1978 85 hp merc what rpms should it be at at idle and wot?

[andrewbabich]

1978 85 hp merc what rpms should it be at at idle and wot?

The cable linkages are all old and showing cracking and rust. I 'm going to put a tach on the spark plug wires and check my rpms. Felt like the boat was doing about 30 mph at wot yesterday.

its a 17 foot tri hull

 

[Texasmark]

Re: 1978 85 hp merc what rpms should it be at at idle and wot?

How about 675 at idle in the water in gear and 5500 at WOT recommendations per current manual.

Rule of thumb, set idle fast enough to ensure that you can shift into F-R and the engine doesn't die. The higher the idle, the rougher it is on the gear train when you shift and if it clunks every time you shift you are too fast.

On WOT, most are 5500 max but I never paid any attention to that on my Mercs (have had half dozen 4.5 to 115 tower); had the 80-84 yearmodel (somewhere in there) in 4 cyl 85 hp. I run em high up on the scale. Currently running a 3 cyl 90 at 5600-5800. Ran my tower at 6000 and it loved it.

Mark

 

[Texasmark]

Re: 1978 85 hp merc what rpms should it be at at idle and wot?

1978 85 hp merc what rpms should it be at at idle and wot?

The cable linkages are all old and showing cracking and rust. I 'm going to put a tach on the spark plug wires and check my rpms. Felt like the boat was doing about 30 mph at wot yesterday.

its a 17 foot tri hull

Don't know what you had for a load, but 30 sounds low to me for that engine's capability. My 85 was on a 16' bass boat and ran 39 regularly.......never could get that one more mph and make 40 (had PTT)....that's how I remember the speed. Back in the '70's a 60 OMC would do 30 loaded on a 15' tri-hull.

Mark

 

[andrewbabich]

Re: 1978 85 hp merc what rpms should it be at at idle and wot?

Sweet thanks for the input guys. I'm going to get a gps and put a tach on it and try it out.

I had four people and a black lab in the boat and 12 gallons of gas so it was a big load.

The throttle didn't do a whole lot until I i was into 80 to 100 percent of the throttle range. It did get right up on plane though. It was stronger then my 1972 50 hp johnson on my 16 ft trihull.

 

[Texasmark]

Re: 1978 85 hp merc what rpms should it be at at idle and wot?

Sweet thanks for the input guys. I'm going to get a gps and put a tach on it and try it out.

I had four people and a black lab in the boat and 12 gallons of gas so it was a big load.

The throttle didn't do a whole lot until I i was into 80 to 100 percent of the throttle range. It did get right up on plane though. It was stronger then my 1972 50 hp johnson on my 16 ft trihull.

With most loads, hole shot means full throttle till the boat planes out and the bow starts to drop....then cut the throttle to maintain the speed you want. If you play around with anything else, you will have hole shot problems as you mentioned. However, if you have a light boat and lots of HP then you can play with a soft throttle and still have a decent hole.

Mark

 

[jimmbo]

Re: 1978 85 hp merc what rpms should it be at at idle and wot?

For what it's worth. 1977 was the last year for the 66.6 cubic inch 4 cylinder 85, in 78 it became an 80hp, then in 1984 it became a 75hp

 

[andrewbabich]

Re: 1978 85 hp merc what rpms should it be at at idle and wot?

Hmm that is interesting and valuable motor info... The boat has a plate that says 1978 on it so I figure the motor was 1978 or newer. Of course they could have slapped an older motor on there but I would not have guessed that.

I will look into that. How would I determine the motor year. Is there some serial numbers or something I could look up. I am curious.

 

[andrewbabich]

Re: 1978 85 hp merc what rpms should it be at at idle and wot?

I just put my tach on (four wrapps around a spark plug wire and grounded style) and it shows between 1000 and 1100 at idle with the muffs on in the back yard.

Is that where it should be? I'm going out for the second voyage today hopefully. We shall see what she does at wot.

I ran it at mid choke and sprayed carb cleaner in it a few times till it stalled out and repeated to try and cycle some carb cleaner through it. I added two gallons of fresh fuel.

I still need to check the model number serial information.

I see something on the removable air box plate that says 'timing at 5500 rpms should be' so I know the rpms should be around 5500 at wot like earlier poster stated.