YAMAHA XR1800 both Engine Siezed after rebuild

[dj100encore]

I recently rebuild both engines on my xr1800. (waverunner 1200GP power valve engines)

re-sleeved 4 of the 6 cylinders and bored to 0.5mm oversize.

After running for 30 mins both engines started having issues, they would restart and run for about 10 minutes, during which all 4 water jets were working.

First the starboard engine stopped. limped back with the port engine.

I have deleted the oil injection and was using premix tc-w3 oil 25/1.

during rebuild I changed the crank seals.

The machine shop told me in hind sight that they had machined standard 0.003" to 0.004" piston clearance, I have read that this should be closer to 0.007"

After opening up the engines to remove the cylinders for another re-bore I found a hole in piston #1 of the port engine.



Any ideas what causes a hole in the piston, I think that the port motor was under more than run in load during the limp back.

Thanks,

David

[Oleg Russia]

I would advise you to make a return line mod. this will save you from piston burnout. and you have problems with carburetors.

[K447]

Welcome.

What was done to the carburetors while the engines were out?
Full carb cleaning and rebuild with OEM carb kits?

Rest of the fuel system serviced? New filters, hoses, clamps, etc?

[Cutlass]

A hole right in the middle of the piston could indicate an air leak from a loose or leaking spark plug. It could cause a severe lean hot spot right in the middle of the piston.

[Myself]

A hole right in the middle of the piston could indicate an air leak from a loose or leaking spark plug. It could cause a severe lean hot spot right in the middle of the piston.

Exactly what I was thinking.

Or could just be bad lean from some other issue. The 1200 pv engines will NOT tolerate anything that's not 100% right. I also see some sort of reddish sealant goop or something. That is not necessary on clean flat surfaces. The .003-.004 clearance should be just fine if warming up engines properly before heavy throttle.

[dunnitagain]

Yup , Loose Spark Plug , makes a Nice Air Jet Torch. I always open up sleeved cylinders to about .0055-.006 piston to wall.

[steve45]

Also, 25:1 oil ratio isn't enough for break-in. That engine normally runs about 32:1. For break-in, I'd run double that.

[jeffg426690]

Also, 25:1 oil ratio isn't enough for break-in. That engine normally runs about 32:1. For break-in, I'd run double that.

I disagree w that. With the oil injection system they run @ 32:1 when wide open, any other throttle position is much less than that. Most folks who run premix on the 66v favor about 40:1

In a given volume of fuel/oil mixture if you add more oil, there is less fuel - which will favor lean condition. Ex; Pour 8 oz of fuel into a glass and 1 oz of oil for a total of 9oz volume, now pour 7oz of fuel into another glass and add 2oz of oil, again total of 9oz volume- which one will run leaner?

There is no need for extra oil during a break-in period, follow the guidelines of varying throttle up to 5k rpm for 4-5hrs runtime.

[steve45]

I disagree w that. With the oil injection system they run @ 32:1 when wide open, any other throttle position is much less than that. Most folks who run premix on the 66v favor about 40:1

In a given volume of fuel/oil mixture if you add more oil, there is less fuel - which will favor lean condition. Ex; Pour 8 oz of fuel into a glass and 1 oz of oil for a total of 9oz volume, now pour 7oz of fuel into another glass and add 2oz of oil, again total of 9oz volume- which one will run leaner?

There is no need for extra oil during a break-in period, follow the guidelines of varying throttle up to 5k rpm for 4-5hrs runtime.

So, you're recommending running less oil at full throttle than Yamaha has designed the engine to use. That make sense!

Extra oil will displace fuel, but it won't make much difference. During break-in, you're typically not running full throttle for long intervals. Something that people don't take into consideration is that oil burns, too.

Yamaha recommends running 50:1 pre-mix in this boat during break-in (I just checked the owner's manual. It's on page 3-14). Adding the extra oil when running oil injection gives a ratio of 20:1. Of course, I'm sure you know more than Yamaha does...

You're certainly welcome to disagree.

[Myself]

On a premix only system I've always run 32:1. Never had any problems. You're not supposed to be running WOT during break-in anyway. After that it's 40:1 in all my stuff.