8hp starts but runs only at high rpms

[MattFL]

I've been going under the assumption this was a 2-stroke, is it a 4-stroke?

First, did you have the throttle wide open when you did the compression test? If no, then repeat with the throttle full open.

If it's a 2-stroke; you're at roughly 1/2 what the compression should be. I would suspect either piston/ring damage or a bad head gasket or some other problem, though it's interesting they're both so close in value.

If it's a 4-stroke, the automatic compression release that makes it easier to start might be interfering with your compression test. Check the manual for the procedure on getting an accurate compression test. Or if it's just as easy, check your valve clearances. On a 4-stroke, if the valve clearances tighten up to the point that the valves don't close tightly then you can get the behavior you're describing.

 

[Plandeck]

I've been going under the assumption this was a 2-stroke, is it a 4-stroke?

First, did you have the throttle wide open when you did the compression test? If no, then repeat with the throttle full open.

If it's a 2-stroke; you're at roughly 1/2 what the compression should be. I would suspect either piston/ring damage or a bad head gasket or some other problem, though it's interesting they're both so close in value.

If it's a 4-stroke, the automatic compression release that makes it easier to start might be interfering with your compression test. Check the manual for the procedure on getting an accurate compression test. Or if it's just as easy, check your valve clearances. On a 4-stroke, if the valve clearances tighten up to the point that the valves don't close tightly then you can get the behavior you're describing.

Thank you for your reply. It is a 4-stroke. I've done the compression test twice once with the engine cold (70psi) and once warm (60psi). I admit I did forget to open the throttle. Checking valve clearance was my next move. It may be a bit difficult because motor is still mounted on the boat on its mooring. The Seloc manual says 128psi is normal. I've been guessing its warn rings, except the spark plugs come out without any oil deposits on them, only carbon. The motor I bought 2nd hand has always performed a bit less power than its supposed to and in comparison with similar boat having same engine, so low compression may be a real issue that has finally gotten to the point where it quits at low rpm (below 4,000). If the choke is closed i.e. rich. she will run at 3,000rpms but still dies out eventually.

 

[MattFL]

Was the motor running OK one day then not OK the next, or did the performance degrade over some time?

The compression being identical in both cylinders makes me think that a leaking head gasket or piston damage is unlikely. Although if the motor has a compression release then the compression test numbers won't be accurate unless you follow the steps in the manual to get an accurate reading. If it were my motor I would check the valve clearances next. If they tightened up then that could prevent the valves from closing all the way, which would both give you low compression numbers and affect performance. If the valve clearances are OK then I would follow the steps to get an accurate compression reading.

 

[Plandeck]

Was the motor running OK one day then not OK the next, or did the performance degrade over some time?

The compression being identical in both cylinders makes me think that a leaking head gasket or piston damage is unlikely. Although if the motor has a compression release then the compression test numbers won't be accurate unless you follow the steps in the manual to get an accurate reading. If it were my motor I would check the valve clearances next. If they tightened up then that could prevent the valves from closing all the way, which would both give you low compression numbers and affect performance. If the valve clearances are OK then I would follow the steps to get an accurate compression reading.

well, I ran the compression test with throttle wide open...maybe an improvement of 5psi to 75psi on each cylinder. So I pulled the power unit and its sitting on my work bench. I will order a set of rings and a head gasket working on the presumption the rings are warn. I did test the rocker arm clearances before pulling the power head. Seemed to me there was plenty of tap in the rocker arms. The exhaust valves had clearance of .008 and the inlet valves about .006 as close as I could tell. I'll look at them again on the work bench. Any warnings about removing the cylinder head and pistons? Thanks.

 

[Sea Rider]

Any warnings about removing the cylinder head and pistons ?

A suggestion, take advantage to remove all salt and carbon build ups on the entire crankcase specially if the motor was running on salt water. Will need to buy new head, head cover, exhaust and base gaskets as well...

Happy Boating​

 

[km1125]

Compression measured in at identical 70psi for both cylinders. Seloc manual says they should be 128psi. Oil added to each cylinder before testing increased psi in one cylinder by 5-10psi and saw no change in the other cylinder. However, introducing oil into spark plug hole was difficult since motor is still in the boat. New spark plugs have carbon coating but no oil which I think means rings are ok(?)...so what is it? Where to start?

So I guess this is a 4 stroke after all.

You have a carburetor issue.

 

[Plandeck]

A suggestion, take advantage to remove all salt and carbon build ups on the entire crankcase specially if the motor was running on salt water. Will need to buy new head, head cover, exhaust and base gaskets as well...

Happy Boating​

luckily 100% fresh water - lake Michigan.

 

[MattFL]

New rings and head gaskets won't hurt I suppose, but I think you're jumping the gun a bit and it might not be necessary. Your compression numbers look low, but if the motor has an automatic compression release (most motors do) then that might be why the numbers are low. The manual will likely have a procedure for disabling the automatic compression release and getting a valid compression reading.

 

[pvanv]

Peter,

My money is still on needing a Proper carb cleaning. I doubt you need rings.

 

[km1125]

Think about this.. for about 50 years motors were built with low compression (due to fuel limitations) and ran just fine below 4000 RPM. You might have (slightly) worn rings, but they are not your problem.

On their smaller engines of the same vintage, the service limit for compression is 113 PSI, measured with the decompression mechanism disabled. With it enabled, the spec is 42 PSI. I would bet your engine specs are similar.

 

[Plandeck]

Peter,

My money is still on needing a Proper carb cleaning. I doubt you need rings.

heres what the valves look like...are those inlet valves abnormally darkened?

 

[MattFL]

No that looks normal. Exhaust valves get hotter so they burn off more than the intake valves. If you want a quick and dirty test of the valve seats; set the head up on its edge, make sure the top valves are closed and fill the port behind the valve with carb cleaner or alcohol. Look at the combustion side to see if anything leaked by. If it's dry then the valves and seats are likely in good shape. If anything got by then the valve and/or seat need some attention.

You can clean the carbon off of the head with soft scrub and a scotch bright pad or something similar, and hot water. Ditto for the piston domes if you're going to re-use them. There's a good bit of carbon, but without knowing the history of the motor it's impossible to say if that's normal or expected. It can be from burning a bit of oil, or from lots of old/bad fuel, lots of slow running,, just a ton of hours, etc..

 

[Plandeck]

No that looks normal. Exhaust valves get hotter so they burn off more than the intake valves. If you want a quick and dirty test of the valve seats; set the head up on its edge, make sure the top valves are closed and fill the port behind the valve with carb cleaner or alcohol. Look at the combustion side to see if anything leaked by. If it's dry then the valves and seats are likely in good shape. If anything got by then the valve and/or seat need some attention.

You can clean the carbon off of the head with soft scrub and a scotch bright pad or something similar, and hot water. Ditto for the piston domes if you're going to re-use them. There's a good bit of carbon, but without knowing the history of the motor it's impossible to say if that's normal or expected. It can be from burning a bit of oil, or from lots of old/bad fuel, lots of slow running,, just a ton of hours, etc..

Test on the valves was blowimg into the ports...all air tight. Those little springs look look a ***** to compress to relese the valve stem...would like to do a "back yard" valve job with a little valve polish and an electric drill but worried about getting it all back together. Springs no longer available!

 

[MattFL]

Check if the valves or seats have coatings that shouldn't be ground. Maybe that's only on performance engines, but it's worth checking. Then if you want to lap them, instead of a drill maybe use something gentler like the suction cup on a wooden stick that you can suck to the valve then lap by hand. Here's a video, I didn't watch it except to see that he's using the tool I'm referring to. That said, if the valves are sealing perfectly, there's no real need to lap them.

 

[Plandeck]

What about honing the piston bores? Look pretty clean to me. No scores only some discoloration at the top and the glazing the manual talks about. Afraid of scratching them if i try honing.

 

[pvanv]

Clearly your valves are seating. No need to lap them. I would reassemble, do a Proper carb cleaning, and then decarbon spray after it's running better.

 

[Sea Rider]

Does the Service Manual states what should be the standard compression and the decompressed values on the combustion chamber if both values are mentioned ? On a 2 stroke motor 70 PSI is way low, don't know about on 4 stroke ones .....

Happy Boating​

 

[Plandeck]

Clearly your valves are seating. No need to lap them. I would reassemble, do a Proper carb cleaning, and then decarbon spray after it's running better.

no carbon deposits on pistons only minimal on tops

Clearly your valves are seating. No need to lap them. I would reassemble, do a Proper carb cleaning, and then decarbon spray after it's running better.

No carbon in ring grooves only minimal on tops

 

[MattFL]

Is this as much a fun project as it is practical by chance?

That carbon on the piston should scrub right off with hot water, soft scrub or similar cleaner and a scotch bright or similar scrubby pad. The hot water will soften the carbon over time and make it easier to remove. Might as well clean up the pistons and head while it's apart.

If you can still see clear crosshatch in the bores then honing is likely not necessary. If you want to pursue honing, first check if the bores are plated. Nikasil plating is popular on small motors and you should not hone it, it can chip off. I'm honestly not sure if it's common on small outboards, but they use it a lot on dirtbikes and that's what I'm more familiar with. If it's not plated and you want to hone them, a bottle brush hone is probably the most forgiving, especially for a used bore that might not be the same size on the top and bottom anymore. Check the manual for what size grit. The purpose of honing is to put tiny scratches in the bore. The scratches hold oil and provide a more slippery surface and better seal than polished metal. I'm not sure if it's worth measuring the cylinder to make sure it's still round and the same size top and bottom, as it is probably not worth the expense to bore it and replace the pistons even if it is out of spec. I'm still not convinced that worn piston/rings/cylinders is the root cause of the problem, but at this point you might as well take the time to clean up what you can and put it back together in better shape than you found it.

 

[Sea Rider]

Have you removed by chance the installed piston's rings ? How about installing a new set of rings on both pistons ? Light cylinder scratches or burns can be removed with water-resistant sand paper of around #400 and #600 to finish.

Happy Boating​