Zodiac Medline 2 underpowered with Suzuki Df 140?

[Beitsta]

I have a new (Dec 05 – approx 40 hours) Zodiac Medline 2 (6 Metre) with a Suzuki DF 140, 4 stroke engine.

My problem is that she will only rev to 5,000 RPM at a max speed of 30knots.

We have checked the throttle cables - both at the hand control and at the engine and they appear to be o.k. – no restrictions, fully extended!

We have disconnected the throttle cables and operated the throttle lever on the engine with the RIB in the water - using a screw driver - yet we can still only get the above figures - which eliminate the cables!

We have disconnected the throttle cables and revved the engine in neutral and we can get the engine to rev to the recommended 6,200 RPM - so now we know that the engine is capable of revving to that figure – but it will not rev to more than 5,000 RPM in motion.

I am running a 21 º x 14” Alloy prop and have tried a 19º prop which gave me an increase of about 200 RPM but no real speed increase.

Speed is measured with a Garmin 198 C Chart plotter.

Does anyone have any suggestions?

Martin

[codprawn]

What prop did you try? Maybe you need a 22 or 23? More likely though your engine is mounted too deep - try raising it a few holes.

[Hugh Jardon]

Originally Posted by codprawn

What prop did you try? Maybe you need a 22 or 23? More likely though your engine is mounted too deep - try raising it a few holes.

i dont think going up the pitch to 22 or 23 will increase the revs! unless i have missed something in all this!

measure the cav plate or take a pic of it and post on here so the experts can cast their eye over it

[sparkey]

prop too big.
A 2" reduction should increase rpm by 200 rev's aprox.
You might also try a diam reduction as well.
When your WOT is correct your speed will increase.

[Richard B]

Without seeing your RIB, it's almost impossible to guess what the problem is!

However, making the bold assumption that it's been sensibly rigged, 30kts from a 6m RIB powered by a 140 sounds a bit low. Meme who posts on this forum has the same Zodiac and the same engine so it may be useful to find out how his performs.

If the engine revs to the max with no load, but won't rev under load, it sounds like there's a problem with the engine. Hugh is right about his prop comments. Codprawn could be right about the engine height. But there may be a fuel starvation problem - check your fuel lines for kinks, make sure there's no obstruction (maybe try running the engine from a portable tank?) and if you have a filter, maybe replace it.

[codprawn]

Originally Posted by Hugh Jardon

i dont think going up the pitch to 22 or 23 will increase the revs! unless i have missed something in all this!

measure the cav plate or take a pic of it and post on here so the experts can cast their eye over it

WHOOPS - swot I meant - teach me to pay proper attention!!!

[pathalla]

try a 13 1/2 inch wheel yet? How is your fuel tank vented? Spiders occasionally like to build egg sacs on the fuel vent screens... have seen this create a problem which is easily overlooked.

[alt]

Check you've got a spark on all plugs. I had a 10HP that would rev really high in neutral, but when under load would hatdly move. Turned out one of the plugs wasn't getting a spark

[Beitsta]

Thanks for all the replies!

Codprawn:
Thanks Codprawn, but I only want to move the engine as a last resort!

Hugh Jardon:
I’ll post a picture of the engine - showing the cav plate - in a few minutes

Sparkey:
I’m sorry now, but as a fellow Cork man I have to ask what does WOT mean. What Diam prop do you suggest?

Richard B:
I’ve emailed Meme with a query on his prop size but am still waiting for a reply.
The fuel lines are under the floor and appear to be o.k. where I can see them

Pathalla:
How do you mean to try a 13 1/2” wheel yet?

Alt:
Hiya Alt! The computer says that she is firing on all cylinders – even under load!

I am awaiting a 17º x 14” prop which I should have next week. It will be interesting to see the result of this!


Keep the suggestion coming – I have to solve this problem soon

[yoyo]

WOT: Wide Open Throtle.