Getting more power from outboard 4 strokes

[haydude]

Hi,

When I was a boy I used to play with 2 strokes motorcycle engines, the quickest and most inexpensive way to get more power out was to change the carburettor jet with a larger one and tune the air intake screw. Next upgrade was the whole carburettor (there is a limit to the jet size a carburettor can take), then the head, piston and cylinder.

I have a 4hp four stroke at the limit of the weight I can carry on my own, but I need more power to be able to plane with 3 people on a dinghy.

Could anyone suggest how to get a tad more power out of it? I looked on the net for carburettor jets but I could not find any supplier.

 

[lw395]

Plenty of places if you google mikuni jets or keihin depending on what carb you have.
Probably not much actual power to be had by just richening the mixture though. All the normal moped-boy stuff of improving the airflow through the airbox, k&n filter might help a bit, but unless your engine is really a 6HP restricted to 4HP, (check the throttle opens all the way!) a 10% gain would be good going.
Find out if the carb size is small for the cc of the engine, compared to say bikes or generators, maybe a bigger one will help,
maybe not, the exhaust may be restrictive, or the cam, valves etc etc.

 

[VicS]

You dont say what engine!

maybe someone will know how to uprate it.
Sometimes small outbords are down rated versions of more powerful units and then it is sometimes , but not always, very easy to uprate them.
Can be as simple as removing a limiter on the throttle or removing a restrictor plate.

Other times it is just not a practical proposition. I have a 6 hp (2 stroke) which is outwardly the same as the 8 hp but detailed investigation revealed not only a different carb but a different manifold and even a different crankcase !

 

[haydude]

It is a Suzuki DF4, same engine specs as DF5 and DF6 (138cc), so it must be downrated and upgradable.

 

[macd]

It is a Suzuki DF4, same engine specs as DF5 and DF6 (138cc), so it must be downrated and upgradable.

The DF5 and 6 seem to rev harder than the 4. Its possible that they may have different camshafts and/or carb settings, but these days the simplest way for Suzuki to vary the output woud be with different ignition curves. I'm guessing, but it may be that the principle difference between the three is in their digital ignitions.

 

[Pete54]

3 people in the right shape of dinghy are going to need 10 to 14hp to plane. That would be a very difficult thing to achieve with your present engine.

Certainly getting more power out of the engine is usually possible, particularly for the lowest power variant of a particular block but trippling it going to need forced induction and a lot of effort. The best approach may be to get a trolley to transport a larger engine.

 

[haydude]

The DF5 and 6 seem to rev harder than the 4. Its possible that they may have different camshafts and/or carb settings, but these days the simplest way for Suzuki to vary the output woud be with different ignition curves. I'm guessing, but it may be that the principle difference between the three is in their digital ignitions.

I am afraid it looks like they are all the same, only the power changes. No electronic injection, only "digital ignition", practically a capacitor is used together with a coil to increase voltage and amps to the spark plug.

I think that the only change is in the carburettor, but I would have to look at a DF5 or DF6 to verify that. Does anybody have one and could post a picture?

 

[FullCircle]

If the DF6 is the same engine, won't they weigh a similar amount? Then sell your DF4 and buy a DF6.

Other than that, it is a case of gas flowing the head on a flow bench to maximise through flow, maybe increasing the compression by skimming the head, probably requiring you to pocket the piston, free flow air filter and rejetting, not to mention carefully flowing the exhaust until you have optimum back pressure and the correct sonic back pulses. Hotter plug probably, together with reoptimising the timing and possibly the dwell angle, and thats without reprofiling the camshafts for highter lift and duration, and looking at the overlap timing to see if that can be optimised.
Of course, you could go for a blueprinted engine to totally reduce the internal friction levels, and ensure the gearbox is also optimised, and a perfectly polished new prop with different pitch to take account of your engine power increases and speed of rotation.
Should be interesting, and a couple of thousand quid at least.....

 

[Wunja]

Of course, you could go for a blueprinted engine to totally reduce the internal friction levels, and ensure the gearbox is also optimised, and a perfectly polished new prop with different pitch to take account of your engine power increases and speed of rotation.
Should be interesting, and a couple of thousand quid at least.....

And I thought a hot outboard was stolen!

 

[macd]

I am afraid it looks like they are all the same, only the power changes. No electronic injection, only "digital ignition", practically a capacitor is used together with a coil to increase voltage and amps to the spark plug.

I think that the only change is in the carburettor, but I would have to look at a DF5 or DF6 to verify that. Does anybody have one and could post a picture?

Interesting, but not correct. The DF5 runs 500rpm higher than the DF4, the DF6 250rpm higher still, which is where much of their extra power will lie. The ignition includes a control processor with a built-in rev limiter, presumably set up differently between the three models. I daresay that the ignition advance curve also varies between them.

 

[lw395]

Even if you can get 6hp from it, will that be enough?
Even a very light dinghy plus 3 people is asking a lot from 6hp to get up on the plane.
Unless the water is totally flat, or the waves are going the right way you may be flogging a seriously unwell horse!
I used to have a 4HP and a very light 8' dory type 'thing' which would really plane only on flat water one-up. It would do 12 knots then.
Still, maybe you can borrow a 6 and try it?
At the end of the day, more cc's is often best all round way to increase performance.
After making sure that gearing ie prop is about right of course.

However, it could be a lot of fun to play around. Maybe investigate old racing outboards from the smaller classes...

 

[JKay]

http://www.ybw.com/forums/showthread.php?p=2430117#post2430117

Billybloater needed more power fron his 9.9 Suzuki and stuck on the 15's carb for £200 in the post above
('e needed the extra power 'cos he's such a fat barsteward :shocked

 

[Seanick]

Ditch the heavy slow four stroke and get a nice light 2 stroke, a 9.9 would do it, and probably weigh the same...........

 

[mike489]

Ditch the 4 stroke

We had a suzuki 6hp and it just couldn't get the revs to plane a 3.2M dinghy with 2 people we chaned the prop still no luck - some friends had the same engine and couldn't plane 2 up with an Avon 2.8M rib we ditched the 4 stroke and bought a Yamaha 8hp 2-stroke - wonder of wonders it planes the 3.2m dinghy with 2 up.

 

[William_H]

Hotting up an O/B

As said the HP may be able to be increased by increasing the revs with appropriate timing and mixture adjustments and improving the airflow of both the inlet and exhaust along with appropriate valve timing. Just like a car engine. However....
Your quest is to get an inflatable to plane. The demands are nothing like a car. You need torque at low revs to get the RIB up to planing speed then power at the higher engine speed to keep it on the plane. In a car you manage the power speed curve by selection of the correct gear however on an o/b you can't change gear unless you fit a variable pitch prop.
I would suggest you fit a tacho to the o/b and compare the revs attained on the RIB with the power curves for the engine then consider a more appropriate prop pitch if there is a disparity. You may get more performance out of the o/b rib combination but as already suggested maybe not enough for what you want.
olewill (paraphrased "forget it")

 

[gianenrico]

...and of course You are aware that any "improuvement" in your engine's performance would void warranties as well as third party liabilitiy insurance (in Italy they are closely linked to engine rating); ah, also such an engine would not comply to regulatory issues......

 

[DownWest]

Inclined to agree with Olewill and the 'get more cubic inches 2T idea'

Around here they are having a blitz on modified mopeds. The max speed allowed (and insured for ) is 45kph. Some kid helped the flics set up the rolling road at his school. So he is first up for the test. Plein gas! (full throttle) and he gets 93kph.... The flics were quite amused, but had to be stern... 3 out of 4 broke the limit.
A

 

[Bosun Higgs]

It is a Suzuki DF4, same engine specs as DF5 and DF6 (138cc), so it must be downrated and upgradable.

In that case go along to your dealer on a quiet day and have a trawl through the parts list to see what parts are different between the three engines. That will show you how to upgrade. Dont forget that it you do manage to get more power you will likely need a different prop.

Alternatively if the engines are much the same then so will be the weight. Why not do a part ex for the bigger motor?

 

[VicS]

go along to your dealer on a quiet day and have a trawl through the parts list

Or view the parts catalog at http://store.brownspoint.com/
The site is not reponding at the moment but is usully Ok. Too many people looking for parts to upgrade their 5 hp to a 6 hp I expect

 

[alanejread]

Just opened up my DF4 to lube the throttle cable and noticed a "4" stamped on the cable clamp that doubles as a throttled restrictor. By bending it back to allow full throttle, it appears to have gained 300 to 400 more RPM and I can now get on the plane 2-up and I clock-in at over 100kg so 3-up a distinct possibility! I have the Suzuki 4 mounted on a generic 3.2m vinyl RIB.