1975 6 hp head gasket replacement -dry gasket?

[BF]

Hi all...

Weather was in mid -30's yesterday, so I figured it would be a good time to tinker in my basement... My 6 hp johnson idles great, but seems to not have the power it should at 1/2 throttle and above. While winterizing it in the fall, I noticed water droplets on the bottom plug (so it came home with me). Compression test showed the bottom about 95, while the top was ~115.

So, I pulled the head off yesterday... the pistons and bores look good. But the bottom piston looks a little too clean with only a bit of carbon (looked partially steam cleaned). It looked like there might have been some water intrustion into the cylinder right at the bottom (the gasket looks too clean there, with no scale deposit).

My Q's:

1) The old head gasket came off fairly easily (in one piece), so I take it that I should probably also put the new one in dry, with no gasket sealer. (?)

2) Is this type of failure common? I know it was idling on 2 cylinders, but suspect that at higher rpms (=more water pressure ?), it would leak a bit into the bottom jug... I found it funny that it would have a watery plug and still the compression didn't look too bad.

Thanks for your $.02

 

[JUSTINTIME]

Re: 1975 6 hp head gasket replacement -dry gasket?

best bet is get a new gasket and do a compression test
this will tell u if have a leak somewhere else

how did the plug look?

 

[BF]

Re: 1975 6 hp head gasket replacement -dry gasket?

plug looked OK, tan center insulator and a bit of carbon. It had a less intense tan colour and less carbon than the other plug, but it didn't look all steam cleaned.

FYI, this engine putts at idle (trolling) for 99% of the time I run it (a kicker beside a 25 hp). I think water only was entering at higher rpms (as I said, I know it was firing on both cylinders at idle pulling either wire made it change)... maybe this is why the compression test doesn't show much, and why the plug colour looks ok (?).

Yes, I'm picking up a new gasket on the way home. The bummer will be waiting 4 months to see how it runs!

 

[mikesea]

Re: 1975 6 hp head gasket replacement -dry gasket?

many may say no sealer,some gaskets are printed no sealer,one boss had me use oil 30 yrs ago,,my last 2 had me use sealer and thats what ive done on many heads for last 16 yrs,one explanation was to put a barrier between the copper sealing ring of the gasket and alum.head and block to help with electrolosis,

 

[OBJ]

Re: 1975 6 hp head gasket replacement -dry gasket?

Later OMC manuals recommend giving the head gasket a LIGHT coat of gasket sealing compound unless the gasket is marked "USE NO SEALER" as Mike posted.

I would recommend that after installation of the new head gasket, take a set of compression readings, decarb the engine, then take another set of compression readings.

If you saw no scoring in the walls of the cylinders and no real evidence of water intrusion, it could be just sticky rings.

At any rate, the decarb won't hurt anything and you'll have a good set of compression readings as future reference.

 

[BF]

Re: 1975 6 hp head gasket replacement -dry gasket?

we'll see what the replacement gasket looks like.. the original factory gasket had no metal in it. Just looked like "regular" black gasket material. In other 2 strokes (sleds and bikes) with the metal seals in the gasket, I've put them in dry.

Leaks are no good, but getting parts apart after they've been baked together with gasket sealant can be a PITA too.

But, are small water leaks only at higher water pressures something you folks have seen?

thanks for the input fella's

 

[F_R]

Re: 1975 6 hp head gasket replacement -dry gasket?

If it still leaks, pull the exhaust cover off the side of the powerhead. Bad gaskets or perforated baffle there will cause water to squirt into the cylinders through the exhaust ports.

 

[BF]

Re: 1975 6 hp head gasket replacement -dry gasket?

OBJ

cylinders and pistons look great.. not excessive carboning. It has received the seafoam decarb treatment for the last couple seasons. I think the fact that the bottom piston was too clean on one side, and there looked to be a corresponding spot on the head gasket that looked a bit too flat (uncompressed if you know what I mean), and that spot also didn't have the bit of mineral scale that outlined the rest of the water jacket lead me to think there was a tiny leak there.

If the compression doesn't come up with a new gasket, maybe I'll give it a deep creep soak before I run it.

thanks F_R... I'll remember that... and when I do run it at higher rpms, I'll definately be checking for water on the plug.

thanks guys

 

[BF]

Re: 1975 6 hp head gasket replacement -dry gasket?

well, new gasket is in... compression is dead even at 98 / 98.

The new gasket has the metal seals around the cylinders and looked to be a bit thicker, so I figure that's why the compression on the top dropped a bit. I'm glad they're even now... hopefully that was the issue.

All I need now is a lake that's not a slab of ice... all in good time!

Thanks guys

 

[OBJ]

Re: 1975 6 hp head gasket replacement -dry gasket?

That's pretty darn good compression for a 75'. Keep in touch and let us know how she runs and if there are still any problems.

 

[BF]

Re: 1975 6 hp head gasket replacement -dry gasket?

well, it's a long time for an update, but I thought I'd post one anyway. The head gasket was not the problem causing the lack of top end power. It still acts the same. Idles great, no power from mid throttle up (sounds like it's labouring, but otherwise runs smooth).

I will go back to basics and double check spark and probably pull the carb to visually clean main jet. (it behaves the same way using more than 1 tanks/hose combo).

I will post again when I have more info.

 

[iwombat]

Re: 1975 6 hp head gasket replacement -dry gasket?

You might also try putting in a new exhaust gasket. That could cause the water intrusion and lack of power you're describing. You really have to address the water intrusion problem. That could easily toast your lower mains.