DF115 runs at 10 MPH full throttle @ 4000 RPM

[Razinkane]

Thanks for the ideas. I’ll give those a try and report back.

[Razinkane]

The old plugs I pulled last week were black on the electrodes and gray on the threads. Like new otherwise

[UrbanBatic]

I am wondering how is this possible. As Harper said this is really wierd from the propeller slip standpoint.

If you are going 10MPH@4000 that is around 8,5 knots. If you are having 17" (with 19" whole thing is even worse) pitch propeller, gear ratio of 2,59:1, you are getting 1550 rpms on the propeller shaft and at 0% slip your boat would go 21,5 knots. !!!BUT!!! you have 60% slip. RPMs are not lying. That's how it is from the phisics and mechanics way of looking. With Alu props we are looking at around 15-20% slip in this rpm area, SS would give around 10% if the choice was good.
It may seem stupid question to you, but is hull clean? You had the boat in the water for how long now? And any picture or video of tests made would be great. Maybe there is something else on a different end and skilled eye of Moonlighter might get it in a few seconds.

[catacom225]

Do you mean Black as if carbon build up on them? The colour of your can tell you a lot on how your engine is running. You should be able to clearly see the white insulator

[redlowrey]

The old plugs I pulled last week were black on the electrodes and gray on the threads. Like new otherwise

going by that statement you haven't got a fuel delivery problem, the engine can't burn the amount of fuel that is being delivered, if it was over propped or too heavy it would do it, but in your previous post you said it struggled and then come good. That is why I was thinking a cyl was dropping in and out.

Seeing you are on a lake remove the cowl and go for a run, in the early stages of exhaust gas leaking from the plug in the engine holder it will be drawn into the throttle plate and the engine cant burn it so you will start loosing power.

[Razinkane]

White insulater was clean

[Razinkane]

Redlowery- To clarify- I don’t lose power. I can’t get it to run above 10 mph. Sorry for the ignorance but your second paragraph isn’t clear to me regarding what might be the culprit. Thanks.

[Razinkane]

Harper- boat is clean on a lift. The prop on the engine is a 3 blade 19 pitch 15d. I have a SS 17 pitch 4 blade prop I. Can throw on but decided not to do since my slippage test (marking prop and hub) was negative

[Razinkane]

Pic of boat image.jpg

[Razinkane]

To clarify- black electrodes seemed like carbon buildup

[Razinkane]

Redlowery- it makes sense what you say re “it’s not a fuel delivery issue it’s a fuel consumption issue”. However is there a chance it could be a bad fuel pump? I’ve confirmed the problem isn’t before the pump so far.

[redlowrey]

Harper- boat is clean on a lift. The prop on the engine is a 3 blade 19 pitch 15d. I have a SS 17 pitch 4 blade prop I. Can throw on but decided not to do since my slippage test (marking prop and hub) was negative

What I should have said instead of saying loosing power, if you have exhaust gas leaking under your cowl and and being drawn into the engine, the engine can't develop it's maximin power that is why I said take it for a quick spin without the cowl, you will know in twenty seconds if it is the problem, it is a very common problem and people don't realize till they loose power or the wiring loom catches on fire from the exhaust gas.

Mate has the engine ran good previously with no PROBLEMS with the prop you have on it now, because being over propped won't spin on the hub it will stop the engine from developing it's maximin power.

[Razinkane]

I’ve run it numerous times without the cowl trying some ideas no improvement. The prop on the engine is the same prop when engine was running good.

[redlowrey]

I’ve run it numerous times without the cowl trying some ideas no improvement. The prop on the engine is the same prop when engine was running good.

Ok now we are on the same page, have you checked and made sure that the throttle plate does go to the wide open position when the shifter is pushed all the way. Just so I can verify that your neutral switch is working properly when it is running in neutral, lift the warm up leaver all the way and when you get to around 3000rpm it should start missing if it does go back to idle, then put it in forward and give it a quick squirt it should go over 4000rpm straight away just be careful, or take the prop off, I am not going to spend half an hour telling you how to check the neutral switch, if it does the switch is working fine and I will move to something else.

If it still misses when you give it a squirt it will sound the same as it did when in neutral. Have you put a timing light on the engine and made sure that the timing is advancing with the throttle opening.

[redlowrey]

I was just looking at the picture of the boat, just drop the lift enough to cover the cav plate in water and do the test then you don't have to worry about the in gear neutral test with the prop out of the water with the muffs.