1994 GTX650 winter project

Baldy63

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92 GTS580. 94GTX650
#1
Rotary timing question. Is the timing disc wear that crittical i did the test with rosin core solder with the disc inside and and they came out squished to upper .015. And lower .013 Specs say .012 + - .002 So if i add the 2 together. Divide by 2 its ok. What i am curious about is does any one have any idea as to how bad the wear gap might be before engine lack of power and poor starting. Another piece of curiosity is the timing why. Is the piston travel about 1” down in the power stroke before the intake is fully closed. When you set the disc. At the book s settings of 130 degrees on the rotary wheel from the bottom of intake port on the mag cyclinder
 


AKnarrowback

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#2
The disk blocks the air/gas mix in the crank case from being pushed back out of the intake as the piston comes down. If the disk is timed properly with the mag side piston you are good to go. Don't worry about where the piston is vs where the disk is because it is a two stroke and they have the inherent nature of having "overlap" of intake and exhaust ports at times. The disk us not perfect in closing off air flow, it just helps decrease the "backflow" out of the intake while the piston is on the down stroke - vs - a motor without any rotary or reed valve. There is a short time where the port is partial open while the piston is on the down stroke.

As for the clearance of the disk. You should be safe with the measurements you gave us. Excess disk clearance will show up as hard starting, poor low end performance and extra hard starting if the motor is at normal operating temps. I had close to .022" clearance on my 94 xp 657x and only noticed low end running problems and hot starting problems. Remilling to .014" cleared things up. Rotary ralve cover wear is common on seadoos since they do get water in the intake at times causing seperation of the oil on the disk and metal to metal contact between the disk and cover.
 

Baldy63

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#4
Question so on ignition timing ? so when i remove the stator to change the crank there is no way to double check the timing You just line up the stator with the little mark on the housing and hope for the best. If you do the static timing you have to rev the rebuilt engine to 6000 rpm. To confirm it s right ..? I have not found or seen any info on low RPM timing is there anything????
 

mikidymac

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#5
Just line up the marks from the stator and the case.
 
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#6
While in the topic does anyone have a trick for putting the rotary flap on? Manual says degree wheel but I was wondering if someone had a trick such as, "it goes here when the mag piston is at the top of its stroke..."

Anybody?
 

mikidymac

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#7
Nope, the correct way is with a degree wheel and a dial indicator to find TDC. You can print the paper degree wheel out from the service manual and use that instead of buying one.
 
Messages
259
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Water Crafts
(2) SeaDoo SPX - 1997 (sold)
SeaDoo Speedster SK - 1999
(2) SeaDoo Spark 90hp - 2016
#8
Nope, the correct way is with a degree wheel and a dial indicator to find TDC. You can print the paper degree wheel out from the service manual and use that instead of buying one.
I just thought someone had a trick lol
 

Baldy63

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#9
I just thought someone had a trick lol
Many sites say 159 degree for all 650 engines. When i took mine apart the rotary disc is a 147 degree. Did the. Previous owner just just put a 147disc in for the heck Or do some 650 engine use the 147. Degree disc. ANYONE KNOW 1994 GTX 650
 

AKnarrowback

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94 xp
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#10
Go to seadoomanuals.net and get the shop manual for the 94's.

The 147 degree valve is correct for the gtx. The 159 degree is used in the 657X motor which was used in the XP only. There are two 657 motors in seadoo history. The 657x was 5 more HP more than the 657 and relied on a different rotary valve, rotary valve cover, carbs, pipe, cylinders and maybe some other things.

Personal observation, since I own both a gtx and xp 94 models, is that seadoo wasted a bunch of time building the 657x since it really wasn't that much more over the regular 657. The 657x showed up in 94, was used in 95 and pretty much abandoned after that. Even when I was setting these machines up out of the crate we in the shop talked of switching a gtx motor into the xp hull since the gtx ran so much smoother, had a holeshot that made the xp look stupid and was only a few mph slower. Even the ski-doo test and tune crew we worked with commented on how bombardier liked their "experiments" that got put into production, cost too much to develop and tool up for only to have them last a few seasons (657x motor, skidoo stratos/escapade - 90's era safari series - 2000 era single A-arm front fan cooled sleds, mach z 1000 twin, formula plus efi.).

A cure for the 657x xp, I found, is to raise the pop off pressure to the mid to upper 20's, take out the stock impeller that was pitched too high and put in the RH impeller from a speedster with a 15/20.5 pitch and enjoy. A simple fix that makes so much difference. The gtx still edges it in a drag race off the line.
 
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Baldy63

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Winnipeg Beach Mb Can
Water Crafts
92 GTS580. 94GTX650
#11
Go to seadoomanuals.net and get the shop manual for the 94's.

The 147 degree valve is correct for the gtx. The 159 degree is used in the 657X motor which was used in the XP only. There are two 657 motors in seadoo history. The 657x was 5 more HP more than the 657 and relied on a different rotary valve, rotary valve cover, carbs, pipe, cylinders and maybe some other things.

Personal observation, since I own both a gtx and xp 94 models, is that seadoo wasted a bunch of time building the 657x since it really wasn't that much more over the regular 657. The 657x showed up in 94, was used in 95 and pretty much abandoned after that. Even when I was setting these machines up out of the crate we in the shop talked of switching a gtx motor into the xp hull since the gtx ran so much smoother, had a holeshot that made the xp look stupid and was only a few mph slower. Even the ski-doo test and tune crew we worked with commented on how bombardier liked their "experiments" that got put into production, cost too much to develop and tool up for only to have them last a few seasons (657x motor, skidoo stratos/escapade - 90's era safari series - 2000 era single A-arm front fan cooled sleds, mach z 1000 twin, formula plus efi.).

A cure for the 657x xp, I found, is to raise the pop off pressure to the mid to upper 20's, take out the stock impeller that was pitched too high and put in the RH impeller from a speedster with a 15/20.5 pitch and enjoy. A simple fix that makes so much difference. The gtx still edges it in a drag race off the line.
So the GTX pop off Specs Says 22 to 29 Do they run better at the lower or higher popoff. Mikuni pop off specs say a 115 gram spring is suppost to pop off at 32 lbs with the 2.0 needle andseat or does the 95 gram do the job at supposedly 25 lbs or the 80 gram which is only 21lbs
 

AKnarrowback

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#12
I can't honestly give you an absolute answer to that question. A basic rule of thumb is lower pop off lets more fuel flow in easier (richer), higher pop off gives a leaner situation. All of this in theory.

I have 35 years of experience working with two stroke small engines, mostly Rotax, and when it came to "dialing in" my 94 xp I had to give up on any part numbers and do it all by guess work when it came to what spring I used for pop off.

IF you are using a machine that is all stock, no mods, then don't mess with the factory settings for the carb. A stock 94 gtx that is in good running condition will jump out of the hole and get on step with little hesitation, it will run up and down through the throttle range without any flat spots or sudden surges. Just make sure the pop off is within the range outlined by the factory and get the pop off on both carbs as close to the same as you can. Mikidymac has an excellent carb thread in the two stroke section that walks you through all aspects of getting the carbs apart and together again, read it all, think about it and read it all again, it is the best place to start understanding these carbs. Take a note on NEVER stretching or modifying the spring, if you have good factory pop off now then hang on to those springs like gold and treat them very carefully when you have them out of the carb. Tweaking the pop off by bending the float arm is the proper thing to do, but not if the arm is above the carb body where the regulator diaphragm "button" will be pushed on keeping it from moving and functioning properly.

The big rule is ALWAYS USE ORIGINAL MIKUNI REBUILD KITS AND PARTS if you rebuild your carbs. On that note I will say I violated that rule when I dialed in my xp for one reason, what seadoo said I had for needle and seats and pop off springs did not match any combination listed in the Mikuni chart. According to the Mikuni chart there was no way for me to have the pop off pressure seadoo called for with the parts they said I had. This I why I said I can't give you an honest answer to your question. What I did have was an aftermarket rebuild kit for both carbs, it was "crap" to say the least, but it did come with 4 sets of springs. Going on complete guess work I found a set that put me around 28 psi for the pop off on both carbs, my stock was around 13-16 psi.
What gram rating they were, I have no idea. Raising the pop off cleared up a huge low end bog/delay in that motor which is why I went into the carbs. I could be sitting at a dead stop and try to do a "drag race" start and my machine would hesitate and sputter for a good second and a half before, weakly, getting off the line where it would then come alive. I cleaned, adjusted, replaced lines, looked for leaks, checked timing, sacraficed a chicken, everything and finally decided to ignore the factory specs and bump up the pop off. I can go from a dead stop to standing my xp on the back end with a blip of the throttle now.

Don't go mixing springs and seats thinking you are going to hit the "Holy Grail" and gain some huge performance gain. The test and tune crew did an excellent job of getting the 657 motor dialed in back in the day. It was the 657x motor in 94 that had some weird a$$ pop of pressure that did not follow any pattern compared to other machines and it had an impeller pitch that was not used again until they came out with bigger motors a few years later.

Read Mikidymac's thread. Most of my 35 years experience is on sleds. This guy knows his stuff on watercraft and got me pointed in the right direction on working with the BN carbs.
 
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