2 Stroke V 4 Stroke

[Seven Spades]

I have a 2.5 hp 2 stroke outboard. It weighs 10kgs, but I would like a bit more power, we are usually 4 up and it feels as thought it is on its limit. I know that I can still pick up new 2 stroke and that they are lighter. However, what is the difference in pulling power between a 2 stroke and a 4 stroke the HP could be the same but they will have different torq.

I presume that with modern ignition systems that there is little difference in starting ability between the two, the 4 strokes are heaver, but which is better in performance?

 

[Gin]

There was a comparitive article in recent months in PBO and the 2 strokes came out with greater punch and of them the Tohatsu 3.5 the best of all.

I've been using a 2hp Honda BF2 4 stroke engine for a couple of years which has pushed my heavy tender or the Avon with me and two teenagers and kit quite well and I wouldn't have changed it out of choice but a careless error on my part damaged the gearbox and I felt it wasn't worth a repair(plus I needed an engine quickly).

So, I gave up the air cooled Honda(which I much liked), not having to flush it out, and the u/leaded petrol advantage
(don't care for messing with 2 stroke mixtures) for the said Tohatsu.

The T 3.5 isn't yet run in but it doesn't sound much quieter to my ear- it is easier to start, even though the Honda was good (it needed a strong vigorous pull or two). The new engine is clearly going to be more powerful; 2hp/3.5hp and even on 1/3 revs it matches now the flat out speed of the Honda.

I used to get up to an hour on the 1.something litre,Honda integral tank and the Tohatsu handbook states about 30-40 minutes for its 1.4 litre tank ,so it looks as though that will be a little more thirsty.

The weight of each engine was much the same

Hope theses observations help in your deliberations

I should have also added that one must be careful if laying down the 4 stroke in the horizontal plane for storage/transporting one must adhere to the manufacturers instructions otherwise crankcase oil can syphon back into the powerhead- the 2 strokes don't have that problem although my Tohatsu handbook says to be sure not to lay the engine horizontal without ensuring the power head is higher than the drive leg to avoid residual water in the leg draining back to the engine

 

[johnphilip]

Why not a 3 Stroke?

(borrowed from a book I read recently where in the last war German masters of forced labour Ukrainian boffins thought that the jump to providing a 2 stroke generator from a 4 stroke engine should be done more carefully by going via a 3 stroke model )

 

[VicS]

In a two stroke the piston goes down, 1 stroke, back up again, 2nd stroke, and fires again. Two strokes: one up and one down per complete cycle.

In a four stroke the piston goes down, the power stroke, then up again, the exhaust stroke, then down again, the induction stroke and finally up again, the compression stroke before firing again. Four strokes: down, up, down and up again per complete cycle.

How would a 3 stroke work? It fires the piston goes down, up and down again then what? Fires at the bottom of its travel?

 

[Kawasaki]

Pulling, or Shoving power in the case of an outboard!
3.5 Tohatsu would do you if you want a bit more poke without too much weight.
2 stroke that is.
I don't think 4 strokes come into their own till you get much bigger.
As Vic says 3 strokes? Canna work.
Rotary 3 cyl maybe A La Wankel/ Mazda etc!
The internal combustion as we know it, doesn't sustain "3 strokes" a bang every other works well , also bang every fourth.
Can't get My head round a bang every 1 and a half strokes.
3 cylinders work well.
I had a 3 cyl Suzuki two stroke bought new in 87 that was a good 65 hp outboard.
Suzuki also made a good 3 cyl Motorbike in the 70's a 380 cc and a 550 cc.
I still have a Suzuki Swift Car with a three cylinder lump in it.
Goes like a Digbat and does 55 to the gallon! (1 litre)!
So 3 cyls are ok but 3 strokes?
Dunno lost on that one.
Masybe a horizontally opposed 2 stroke?
Hmmm , twat, you,ve got me thinking now!
Too old for this stuff.
Think the Times Crossword would be easier to work out!

 

[Gin]

Rather a disappointing addendum.

Today, still in its run in period at 50-1 ratio, the T 3.5 at 1/3 revs managed to get through a whole tankful of 2 stroke mix in 17 minutes.-

Result? a bloody horrible row of 1/2 a mile back to base after a tiring two races. Seriously not impressed and no evidence of a remote tank connector to avoid a recurrence.

At the moment I really wish I hadn't been so frigging stupid as to wreck the Honda's gearbox

 

[alan_d]

I'm sure my Tohatsu 3.5 specified a running-in mixture of 25:1. (Doesn't help your problem, though.)

Alan

 

[Gin]

Definitely 50-1 in the latest user manual, but tell me how long do you reckon you get at say 60% revs on a full tank- I am hopeful that after the run-in period my engine may become rather less thirsty

 

[Pasarell]

My Tohatsu 3.5 was new in June last year. Handbook says 50:1 while running in then 25:1 afterwards - or do I mean the other way round? Haven't got the hand book here.
Great little engine. Pushs the tender with 3 up and gear at 4 knots with half throttle. Not sure how long the tank lasts but once did about 30 minutes flat out in a rigid tender just for fun and only used about half a tank even though the book says 40 minutes running time

 

[Gin]

Yes you're right 50-1 for the first 5 hours.

Anyway now a confession- but first thanks for the tank time comment, which I am sure, now, that I too should achieve- see below.

Yesterday when the engine died and I looked down th filler I could see no reflection from fuel moving around and without a 'dipper' I thought the tank was dry.

This morning in good light I can see lots of fuel and the engine started up first time (it kept dying yesterday when trying to start several times)


I can only conclude, that perhaps it flooded (no smell and no choke used as we had done a couple of minutes motoring already) or perhaps a bit of dirt in the carb.

All looks and sounds well now, so big relief

WHOOPS! got the actual mixture right for running in the engine but posted here incorrectly

25:1 for running-in; 50:1 thereafter