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Carb rebuild or compression

jemery.wa

New Member
Messages
17
Water Crafts
96 Seadoo GTX
94 Polaris SL 750
I have a 96 GTX that I have been trying to get running right. I have rebuilt the carbs and been through them 2 more times (due to inexperience). My pop off is right around 25lbs each and they seem to hold pressure over 10lbs. the compression is 120 psi in front and 110 psi in back, I know that is somewhat low. It will start and run on the trailer but in the water i can get it started but it seems to be flooding out at the low end, the low end needles are almost all the way in. to restart you have to shut the gas off and let it crank for awhile. I have a top end rebuild kit but want to know if the starting problem is a result of low compression or carb problems?
 

Mazachs

Member
Messages
47
Location
Levis, Quebec, Canada
Water Crafts
95 GTX657x
Did you rebuild your carbs with GENUINE Mikuni parts? If not, you have to start here. Always use genuine mikuni parts for carbs on Sea-Doo. It's more expensive but you will save a lot of issue relate to bad parts.
 

JeremyD615

Well-Known Member
Messages
1,426
Location
Nashville, TN
Water Crafts
1995 Sea Doo Speedster
1995 Sea Doo XP
1996 Sea Doo XP
Unfortunately I’d say the compression is your problem on this one. Get that new top end on it and reset the carbs to factory settings and it should be good to go.
 

Ckrawiec

Well-Known Member
Messages
882
Location
Rochester, NY
Water Crafts
1996 GTX
2017 Spark 2up
2018 GTI
2006 Bayliner 185BR
2019 Misty Harbor 22' pontoon
According to the MIKI master....150 psi is ideal, 130 psi is marginal, 120 psi and the engine is done. 10 psi difference between the cylinders on the extreme low end tells us the engine is past it.
Top end rebuild for sure.....I believe that even IF you could get her going in the water...the LOW compression wouldn't get you very far. Good luck with the rebuild.....keep us posted.
I have a '96 GTX and it is my favorite ski....still holding at about 142 psi each......150 hours so far.....and this ski has had it's share of troubles (sank once).

FIRST thing I'd do though......get another compression tester and double check those numbers.....I hope you weren't using a Harbor Freight guage (POS).....borrow one from AutoZone.

If the compression is NOT what you thought and is better....then CARBS are your issue...they are very finicky....took me a bit of work to get mine dialed in properly!
 
Last edited:

jemery.wa

New Member
Messages
17
Water Crafts
96 Seadoo GTX
94 Polaris SL 750
Thanks for the replies. I didn't use a Mikuni kit and because of that I wasn't sure if compression was my main problem. I will expand on my carb rebuilds further, an important lesson that I should have know is to read forums before tackling this procedure, I bought a non Mikuni kit and did the work and made mistakes. The first and largest mistake I made was replacing the OE springs with the ones in the kit as well as the arms, and I can only find one of them now. I didn't replace the needles or seats, just the o-rings. Another mistake was using a extra gasket in the pump. after the first rebuild I got the ski started and it actually ran, it didn't when I bought it, although it would bog down and flood out. That is when I noticed that they were leaking gas. When I pulled them I found that I had used an extra gasket in the pump but the leak on that carb (Mag) was the rubber o-ring style gasket, it wasn't thick enough to get a proper seal, I fished the original out of the trash and re-installed and no more leak. The PTO carb was leaking from the diaphragm and i am guessing that i just didn't have to screws torqued down enough because after reassembly it doesn't leak from there anymore. At this time I made my own pop-off tester and checked the pressure, that is also how I found the leak in the pump, My pressures were right around 25psi and the pump would hold pressure. After re-installing the ski would start and I adjusted the idle and it would run at 3k with water connected for quite a while. The problem is that when you shut it off you would have to start it with the throttle wide open, as well as when it was in the water and would bog down and die out the only way to start it is to shut the fuel off and crank until it starts. I pulled the carbs for the 3rd time after more research to adjust the arms because I hadn't done that properly either. After re-installing I was able to start it, I took it to the lake and had it in the water on the trailer, started it and had it idling at around 2k, since I had both of my skis there I pulled them off of the trailer and decided to test out the Polaris first since that one needed the impeller replaced when i bought it and seemed to run fine. After a nice ride followed by a good swim after it died (probably due to bad gas) I decided to try the Seadoo but it wouldn't start after sitting in the water waiting. I did empty the gas in the Seadoo before the first rebuild, was going to before taking the Polaris but it was raining that morning.

The first time I checked the compression I didn't hold the throttle open and my numbers where 90 and 80, after doing more research I found the way you are supposed to do it it to hold the throttle wide open. After That test with throttle open I came up with the 120 and 110 numbers, that is with a brand new Harbor Freight tester so I am sure there is a margin of error.

Sorry for being long winded, last night I wrote the post quick so i could go to bed, I was looking to find out which direction to go first, I wasn't sure if the compression was low enough to cause these problems and didn't want to rebuild the top end just to find out that it wasn't the problem.

The other question that I have is; Is there a need to rebuild the whole carbs with Mikuni parts or just replace the diaphragms and springs since the gaskets seem to be working and the pump is also fine?
 

JeremyD615

Well-Known Member
Messages
1,426
Location
Nashville, TN
Water Crafts
1995 Sea Doo Speedster
1995 Sea Doo XP
1996 Sea Doo XP
Okay, so we’ve got a lot more information now! The honest truth is, you have no idea where this thing sits because you’ve used bad carb parts and a harbor freight gauge. You can’t trust anything of the information you have right now.

First, throw that harbor freight gauge in the trash. They don’t work at all. I’ve tested one on an engine that reads 148psi with a Snap On gauge, and it read 100psi on the HF one. They just aren’t reliable at all. Go rent a gauge from the local auto parts store and check your compression with it. I guarantee you’ll get different numbers than you saw before.

Second, quit trying to get the carbs to work with the aftermarket parts. You’re just going to frustrate yourself, and even IF you can get it to run that way, the carb settings won’t be right. You may have to wait a bit, as there is a parts shortage right now, but buy the “Back to OEM” kit from OSD Parts with new needles and seats. They can get you the correct springs as well. It sucks to have to do the complete job again, but you’ll know it’s correct and can eliminate the carbs as an issue that way.

Third, check the rest of the fuel system. Replace the fuel selector, and the o-ring on the fuel/water separator. They are common places for leaks, and you want to make absolutely sure your fuel system is in good shape. Nothing kills these things faster than poor maintenance on that part of the ski.
 

dr00andrew

Premium Member
Premium Member
Messages
47
Location
Jersey Shore
Water Crafts
1996 SeaDoo HX (sold)
1996 SeaDoo GTX
It has been answered but I will throw in my response anyways. You will definitely need a new top end. It also wouldnt be a bad idea to have your cylinders sent off the be re-plated or sleeved. If someone else could answer that, please do. For example, the cylinder in my YZ 125 is plated and you have to send it off to a cylinder repair company to get re-plated after it has worn out enough. If it is sleeved, than they will press the old one out and install a new one.

Since you have such low compression, I would definitely imagine some sort of scarring or damage to the cylinder walls. Attach some photos if possible so we can see what they look like. It is also important you have a professional size your piston correctly to your cylinder. if it is sized to small you will get piston slap in a loose bore and eventually it will explode in your cylinder sending chunks of piston everywhere. If it is sized too big, it will expand too much when hot and seize in the cylinder. either way, make sure you have someone knowledgable do that. If you send them off to be repaired, they will take care of that for you.

As for your carburetors, the needles, seats and o-rings are essential to rebuilding the carb just like everyother piece. If you did not use genuine mikuni parts, they will fail the pop off and leak tests. I too have a 96 GTX meaning the components in your carburetor have the potential to be 24 years old at this point. The internal parts of a carburetor are considered a wear item and simply will not hold up for 2 decades. Unless the PO showed you paperwork or a receipt that showed he rebuilt the carbs, assume they have never been touched.

Just to sum it up, you will need new top end parts and most likely repair to your cylinders. Also make sure you replace every internal part of the carb with genuine mikuni parts and you should be good to go
 

JeremyD615

Well-Known Member
Messages
1,426
Location
Nashville, TN
Water Crafts
1995 Sea Doo Speedster
1995 Sea Doo XP
1996 Sea Doo XP
It has been answered but I will throw in my response anyways. You will definitely need a new top end. It also wouldnt be a bad idea to have your cylinders sent off the be re-plated or sleeved. If someone else could answer that, please do. For example, the cylinder in my YZ 125 is plated and you have to send it off to a cylinder repair company to get re-plated after it has worn out enough. If it is sleeved, than they will press the old one out and install a new one.

Since you have such low compression, I would definitely imagine some sort of scarring or damage to the cylinder walls. Attach some photos if possible so we can see what they look like. It is also important you have a professional size your piston correctly to your cylinder. if it is sized to small you will get piston slap in a loose bore and eventually it will explode in your cylinder sending chunks of piston everywhere. If it is sized too big, it will expand too much when hot and seize in the cylinder. either way, make sure you have someone knowledgable do that. If you send them off to be repaired, they will take care of that for you.

As for your carburetors, the needles, seats and o-rings are essential to rebuilding the carb just like everyother piece. If you did not use genuine mikuni parts, they will fail the pop off and leak tests. I too have a 96 GTX meaning the components in your carburetor have the potential to be 24 years old at this point. The internal parts of a carburetor are considered a wear item and simply will not hold up for 2 decades. Unless the PO showed you paperwork or a receipt that showed he rebuilt the carbs, assume they have never been touched.

Just to sum it up, you will need new top end parts and most likely repair to your cylinders. Also make sure you replace every internal part of the carb with genuine mikuni parts and you should be good to go
While this certainly could be the case, wait until you get a new gauge on it and double check the compression Before you start thinking about a top end. Those Harbor Freight gauges really do suck that much, and you may not have a compression problem at all...
 

mikidymac

Moderator
Staff member
Moderator
Messages
14,048
Location
California
Water Crafts
2003 GTI
2001 XP
1996 XP
1996 HX
1991 Superjet
2001 Superjet
Yes to all above.
1st check the compression with a good gauge and even Autozone will loan you one if you don't want to buy.
2nd you are going to have to do the carbs with Genuine Mikuni parts no matter what you do with the compression.
 

dr00andrew

Premium Member
Premium Member
Messages
47
Location
Jersey Shore
Water Crafts
1996 SeaDoo HX (sold)
1996 SeaDoo GTX
While this certainly could be the case, wait until you get a new gauge on it and double check the compression Before you start thinking about a top end. Those Harbor Freight gauges really do suck that much, and you may not have a compression problem at all...
yeah, thats a good point with the HF gauges. Autozone let me rent free of charge
 

jemery.wa

New Member
Messages
17
Water Crafts
96 Seadoo GTX
94 Polaris SL 750
Ok, bought a new compression gauge from Autozone, it is an Innova with the braided line did several tests and this gauge reads an average of 145 (mag) and 125 (pto). I am not sure of the quality of the one I bought, they didn't have the rental unit so I figured at least this would provide a 2nd opinion.
 

mikidymac

Moderator
Staff member
Moderator
Messages
14,048
Location
California
Water Crafts
2003 GTI
2001 XP
1996 XP
1996 HX
1991 Superjet
2001 Superjet
The 20 psi difference is concerning and points to a mechainical problem it the low cylinder.
 

jemery.wa

New Member
Messages
17
Water Crafts
96 Seadoo GTX
94 Polaris SL 750
also for disclosure, the fuel lines have been replaced by PO except for the return line and vent lines, they are still gray. I did replace the filter and o-ring.

Maybe the 20 psi difference is due to the PTO carb running too lean before.

I will be buying a Mikuni rebuild kit and since I don't have both of the original springs I will have to use the ones in the kit, I am not exactly sure how the arm should look, from everything I have been reading it seems that it should be parallel with the carb. is there a reason to remove the jets if I am not changing the size?
 

jemery.wa

New Member
Messages
17
Water Crafts
96 Seadoo GTX
94 Polaris SL 750
I did look with a bore-scope and didn't see anything alarming but that view is limited.
 

mikidymac

Moderator
Staff member
Moderator
Messages
14,048
Location
California
Water Crafts
2003 GTI
2001 XP
1996 XP
1996 HX
1991 Superjet
2001 Superjet
Do not use the springs in the kits. Order the correct ones and follow my carb rebuild thread. You can pm me if you have questions.

Yes you have to remove everything to clean it correctly.
 

JeremyD615

Well-Known Member
Messages
1,426
Location
Nashville, TN
Water Crafts
1995 Sea Doo Speedster
1995 Sea Doo XP
1996 Sea Doo XP
He needs the black 80gram springs, correct? I thought that’s what OSD included in their back to OEM kit for the 787’s... Am I mistaken?
 

dr00andrew

Premium Member
Premium Member
Messages
47
Location
Jersey Shore
Water Crafts
1996 SeaDoo HX (sold)
1996 SeaDoo GTX
He needs the black 80gram springs, correct? I thought that’s what OSD included in their back to OEM kit for the 787’s... Am I mistaken?
No you are correct with the 80 gram black springs. eugene has a chart for pretty much every 2 stroke sea doo there is and said the 787 GTX takes the 80 gram black springs.


Although I cant verify the accurateness of it, here is a mikuni chart i stumbled upon recently. I am by no means claiming this is correct, but I feel its atleast a good starting point even if it doesnt list makes/models
 

Attachments

jemery.wa

New Member
Messages
17
Water Crafts
96 Seadoo GTX
94 Polaris SL 750
The springs that came out were dull silver, I still have one of them just have been unable to locate the other one, must have fallen into the mystery hole that so many parts go to die. I can't verify if they were original although the old diaphragm did have the red nipple. Do I need to buy the black ones or the dull sliver?
 

jemery.wa

New Member
Messages
17
Water Crafts
96 Seadoo GTX
94 Polaris SL 750
Do not use the springs in the kits. Order the correct ones and follow my carb rebuild thread. You can pm me if you have questions.

Yes you have to remove everything to clean it correctly.
I just finished reading your rebuild thread and REALLY wish I would have seen it before the first time, it is an EXCELLENT write up.
 

mikidymac

Moderator
Staff member
Moderator
Messages
14,048
Location
California
Water Crafts
2003 GTI
2001 XP
1996 XP
1996 HX
1991 Superjet
2001 Superjet
96 GTX uses the black 80 gram springs.
 

jemery.wa

New Member
Messages
17
Water Crafts
96 Seadoo GTX
94 Polaris SL 750
everything is ordered, based on my compression will I be able to run it before the top end rebuild?
 

mikidymac

Moderator
Staff member
Moderator
Messages
14,048
Location
California
Water Crafts
2003 GTI
2001 XP
1996 XP
1996 HX
1991 Superjet
2001 Superjet
If it was me I wouldn’t.
That 125 psi has something going on.
 

jemery.wa

New Member
Messages
17
Water Crafts
96 Seadoo GTX
94 Polaris SL 750
So between rain storms today I managed to get the head off. The PTO cyl (125psi) seems to have quite a bit of play, the MAG doesn't have as much but I am not sure if it is because it is in a lower position. I couldn't get any further due to the rain. This is the pic of looking down into the ski, the PTO is on the right. I didn't feel any major scratches in the sleeves.
seadoo cyl.jpg
The head, PTO on the right.
seadoo head.jpg
 

jemery.wa

New Member
Messages
17
Water Crafts
96 Seadoo GTX
94 Polaris SL 750
Another update, got the cylinders apart and this is what I found.

20200607_111031.jpg
Going to have to send the top end kit back and go over plus have the sleeves bored out, I measured the sleeves and they are currently at 82mm which I assume is stock. How much over should I go?
 

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