Low Revs

brinky · 06 June 2010, 12:05

Hi, can anyone give me some advice on why my 50hp 2 stroke doesn't rev over 4000 rpm,
the manual says it should go to 5500 rpm.
Its seems to hit 4000 rpm but I can still move the throttle forward a couple of inches.
Engine starts fine but does splutter a bit on acceleration.

Thanks

doggypaddle · 06 June 2010, 12:32

sounds like it could possibly be over propped. did it ever get over 4000 rpm with the current setup?

brinky · 06 June 2010, 12:43

Not sure really, have only used the boat half a dozen times, it had a faulty engine control lever when I got it so I had to peplace it and put new cables on at the same time.

doggypaddle · 06 June 2010, 14:22

i would just take the air box off, and check you are getting full throttle on the butterflys, and that the timing fully advances, ie, theres no slack in the throttle cable, as you have disturbed it all.
Do a compression test to make sure your not down on compression and therefore power, If not it sounds like you may need a much finer pitch prop, this should improve acceleration, top speed and possibly economy.
i would take it to a good outboard dealer and have a full service to rule out mechanical/electrical problems.
then do some ebay prop purchasing

brinky · 06 June 2010, 16:19

The manual says it should have a prop ratio of 13/27. The prop on the outboard has TDA13
stamped on the outside, and appears to have PC 2152 stamped on the inside!!!!!!!
Does this make any sense ???

Nos4r2 · 06 June 2010, 17:52

Quote:

Originally Posted by brinky

The manual says it should have a prop ratio of 13/27. The prop on the outboard has TDA13
stamped on the outside, and appears to have PC 2152 stamped on the inside!!!!!!!
Does this make any sense ???

Yes and no. 13:27 is the gear ratio. TDA13 could be a pitch (sounds like it),as PC2152 could be a part number.It's definitely not a pitch.

brinky · 06 June 2010, 18:37

Ok, in the specification table, under propeller it states : Ratio 13/27 anti-weed with three blades and silent block incorporated....

What size and pitch would people on here recommend, I intend on pulling a ringo and maybe skiier.

doggypaddle · 07 June 2010, 12:25

you will just have to find what props fit your engine, from selva and other manufacturers, and try some out. removing your old prop and counting the splines and measuring shaft diameter would be a good start, or ask someone at a prop place like steel developments what props will fit.

If the only problem is the prop then you will need to go down a few pitches to gain 1000+RPM. Rule everything else out first!

I had a very similar problem years ago on a crescent outboard on a small speedboat. The prop was supposedly about right for the engine size and boat size, but it was way overpropped, making only 3800 rpm. turned out the gearbox had been changed for one off a bigger engine and was a higher ratio, so had to drop the prop from 21" to 17" to compensate

brinky · 07 June 2010, 21:03

Tried an 11 3/4 x 17 pitch prop today, only got 3200 rpm. Will have to keep trying.....

Nos4r2 · 07 June 2010, 21:11

What's the engine pushing?

brinky · 07 June 2010, 21:13

Its a 4.6m Selva.

Nos4r2 · 07 June 2010, 22:49

Is it kept in the water?

brinky · 08 June 2010, 07:16

No I have the fun of launching and recovering.......

Nos4r2 · 08 June 2010, 07:55

Quote:

Originally Posted by brinky

No I have the fun of launching and recovering.......

Ok, so it's not weed buildup.

In which case you need to
1) find the pitch of your original prop.
2) Make sure your engine is running right.

doggypaddle · 08 June 2010, 11:56

Quote:

Originally Posted by brinky

Tried an 11 3/4 x 17 pitch prop today, only got 3200 rpm. Will have to keep trying.....

so your original prop could be a 13" then?
in which case you need to be trying an 11" and a 9"
have you done a compression test, stripped and cleaned the carbs, and checked the timing advance at WOT yet?
If not then It might be an idea! and is your tacho correct?

Nos4r2 · 08 June 2010, 12:12

Quote:

Originally Posted by doggypaddle

so your original prop could be a 13" then?

That's what's confusing me. I thought that too, but the revs are very low for a 13" prop.Makes me think the engine isn't right somehow, or the revcounter is set up wrong.

Quote:

Originally Posted by doggypaddle

in which case you need to be trying an 11" and a 9"
have you done a compression test, stripped and cleaned the carbs, and checked the timing advance at WOT yet?
If not then It might be an idea! and is your tacho correct?

What speed is it doing at WOT with the original prop?

brinky · 08 June 2010, 15:37

Hi,
I have looked at carbs, they seem ok. I will do a compression test when I can borrow the tool.
With the original prop at 4000 rpm the speedo shows about 28 mph, not sure how accurate this is.
As for the tacho I would not have a clue if its set up right.
I have spoken to a local outboard service man, who thinks its probably prop related as being a 2 cylinder engine it wouldn't reach 4000 rpm if there was anything major wrong.
Thanks for all the advice, I will keep trying....

Nos4r2 · 08 June 2010, 17:59

Quote:

Originally Posted by brinky

I have spoken to a local outboard service man, who thinks its probably prop related as being a 2 cylinder engine it wouldn't reach 4000 rpm if there was anything major wrong.

He's probably right if it's a twin. I had it in my head it was a triple for some reason.

brinky · 08 June 2010, 19:43

I am now trying to find an 11", or 9" prop.
Props for a Johnson should fit.