1998 Challenger 1800 starving for fuel on high speed circuit

[tingraham]

Hi all... I am in need of some help and have been browsing endlessly for the past month to find a solution to this very obnoxious problem. I have found lots of good info but nothing has solved the problem yet.

Story: Purchased the boat for very cheap not running but pretty much all was there. Shortly after digging into the motors, I found that there must have been water sitting in the crankcase for quite some time obviously ruining everything in the lower end. So I forked out the money and got 2 SBT engines and did the swap. While waiting for the motors I went through the carbs and replaced all the basic parts and gaskets. Also went through the accelerator pumps and got them working nicely.

Once I received the motors, I began assembly and everything went back together nicely. Cranked it over and she fired up on both sides and seemed to throttle up decently out of the water (of course not over doing it). Adjusted all the cables and idle out of the water.

Once I put it in the water is when all the crap hit the fan. Found that it would only rev to about 3500ish on both sides but sometimes could get it to spike for a moment. Failed trip.

After searching, I came across the infamous rectifier issue. So I purchased 2 new rectifiers and threw them on with zero results. So, back to the drawing board. I pulled carbs back off and checked all the internal filters again and looked for anything visibly wrong. The filters were spotless inside all of them. When I installed the carbs back on the engines, I decided to swap the carbs between the motors because one engine seemed to be able to rev a bit higher than the other the first time I tested it in the water. Took it back to the water and it ran pretty horrible all the way around once again.

Through this whole time I have noticed that the clear new wix fuel filters I have barely have any fuel flowing through them when it is running. So that led me to believe maybe the external fuel pumps were shot. So I proceeded to replace the pumps with the Mikuni dual output high flow pumps that others have used. Took it out after that and it idles and revs to 3500 very smoothly but STILL does the same thing on one motor and the other revs nicely to 7k+ but I believe the high speed circuit needs adjusting slightly but didn't bother yet because of the one side still not revving. The fuel filter on this same side still shows very slow flow and isn't even 1/4 full when running while the other side seems to be flowing much better. If you let the one side idle for awhile and then spike it, it will rev the snot out of it until what seems to be that it runs out of fuel and falls on it's face and wont rev again.

I also checked the fuel baffle filter which was clean. There are zero old grey lines on the boat.

I once again have the carbs off on the one side and have run out of things to try.

Please provide any input you can possibly think of. I am getting sick of throwing money at this without having any results.

thank you!
Tim

 

[Sportster-2001-951C-Stock]

Are you absolutely certain you have the fuel pump connected correctly? These carbs low speed circuit is famous for being difficult to clean, if you have the mikuni manual it shows the fuel flow paths through the carb and explains the theory.

 

[tingraham]

Thank you for the quick input. I am 100% positive the fuel pumps are installed correctly. I do not have the Mikuni manual but I have a pretty good understanding of how they work. I will try searching to find the manual online.

I am going to go out and check all the return lines now because that is the only thing I haven't gone through. I know they are routed correctly, just not sure if there could be some kind of plug somewhere.

thanks!

 

[tingraham]

Well, today was another failed test run. I have replaced all fuel lines and even tried taking the fuel filters out of the equation (by going directly from baffle to pump) with absolutely zero results. So I pulled the carbs off and tested the pop off pressure, both carbs came in at 40 PSI which according to the manual, it should be 38 plus or minus 12... I don't know what else to check or replace... I tried backing the high speed screws off 1 turn and it seems to fall on it's face at even less throttle. My theory is that the carb adjustment and jetting is good in the stock arrangement but once the fuel runs out in the carb, it completely leans out and falls on it's face around 1/3 throttle as well.

The only other thing I can think of would be poor crankcase pressures, but that seems doubtful...

Please, any help someone can provide would be helpful. Thanks!

 

[ialonso]

What fuel filter are you using? OEM? make sure no air is getting in there. I moved on to a spin on canister filter long ago and have not looked back. Air on the lines will cause unable to rev past a certain point, as you are fuel starved. Fuel Filters are a good candidate for air entry. Also, New carbs fix my fuel issues when all else fails. since one is working, are you willing to swap them to validate (one engine to the other)?

One time to troubleshoot the fuel pump I hooked up a high pressure electric fuel pump to remove all doubt. It worked! I had moved to aftermarket filters and I had picked the crappy one. Mikuni high flow fixed it.

 

[tingraham]

I am using wix clear filters:


There is mostly air in them just a stream of fuel on the bottom flowing through. Both motors are now running identically as I found that the diaphragms from the carb kits I got had a very short stud on them that push down the pop off lever not allowing them to fully open so I put the originals back in. So we took the boat back out and now they both only rev to about 3K RPM. I tried turning out the high speed screws a bit and got zero results. They start and idle perfectly...

I saw a post regarding the Mikuni high flow pump crushing similar filters to what I have. Clearly my Mikuni high flow pumps aren't pushing that kind of fuel through the filters.

 

[Sportster-2001-951C-Stock]

Well, today was another failed test run. I have replaced all fuel lines and even tried taking the fuel filters out of the equation (by going directly from baffle to pump) with absolutely zero results. So I pulled the carbs off and tested the pop off pressure, both carbs came in at 40 PSI which according to the manual, it should be 38 plus or minus 12... I don't know what else to check or replace... I tried backing the high speed screws off 1 turn and it seems to fall on it's face at even less throttle. My theory is that the carb adjustment and jetting is good in the stock arrangement but once the fuel runs out in the carb, it completely leans out and falls on it's face around 1/3 throttle as well.

The only other thing I can think of would be poor crankcase pressures, but that seems doubtful...

Please, any help someone can provide would be helpful. Thanks!

I think on that setup, the large venturi begins to take over and begin flowing about 3,000RPM in water under load (787 dual carb engines), best I can tell. Do if it's running out of fuel there I'm thinking fuel volume isn't enough to keep up or as you investigated, the metering needle isn't coming off the seat enough.

So I'm curious to know which size orifice metering needle seats are in your carbs. 2.0 is the largest I know about Seadoo using, even some 717 engines used that size along with the bigger bore 951.

But if your pop pressure is at spec (BS to the 12psi range, the Mikuni chart is 1st reference and the acceptable range is withing a couple psi, not 12!) then it's possible you have the wrong spring AND the wrong metering seat orifice (too small).

Anyway, it sounds like it's running out of fuel about the time the large venturi should begin flowing, those are large passages so doubtful those are clogged.

Two things pop up, already mentioned, metering orifice might be too small or the fuel pump might not be capable of producing the volume necessary.

As far as filters having air trapped in them or being crushed, it's normal for air to take most of the space and I highly doubt the mikuni pump produces enough volume or pressure to crush the element even if the element was full of sand and trash.

One more thing, leak check your fuel lines to make sure air isn't leaking inwards into the lines somewhere upstream of the fuel pump as the pump is sucking fuel from the tank.

I'm thinking you have mikuni Super BN carbs, here's a link to the manual which applies to any eneing equipped with these diaphragm carbs, including Yamahas, so keep that in mind as the DETAIL of calibration is application specific.

http://www.mikuni.com/pdf/sbn_manual.pdf

Are you sure your fuel pump is up to the task for supplying fuel to twin engines or do you have dual pumps? ie: Do you have two of them or a single pump feeding both? I don't know your application specifics, nor have I worked on a twin Seadoo, FWIW. just tossing out ideas.

Also, the neutral switch rev limiter on some 2-stroke boats kicks in somewhere around 3,000 RPM so you'll notice the ignition goes dead at this RPM if the neutral switch is detected (maybe someone jumped it out?), the engine starter won't crank unless the neutral switch is made.

Just tossing out ideas of things that might apply....

 

[tingraham]

The carbs have a 1.5 needle and seat. I now have tested the pop off on all carbs and the 2 port carbs are at 40 PSI and the starboard are right on the money at 38 PSI and do not leak at all. As you said, the passages are large and are definitely not plugged as I ran carb cleaner through them to verify. I have 2 Mikuni High Flow fuel pumps installed (1 for each engine). Others have used this pump for the same boat, that is why I went with those. I just tried taking a fuel line directly from the filter into a gas can to make sure there aren't any air leaks in the baffle somehow, same results (as far as the visible flow goes). They are indeed Super BNs, thank you for the link.

As far as the neutral limiter goes, the neutral limiter does work properly and distinctly cuts the ignition.

I will browse the manual and see if there is anything new I can come up with from it.

thanks!

 

[racerxxx]

Out of water will both engines hit 7k rpm?

 

[tingraham]

They sure do.

 

[racerxxx]

So you switched carbs and the same engine still has the same problem correct? Have you completely disconnected both rectifiers from the system? Only other thing I could think of is switch the wiring from one engine to the other on the front of the mag housing. Possibility the MPEM is bad? Why did the PO park the boat? Have you unscrewed the spark plug boots and cut the wire back a healthy 1/4 inch? When you swapped the engines did clean the stator and pick up? Did you remove the mag cup from the flywheel when you pulled the flywheel and did it go back on the same way? Just grasping for something right now.

 

[tingraham]

When I switched carbs it followed the carbs, so it has to be a carb issue. At this point though, both engines are running exactly the same... crappy... I tried disconnecting both rectifiers, still does it. The boat switched all sorts of hands until it got to the PO and he never had the money to get it going. I have not tried unscrewing the spark plug boots and cutting the wire back, but will try it. I did clean the stator and pickup. I didn't remove the mag cup from the flywheel.

I really appreciate the fresh ideas, keep them coming!

 

[racerxxx]

Well, if you swapped carbs and it went to the other engine you found the problem. When I clean carbs I will spray thru them but I will go back and use ATF (automatic trans fluid) in a pump bottle and pump it thru the carb ports. Follow the Mikuni manual breakdown diagram and you can follow the "path" where the oil should come out. The reason I use ATF is you can see where it is bleeding from to verify those little tiny orifices are open. What did the carbs look like inside? Did you verify you used the same gaskets as the good running set? You could have a wrong gasket on the one side??? Just a guess, usually more confusing on a fuel pump set up, but since the fuel pumps are divorced from the carbs its kinda hard to screw that one up.


Plews 50-595 is the one I have. About $8

 

[howie099]

Did you make sure the needle arms are adjust properly? You used all Mikuni parts and needles/seats in the carbs? Your pulse lines good? A set of new carbs are $400 so if you have only one messed up set you might just want to replace them. Something has to be plugged or wrong in that setup if you swap the one around and its ok.

 

[Sportster-2001-951C-Stock]

I'm pretty sure the dual carbed 787 did use a 1.5 seat, but I'll double check that.

According to Mikuni's chart, the black 80 gram spring would produce a 38 pop pressure. If you're within 2psi, that shouldn't have much effect.

Next time you're in there, get the numbers off the high and low speed jets, too. Maybe it's got the wrong high speed jets in there (doubtful, but hey...).

Sure there isn't water in the fuel?

And if you're running without the factory air boxes, that's gonna make it run lean.

Okay, so it looks like Seadoo went with a 142.5 HS jet and a 67.5 LS jet for the 787 engines dual carbs. There were a couple different springs though, those numbers might both cross to the 80gr spring, not sure. Hmm, you should be in the ball park.

Are you sure the throttle butterflies are fully opening? Some bell cranks have two holes and it's easy to get the throttle cable inner wire in the wrong hole.

And since you have two separate MPEM's (I think you said this), might want to verify the grounds are okay and make sense.

 

[racerxxx]

Did you make sure the needle arms are adjust properly? You used all Mikuni parts and needles/seats in the carbs? Your pulse lines good? A set of new carbs are $400 so if you have only one messed up set you might just want to replace them. Something has to be plugged or wrong in that setup if you swap the one around and its ok.

Jet boat carbs are not 100% the same as ski carbs. The fuel and return nipples are different. Would they work, yes, but he would need to change the fuel line configuration.

Sent from my SGH-T889 using Tapatalk

 

[racerxxx]

Howie brings up a good point, did you use genuine Mikuni parts. And which parts did you replace?

Sent from my SGH-T889 using Tapatalk

 

[tingraham]

Well the problem is now that there isn't a noticeably worse set of carbs as they both run the same now... So that tells me it's something that each setup has in common.

-The carbs were surprisingly clean inside except for on the valve assembly side had some rustyness about 1/4 up because water must have got in and sat.
-The only things that have been replaced in the carbs at this point is the tiny check valve on the valve assembly 2 of the o-ring gaskets on the fuel filter side, which btw the little fuel filters were spotless when I originally took them apart. I didn't replace anything with the needle/seat/arm or spring as they are working properly and not leaking or anything.
-The arms look like the dull silver ones I believe and the springs look shiny.
-The pulse lines have been replaced when I put the new pumps on.
-The jets are all the correct size and in their correct carbs (PTO vs Mag).
-Butterflies are definitely opening all the way.
-Also, this only has a single MPEM, and it better not be the issue...
-The gas is fresh as I pulled the tank out while the motors were out and cleaned it.
-I currently just have the flame arrestors on and not the airboxes as I would think that it may run a little lean but not completely fall on it's face lean. We did try putting them on several test runs ago and didn't get any results.
-The lines are definitely routed correctly, I know these carbs pretty well inside and out at this point lol.
-Thanks for the ATF suggestion, I will give it a try.

I think the reason one set of carbs was originally running differently than the others was due to that diaphragm issue I had, the one side must have had the arm slight more bent up allowing the needle to open slightly more with the short stud on the diaphragm.

The stock jetting should work properly for an SBT reman, correct?

We were able to get them to rev out a bit if you feather the crap out of the lever just right, but it would eventually fall back on it's face.

 

[howie099]

I would do the full rebuild just because you don't know when was the last time it was done on the carbs. I would just do one side and see if its ok since if you needed new carbs you are not throwing away extra money. If you used stiff hose for the pulse lines those should be ok. I can tell you for use the aftermarket mikuni's pumps are fine on the cars since I run them on my boat. If you are sucking air at the OEM external filters you can bypass for a test but if you just get a $4 gasket for them you should be ok. Also are your vents working properly on the fuel system? What sucks in your situation you don't know for sure what worked and didn't work.

My carbs look pretty much the same as my PCW they both have the same engine(717) and same carb setup(single mikuni). They just use a different seat and needle and different fuel pumps which I think is weird that they have different seat size since they are the same engines and flame arestors.

 

[tingraham]

Interesting point regarding the stiff hose for the pulse lines. I just used regular fuel line, is there something different that should be used for these??? I am able to pinch the pulse line with my fingers and cut the pulse off and stop the pump. When I do this, I don't noticeably feel a pulse while I pinch.

From what I can tell the fuel system vent pretty much free flows air to the atmosphere by design. If I blow air into the tank from one of the fuel lines it just vents out.

I do not have the OEM filters and from what I can tell these particular boats didn't have those small screw on type filter things like my XPs have.

The problem with the carb kits is most of the parts are for the fuel pump on the PWC carbs. You are basically getting a diaphragm, o-rings, metering arm and filter for the boat. All of which are usually visibly bad/leak and currently nothing leaks.

I do have a 97 XP that I could pull the carbs off of if I really want to put the carbs to the test and do the swap. I will have to swap all the jets, metering spring and remove the built in fuel pump off the side.

 

[howie099]

If you use standard automotive 1/4" fuel line that won't collapse on you. Some of the Ebay ones will if they are real cheap. I have those aftermarket universal clear see through fuel filters after the OEM ones. My boat has the standard seadoo fuel filters. OSDparts sells custom kits without the fuel pump parts to save you do cash its like $95 with the good needles per engine for you. Its mostly the needle/seat and diaphragm that need replacing those are money parts in the kits.

I wouldn't pull the ones from a good working ski if you have to do all of those changes that is just a bunch of extra work that you might damage a gasket or jet moving them all around.

 

[tingraham]

I put some nice stiff marine grade on it, we'll see how that goes.

I cut back the all the wires about 1/4 inch, see if there are any results from that as well.

Another thing I forgot to mention is that I usually have to start the port engine before the starboard. If I try it the other way, it stalls the running starboard engine as soon as I hit the port start button. I am assuming this is related to a voltage drop cutting the spark on the one engine for some reason and it's unrelated to the other issue.

Probably will go for a test run on Saturday. Hopefully get some results.

 

[enut]

When in the water, I noticed the same thing with having to start the port engine first. I did not notice any thing that looked like a bad connection on any of the wires.
Keep us posted if you find anything out with that. My boat is winterized and put away for the winter, so I can't experiment with it.

 

[ialonso]

I put some nice stiff marine grade on it, we'll see how that goes.

I cut back the all the wires about 1/4 inch, see if there are any results from that as well.

Another thing I forgot to mention is that I usually have to start the port engine before the starboard. If I try it the other way, it stalls the running starboard engine as soon as I hit the port start button. I am assuming this is related to a voltage drop cutting the spark on the one engine for some reason and it's unrelated to the other issue.

Probably will go for a test run on Saturday. Hopefully get some results.

All this reads to be as a fuel issue still. I'm sorry to be stubbornly stuck on fuel, but it ties both issues together. Any chance you are willing to experiment by feeding the engine from a portable tank to troubleshoot while starting the other? to me it's all fuel starvation along with the comment that the filters are mostly filled with air...I am just stuck on fuel. Using an electric high flow pump to test would be a great troubleshooting aid, as it was for me long ago when fighting something similar. Good Luck!

 

[tingraham]

Well I definitely feel like it is a fuel issue as well. Took it back out Saturday and all was the same. I ordered all new diaphragms for it from osdparts, should get them Wednesday. The originals I put back were a bit stretched out. seeing as when I put those back in it made a huge impact in the way of performance, I figured I would start there. I'm not convinced of the needle and seat yet because it appears to work perfectly when testing the pop off.

Will post back with results when I get the new diaphragms in. If it fails, it will probably be electric fuel pump test time.