replicate.

 

[Sportster-2001-951C-Stock]

FWIW, in my 2001 service manual both single carb 717's (GS and GTI) use the 167.5/75 high/low jets with GS model=130gr spring and GTI model uses 115gr spring. Seat orifice and pop aren't given but assuming a 2.0 seat the 115gr spring without shim should pop at 32psi, this is factory spec spring for GTI according to my 2001 manual.

That might be high enough pop to allow the idle mixture screw to work properly but if not I'd increase pop (using factory original "130gr spring" preferably) until idle mixture is controllable. not sure where a 130gr spring comes from.

If still idle mixture is too rich and screw won't control it, you might be forced to use 1.5 seat with 80gr black p/n Mikuni 730-03033, BRP 270500267, this gives 38psi pop.

Basically, if pop is too low, the idle mixture adjuster might not not be able to keep the idle mixture from being too rich.

 

[Sportster-2001-951C-Stock]

This is a jet ski, nothing is that simple. lol.

Well put, applies to boats as well. Gotta be careful and pay attention else $$ stuff can break.