4 stroke/2 stroke outboards. Difference?

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swanny

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Too lazy to open a book I guess but whats the difference between 4 stroke and 2 stroke engines. Just bought a 12ft Porta-bote and want a 5hp engine now. Got a 3.3hp Mercury and goes very well on it but need a bit more umph with two up! Maximum weight must be no more than 25kilo for engine. Is it true I don't need to mix oil with petrol with a 4 stroke? Are they quieter or Wot? The Porta-bote is great by the way! Caught eight Cod on from it in Flensburger Fjiord on Monday!! Kewl!
 
Four strokes have valves, oil in a sump, basically like your car and do not need oil and petrol mix, they are quieter, in the larger sizes, however, they are also heavy for their power. Weight is obviously a factor with your boat, so stay with the two stroke, it doesnt take long to mix the two stroke mixture and with 3.3hp, it doesnt use a lot, 5 hp wont use a lot either, so you dont have to mix it often. IMHO
 
2 strokes are reckoned to be much less environmentally friendly than 4 strokes - hence they've pretty much been banned. As has been said you might struggle to find new 2 strokes these days. 4 strokes are (as has been said) less powerful weight-for-weight and less compact but you don't have to mess about mixing fuel (although you do have to change oil!) They are also supposed to be quieter than 2 strokes and better for low speed operation (as in low engine speed). Finally, I think people generally reckon them to be more economical but I'm not sure about that bit!
 
You can still buy new two strokes - all of this year, and any unsold 2006 stock into next year. There seem to be plenty about at the moment - shouldn't wait though, as they may start to attract a premium if too many people want to buy them up in the summer
 
Also, IIRC 2-strokes can be happily laid down on their side for storage/ transportation, which isn't neccesarily the case for 4-strokes.
 
My Yamaha 4hp 4 stroke seems to have more grunt than the Honda 5hp 4 stroke I had before. And it is light for it's type. The economy is undeniable. As well as using less fuel per se, the cost of the fuel is as much as 2/3rds that of 2 stroke after you've paid for the oil to mix in.
You can lay 4 strokes down, but usually only one one side.
 
Oil - in a sump - which is basically a bowl in the bottom of the engine .... lay it down the wrong way and it starts to leak into all the undesirable places ...
 
Noise? Small air cooled 4st are definitely noisier than a small water cooled 2st. At tickover 4st water cooled are a bit quieter. At full chat there is not a lot of difference IMO, but the type of noise is different.

Weight? 2st lighter hp for hp every time.

Servicing? 2st - clean plugs once a year, de-coke once every 10 years (though I have never, ever done this, my current engine is an '89), change oil every time you fill up with fuel (2 stroke mix). 4st clean plugs once a year, de-coke once every 10 years, oil change once every year, valves to adjust, v.v. complicated.

Costs? 2st cheaper to buy, cost of 2st oil marginal, I don't use a bottle in a season. 4 st still needs a sump full oil each year, and a filter I guess, so the cost of keeping them lubricated can't be much different.
 
I also would go for a 2 stroke. Lighter is the biggest factor. Don't think they are as expensive to buy. If buying a new engine I would buy quickly before the the government chooses for you. If the government are banning 2strokes they have got to be best as the gov. never gets anything right. Probably find ministers husbands/wives/boyfriends/girlfriend/mistresses...etc.etc.etc have all got the import rights to 4 stroke engines. IMHO.
 
A few more points:

* Spill a bit of 2 stroke mix and it's much harder to clear up. With straight petrol any residue evaporates quickly.

* 4 strokes seem to take more welly to start - I think the compression is greater. Many people find our 15hp 4st Yam hard to start, whereas a friend's 25hp Mercury 2 stroke takes about half the effort. My wife simply can't start ours and I would not replace it with a 4st unless I went electric start for this reason alone.

3. If you are going blue water 2 strokes rule. It's harder to get parts for 4st in the Caribbean, and mechanics are less experienced at working on them.
 
excellent points and, taken with Andy's above, provide most of the behind the scenes issues.

If you plan to let small kids loose in it a 2 stroke is a significant advantage as they can start the damm thing!
 
Starting.

4 st tend to have lower gears on the recoil starter. A good pull might provoke 1, if your lucky 2, power strokes.

With a 2 stroke a good pull will provide several revolutions, with a (potential) power stroke for each rotation.
 
Re: Starting.

WOW! Well that's sorted me out thanks very much! All I knew was that the 4st didn't need an oil mix. It kinda put me off 2st and was seriously considering a 4st. I had heard that 4st's were quieter but if it's marginal then that's not important. Being able to lie it down anywhere is a major plus as sailing people will appreciate - I don't like to leave mine on the pushpit when I'm not around. 'Leaking oil', 'more servicing', 'more weight', negligable noise benefits' - why is the government banning them??????? Off to buy one quick! thanks.
 
Bought a Yamaha 4HP 4 stroke last year. Had real problems starting it, especially after it had been running. Previous experience with a Suzuki 4HP 2stroke and Yamaha 2hp 2 stroke made us think there was a problem with the new one. Tried the usual-swearing, threats, rest, sneaking up on it. Nothing worked. Took it back to dealers-nothing wrong with the engine-we were wimps-more welly/pull/better technique needed. It starts now. Because we believe it will? (fingers crossed and regular visits to the gym). Goes brilliantly when it works.
 
I got rid of my 6hp 4 stroke, oil on decks, heavy, and when you carry it you end up walking like a pregnant duck /forums/images/graemlins/grin.gif
 
I have same motor. Cracking engine, but as you say, make sure no crew stood behind you when you are pulling the cord.


If you have oil leaks froma 4 stroke when stowing it, you can't have read the manual correctly.
 
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