A two stroke or four stroke outboard engine ?

Introduction – Two Stoke or Four Stroke Outboards?
Until the turn of the millennium, almost all marine outboard engines were two-strokes. However the marine industry has moved with the times. Like the major car manufacturers, the boating world has been forced to adapt to a world of increased regulation and environmental controls. Therefore since the turn of the millennium, four-stroke outboard engines have dominated the new sales of marine outboard engines in the UK.
This article from us here at Pennine Marine is a bit of “friendly advice”. The paragraphs below are aimed at simply explaining all the issues around two and four–stroke outboard engines to the average UK boat owner.
The reasons for the change away from two-stroke outboard engines
At the turn of the millennium, new rules for leisure boating were introduced across the EU. These rules were mainly aimed at improving the safety standards of all new leisure boats. However at the same time they dramatically changed and improved the environmental credentials of outboard engines.
This set of EU rules is called the Recreational Craft Directive, or the RCD to give it the correct TLA (Three Letter Acronym). These “new RCD” rules took a few years to work through the outboard engine manufacturers’ design systems. Soon after the RCD rules came into effect, the modern designs of four-stroke outboard engines started to appear in the mainstream boating world.

The thinking behind the RCD rules was mainly driven by three factors. Firstly trying to get significant improvements to fuel economy; secondly to reduce exhaust and C02 emissions and thirdly to cause far less noise pollution.
The RCD rules forced a major change in the technology used in the design, manufacturing and servicing of marine outboard engines. All the model ranges- from all the outboard engine manufacturers - have now been altered to suit these new RCD rules.
Modern Outboard Engine –The Advantages and Disadvantages
The imposition of these fuel, noise and pollution standards dramatically changed the way all outboard engines were designed, manufactured and serviced. Many modern outboard engines now sold in the UK are now four stroke designs.
The three big, premier league Japanese outboard engine manufacturers - Yamaha, Suzuki and Honda - all base their outboard engine technology on their well proven motorcycle and ATV engines.
However a few other outboard engine manufacturers decided to stay with their heritage engine designs, which they improved by adding on more sophisticated control systems. They therefore managed to develop more sophisticated two-strokes which complied with the latest RCD rules and regulations on emissions and fuel economy.
All the major outboard engine manufacturers – both for two-stroke and four-stroke units - have (to a greater or lesser extent) adopted more sophisticated electronic and computerised control systems. These changes were essential to help them comply with these RCD emissions and fuel consumption standards. Many larger outboard engines now have computerised fuel injection systems.
For details of who makes the various different types of outboards, please see our separate section “Guide to Outboard Brands” on this website.

Advantages and Disadvantages of Four Stroke Outboard Engines
The positives to the average boat owner from the changeover to using the latest four stroke outboard engines are definitely less noise, fewer fumes and better fuel consumption. Therefore modern outboard engines now have a lot more respect for the environment than they had even ten years ago.
The much improved fuel economy benefits boat owners by giving them significantly lower fuel bills. This means (excuse the pun) that there is more “brass in a Yorkshireman’s pocket at the end of a day out on the water”.
An unexpected benefit from the changeover to four-stroke outboard engines was a massive improvement in outboard engine reliability. The change in design standards meant that all outboard engine manufacturers had to use more sophisticated design and manufacturing techniques. This has meant there are far fewer unexpected breakdown and warranty issues now coming to light during the operation and use of modern four-stroke outboard engines.
Most warranty issues that arise with modern four-stroke outboards are now caused by unauthorised modifications, incorrect replacement parts being fitted during a service, or also (and by no means least) “home-servicing”.
There were however two negatives from the changeover. One is that many modern outboard engines are now too sophisticated for home servicing (except for doing the basics). Secondly is that four stroke outboards are heavier to lift than the older two stroke models. The heavier weight of four stroke outboard engines has affected the portability of some medium-sized engines.
Who can still buy two–stroke outboard engines?
The rules are now quite simple;
“A new two stroke outboard engine that does not comply with the RCD rules is not legally available for any leisure/recreational user to buy in the UK or EU.“
If you are a leisure user it is illegal for Pennine Marine to sell you a new two-stroke engine that does not comply with the RCD noise, fuel and emission’s regulations. The definition of a recreational user is quite wide. It includes all watersports clubs and even professional recreational watersports centres.
However leisure and recreational users (of all types) can still use continue to use any two-stroke engine that was first sold new (or originally imported) in the UK before the cut-off date. These older two-stroke outboard engines can quite legally be used, right up until they ultimately expire or die. Preowned two-stroke outboard engines originally made and first sold new before the RCD rules came into effect can also be legally bought and sold second-hand.
New two-stroke outboard engines (of the older designs) are still available, however only for bon-fide commercial, professional and government operators.  There is also a special exemption for RYA powerboat racers.  However new two-stroke outboard engines of the older types are only manufactured in Japan or Thailand to a customer’s own special order. Therefore they take months to be delivered.
Frankly for commercial users, the two-stroke outboard engines now offer few advantages over the four stroke outboards. We have found that most commercial boat users are rapidly changing over to four stroke outboards, especially because fuel prices keep on rising. As four stroke outboards are more fuel efficient than the two strokes, commercial users value the saving in fuel that come from using the modern engines.
There are now very few two-stroke engines sold in the UK (to any of the categories noted above).
Modern two-stroke designs that comply with the RCD rules, i.e. Johnson/Evinrude outboard engines and some models in the Mercury/Mariner and Tohastu engine ranges can still be sold to leisure users.
Older Preowned Two Stroke Outboard Engines
Many good pre-owned two-stroke outboards are still available second-hand. Many of these older outboard engines - particularly those made by the premier Japanese brands - continue to give very good service to their owners.
In very general terms, the older two stroke outboard engine ranges were fundamentally good engines, which ran for years. Whilst all the various different two stroke manufacturers and models all had their idiosyncrasies, many outboards engines ran for years with no - or a few known – vices and quirks.
We have looked after a large number of these two stroke engines over the years. Whilst they are getting fewer in number, the older ones will keep going provided they are given a bit of tender loving care. Whether your two–stroke outboard engine will keep going all depends on the care and maintenance, in particular whether and how professionally it was regularly serviced.
If there are major problems with these older two-stroke engines it tends to come down to one of three things poor (or no servicing) , not flushing through the cooling systems and especially a lack of oil lubrication.
If there are any serious problems with this type of two stroke engine it will be probably/almost certainly be caused by poor lubrication and lack of oil circulating around key components.
Two Stokes verses Four Strokes -the positives for changing over
The modern four stroke outboard engines are a lot more sophisticated designs than the two strokes engines ever were.
Assuming you want to buy one of the good Japanese or North American outboard engine brands, with a modern four stroke engines there are five key differences when compared with the older two-stroke engines. These five differences will be;

  • Four-strokes are very trouble free and far more reliable. We only see four stroke engine leisure owners once (or maybe twice) a year for their routine servicing.
  • Four strokes rarely break down between servicing.
  • Power delivery and throttle control is far smoother. This makes boat control easier.
  •  Fuel consumption will be a lot better on the newer engine, probably 30-40% better on an average day out.
  • Noise will be noticeable less on the new engine, which is particularly noticeable for passengers sitting in the rear seats of your boat.

Therefore there are a lot of positives to changing from a two stroke to a four stroke, mainly for noise, fuel consumption, noise and reliability.
Two Stroke verses Four Stroke Outboards - the Reliability and Servicing Issues
As noted above, the key difference between two strokes and four strokes is that the modern four stroke engines are now all far, far more reliable. When the new generation of engines were introduced and had bedded in, reliability massively improved.
We know from being a Yamaha and Suzuki premier dealership that the number of warranty issues and claims from both of these leading manufactures dropped down to a fraction of the old numbers. In terms of engines sold, our warranty claims dropped from one claim for every approximately 5% of engines sold right down to 1% now - i.e. outboard engine reliability improved by 400% .
Quite frankly it is a rarity to see a new Yamaha or Suzuki back in here at the Pennine Marine workshop for a warranty claim - like “nearly never ever”.
However the down side of introducing the new four stroke technology is that modern four stroke engines need to be maintained by a professional service dealership, with the right tools and computerised diagnostics. The electronics on board these newer outboard engines are just too sophisticated for home servicing. Many modern outboard engines now require plugging into computerised diagnostics during their servicing, simply to ensure they are running properly.
Two strokes verse Four Strokes - the Weight and Portability Issue
The weight and therefore portability issue is often cited as being the one main reason for not changing from a two-stroke to a modern four-strokes outboard engine.
For modern boat, with a permanently fitted engine. If yours is a newer boat, which was designed and built with the newer four stroke engines in mind, this question of a two or for stroke engine really is a “no-brainer”. If your boat was designed to carry a four stroke engine, there is normally an advantage to fit a four stroke engine.
However this weight issue of four stroke engines is normally more of problem with boat owners who have handheld/portable engines and carry them around with them (rather than those engines mounted permanently on the back of a boat). Carrying the extra engine weight by hand is what makes people really moan about four strokes. Having said this, the weights of the more modern and thus more portable four strokes is now far better than when they were first introduced.
Both Suzuki and Yamaha have gone a long way to address this weight and portability issue on their latest engines. A careful check of the weights will show that most four-stroke outboards are now not substantially heavier than the older two stroke engines. If you would like specific and detailed advice about the weight of a new engine you are thinking of buying, then please call us here at Pennine Marine.
Performance Issues
What we here at Pennine Marine in Yorkshire find from our experience is that what you slightly lose in performance on the extra weight of the new engine will be completely offset by the fact that your new engine would be running at full power as soon as it is purchased new.
Your older two-stroke engine will have degraded in performance when you have been using it over the last decade. This degradation will have more than compensated for the very slight increase in extra “new” engine weight now being carried on your boat.
Fitting a modern four stroke engine to an older rib or powerboat
If you are thinking about changing your outboard engine(s) and fitting them onto an older boat (one built before 2000), there are two key pieces of information we would need to know.

  • What the engine weight limit is on the transom
  • The design limit of the overall weight on the boats stern.

If the transom on your older rib or powerboat is not strong enough, even this slight ten percentage increase of an extra 20kg or 40kg (for a big new four stroke engine) can be a serious issue. It will literally weight you down on the stern.
If you are thinking about changing your engine on an older boat, please carefully check these two limits with the original boat builder-for your particular boat model - before you buy a new outboard engine from us.
If one or both of these two key weight limit(s) are exceeded, it is almost impossible for us to strengthen the boats stern to take a heavier engine. We really must stress that if you are thinking of changing the outboard engine on an older boat, then knowing the boat strength/ allowable weight information is really critical information for us.
 Conclusion - Two Stroke or Four Stroke Outboard Engine
To conclude, we would say that the hard part is persuading somebody to change from an older two-stroke outboard to modern four stroke outboard. We are going to have to say that almost everybody, once they have changed over from two to four strokes will not go backwards. Our problem is persuading them to change!