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Yamaha 1800 SVHO marine engine failure analysis

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Link to pictures

https://workupload.com/archive/hWcbhMb8

Hallo,

I would like to show my Yamahas 1800 SVHO marine engines condition and discuss the failure. It is a 4-cylinder 86 x 78mm, 1812cc super charged engine. It made up to 9200 rpm while OEM is at 7500.

It was driven hard for approximately 30 hours of its total 50 hours. Heat management is a problem on this Jet ski since cooling cannot be upgraded due to stock class racing rules.

During the last weeks of its use compression dropped especially on cylinder two:

1 135psi

2 110 psi

3 125 psi

4 135 psi

The oil consumption increased dramatically.

On its last day, it lost sufficient power as soon as it warmed up it lost more than a 1000 rpm on WOT. (Jetski has only one gear defined by the impeller pitch and no incline when riding since water is flat)

Disassembled engine shows:

Cylinder 1:

- Polished appearance on middle of skirt. I consider it as normal

- Depp scores on the piston pin side as well as marks of it on cylinder block. What could be the cause for this?

- Stripes in the cylinder not fellable

- On the bottom of the piston I think to see some brown color what should be burned oil

Cylinder 2:

- Edge of Piston started to melt on intake manifold side. Head gasket was leaking to water channel on exhaust manifold side. I think this was the reason for the failure. Second cylinder got no cooling what caused detonation and the ECU to reduce ignition advance. This engine will not build up pressure in cooling system since it is open loop using water from the lake.

- Polished appearance on middle of skirt. I consider it as normal

- One deep scores on the piston pin side as well as marks of it on cylinder block. What could be the cause for this?

- Stripes in the cylinder not fellable

- On the bottom of the piston I think to see some brown color what should be burned oil

Cylinder 3 and 4

- Polished appearance on middle of skirt. I consider it as normal

- On the bottom of the piston I think to see some brown color what should be burned oil

- Stripes in the cylinder strongly FELLABLE. Reason for this might be a piston to bore clearance that is tighter than oem specification. I measured 0.08 mm on cylinder 1 and 2 but only 0.07mm on cylinder 3 and 4, while specifications 0.08 – 0.09

Looking forward to any comments and suggestions.

I am no thinking if I should sleeve it and use OEM pistons or go 1 millimetre oversize. Since oversize increases cc will I gain more power on a supercharged engine?

Hello, it is a little hard to tell from the description but try adding some pictures. this will help

and as for the pistons i would if allowed use a forged piston and clearance as per the spec card

when revving higher than standard and racing it you are always recommended to allow for more expansion

Regards Ross

Hallo Ross,

You can downloaden pictures following this link. If you are not comfortable let me know i will upload to forum.

https://workupload.com/archive/hWcbhMb8

I can't change shape and material of the piston but was told that Yamaha and seadoo oem pistons are forged