Mercury 80HP triple rebuild.

[rev.ronnie]

Since its slow in the tech dept. I'll throw this out there.

I am rebuilding a 1990ish 80 HP triple for the Viper. Looking at the exhaust chest, it looks like the exhaust port is begging to be wrapped around to the front about .250" and raised a little... Anyone have any experience with these? Any porting tricks that have worked for other guys? I'm not gonna get too carried away, and I'll do most of it in the milling machine.

I am going to bore it, polish and balance the rods, and crank and flywheel, put in Wiseco pistons and a set of Carsons reeds of course. I am going to try to locate a tuner and carbs from a 90 HP as well.

[Fast Fred]

they are not known to respond to mods well.

[Cavit8]

I'm curious how this turns out, I bought a little 15' Renegade flats boat that has a '92 75 Mercury. I've read conflicting stories about the difference between this and a 90 horse. Some say timing and jets are the only difference, others say carbs and exhaust. Good luck with it.

[Fast Fred]

the port maps are more than likely not the same , even for 80 to 90hp. crisp stock is best.

[rev.ronnie]

I'm curious how this turns out, I bought a little 15' Renegade flats boat that has a '92 75 Mercury. I've read conflicting stories about the difference between this and a 90 horse. Some say timing and jets are the only difference, others say carbs and exhaust. Good luck with it.

I was told carbs and exhaust as well. The 90 HP has a bigger venturi. I am going to put a set on this one if I can find them before it goes together.

http://www.mercurypartsexpress.com/u...:mc28636mr1012

Here is the parts list. The 90 uses a WME 15 carb set and clearly a different tuner. I would like to see one, I'll bet it's a cut down 80 tuner.

[rev.ronnie]

Cavit8,
i just picked up a set of later model 90 HP carbs and the butterfly and Venturi size is the same..I'll dig into them and see what is different. I may swap them just to get rid of the plastic linkage junk.

[njj502]

The later 3cyl 75/90HP motors are the same except for Carbs and timing. I've thought about swapping dad's up to a 90 several times but just never did it. I belive the 90 had 4* more timing at WOT.

I'm not sure on the older ones, I thought the 80HP was a 4cyl.

[Cavit8]

Cavit8,
i just picked up a set of later model 90 HP carbs and the butterfly and Venturi size is the same..I'll dig into them and see what is different. I may swap them just to get rid of the plastic linkage junk.

This is great! I feel like I have my own R&D department here. Thanks for posting this stuff. I'll check mine tomorrow for grins. Is it written on them like a heavinrod carb or did you measure them?

[Cavit8]

The later 3cyl 75/90HP motors are the same except for Carbs and timing. I've thought about swapping dad's up to a 90 several times but just never did it. I belive the 90 had 4* more timing at WOT.

I'm not sure on the older ones, I thought the 80HP was a 4cyl.

I've got some pretty old junk around here, which ones do you consider "later"

[rev.ronnie]

The care were of like a late 90's. So olde, but not like 70s vintage. My 80 HP is a triple too.

Im gonna compare some reed stops tomorrow. I think that may be a difference in the HP ratings...if they limit the reeds opening, it will limit power. Kind of like an internal rev limiter...

That would explain the 90 having a larger jet combination.

More on this tomorrow. I bored the cylinders a little while ago and they cleaned up at .020" so I have to order pistons and parts tomorrow. I did some work to the ext. ports as well. Nothing major, and I have to make some bearing blocks for the balancer because he mains are a different size on each side.

Let the fun continue..I'll keep you posted.

[njj502]

I was thinking the 90's and newer of the "later" engines. Maybe that 80 was powerhead rated before propshaft rating?? That's prob soak up 5 hp...??

[rev.ronnie]

I was thinking the 90's and newer of the "later" engines. Maybe that 80 was powerhead rated before propshaft rating?? That's prob soak up 5 hp...??

That could be..and you may be on to something with the timing as well. It it has more timing, it could be because it is jetted bigger, and by the Merc parts list, it is.

Actually, now that I think of it,I thought they went from powerhead to Propshaft rating in the early 80s. That is why the 115 HP inline 6 got re-rated to a 90.

This is a neat motor, it's got v6 parts in it. It's the same configuration as the 3.375" bore v6. The pistons don't have all the clearancing though.

[Capt.Insane-o]

Make sure the sleeves are still seated all the way in the block, they had a bad bad habit of moving around especially if overheated. I would'nt use wiseco pistons in it and they were generally know as being a piece of crap. I've modded a couple of the big bore (even worse) motors with 2.5 sleeves and pistons to sucess but it is a **** ton of work. I'd just weight match every thing and a nice set of Vertex pistons with some reeds. Any weight you can safely take off the flywheel will be a big help.

[rev.ronnie]

I checked the sleeves and they are still staked at the bottom and the compression relief slot machining is still lined up perfectly with the aluminum behind the liner. They look to be machined afterwards, so the are a good indicator. I was thinking of putting some loctite 290 around the bases, it leeches and if there are any voids, it will find its way in as extra insurance.

I already have the wisecos coming and the block is bored and in the honing machine already at .020". Just waiting for them to hone the last .001". I really like wisecos and have used them in most every application for 30 some years. I will agree that they are failure prone if the engine is not warmed up before being brought to load. The kids will be very aware of this when they use the boat. The 12 y.o. Is helping rebuild the engine and she would not look forward to all that disassembly and gasket scraping again!

I was gonna lighten the flywheel, but I tried to balance the crank and I need bob weights to do it, which I don't have in that size journal yet. Using the crank as an arbor would be the only way I could rebabalce the flywheel, so I'm going to leave it. I'll just balance the parts the same.

Thanks for the info.

[rev.ronnie]

Cavit8,

I didn't answer your question on the carbs, I did measure both side by side.

Today's tinkering got me to the reed plates. There are 9 petals on a "daisy" type plate. The holes measure .675". There is a chamfer and then it is smaller inside. I cut one of the plates out to .700" holes and blended and radiused them.

Going by measurement, there is 3.22 sq/in of area as stock. My modified plate now has 3.46 sq/in of area. That is a 7% increase in area.

So, I am wondering what this will actually do...then I notice that I still have a custom cylinder head adapter bolted to the flow bench for a Rover head that the reed plate fits perfectly on...

Anyhoo, I put the stock plate on there, sealed it up with clay on the rubber ring and cranked it up to 72 CFM as measured on the airflow meter. With out changing the depression I put the modified plate on there and it flowed 74.6 cfm. That is a 3.5% increase already. This is just a raw number from the meter.

To properly compare, I need to adjust the depression to the same on both plates, but the computer would not boot up and I'm tired. I can't get actual depression from the flow meter, I need the software program running to get that. I'll re boot it later and recheck it. I'll bet it's close to 7%...

One last thing, the reeds, in stock form, were howling and vibrating like mad. On the modified plate, they were open smooth with very little movement. I will be installing Chris Carson's reed on this and I will test those too..

This is getting a little fun.