Low Compression on 4hp. What am I missing?

[Wee Hooker]

Looking for thoughts on what to look for next. So I have an 80's vintage Tohatsu built Mariner 4 hp motor that came back to me with 32# or compression in the one cylinder. I just (this weekend) pulled the head off in an attempt to fix and here is where I'm stuck. So the head looked good when I pulled it, no warping, cracks etc that I can see, Likewise the piston face looked good and the cylinder walls shiny and smooth as a baby's butt with no gouging, drag marks etc that would point to a bad ring. Cylinder appears to fit snug with no play too. There was a fair amount of salt deposits in the water jacket so cleaned it all up, replaced the head gasket and retorqued the head . (Thanks Paul!) Still getting 32# on the compression tester so I layed the motor carb down/head up , put a little oil in the plug hole to coat/test the rings and retested. Still 32#. No improvement.
So at this point, I'm scratching my head. Where could the pressure be escaping ? Is it just that the reed valve is open/leaking and the 32# is the result of just the end of the stroke? Will a bad reed valve give me this symptom? Could we be leaking into the water system? I'm pretty decent ith these things but at teh bottom of my bag of tricks. Ideas welcome before this otherwise nice little OB is delegated to the status of spare parts queen .

P>S. Love these Little Tohatsu 2 stroke OB's I have two others in 5 hp. To me, there is nothing today that comes close to the power to weight and feature ratio that these have. I repositioned the controls on one of them to use on my 17' Old Town Sprot Canoe and it is a perfect fit. ( That boat will fly at 23 MPH (per the GPS) with that 5 hp btw!)

 

[Scott Danforth]

Did you measure the piston diameter and the bore diameter? Of they are both in spec, I would suspect rings or a bad reed.

Yes, a bad reed valve would give you the symptoms

 

[Wee Hooker]

Thanks for weighing in Scott. It's appreciated. I did not measure the diameter of either as I honestly thought this was going to be cured with the head gasket. Guess I could pull the head again and measure the bore to see if its in spec but I'm thinking I would have to split the crank case and disconnect the piston rod to measure the piston, no?
That said, since I wet the piston face /cylinder walls with oil and the pressure didn't move, I'm thinking it's not rings. From my experience with Car engines, the pressure should have jumped (at least momentarily) with the oil trick if it were rings. ( in a car, once you squirt oil into the cyl, if pressure goes up, you need rings, if it doesn't you need valves (which are on top and therefore not covered in oil .)

So, how do I inspect reed valves? Are they hard and /or very costly to replace?

 

[pvanv]

In order to access the reedd on that model, you must separate the block:

 

[racerone]

Reed valve has NOTHING to do with compression in the cylinder.-----Try another compression test device.

 

[Sea Rider]

Yep, reed valve is other story, could compression tester be in bad shape ? check with other. Compression for 4-5 HP Tohatsu OB's is 78 PSI. Turn throttle grip to full wot, perform at least 4 full consecutive vigorous rope pulls, check achieved compression. Let tester to read for some minutes to see if achieved pressure is lost. Is OB difficult to start ? Cylinder wall must be perfectly round and inside tolerance specs or some compression will surely escape.

Both crankcase and cylinder head mating surfaces must be immaculately clean & even for a perfect gasket seal, at which NM setting was head torqued to ?

Happy Boating

 

[Wee Hooker]

My compression testr is an inexpensive harbor freight set but it has worked well in the past. I will make it a point to verify it on another of my 5b's when I get them out of the basement next month. Meanwhile, the motor continues to have both spark and fuel and the pull start seems a bit easy. Verification of the tester is a good idea but I don't expect it is the tool. Head torque was done to 220 in-lb. ( Paul, thanks for the manual dumps but I actually found my factory service manual so am good.) FWIW, The head gasket (not my first,) was put in correctly on a squeaky clean set of surfaces.

 

[pnwboat]

Sometimes excess carbon build up in the piston ring grooves will cause the ring(s) to "seize" in the groove. The ring(s) must have enough play to conform to the cylinder walls to seal properly. This is not uncommon with motors that have been overheated.

 

[Sea Rider]

When OB is cold, is it easy or hard to start ? How many NM would would 220 In.Lb be ? Check that all bolts on powerhead are adjusted to its factory specs.

Happy Boating

 

[pnwboat]

220 inch LB's = 24.86 NM

 

[BEALS]

HELLO EVERYONE NOT SURE IF I'M DOING THIS RIGHT?

WHAT i'M LOOKING FOR IS A HONDA 50HP LS NO TILLER 2007 OR 2008 WE HAVE A 2008 ON ONE OF OUR HULLS AND WE ARE TRYING TO PAIR THEM UP. BEALS_MIKE@YAHOO.COM SOMETIMES FOLKS ARE TRADING UP ON POWER SO IF YOU CAN HELP thanks..

 

[Sea Rider]

220 inch LB's = 24.86 NM

You are at the minimum torque adjustment. for M10 head bolts specs are 23-31 NM, adjust to at least half of mentioned numbers.

Happy Boating

 

[Wee Hooker]

My manual said 215 in-lbs . I can tighten it a bit more but given that the compression is exactly what it was before I started, I'm betting the issue is elsewhere. This motor pulls easily but won't even cough.

 

[Sea Rider]

Those torque numbers are from Tohatsu 1-2 cylinder 2 Strokes Service Manual. If OB pulls easily with minimum resitance has definitely low compresion.

Happy Boating

 

[Wee Hooker]

I'm going to try replacing the reed valves for lack of a better trouble shooting plan. Looking for a source now.

 

[Sea Rider]

If reed valves has not been fingered by a tree shaded mechanic it's dumping money through the drain.. To change reeds will need to tear powerhead down, take advantage to pull piston out, check if with stuck rings on grove along cylinder wall shape, must look evenly clean and polished and perfectly round. Has that OB ever overheated badly due to impeller failure or 2 strokes poor fuel/oil mixture lubrication ?

Happy Boating

 

[Wee Hooker]

If reed valves has not been fingered by a tree shaded mechanic it's dumping money through the drain.. To change reeds will need to tear powerhead down, take advantage to pull piston out, check if with stuck rings on grove along cylinder wall shape, must look evenly clean and polished and perfectly round. Has that OB ever overheated badly due to impeller failure or 2 strokes poor fuel/oil mixture lubrication ?

Happy Boating

I don't know the history on the failure for sure. Guy said he pulled it out of the shed in the spring and pulled the cord a couple of times on the stand to see if it would cough. He said it started right up to his surprise but died before he could get to the kill button. He said it never even coughed again. He knows better than to run it out of the water for any length of time or without proper fuel mix. That said, I wasn't there.
I know he didn't do anything inside the motor as he's not inclined enough to even attempt it. He had a local marina "tune it up" and when that failed, I got the motor back.
I don't "Need" this motor but admittedly want to figure this out for the sake of learning. I like working on outboards and have been pretty successfull so far. As mentioned above, I had the head off and the cylendar looked good from that angle. Maybe a microscopic crack? Is there any practical way to look for something that small? If I do get into the valves, I'll likely take your advice and pull the piston completely for a better inspection of everything.

Right now, I'm having trouble finding the valves and gasket set.

 

[ondarvr]

You don't need new reed valves, they have nothing to do with compression. Those gauges are known to be inacurate, but many brands are, and few read exactly the same, so try another gauge.

Try some starting fluid to see if you get any response, check timing and flywheel key.

 

[Wee Hooker]

You don't need new reed valves, they have nothing to do with compression. Those gauges are known to be inacurate, but many brands are, and few read exactly the same, so try another gauge.

Try some starting fluid to see if you get any response, check timing and flywheel key.

Thanks, I did try two gauges and they were pretty close around 35#

I guess I need schooling on reed valves. I was lead to believe they could cause this. My internet research hasn't turned up anything too definitive on the topic.

I havn't tried starting fluid but have tried wd-40 and 50:1 gas mix right into the plug hole without any response. Guess I could try the fluid, I've just always heard nightmares of what it can do.

 

[ondarvr]

Reed valves stop the fuel and air mix in the crankcase from flowing back into the carb when it's compressed and forced into the combustion chamber.

Starting fluid can be abused, but it can also be used in a responsible way. A small squirt to see if it fires is fine, continuos use to keep it running, or spraying too much at one time can be a problem.