Mercury 9,9hp 4sk Poor performance on SIB

[Fenlander]

That's far too low so you need to pitch down, remind me did you identify your existing pitch?

[Steve509926]

It is quite low.

Was that rpm reading taken at wide open throttle? What speed was it achieving at that wot? What trim hole did you have the OB in?

Dropping down to a 7.5p prop will bring the rpm up to somewhere closer to where it should be - 5000. to 6000 rpm.

[jimbo341]

F8.5 so 8.9x8.3 prop, not sure what the speed was, probably around 12 knots, trim hole was 1 (closest to the transom).

[Steve509926]

You need to do some proper tests. Trying different packers on the transom to raise the outboard, use different trim holes - I'd suggest No 2 is probably where you should start, record what speed and rpm are you achieving at wide open throttle. Are you getting lots of spray over the transom, is the prop ventilating or losing grip? You need to know where you are know if you want to get the best out of your set up and at the moment you don't have enough information.
And pressure is key in a small sib with a small OB, make sure the pressure is absolutely spot on when you launch and check it half an hour after you've launched. I know my sib and over inflate it, depending upon temperature, before I launch.

Take a look at what I recorded when I did my tests (it is a bit over the top but that's the way I am)

https://www.rib.net/forum/f36/tohats...bie-91068.html

When you have that type of initial data, you then know what is the best height for you OB, what trim hole you should be running in (not always the same if you are 1 or 2 up) and what prop you should have on your OB.

[Fenlander]

Gut feeling is would be well worth trying a 7.5" pitch prop. Probably Polastorm for a decent price.

[Steve509926]

Originally Posted by Fenlander

Gut feeling is would be well worth trying a 7.5" pitch prop. Probably Polastorm for a decent price.

Can't disagree

[jimbo341]

I've had some time to look into this again. I found a good price for the service manual specifically for this motor and considering the boat size and weight the propellar is beneath the recommended pitch, which is ok but I dont think dropping down further is the solution.

Meanwhile, I re-checked fuel lines using clear hosing and found there was a substantial amount of air entering prior to the fuel pump. The fuel tank hose was severely degraded inside limiting flow and the attachment to the engine was slightly loose letting in air. I replaced these and rpm now is around 5000rpm which is better.

(Let me know if I should continue here or start a new thread in engines)

However, the service manual also mentions that if compression test is below 120psi, it indicates a worn engine. Mine is 125psi and when I did a wet compression test it showed 165psi leading me to think that the piston rings are worn or cylinder damaged. I therefore purchased a cheap USB micro camera to look inside the cylinder and this is what I see. Can anyone help with diagnosing these photos? Would just changing the piston rings be sufficient or would the cylinder walls need re-honing or re-boring?

[beamishken]

Originally Posted by jimbo341

I've had some time to look into this again. I found a good price for the service manual specifically for this motor and considering the boat size and weight the propellar is beneath the recommended pitch, which is ok but I dont think dropping down further is the solution.

Meanwhile, I re-checked fuel lines using clear hosing and found there was a substantial amount of air entering prior to the fuel pump. The fuel tank hose was severely degraded inside limiting flow and the attachment to the engine was slightly loose letting in air. I replaced these and rpm now is around 5000rpm which is better.

(Let me know if I should continue here or start a new thread in engines)

However, the service manual also mentions that if compression test is below 120psi, it indicates a worn engine. Mine is 125psi and when I did a wet compression test it showed 165psi leading me to think that the piston rings are worn or cylinder damaged. I therefore purchased a cheap USB micro camera to look inside the cylinder and this is what I see. Can anyone help with diagnosing these photos? Would just changing the piston rings be sufficient or would the cylinder walls need re-honing or re-boring?

Why do you think you have a compression issue when your compression is well above spec? Those bores aren't even worn, you can still see the honeing marks from manufacture. I think you need to look elsewhere.your still down on rpm so looking at propeller type & pitch might be a good idea

[Fenlander]

>>>still down on rpm so looking at propeller type & pitch might be a good idea

My thoughts exactly. Those bores look fine, compression figures against a theoretical spec can be misleading (such as your gauge being out).

You need to resolve the low maximum revs first so I'd still say try a lower pitch prop and don't think of stripping the engine.

Did you check the throttle linkage is fully opening the carb butterfly?

[jimbo341]

Originally Posted by beamishken

Why do you think you have a compression issue when your compression is well above spec? Those bores aren't even worn, you can still see the honeing marks from manufacture. I think you need to look elsewhere.your still down on rpm so looking at propeller type & pitch might be a good idea

Service manual states that "a reading below 120psi might indicate an engine wear problem". Mine is 125psi so close to this although I cant verifty with another gauge.

Also, regardless of the original reading, when i add some oil to the cylinders and reperform the test i get a reading increase of 165psi. The manual suggests "if compression is higher with oil added to the cylinder, suspect a worn or damaged piston, the cylinder and/or rings".

The engine is using alot of oil, and when I open the oil tank cap when the engine is running, there is some smoke/mist and a strong smell of petrol possibly indicating blow-back.

Finally, the service manual propeller chart states that for a gross boat weight of 272-453kg and a length of 3.9-4.9m, 8.9x8.3 is the correct prop. I am running a 3.3m boat with a weight between 200-275kg so it looks like the prop I am running or even higher is the correct match.

I am obviously a novice at this so if you think that the problem is the prop thats an easy solution, but I am a little bit worried that there is a problem with the pistons or rings, if you think the cylinders appear to be in good condition.

Boat is off the water now so I have time for a rebuild if it is worth the effort, but dont want to do it unnecessarily.

[beamishken]

You've obviously been running it overloaded pulling the rpm back which could account for the fuel in the oil & backpressure but your compression is within spec & you'd always expect a higher compression reading with oil in the bore, you reduced the size of the void for a start. Personally I'd get the rpm to the correct reading & give it a few good thrashings with some clean oil in & see how it goes