increase rpm

[fireman57]

1978 85 Evinrude on 24ft toon. Runs beautifully but only turning 3900rpms. Not complaining about speed as it is around 21mph but would like to turn higher and not lug the engine. I have a 17pitch prop on now and even dropping to a 14, which is about as low as they go won't do it. Is there a way to get her up and let her whine without advancing timing and blowing it up?
It does sit pretty low in the water as the back end is pretty heavy with me (180lbs), 24 gallons of fuel, and two batteries. I am afraid if I lift it two inches it won't run deep enough behind the transom pan.

 

[jakedaawg]

If engine is sound and running well decrease pitch or diameter or both.

 

[jimmbo]

Is the tach accurate? Have you checked the condition of the engine, as in a compression test?

 

[fireman57]

Compression is 120 on all 4 cylinders. The tach is properly installed. Took it off one of my old boats and it was accurate on it. Increasing pitch and diameter will not get me to 5200rpms as you only gain a hundred or two with each step in pitch.

 

[randyray41]

It seems to me a heavy 24 ft pontoon boat with a 85 hp motor would have a hard time cruising 21 mph at 3900 RPM. I bet tach is reading wrong.

No expert, just my thoughts.

 

[jakedaawg]

Compression is 120 on all 4 cylinders. The tach is properly installed. Took it off one of my old boats and it was accurate on it. Increasing pitch and diameter will not get me to 5200rpms as you only gain a hundred or two with each step in pitch.

I said decrease pitch or diameter or both. On a 24 with that old motor I would expect an 11 or 13 pitch at most. Remember it a 78 so maybe 50-55 hp at prop. Then have the prop shop cut it down 1/2" . Good to go.

 

[fireman57]

Having a shop cut the blades down a 1/2" makes sense. I had a 14 pitch on there from another boat and it only turned about 4100 and kept the same speed. Speed verified by GPS.

 

[Joe Reeves]

1978 85 Evinrude on 24ft toon. would like to turn higher and not lug the engine.

A side note..... Yes, best to get away from that lugging condition ASAP as running a engine full throttle with full spark advance with the WOT rpm dangerously below specifications as you mention quickly results in a condition know as coking (coke-ing).... a sort of gray white waxy substance resembling the appearance of a carbon like baseball in size, to appear in the combustion chamber. I'm sure that other technicians and boaters have come across this happening... just wanted to mention it for those that haven't.

 

[fireman57]

Joe it is so good to hear from you. Someone said you had passed and I am so glad you haven't. When I joined here many many years ago you were the one that guided me through things step by step and I have never forgotten it. You even sold me parts from your stock at unbelievable prices. I thank you so much for your years of service here and being the great person you are.
I learned 40 years ago that lugging an engine creates problems in a hurry. This one runs so nice and I want to keep it that way.

 

[Joe Reeves]

Joe it is so good to hear from you. Someone said you had passed and I am so glad you haven't. When I joined here many many years ago you were the one that guided me through things step by step and I have never forgotten it. You even sold me parts from your stock at unbelievable prices. I thank you so much for your years of service here and being the great person you are.
I learned 40 years ago that lugging an engine creates problems in a hurry. This one runs so nice and I want to keep it that way.

Thank you so much my friend... Good to hear from you. it feels good to be remembered and appreciated. Yes, I'm still around, just turned 85 August 6th but feel like I'm still 38. Had a bout with bladder and prostate cancer, items removed in Feb 2016 but bounced back as strong as ever with intentions of being around until at least age 122. .... I still have a lot of inventory left and will be getting back to the eBay routine soon.

 

[Chinewalker]

14" is pretty tall prop for a toon. I'd drop several pitch sizes.

 

[fireman57]

Glad to hear you are doing better and plan on being around. It was a pleasure to talk to you on the phone a few years back. I turned 60 last month and am starting to feel it but it's not so bad. When I refer to the little blue pills I am talking about ALeve. Love that stuff for the aches and pains.

 

[fireman57]

Chine I meant 14 pitch. Can't get much lower than that.

 

[jakedaawg]

Michigan wheel shows an 11 and a 13. I don't know how to do links. Sorry.

A good prop shop should be able to switch your 14 to a 12.

 

[Chinewalker]

Chine I meant 14 pitch. Can't get much lower than that.

Sure you can. You can go down to 11 pitch in OEM. Might find something else in aftermarket. As noted above, verify tach accuracy first...

 

[jimmbo]

iboats shows several props below 14"

http://boatpropellers.iboats.com/Ev...lers/?chart=18&engine_id=79&cart_id=258806131

 

[jimmbo]

with intentions of being around until at least age 122.

You should have it made then, very few people die after that age