Nissan 50 compression issue

[Mac300]

I have a Nissan 1998 50 2 stroke my son just acquired that has issues. No history on the engine but this is what we have determined. The motor had a oil reservoir so the oil and gas did not have to be mixed but it has been bypassed and has a "standard" direct fuel oil supply to the fuel pump now. The main fuse was blown when we got it. We got it cranked but no water was coming out of the pisser. Pulled the hose and it was stopped up. Cleared the line and it had water flow. Fair amount of white smoke. Did a compression check and had 70lbs on 2 cylinders and 90 on the third. When I pulled the plugs all were milky around the base (water in combustion chamber?). Pulled the head and cylinders are scored some and top of heads are milky. There was not an obvious place that I could see on the head gasket where it failed. I tested the spark and it was weak but that might have been a bad ground on my tester. It cranks and runs fine but obviously has issues. It is not on a boat so I have no idea how it does under a load. I'm no mechanic and most of my experience is rebuilding carbs, replacing impellers, fuel delivery, etc. I've never gone into a motor before so I don't really know if the scoring in the cylinder is really that bad or normal wear. I guess my main questions are why the low compression...blown head gasket or the scoring? Where would the water in cylinders be coming from? Is it usually pretty obvious if the gasket has failed? Don't see any evidence of cracks in the block when it is running. No obvious warping. Any input or guidance is appreciated.

thanks

 

[racerone]

No short cuts here.-----Time to rebuild it if parts are available.

 

[pvanv]

Hmm. Assuming your model is NS50D2. Year is irrelevant. Beware the money pit. If you have scoring on the cylinders, at a minimum, you will hone them and install new rings. If a telescoping gauge shows that you need more, you can bore 0.5 mm oversize, and install the oversized pistons and rings. That will require a machine shop. Odds are the scoring is giving you the low compression. As for the water, the head gasket was likely never re-torqued, and it was likely weeping a little.

If the oil mixing system is still there, you should reconnect it; They almost never fail, and they reduce oil flow at light loads and low RPM, which is cleaner, cheaper on oil, and makes the plugs last longer.

What you could try is to get a new head gasket properly torqued, fire it up, and run a couple of cans of decarbonizing spray through to see if the rings free up. If the scoring is minor, that may get you by.

 

[Mac300]

Thanks for the advice. Will certainly replace head gasket. For my education, what normally causes the scoring? I was wondering if the previous owner had an oil issue thus bypassing the automatic oiler. Or if it had overheated with the pisser hose stopped up. There is a lot of corrosion in the water jacket, I believe it saw a lot of salt/brackish water. Thanks so much.

 

[Mac300]

Paul, the decarbonizing spray you are referring to, is that done by removing the plugs and soaking the cylinders or by spraying in the carbs while running? I have also seen people adding some additive to a nurse tank and running the engine for a short period to do the same thing I think??

 

[Sea Rider]

Was that nice OB used in fresh or salt water ? if plan rebuilding the entire powerhead, remove side exhaust cover, surely both gaskets are shot, dried, burned, leaking droplets of water through pistons inside combustion chamber specially being a 20 year oldie.

Replace both internal gaskets along cylinder head gaskets and torque them to specs, will need to be retorque soon after 10 worked hours have elapsed. Gaskets cede a bit with heat, that's why need to be retorqued to specs for a second time.

Decarbon spray sprayed through carbs will work much faster than any other product poured into the tank. Engine must be ON while product is sprayed. Follow can instructions. Get a can of CRC Engine Tune Up Carbon Remover, it's an amazing product. If cylinder scores are not compromising too much the internal compression, product should restore some of the lost compression after the decarbonizing procedure.

Note : Must remove the plastic air silencer to fully access carbs..

Happy Boating

 

[pvanv]

Luis is correct. The CRC works well. Stuff in the tank is more preventive than corrective.

 

[Mac300]

Thanks guys, don't want to throw away much money on this old motor. Will replace head gasket and try the other suggestions and hopefully have some luck. After looking at pics of other scored cylinders these really don't look to bad. I bet the previous owner had an issue with the oiler and overheated it. Maybe I'll get lucky. If not, maybe I can sell it for parts. Thanks to all for your advice!!

 

[pvanv]

You can DL your owners manual from http://www.nissanmarine.com/tech_talk/owners_manuals.html
Pay attention to how to bleed air from the oiling system.
Minor scoring is likely normal, due to crud in the air.
Plugging the telltale will not affect cooling system operation.

 

[Mac300]

Thanks Paul!!

 

[Sea Rider]

If plan achieving the max water performance out of that OB along the least investment in parts. Will be good to know if that nice OB was used in fresh or salt water as both are completely different escenarios in what will find inside the power head once cylinder and exhaust gaskets are removed.

Happy Boating