Seahorse 4hp - Replacement Head Gasket Leaking?

One of those self-doubt gremlins has hit me. I've been reviving a 1978 Seahorse 4 hp twin that was mainly run at low speed and has been sitting for 10 years. So far:

-Run Powertune through to decarb
-Cleaned and rebuilt carb
-Replaced impeller
-Replaced plugs
-Pulled the head and exhaust cover gaskets, cleaned passages and replaced

The engine starts and runs fine with good spark, cooling, etc, with only a little acceleration delay noticeable.

I'm beginning to second guess my head gasket job. When I run it in the tank until fuel starvation, there is still about a teaspoon of oil/gas (and perhaps water) that runs out of the the upper cylinder when I pull the plugs, and some oily sludge exits from the exhaust/cooling ports over the next few days when it sits.

I'm assuming the sludge from the ports is residual carbon blowing out the exhaust tube, but should there be anything left in the cylinders? The plugs are too new to tell if they're being washed clean by water. Should I continue to run it and watch the plugs, or is there a better way to tell if I'm actually getting water in the cylinders? Thanks.

Re: Seahorse 4hp - Replacement Head Gasket Leaking?

If your head gasket is leaking there will likely be evidence of water on the plugs. Most likely a whitish emulsion of water and oil. It's pretty easy to tell if the head gasket on that one is leaking because it's never slight. Put your finger over each plug hole, turn the flywheel into compression and feel. If there's a major varience between the two suspect the gasket.
Always retorque after your first warm up/cool down cycle. It will need more.

I always get the unburnt oil drip after running out of gas on mine. The last few seconds the engine is running it misfires a lot so a lot of unburnt fuel and oil is left around. The top plug business could be normal if fuel tends to leak from the carb down the top reed when you carry it, or just some minor variance on the top cylinder at idle. Check and compare how far you can get the spark to jump.


If you have a hesitation at take off, it's probably becuase you have the low speed mix adjusted a little lean. Try richening it and see if that helps. There's a temptation to set the low speed too lean since it idles better like that, but it strongly affects acceleration and starting.

Re: Seahorse 4hp - Replacement Head Gasket Leaking?

Thanks, Paul

I'd retorqued the head already, compression seems OK (I assume it would run pretty rough if there was much of an imbalance between cylinders) and there's no emulsion on the plugs that I can see. I'll run it a bit and see if the plugs tan out properly. It's a pretty simple motor but I need it to be fairly reliable.

Re: Seahorse 4hp - Replacement Head Gasket Leaking?

You're welcome. My one ('72) has been the picture of reliability. The only trouble I had was a blown head gasket, but that was definitely my own fault. It was a hand made gasket I was experimenting with, which was much thinner than the aftermarket head gasket (only a little thinner than the stock one).

I've done essentially what you have - carb, impeller, pulled covers to clean water passages. But I also had to replace the ignition coils since mine were cracked from age.

The 4hp design goes way back to '52, and really didn't get a major revamp until the late 80s. The entire family are well known for their reliability and smooth running. The only thing lacking is that 50:1 is pretty thin for them. Have a look at the Oil Mix FAQ in the FAQ forum for details. I run mine at 24:1 which is probably excessive, but it seems to like it fine. TC-W3 and fresh or stabilized 87 octane gas.

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Re: Seahorse 4hp - Replacement Head Gasket Leaking?

I have a '70 as well that I needed to do the coils on, too. This '78 has one of the early CDI ignitions with coils mounted on the cylinder head. The CDI seemed to revive fine on an early test run but presumably one of the capacitors gave out after 28 years and I had to replace the unit in addition to the mechanical items already discussed.