long shaft vs short shaft

[rosbullterier]

Just wait till I get my fast boat on the water and join the Nouveau Poor.

Takes 3/4 hour to get to Plymouth x 3.8L (x2)

- can you imagine the headlines:
'Armed Petrol Station Robbery "give us 90gallons demanded - not cash" - stressed fugitive sought'

[atoyot]

I had a 1974 model 4hp Johnson that was a real workhorse - having an external fuel tank was great for range, too. That was on a sailboat with a total weight with crew around 470kg total if one includes the necessary beverages. This engine would push along very nicely up to about half throttle, perhaps 2/3, but much more than that would give only marginal benefit. I attribute most of this to prop pitch although it did get me up to hull speed on very flat water.

The same engine on an aluminum, flat-bottom Johnboat might do better; On a boat that has to displace a bit of water (even my planing hull at 4.7knts hull speed), the Johnson original prop tops out and even this smooth-running two-cylinder torque-beast will overwork itself. That is, this or any engine produces it's rated HP at a certain RPM and it's possible that your engine's not being allowed to run at the RPM necessary to produce the full 4hp and is being overworked.

So I guess I have to stop and ask at what throttle level your engine is run and for how long, before signs of head damage are evident.

I don't know if one can find a lesser pitch prop for these or not; I found that, If run just up to max torque and then throttled down by the minimum noticeable rpm drop, they'll run OK for a good while. It may not be producing 4 hp but the boat keeps moving, at least.

If you've overheated too many times (i.e., twice?), you may want to lap the head. This page describes the technique pretty well - http://www.maxrules.com/fixtuneitup.html had has other advice for similar engines in that series of two-cylinder power heads.

-ted

[markle]

Thanks for that, lots us useful info on that site

The head blew first time after about 3 minutes use, and after that after about 5 minutes use, think the first was an allready flawed gasket (it had been sat for 5 years!) and the next two times my cereal packet gaskets were not up to the job! Still not sure if the over heating blew the gasket, or if it over heated because the gasket was blown (hoping for the latter as they seemed to suddenly warm up), All times were after a bit of full throttle, which makes me think the gaskets were not up to it ....

Cant see any info on lapping the head on that site but will have a longer look soon, working 13 hours a day at the moment is helping me repair the motor!

I generally dont run them flat out for long, 3/4 throttle maybe, have to watch out for rogue waves and the like, not salty enough to charge right through yet

Thanks again

Mark

[atoyot]

Here's another home rebuild demo - http://www.shareaproject.com/pages/proj ... 82,00.html

The making of a gasket from an air filter carton, etc is a neat trick. The real Johnson/Evinrude gasket has stainless steel rings around the cylinder holes, far superior to a thin sheet of cardboard. Make sure your replacement has this, or it's not a good replacement.

Lapping can be done with a piece of window glass and some lapping compound from an auto parts store. Do a figure-8 with the head over the glass, applying light, even pressure. It doesn't hurt at all to use a micrometer or calipers to ensure that you're taking off an even amount on all sides. Take your time and take off only enough metal to make it flat and even. I've even used 600-grit wet&dry paper and light oil (Break Free). Worked quite well.

If you're into reading about Seagulls, follow this guy's writing on this forum http://bbs.trailersailor.com/forums/tsa ... ead,gasket and you'll read where he discribed having done just what you did to fix your engine.... except he did it at sea. Neat stuff.

[Charles uk]

If you take the head off you will see why it's not possible to lap it on any flat surface.

Seagull Centurty heads are not of the strongest construction & can bend, through use & over torqueing of head bolts.
To straighten place on spare cylinder without the gasket, give a firm clump with a hide faced hammer right in the middle.

It's the cylinder that ought to be flat, use a new or a clean annealed gasket & silicon RTV goo & your problem should be solved, head gaskets don't tend to fail in use, the damage is usually done on disassembly.

[atoyot]

Charles, you're correct regarding the head on a large-block model Seagull. The engine our friend's playing with is another type entirely and not a British Seagull. Being as he's willing to perform this type repair (head gasket, cooling service etc.) perhaps we can win his heart and recruit another Seagull collector.

RE: lapping a Seagull head, I've done the oiled emory paper trick on my 40 Plus l/s in the past year, runs quite well. For those readers new to Seagull maintenance, the ridge found on the larger, i.e. Century head is, on a 40-size engine, on the iron block instead. That makes lapping a 40-Plus head possible although I have to admit that there's really no real need to do that on British Seagull heads. I was in an experimental mood at the time.

Back to Johnson/Evinrude - the six head bolts only need to be wrist-upon-socketwrench-tight... and can probably measured in inch-pounds; tighten in a diametrically-opposing pattern of course.

[atoyot]

Would be fine if it were not for the speed boats charging past with their huge wakes ... nouveau riche I say (along with other words that start with f).

Gotta love it when the stinkpot owners drop down from a harmless plane, to maximum wake speed and think they're doing us a favor. Such is the mentality of thse from the nearby metropolis with much more money than brains & their floating castles, whom we have branded the "Philadelphia Navy".

[charlesp]

Over here in Poole we call them the 'Birmingham Navy'! I have to admit I first noticed the lack of wake on the plane when being carried at full chat aboard a Scarab with twin V8's down the Wareham Channel. It was dusk, we were the only people on the harbour, no breeze, no ripples and it was a magnificent blast.

The worst offenders, and those I can't possibly blame, are the Royal Marines who wreak havoc whilst performing prodigies of heroic marine evolutions in the main channel. You don't mind them, however, they are after all doing a job, and it's worth just hanging on and enjoying the spectacle of people whe really know what they're doing..

[atoyot]

The worst offenders, and those I can't possibly blame, are the Royal Marines who wreak havoc whilst performing prodigies of heroic marine evolutions in the main channel. You don't mind them, however, they are after all doing a job, and it's worth just hanging on and enjoying the spectacle of people whe really know what they're doing..

Aye. I'd consider it a privelige to get soaking wet, watching people doing stuff that I'd never be able to do even on a dare. Now, these "gin palaces" do have one redeeming feature: Sun-bathing surfaces, to wit -

Borrowed from www.sailinganarchy.com

[Hugz]

Definately not an aussie lass, she has clothes on........... yay, come on summer.

[markle]

ok so on a similar note for all of the 'rude; enthusiasts out there ...

anyone know what gearbox oil a 1984 would take and how to fill it?

Mark