5 hp Low Compression ? Nissan NS5B will not start - has good spark

[fishon71]

I have a 5 hp Nissan NS5B serial# 369-1 76120 that will not run. It has good spark and even if I spray starting fluid the in carb ? nothing. Compression seems low at 30 pounds. What should the compression be and what is minimum to start? Also do you know what year this is? Thanks

 

[Tig]

Re: 5 hp Low Compression ? Nissan NS5B will not start - has good spark

You are right 30 lbs is very low. Starting fluid will make it lower by accelerating cylinder wear (rings). I don't know your specific engine but the 100 lb range is typical for small engines.

 

[fishon71]

Re: 5 hp Low Compression ? Nissan NS5B will not start - has good spark

Ya, thats what I thought, so a new set of rings will get my compression back up I assume? I pulled the head and it all clean inside, no scratches.

 

[Tig]

Re: 5 hp Low Compression ? Nissan NS5B will not start - has good spark

Is this engine a 2 stroke? A leaky (damaged/stuck) valve would also cause poor compression on a 4 stroke.
A stuck ring would is also a possibility. When you have the piston in your hand the rings should be somewhat loose in the groove.
If you do the rings, get the cylinder honed while you are at it.
A maintenance manual would be handy to cover those little details that get missed like exactly how the ring gaps should be offset upon assembly.
Back on the topic of starting fluid. I dribble a bit of premixed fuel directly into the cylinder via the spark plug hole to coax a stubborn two stroke engine to life at the start of the season.

 

[Tig]

Re: 5 hp Low Compression ? Nissan NS5B will not start - has good spark

When you say you pulled the head and it's all clean, do you mean the head (compression chamber) is shiney clean? Like new?

 

[fishon71]

Re: 5 hp Low Compression ? Nissan NS5B will not start - has good spark

This is a two stoke, I did look at the valves first just be removing the carb, they appear both be closed, didn't see any crap. I put my thumb over the carb inlet and there was no suction when I pulled the cord.
As far as clean, the top of the piston and the the internal side of the head where shinny clean. Either some has been in there or sprayed an awful lot of starting fluid in there. I always squirt a little spray oil with the starting fluid, better than nothing. I suppose if the rings where bad enough and it ran gas could have gone by and cleaned everything too.

 

[Tig]

Re: 5 hp Low Compression ? Nissan NS5B will not start - has good spark

On a two stroke the valves (reed or rotary) are of no consequence to compression, so no worries there.
You may be right about when you say that starting fluid may have cleaned the engine. In my experience the only time I have seen shiney clean is on fresh rebuild or water in the cylinder. Leaky head gasket also means very low compression. Inspect the old head gasket carefully.
I take it this engine is new to you.
Any chance it was rebuilt and they messed up on the rings? (stuck, broken, wrong size) No scratches is a good sign. Do you see any sign of the cross hatch pattern left by a cylinder refresh?

 

[pvanv]

Re: 5 hp Low Compression ? Nissan NS5B will not start - has good spark

We have seen this 30-pound compression when a 2-stroke was run on straight gas (no oil). You would like 100 pounds compression. On a 2-stroke, no compression equals no suction at the carb.

No suction when cranking can also be due to bad crankshaft seals.

Shiny clean combustion chamber is usually due to water intrusion.

I would mic the cylinder and piston to determine whether stock rings will work (and to see if the cylinder is warped). If it's out of spec (see a service manual, available at any dealer, for specs), then you will be looking at a new (oversize) piston as well as rings, and a cylinder re-bore -- probably uneconomical to repair in that case.