Suzuki 2 stroke 4hp not delivering more power over 1/2 throttle

[Franziska]

Hi,

we have a small 4hp 2stroke Suzuki outboard for our dinghy.
The outboard usually starts very well and runs reliably.

It's about 19 years old and has only seen very light use in freshwater before we bought it.
Still looks almost sparkling new.

Still, we have one issue with it. If you turn the throttle up more than 1/3 you only get more noise but very very little extra power.
So we usually run with 1/3 throttle.

Any thoughts how we can get the rest of the power out of our little friend?

[a64pilot]

Your probably tired of hearing this but it’s very likely the carburetor, specifically the main jet.
It could also be grossly overpropped too, so check that as well

On edit it could also be the timing advance, on many older engines the timing advance worked by moving the magneto coil with the throttle. It could be stuck. Take the cover off and with the engine off, see what moves with the throttle and see if something under the flywheel moves or is connected to a mechanism that should move it with the throttle.

[Lake-Effect]

Originally Posted by Franziska

If you turn the throttle up more than 1/3 you only get more noise but very very little extra power.
So we usually run with 1/3 throttle.

When you say "more noise" when you go past 1/3, do you mean it revs up faster, or that the revs stay the same but it sounds "wrong"?

If it's the latter, then it's probably something like carb as a64pilot suggests. We have a 36 yr old Suzuki 5 hp, well-used, that continues to run reliably. I swear by new, ethanol free gas, good 2-stroke oil, and the occasional shot of SeaFoam fuel treatment.

If it is still new-looking, perhaps it sat unused for a while... which also suggests that it would benefit from some carb cleaning.

[Compass790]

What other posters said but would just add always use a filter funnel when filling the fuel & fit an inline fuel filter if possible. They are very cheap in auto stores or online.
Most small outboard problems are carby dirty or kill switch

[Franziska]

Dear all, thanks for the suggestions. Will definitely try them when back on the boat.

One more question for A64, as it's a 1cylinder 2 stroke engine, how many injectors are in the carburetor?
I'm afraid we might have missed something when we had it apart last time.

When I throttle up over 1/3 it has a chocked louder noise to it, revs seem to go up by not much.

[Lake-Effect]

Has the outboard always been like that since you got it? If yes.... hmmm. There's the possibility that something happened to it before you bought it.

Definitely try inspection and cleaning of the carb, new fuel, etc. A compression test might be advisable. If that brings no clues or improvements, it's beyond my meagre skills and it might require the efforts of a good mechanic.

[a64pilot]

Normally any small engine regardless of size or 2 or 4 stroke will have an idle jet and a main jet. The idle jet is of course mainly low throttle operation and it’s usually what is adjustable by a mixture screw if you have one, once much off of idle you begin to transition to the main jet, it’s not an all on or all of type of thing but occurs gradually, but your usually well onto just the main jet by 1/3 throttle.
Mixture screws rarely the main jet. Of course if it’s restricted, then if you try to go higher you pretty much run out of fuel, usually engine begins to get rough and will die if you take it wide open.
But as jets often get a little restricted and not fully clogged it’s tough to say exactly what will happen.

I’d check prop pitch and the timing advance myself, if over propped it just can’t accelerate even if the motor is perfect, not without getting on plane and unloading the prop of course.

But take the cover off and see if something under the flywheel move when you advance the throttle, that is the easiest thing to check and I like to do easy / inexpensive first.

[a64pilot]

Originally Posted by Franziska

When I throttle up over 1/3 it has a chocked louder noise to it, revs seem to go up by not much.

That is normal usually and is called intake honk, and does sort of point to being overpropped

[blumitzi]

With so many people suggesting carburetor and fuel issues, here is another seldom considered issue with 2 stroke engines. The exhaust may be partially blocked. If the exhaust is blocked it will start and run but as the throttle is increased the extra exhaust backs up and wont let the piston draw fuel. It sounds louder and may even rattle a little bit. Bad fuel and too much oil can plug the exhaust ports. Remove the exhaust pipe from the cylinder head and look in the hole and then move the crankshaft (with plug removed) to see if you can see the piston move. If not you will need to open these ports.

I hope this helps.

[Franziska]

Hey thanks for the answers.

Will look into this once I am back onboard.

@A64 with overpropped you mean it's the wrong type of prop, correct?
Not sure what is on there right now, guess it is what the OB was factory shipped at the time.

On the web I found as sizes available:

7 1/2 x 6
7 1/2 x 6.5
7 1/2 x 7

All 3 bladed.

Our dinghy is fairly heavy and we do not get it to plane at the moment, which might be due to power output and prop.

Suggestion which on I should try?

Guess actually best would be to go to a coastal Suzuki dealer and try different ones if they allow it...

[SwellGuy]

Pretty easy to test the overprop theory.


Does the engine reach way higher RPMs if the prop isn’t doing work? (Put it in neutral, or take the prop off, or hoist it out of the water for a quick few seconds)

Careful not to overrev an unloaded engine or you’ll be posting in that ‘how does an outboard die’ thread.

[a64pilot]

If all the change available is 1” I’m not sure that’s going to make a whole lot of difference.
I know your small and light, but I’m not sure I’d expect to plane on 4 hp, or I wouldn’t think so.
By Overpropped I mean a prop with too much pitch, it’s exactly like trying to drive up a hill in a car in too high a gear, you can open it wide open and it may rattle etc and make more noise but that car isn’t going to accelerate unless it can get into a lower gear.
Same thing for outboards, we get away with only one gear on boats largely because the propellor can slip, and that helps a lot, but if you have too much pitch, the engine will bog.
But 6 to 7 is only one inch.
However if you look the numbers should be stamped into the propellor hub, or they usually are

It could as bluemetzi pointed out be a clogged exhaust too, clogging an exhaust will not one down for sure

[Compass790]

Originally Posted by blumitzi

With so many people suggesting carburetor and fuel issues, here is another seldom considered issue with 2 stroke engines. The exhaust may be partially blocked. If the exhaust is blocked it will start and run but as the throttle is increased the extra exhaust backs up and wont let the piston draw fuel. It sounds louder and may even rattle a little bit. Bad fuel and too much oil can plug the exhaust ports. Remove the exhaust pipe from the cylinder head and look in the hole and then move the crankshaft (with plug removed) to see if you can see the piston move. If not you will need to open these ports.

I hope this helps.

Yes another vote for this possibility.It happened to us years ago on a 9.9hp Johnston. Not as common as the other possibities but should be considered if others don't fix your problem

[Franziska]

Hey once more thanks, especially for the good explanation A64.
Once I'm back I'll call up this thread and give it a try. I think there are good chances to get some more out of it.

Agreed 4hp probably won't make it fly :-) or get on a plane.

[techmans]

Most likely on a 2-stroke engine that old is that the exhaust is plugging up with carbon deposits.Anywhere from the cylinder ports themselves down to the under water exit.
Since the engine is regularly used year round I would think carb problems to be unlikely.