Could Detonation be the cause of low compression?

[darrylgood]

Hello All,

I have a 97 seado xp,

My rear cylinder has horrible compression. I opened it up and found that the front cylinder head is perfect but the rear (where i have low compression) has strong signs of deto. Pics attached.

Is this why I have bad compression?

Is my ski done/garbage? Am I able to replace that head and rebuild my carbs/line. Will that fix my compression issue?

Thanks so much!

 

[Bills86e]

No. A lean carburation condition caused your problem. The pto became starved for fuel, but only had air, heated that cylinder to the point lubrication oils dissapated
warped the piston/damaged the rings. Next the piston top will melt. Remove the cylinder & piston and have a machinist evaluate. Rebuild the carb(s) and fuel delivery
system. Don't drop any wrist pin bearing into the case. Inspect that for metal bits.

 

[dsw222]

judging from that pic, it looks like your piston already started to melt (left hand side of pic)

what is the compression? how low?

 

[darrylgood]

Thanks Guys,

The compression on the front was fine, around 150, the rear was roughly 20 I believe.

Now that I have this low compression, I obviously cant get it to start.

Thanks so much Bills86e, I am aware how the Deto occures and yes i do have the grey fuel lines still. I will be switching those out and re-building the carbs.

I guess my main question is: is my poor compression most likely because of what has happened to this one piston (it doesn't looks that bad and the walls seem fine) or could it also be something else.

I just don't want to tear the top end apart and put in my new piston. re-do the carbs and find i still have low compression.

Thanks!

 

[dsw222]

Thanks Guys,

The compression on the front was fine, around 150, the rear was roughly 20 I believe.

Now that I have this low compression, I obviously cant get it to start.

Thanks so much Bills86e, I am aware how the Deto occures and yes i do have the grey fuel lines still. I will be switching those out and re-building the carbs.

I guess my main question is: is my poor compression most likely because of what has happened to this one piston (it doesn't looks that bad and the walls seem fine) or could it also be something else.

I just don't want to tear the top end apart and put in my new piston. re-do the carbs and find i still have low compression.

Thanks!

20 is bad.

Basically, "compression" is a measure of the pressure above the piston in when the piston pushes as far up as it can to squeeze all of the air/gas together right before the spark from the spark plug is ignited.

If you have low compression, then that means air is escaping somewhere inside the cylinder. In your case, the piston melted so now air rushes out below the piston instead of the piston pushing the air upwards.

You need a top end rebuild. I would do both at the same time because you don't really want them to be different. So you will need 2 pistons, new rings, new clips, wrist pins, installation gaskets, and you will need to have your cylinders bored and honed by a machine shop.

OR you can go the route I did and buy a complete top end rebuild kit from SBT, where they will send you everything you need, and you send them your old cylinders back (so they can fix them and give them to the next customer). Or Full Bore also offers this service but they recondition your cylinders instead of giving you someone else's.

So you're looking at about $500 or so and a day's worth of engine work.

 

[darrylgood]

Thanks so much dsw222

My cylinders look perfect, do you think I would be ok with just replacing the piston head/rings?

Thanks

 

[dsw222]

Thanks so much dsw222

My cylinders look perfect, do you think I would be ok with just replacing the piston head/rings?

Thanks

I would think that at a minimum you would need that cylinder bored and honed because it might look perfect but you dont know exactly how the detonation would have effected it. Maybe I'm wrong about this... perhaps Dr Honda or someone could chime in on this.... but I don't think you want to have a factory sized piston in one cylinder and an aftermarket oversized piston in the other (they will have slightly different properties)

Plus... is your other piston showing signs of detonation too? If one is, the other could be too. In that case, you could buy a piston, put all that work into installing it and getting your cylinder honed... then the other piston could melt too, and you are back where you started.

Like I said... If it were me (and it actually IS because my port engine's rear jug currently has 0 compression and I will be fixing it over the winter)... I would do a top end exchange from SBT or Full Bore.

 

[darrylgood]

The other one is perfect, not any signs at all and the compression is perfect.

So what your saying is if i buy a pistone set ( http://www.shopsbt.com/sea-1997-xp/47-107.html ) from sbt, it will not be factory size? It will be a touch larger?

I just saw that it gives me 4 choices for size:

Standard
0.5mm
1.0mm
1.5mm

I assume Standard is factory.

I do understand what you are saying about not being able to tell if the sleeve is damaged or not.

I think it might be best for me to get it bored.

Hmmmmm I will have to do some calling around

 

[dsw222]

If you get it bored, it will no longer be standard size. Therefore the two jugs will have slightly different displacements.

 

[dsw222]

But I dont know if that matters or not...

I'm doing the cylinder exchange for my boat because I don't want to deal with ripping it apart again if the other piston decides to melt too

 

[darrylgood]

I think i will get my original cylinders bored - http://www.fullboreonline.com/sea_doo_800_top_end_rebuild_kits.htm

Cant beet the 295 price

 

[dsw222]

yep... good deal!

you should be happy with them, there has been a lot of good feedback here

 

[darrylgood]

Thanks again every one

 

[darrylgood]

New problem

Yikes new problem,

just decided to do a last compression test again before I took the whole top end apart. Again my bad piston is 20 psi, we knew that, the good one is now 250!

What is going on here?

What is the cause of to much compression? Is that something that is only cause by pistons/cylinder or can there be another issue in the engine?

Thanks again