1977 Mercury 850 parts from 1980 90 hp

tallcanadian

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I was wondering if the lower unit and midsection from a 1980 90hp will fit on my 1977 850. I know you Merc guru's will have the answer. Thanks.
 

Texasmark

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Midsection will fit but lower unit is a different gear ratio.

Just curious, what's the 850? I had one but never guessed at the gear ratio. 90 is probably a 2.33:1? Maybe that's not a problem considering the cost of a prop vs what he's looking for? Whadda ya think man?
 

Texasmark

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Sounds like the 90 and 115 were the same back then. Back in the '70's I had a 4 cyl 85 and in '88 a 6 cyl 115 and it was 2:1 and a 2004 90 triple and it was 2.33. Difference in the props was 3 inches of pitch.....19 on the 115 and 21 on the 90 with different boat sizes to match the engine sizes. Don't remember what I ran on the 85.....19 comes to mind.

If he has a good Merc marine store handy they may have an assortment of used props for a reasonable price, like one near where I boated did. So for the same boat, he'd have to cut the pitch of his prop about 15 %. So if he's running a 19 now, he'd have to drop to a like kind prop at 17 for the nearest standard prop. Course if he was running at top rpm with what he had, he could stand 15% of extra load and a somewhat slower hole shot.

If he's running 5500 or more, 15% would put him at the low end of the 4500-5500 rpm range.....course personally, I'd keep the rpms up there with a shallower pitched prop.
 
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tallcanadian

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Wow. Great information. So if I went with the 90 hp lower unit, do I need to worry about blowing up? I have a 17" prop I'm there now. And I guess I should pick up a tach.
 

Faztbullet

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With a 2:1 ratio it will be a dog out of the hole even with lower pitch prop...been there seen it. It will be ok at the upper end 4k and above.The 4 cylinders need that extra ratio to build torque on low end..
 

Texasmark

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With a 2:1 ratio it will be a dog out of the hole even with lower pitch prop...been there seen it. It will be ok at the upper end 4k and above.The 4 cylinders need that extra ratio to build torque on low end..

Interesting and a question I have had for many years. Since we are talking about gears of a common gear train, I can't understand how changing the gear within the engine is any different from the gear that rotates in the water, other than prop slip. Obviously you are correct and I'm not questioning that as justified by the manufacturer making the 2.33 like was on my 2002 90 triple when the big brother, I4 of the same vintage had a 2:1. Just from a common sense, logical stand point, it just doesn't make sense to me.
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Ok Mr. Canadian, what boat is this on? How about a picture. Maybe you aren't all that bad off anyway. If you don't have an easily planing hull that isn't stern heavy, it may not matter as hulls like that aren't "hole dogs".

Even if it is a hole dog, a ported prop will unload the engine in the hole and hook back up once on plane.....big fatassed bass boats use them all the time to get their lardasses out of the water. Once I found about them back in 1989, I never ran anything else but ported SS props.
 
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tallcanadian

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The boat is a 15'2" tri hull. It will also have power trim, if that helps. And if I have to install trim tabs, I will.

 

tallcanadian

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Thanks again for all this valuable info. It's very much appreciated. I will be going this to week to have look at it.
 

Faztbullet

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You will need to drop to a 15pitch prop to get proper RPM and forget about skiing and tubing even with vented prop
 

Texasmark

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Back in the early 1970's when the family was young and we boated with friends a lot, I had a 18' Caravelle tri'hull with a 125 Johnson. Friends and family had boats very similar, if not exactly like the one pictured herein, trihulls, and the same 15' rated length. I needed the bigger boat as I had 4 kids. Back then the OMC 3 cylinder was on two of their boats and came in at 60 and 65 hp. I specifically remember my BIL's boat which was a red Invader 15' with a 1971 60 Johnson in seafoam white cowl and sea green rest of the engine. He was running a 15P. I specifically remember that because his rig was acting like a dog when he bought it and I talked him into changing out his then current 17P to a 15 which worked a lot better. I'll grant you that it was no speed demon but it got his kids up on skis and they had a great time.

I don't know the gear ratio of that engine but probably not all that far from the engine in this post. He didn't have 85 hp nor did he have PTT. Now that I think about it, something here is dreadfully wrong. To have to drop to a 15P on that boat with that engine is just not right.

My Merc 85 (79-84ish time frame....decal on cowl was 3 stripes red, baby blue, and silver that ran straight back then dropped down at the rear, like the Ranger Comanche LOGO and Mercury was spelled with a lower case m which I despised) ran a 19SS on a 16' bass boat when I had it. I specifically remember that because I was sitting at 39 on the speedometer and I wanted 40 out of that thing badly. Sent the prop to a prop shop in Dallas to have it tweaked and when I got it back it was missing some material........as you would surmise, less material meant less surface area to connect up and I did loose some of that 39, forget how much. It had the PTT with the pistons outboard of the transom bracket where shock absorbers of earlier editions mounted. I later had a 75 Rude on a 15' Kingfisher bass boat and it ran a 19SS. I didn't have a tach back then so I have no idea what any of these engines were running in terms of rpms, but I do remember the prop. Thought I had too much till I put a piece of 1x2 redwood under the engine (jacked it up one bolt hole) and wow, did that make a difference in that rig. That's the engine I blew on that boat because the thermostat stuck shut and it locked up on me at WOT. The engine had been acting funny but I had no idea that it was the stat. I was young and stupid then.....now I'm old and ...........better educated. Ha!

Enough blabbing for now. Let's get this problem solved.

Mark
 

tallcanadian

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A lot of great information and I appreciate it all. I've included a pic for you Texasmark. Was it like this?

 

jimmbo

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Tex, you just described the decal on your 85 merc as being a 76-77 vintage, just like the one in the pic above
 
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Texasmark

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Tex, you just described the decal on your 85 merc as being a 76-77 vintage, just like the one in the pic above

OUTSTANDING!!!!!!!!! Nice and shiny like mine was too, course I bought mine new and kept it garaged, should have been shiny. For your's to look like that today is remarkable. I'll try to remember the year model. I bought the Caravelle new in '72 and the family lost interest in boating. I liked to fish and wanted a fishing boat so I sold the Caravelle and bought this. Didn't realize I sold it so soon; thought it was 7 years old when sold, but that's pretty close.

That was the finest engine I ever owned with the Johnson a close second or maybe vice-versa....never had a minutes trouble with it. Had zero problems with it either and it was a walk in the park to start and run. I used to fish a lot in the winter around Dallas and I could launch that sucker and be on my way in a matter of minutes while a lot of other guys were fiddling with their engines trying to get them to start.

The air box on it and the 4 cylinders running at WOT made some beautiful music. That was my first engine with the air box (the plastic box covering the carburetors).
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I think the Canadian's picture nailed the potential problem and may be the reason why a shallower pitch would be right on that rig. First of all there is a lot of stuff in the boat as compared to the boats used back when I was boating. Second the hull looks like about a 20 or so degree dead rise and some other disturbances in the stern. If there are stern seats in front of that splash well, bet they are usually occupied and that adds to the problem, even unoccupied, weight in the stern is weight in the stern. Hulls like that ride beautifully (soft), even in a tri hull, (sponsons catch waves especially when quartering into the wind) but it comes with a price. Deep Vs require more HP to get up (dogs in the hole shot) and on the water and don't really plane out like a flat transom semi V. Course that's why they are smooth riding. A semi V rides on top of the water not in it like that boat and as a result in rough water they just bounce from wave crest to wave crest.....but they are fast hole shooters and run faster on less power. At my age the deep V IS the hull of choice.


Fazt.....can't argue with you when you were there and saw it, but couldn't it have been caused by something else/ Did the prop get swapped out in the same type and ratio as the gear ratio changed? If so then the dog had to be associated with prop slip and was worse. Was everything associated with the boat the same, same trim position, and all that??????? So my next question (gee I'm full of questions) is, why did a change of 15% in a gear train, matched to a countered 15% change in prop pitch cause the prop slip to change to the point of making a dog out of the rig?
 
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