No Compression - 6hp 4stroke Nissan - Tohatsu

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A friend of mine gave me a nissan 6hp 4 stroke that he had purchased in the B.V.I. since I had lost my job and needed an OB for my tender and I had done him a couple of favors. It was missing the carb, so I purchased one and installed it, but it would not start. Another friend of mine allowed me to test it with a Spark tester which I hooked up and it was lighting up. Because the OB would not start, I took it to a mechanic who diagnosed it as having low compression; around 50 psi. The mech told me it would probably cost more money for him to take apart and diagnose, than the motor is worth.

I would like for someone to tell me, if I can take apart the motor and find out how to diagnose and fix it and what steps to take. I have a lot of time since being laid-off and I really need this OB, since me and my wife are live-aboards (I can deal with rowing, but it's hard on the wife!) Please TohatsuGuru, if you are reading please help!

 

[pvanv]

Re: No Compression - 6hp 4stroke Nissan - Tohatsu

Start with a Factory service manual and proceed from there. 50 pounds compression is pretty low, but it should at least start and run, albeit poorly. The 4/5/6 has a decompression weight on the cam, so depending on how the compression test was done, that figure may be falsely low.

The compression test only tells part of the story. If low, then do a leak-down test to determine the cause of the low compression. If just a carboned-up valve, a de-carbon may help. If a cracked ring, you will be in for a new piston and ring, as well as at least a re-hone of the cylinder, or possibly a new short block. That might not be cost effective.

 

[siliconavatar]

Re: No Compression - 6hp 4stroke Nissan - Tohatsu

Thanks for your response.
I ordered the manual. Will ask the mechanic if he has the leak-down tester and get back.

 

[siliconavatar]

Re: No Compression - 6hp 4stroke Nissan - Tohatsu

Got Service Manual. Mechanic stated that leaked down test showed escape from several sources; exhaust and out of oil pan plug.

I then took engine apart, though not the oil pan. When I removed the head I saw right away oil and dirt. I cleaned it all out and turned the flywheel. No oil or dirt was coming from the cylinder walls. I then checked the valves and sure enough there was the same soot on exhaust side. and the bolts were loose on the pivots. I cleaned the head and pushed in the valves since I didn't know how to take them off. I cleaned the edges of the valve seats and the valves themselves as best as I could.

At this point I placed my hand over the opening of the cylinder wall and turned the flywheel and noticed pressure both pushing and sucking. I noticed a very small amount of clean oil around the cylinder walls, less than a quarter of a drop all along the wall very evenly except at one point where it made a very thin line. I am assuming that this may be normal since I was not letting any air in through my hand and the tolerances would allow this. You could hear air rushing from the oil pan when I put my ear to it. I touched the sides of the walls and could feel no scarring or vertical lines.

Any suggestions on how to proceed or what other tests I can make?

 

[TOHATSU GURU]

Re: No Compression - 6hp 4stroke Nissan - Tohatsu

The safest thing to do is a complete dis-assembly, inspection and cleaning. Otherwise you run the risk of missing something. Anytime you inherit a non running engine its wise to spend the extra time.

 

[siliconavatar]

Re: No Compression - 6hp 4stroke Nissan - Tohatsu

Cleaned everything out. Took the head to a mechanic to inspect as well as the cylinder/piston. Mechanic stated all is good. Got head-gasket, installed everything. Got feeler gages and adjusted valves. Noticed suction on intake manifold, when turning flywheel. Re-assembled everything. Now engine is exhibiting a little different behaviour; When pulling starter rope, smoke comes out of carb when choke is open. I still can't get it to run.

 

[pvanv]

Re: No Compression - 6hp 4stroke Nissan - Tohatsu

Did you disassemble/clean everything? If so, did you verify valve/cam timing?

When you had the head off, you "cleaned the valve seats as best you could" Did you check the valves for leakage? An old-time 4-stroke valve leak check was to set the head chamber side up, pour mineral spirits cleaning fluid in the chamber, and see if any leaked out through the closed valves.

Did you get a new carb, or a used one? If a used one, did you completely disassemble it and soak it in carb dip, then blow out all passages with air, before installing it? A lean carb can cause popping through the intake.

 

[siliconavatar]

Re: No Compression - 6hp 4stroke Nissan - Tohatsu

Previous mechanic told me valve cam timing was good.
I did not know how to set the valve clearances, but according to what I could understand from the factory manual, I first find the tdc by looking into the gas pump hole until I see the notch mark, and then I verify this by looking at the timing mark on the flywheel, which should be aligned with the first mark on the casing. (the other marks are 5 and 25 degrees) I then use a feeler gage and adjust the intake valve to .06 to .14 mm and the exhaust valve to .11 to .19mm. After tightening the lock nut, I checked again.

I did check before putting on the carburator to see if I had suction by placing my hand in the intake manifold and could feel pressure suction, by turning the flywheel. But I also felt a push of air at one point while turning. not sure if that was normal.

* I checked the valves for leakage by using gasoline instead of mineral spirits.
* The carb was new from the factory.

Thank you for helping me.

 

[pvanv]

Re: No Compression - 6hp 4stroke Nissan - Tohatsu

Sounds like you set the valves correctly. You just need to be sure that you were at TDC-compression, not TDC-exhaust. That's why you check for the crown of the fuel pump cam lobe first. Not unusual for the piston to start into compression just before the intake is fully closed. At running speeds, the intake flow continues, and there shouldn't be much spitting from the carb.

If in fact the compression is low, it will need extra choke and a slightly higher throttle to start from dead cold. I would try starting it again. Verify that you have fresh fuel, throttle up to about 1/3, crank briskly twice with choke fully closed. If it doesn't fire, then partially open the choke and crank once or twice like that. Should at least fire. Then you can modulate the choke until it comes to life and continues to run.

 

[siliconavatar]

Re: No Compression - 6hp 4stroke Nissan - Tohatsu

for a minute there I was doubting the mechanic's assessmenet of the timing, but he assured me of it. From what you are telling me I should be able to try two positions on the choke knob. First, try it with the choke in, but with the throttle 3/4 and then try to extend the choke out.

I must admit that I have a friend that has the same engine, except nissan instead of tohatsu, and he allowed me to replace all of his electrical parts with mine,execpt the ignition coil and the charge coil. They all work fine in his engine. His engine however does not start to factory specs. Instead he starts his, by pulling the choke all the way out, and the throttle all full. Once its starts, he puts the choke in, reduces throttle, and then, he puts it in gear.

If I have to pull the whole engine apart again, it is not a problem, since I have been laid-off from my electronics job, I have a loty of time. I still thank you for the attention you gave to me. Until you told me, I thought that maybe I would have to adjust the valves at 5 or other degrees. This makes me more confident. I would assume that other from your input, I would have to question the mechanic's assessment's, and to unravel the engine and check the timing. (which I do not know how to do. - to be honest with you -

If you wish for me to take the engine apart again in order to look at the timing, I do not mind, so please tell me.

Thanks again,
charlie.

 

[pvanv]

Re: No Compression - 6hp 4stroke Nissan - Tohatsu

Charlie,

You're on the right track.

The cam timing on these is gear driven. So unless the pan was apart, you should still be in time. Of course, who knows what the previous owner did?

What I was suggesting is that low compression equals low intake suction, which requires richer settings and/or more air/fuel charge to start from cold. For example: I have an ancient McCullough 2-stroke chain saw that runs fine, but must be cold started at WOT. I also have a Kawasaki-powered 2-stroke snow blower that runs OK, but must be cold started with the electric starter, in order to get enough charge into the cold cylinder to fire at first. Neither of these old, beat-up motors deserve even an hour of my time to repair them, and they will be retired as soon as reasonable.

If your friend's carb has some varnishing in the low-speed passages, or if it has slightly low compression, his technique for starting is a legitimate way to get it running. Same goes for your motor. Since you have the luxury of a known-good spare motor, and are baffled as to what is wrong, you can swap carbs, etc., and use the same starting techniques that work on the other motor. However, a healthy NSF6A2 shouldn't need those tricks.

 

[siliconavatar]

Re: No Compression - 6hp 4stroke Nissan - Tohatsu

Should I have felt pressure coming from the intake manifold when I was turning the flywheel? Is a little pressure ok, or should proper timing have prevented that? Like I said I felt sucking of air, but as I turned the flywheel, I also felt air pushing out throught he intake manifold.

Regards,
Charlie

 

[pvanv]

Re: No Compression - 6hp 4stroke Nissan - Tohatsu

I wouldn't expect that on a 4-stroke. You might want to check the marks on the timing gears to make sure you are in time correctly.

 

[siliconavatar]

Re: No Compression - 6hp 4stroke Nissan - Tohatsu

Will do that and get back.