1981 Mercury 7.5 2 stroke

[keithb7]

I recently purchased a used 1981 7.5 hp outboard. I plan to use it with a small SIB. I am new to outboards but not new to mechanical repairs. I’ve rebuilt multiple other types of 2 and 4 stroke engines. I tend to source an OEM service manual ASAP. I’ve not found one yet but I will.

Is anyone able to tell me what year did Mercury start using the breakerless spark box on the 7.5? I might pick up a spare parts engine or two. Prefer years that are very similar to my ‘81.

I also curious to learn how many cc’s the engine is. If you have a manual you’ll sell let me know. Thanks. Keith

 

[Scott Danforth]

you can get the manual from Mercury or find them on-line at sites like oldmercs or AOMCI for the 7.5/9.8. look at the 1980-1985 motors as they are all the same.

the first thing you need to do is pull the leg, measure the vertical drive shaft diameter, then order a raw water pump kit. the reason you have to measure the driveshaft on that motor, is it could come with one of two sized shafts.

 

[keithb7]

Thanks. I will try those web sites.
I have already pulled the lower leg measured, the shaft and ordered a water pump. Its on the way.
The thermostat was removed. I also ordered one.

 

[keithb7]

The Mercury Shop Manual I ordered arrived. It covers the smaller Mercury outboards, including my 7.5 HP. I was hoping to find stock compression numbers. They don’t publish them in the book. Can anyone tell me what the compression should be on my ‘81 7.5 HP engine. I have measured my engine and got 75 psi, WOT. Same number with engine cold or hot.

I have read others mention these are designed as low compression engines. How low? Wondering where it should be. Thx.

 

[Texasmark]

I had a mid 70's 7,5 and never had to go into it, so I can't advise on that. For the 3 and 4 cylinder engines, the manual states that one should expect problems if cranking compression (about 200 RPM) was less than 120 PSI.

I don't know why the smaller engines would be lower compression by design. The combustion physics is the same for that type of engine and volume of the combustion chamber I doubt would have anything to do with compression....dimensions would be downsized proportionally....after all the design is to explode gasoline/oil mix and compression to operate on today's regular grade gasoline fuels takes about 8.7:1. regardless of HP.

 

[racerone]

The 7.5 ( COMET ) model has smaller intake ports than the 9.8 version.-----Throttle does not need to be open for a compression test.------Remove bypass cover to look for possible piston / ring damage if you feel compression is low.

 

[keithb7]

My understanding is piston rings wear over time and compression slowly drops. There does
not need to be any visual damage to piston or rings for compression to be low. Could just be a tired engine. Maybe 70 or 80 psi is all these 7.5 hp engines ever made. Looking to confirm.

The 9.8 and the 7.5 back in those days
entirely used the exact same engine parts. Except the block. Just the port timing is different to produce lower horsepower power rating for the 7.5. It could very well be that the 7.5 made 75 psi new and the 9.8 made 100. I don’t know. Just deducing and making guesses here at this point.

Thx.

 

[Scott Danforth]

the 7.5 and 9.8 only differed by the carb.

PSI was 120 ish on my 9.8's

run some ring-free thru the motor first to see if the compression comes back

 

[racerone]

???-----The intake ports on the 7.5 are smaller !

 

[keithb7]

???-----The intake ports on the 7.5 are smaller !

Sorry. Yes smaller ports on 7.5 compared to a 9.8.

I did some internet searching. I found some posts where folks were claiming 100, another 165.

compression for a 7.5 hp mercury

Can you tell me what the compression should be for a 1973 mercury 7.5 hp outboard?

forums.iboats.com

Vintage Mercury 7.5hp Thunderbolt restoration - RIBnet Forums

Picked up this old Mercury, no spark on either coil but compression of 165psi in both cylinders, I've not had time to look at it yet, I'm hoping it's the kill switch as on this model the switch breaks

www.rib.net

I’ll assume I’m quite low at 70 psi and plan to re-ring it.

 

[racerone]

I doubt you will ever find one at 165 PSI.-----Remove bypass cover and have a look at pistons / rings.----Just 3 screws and make a new gasket.

 

[keithb7]

Will do. Thx.

Any chance that piston ring end gap would be measurable from that by pass cover? I suspect no, that end gap pins will be 90 from the ports. I will inspect rings and test them for spring tension.

 

[keithb7]

Reporting back. Removed bypass access cover.
All rings are still sprung. None broken. I see fine scoring patterns on the rings. They look pretty worn to me. I’ve limited experience with this. I attempted to take a few photos. Difficult to get clarity with cell phone camera. Hone and ring time?

 

[keithb7]

 

[racerone]

I see the scoring.----Motor must come apart to be refurbished.----Inspect all parts and decide on a path forward.----Note----I own more than one of this motor type.-----Last one i got for free.----Most shops do not want to work on them anymore.

 

[chrisgt]

How do the cylinders look? You should be able to see them through the bypass cover as well.

In all likelyhood the engine will run with 75psi of compression; give it a try and see how it does. If it's been sitting for a long time it might just have some sticky spots in the ring grooves and it'll free up and gain compression after a good run.

 

[Texasmark]

How do the cylinders look? You should be able to see them through the bypass cover as well.

In all likelyhood the engine will run with 75psi of compression; give it a try and see how it does. If it's been sitting for a long time it might just have some sticky spots in the ring grooves and it'll free up and gain compression after a good run.

That wouldn't hurt. Pour 2 oz per gallon of Sea Foam (available everywhere) into the tank and then put in your normal fuel-TC-W3 oil 50:1 ratio. Run it for an hour if you can and let it sit over night or more. If you run it in a barrel, just remember that combustion temps are in the water that the engine returns to the barrel and you will have to keep fresh water added to keep the water at a decent temperature. Come back and run it hard on a boat...as hard as it will run. The SF will remove gum and varnish that may have your rings stuck to the pistons, freeing them up so that they can expand and seal off the combustion chamber better. It will clean up your complete fuel system too. I have it in every tank of fuel I run, at the normal 1oz per gallon maintenance ratio. I just don't have fuel problems adding that to my fuel mix.

I don't remember when that engine was converted from cross flow to loop charged. Only seeing 2 rings, I am supposing its a looper. Those 2 rings are wedge shaped and are designed to expand under compression pressure to seal off the combustion chamber. If they are stuck to the pistons, they can function as designed and this, if the problem, may save you an overhaul even though wear is evident.

 

[racerone]

I see evidence on scoring in cylinders.----No magic potion can fix that.----Yes the motor can run the way it is.----There is a difference between " running " and running as designed.

 

[chrisgt]

Light scratching on a 2 stroke isn't really a big deal. Poke a toothpick or something soft in there and see if you can feel them, if not they probably aren't an issue.
It's a wonder what fresh gas and a WOT run up and down the lake can do for an old motor, they live to be used.

It sounds like you have your heart set on rebuilding it, we're just trying to help you get the most out of it as is.

 

[keithb7]

The rings are sprung. I’m not convinced they are stuck. I see enough scoring on the rings to lead to low compression numbers. Ideally I’d like to get full power from this engine. It’ll be going on a 10 ½ foot SIB. It won’t be mainly trolling around fishing. This project is my first time digging in to an outboard engine. I look for to doing so.

Not knowing the history of this engine I am guessing at this point, what caused this scoring. Could be an air leak on a crank or case seal leading to a slightly lean condition. Could be from running a bad oil mix ratio. Not enough oil. Or poor quality old oil. Maybe someone topped up the fuel tank and didn’t know to mix oil. Who knows. It runs pretty good actually. Under a load it will likely under perform a bit.
When I bought it, I found the thermostat was removed. Maybe it got stuck and it overheated? Causing some scoring. Then they removed thermostat? Maybe.

I’ll open it up the engine, see what I find and plan a repair from there. I will keep updating this thread as I progress. Those who want can follow along.

I also see a deal on a 9.8 Mercury, same era. I may buy that one too. Same weight and size just a bit more power than my 7.5.

Double down and when I’m done I’ll have 2 decent engines? Hmm…