1995 seadoo xp 720 compression issue help

[cstuteville]

Seadoo Compression Issues
I have a 1995 Seadoo XP 720 that im having some issues with. I original bought the ski and it ran lean and lost compression in both cylinders. I replaced the following after that happened.

Replaced all fuel lines
Replaced Fuel Selector Valve
Rebuilt Carbs with Mikuni Rebuild kit
All new top end gaskets
New bored .5 over cylinders with brand new .5 over pistons and rings

After all this was completed, I tested compression before I test rode it. It read 130 PSI in both cylinders with a harbor freight tester. The compression tester seems to be good because I used it to test my Polaris and it reads fine. After that I took the ski to the water for a test. I took it easy on it and did plug chops to assure it wasn't running lean and tuned it where it was running great but was really hard to start after it was shut off. I rode it for about a half hour. Once I got back home, I tested the compression again and it dropped to 90 PSI in both cylinders and is still hard to start.

The ski ran great and hard in the water so im a little confused on what to do next.

Can anyone help me on what to do next or what might fix it?

UPDATE: The seadoo sat over night and i restested compression and the compression is now good. 120-130 psi. I'm confused and can't figure out why after the test it read low and after it sat it reads normal. Help please!

 

[LouDoo]

Hi and welcome to the Seadoo forum. 120-130 compression is not good, for a fresh rebuild you should be in the 140 to 150 range. A couple of things come to mind, the first thing I would do is to make sure you are getting accurate compression readings, compression should be taken on a cold motor with the throttle open. I have a HF compression gauge which seems to be accurate but there's a lot of guys out there that have had problems with the HP gauges, re-check the compression with a known good gauge, both AutoZone and Advance have gauges that they will lend you.

If the compression is still low you should check the squish, the procedure is in the service manual, this may require changing either the base gasket or head gasket also you should check and make sure the cylinders were bored out correctly.

Lou

 

[Matt Braley]

Yeah I was thinking squish also. Your loosing 20psi somewhere. Did you double the base gaskets under the cylinders or something like that? Have the domes in the head been recut(like SBT)?

Sometimes it takes a minute to seat those rings in. Especially if your running double oil while breaking in or something. Hopefully the inconsistency is just the cheap gauge messing with you.

 

[cstuteville]

Thanks for the reply. I was very close to 140 psi before first start after rebuild. I will look up the squish test. I am currently using a 5 hole base gasket and no my head has not been recut. I will see if I can get a different gauge today to test that out as well. The ski ran great as I said but became hard to start where I had to feather the throttle to make it start

 

[mikidymac]

When you rebuilt the carbs did you reuse the spring or a new one from the kit?
It should not be hard to start.

 

[cstuteville]

When you rebuilt the carbs did you reuse the spring or a new one from the kit?
It should not be hard to start.

I used a new springs which I heard was not good but I don't have the old springs anymore.

 

[mikidymac]

I would order a new spring from Minnetonka or OSD. It will not run correctly with the wrong spring.

 

[cstuteville]

The spring was new from mikuni in my carb rebuild kit. Are you saying that the one in the kit is not good?

 

[CReynoldsMIZ]

He's just saying that sometimes it is not the exact one you should use. Look up the part number for the spring that was originally in the ski, seadoopartshouse(dot)com should get you that. Then scour eBay searching that prt number and you'll find the right ones

 

[cstuteville]

Ok. On that website it only dates back to 1996 model. I may have to go with that or find another website. I will be checking compression again today with a new compression tester. I will update results. As far as the starting issue. This afternoon I came out to start it and it started first crank. Seems to be a issue when it's warm or hasn't been sitting.

 

[cstuteville]

Update: Rented a different compression tester and it still reads dead on 120 psi in both cylinders.