1981 Mercury 7.5 2 stroke

[keithb7]

I spent some time researching the air coming out the end of the crankshaft. I studied the parts and technical book and a few other similar models. Nothing confirmed my inquiry. I sprayed soap in there and pressurized it to maybe see where the air was coming from. Inconclusive. I tried applying compressed air in at the crank. No go. A clue perhaps? The reeds open when air pressure is applied via the carb opening. When applied from the crankshaft end reeds are forced closed. For whatever reason, air comes out the center of the crank. I’m not going back in to rip the crank apart to learn why. My guy tells me it was designed this way.

I greased the splines. Installed the felt washer and re-installed the power head. I hooked up the flywheel and starting rope system. I pulled it over. I get 80 psi. Rings are not seated. I can assume compression will rise some once seated. Other sources tell me these are low compression engines. Next is carb ultrasonic cleaning and rebuild. Then I’ll flash it up.

 

[racerone]

Did you read post #40 ??----Or are you simply ignoring some input by me?

 

[keithb7]

I’m not ignoring you @racerone. You have provided some valuable info. I’m not sure what info I am to glean from your post #40. Starting a paragraph with a bunch of question marks is interesting. I suppose I am to try and read between the lines and guess what you are attempting to say.

You state some motors have a seal on the drive shaft. Some do not. When my engine was removed there was no o-ring present on the drive shaft. When I review parts manuals and Mercury service info, there is no o-ring listed for my engine. Am I to round up various o-rings and find something that might work to seal it? Maybe it won’t seal it. There’s no simple way to tell if it’s sealing once the engine is re-installed and bolted down.

My questions and posts here are not directed solely at you. My intention is to ask anyone of the members here who may have experience with this engine. I’m posting here so people in the future may reference it. Hopefully my info will help them as well. There is so much inaccurate info online these days. I have found lots of mis-information about these engines. For example one person states they had a 7.5 and it made 110 psi. Someone else states they are low compression engines.70-80 psi is good. Someone else states that all you need to do is bolt on a carb from a 9.8 then the 7.5 becomes 9.8. I say BS. The two have different block part numbers. We discussed larger transfer port sizes in the 9.8 earlier in this thread.

In today’s world one must filter through tons of garbage to find the facts. One must be weary about what they read online. At this point, I have yet to see facts or further clues from Mercury that support my 7.5 needing an o-ring on the driveshaft. Perhaps I will seek out an o-ring that appears to fit, and install it for good measure. Hope that it’ll do something good for the motor.

 

[racerone]

Good luck with your motors.

 

[keithb7]

Removed the motor again. I dug through my shop supplies. Found an o-ring that fits. Installed it on the driveshaft. There is a groove in the shaft to retain it.

 

[keithb7]

I visited the local Mercury Marine dealer today. I spoke to a technician. I asked him about the air leak out the end of the crankshaft. Also the o-ring situation on the driveshaft. He recalled this. He said for whatever reason, whoever was making cranks for these engines back in the day, they built a crank with this air passage. He estimated about 30% of the cranks for these engine had this air passage. He said they had an o-ring on the driveshaft when new. Whenever the engines came in for service the o-ring was usually gone or chewed up. They never replaced the o-rings. They just greased the driveshaft splines and put them back together. This sort of explains why there is no part # listed for this o-ring, and nothing mentioned in the service manual that I have. Tech told me that with or without the o-ring there will be no detriment to the engine.

That sorta closes this up for me. Good enough, with or without the o-ring. I have an o-ring in mine now for whatever that's worth.

If anyone is interested, I did produce a video of my re-ring job. I posted it on YT for others to reference as needed in the future. Here it is:

 

[racerone]

The o-ring for the driveshaft is clearly shown.----Item 35 on the parts pages.-----Simple facts for you.-----There were many issues with rusted splines on older Mercury motors.-----On these they broached the spline to go into the crankcase.----This allowed the oil that you mix with gas to lube the splines.----O-ring seals the crankcase compression !!!----Make sure you use the factory o-ring.

 

[keithb7]

That’s an interesting place to illustrate the o-ring in the parts book. The adjacent felt washer is shown with the crankshaft. Hope my o-ring is close. Lol. The powerhead is not coming off again at this time.

Thanks for pointing that out @racerone. Interesting how it is designed to lube the driveshaft splines. I guessed at that idea back in post 39.

 

[keithb7]

An update:
Running good. Easy start, accelerating well and idles nicely. Seating rings here now. It’s below freezing here today. That’s not smoke you’re seeing.

 

[keithb7]

Double post error.

 

[keithb7]

I picked up a ‘74 9.8 for $65. Owner stored it on its side for the past 8 months. Rings are rusted to the cylinder walls. I had an opportunity to compare intake ports on my 7.5 to my 9.8. Thought I’d share a photo of each for comparison. 9.8 is the lower pic, ports are considerably larger.





I’m currently soaking the 9.8 with penetrating oil and ATF. Tapping piston tops lightly with a wood dowel and a hammer. Time will tell how that works.

 

[racerone]

See post #6 re port size.----Piston rings in the lower picture appear to be like new.----

 

[keithb7]

The 9.8 engine free’d up after about 10 hours of soaking. I’m thinking I’ll take it apart. Clean it all up. Scotch-brite any remaining rust. Check seals, bearings, rings. Take some measurements. Inspect cylinders & pistons and settle on a plan from there.

 

[keithb7]

I’ve gotta stop looking at these engines on the used market. I’ve now acquired 4 small 2-stroke outboards. All Mercury. All very low prices.

‘90 8.0
‘81 7.5
‘79 9.8
‘74 9.8

That happened quick.

 

[Texasmark]

An update:
Running good. Easy start, accelerating well and idles nicely. Seating rings here now. It’s below freezing here today. That’s not smoke you’re seeing.

View attachment 405189

Normally you have the water level ABOVE the water pump. I notice that your normal Pee Stream indicating that water is being circled through the powerhead properly is just sputtering.......

 

[racerone]

Agreed----Water MUST be 6 or 8 " above the impeller !

 

[jimmbo]

I picked up a ‘74 9.8 for $65. Owner stored it on its side for the past 8 months. Rings are rusted to the cylinder walls. I had an opportunity to compare intake ports on my 7.5 to my 9.8. Thought I’d share a photo of each for comparison. 9.8 is the lower pic, ports are considerably larger.

View attachment 405280

View attachment 405281

Even with the Evidence shown in your Pics, many will still insist they turned a 7.5 into a 9.8 with a Carb swap, and even though it now runs worse at low and mid rpms, it gave them 5 or 6 more mph....
Heck, some even claim just the Carb Jet alone will does it
Sorry for that Rant, but...

 

[keithb7]

Point taken guys. I will ensure I run it in a larger bucket with more water whenever I fire it up at home.

I thought I read somewhere that it will sputter until the thermostat opens. This one has a thermostat.

 

[keithb7]

Hi @racerone I want to apologize if I offended you in a response in this thread. You seem to have a ton of knowledge and experience to share on these old Merc engines. I appreciate your posts in my threads. I believe that both of us are not the “clueless beginner” when it comes to mechanical repairs and theory in general. I’ve been wrenching on my own stuff for about 38 years. I just never got into outboards until recently.

I believe that the internet is full of mis-information. It’s important to figure out the good from the bad. I’ll go out on a limb and guess you’re over 55 and likely have decades of experience. Your 2 stroke knowledge is disappearing quickly from the internet as folks age, or give-up on new forms of social media.

Thank you for being so active and posting, often sharing your knowledge on the iboats forum. I’ll continue to post as well and ask questions or share my learings. I do invest in my own education as well. I prefer not to just turn to the forum for easy answers. So far I’ve purchased a couple of different Mercury shop manuals, as well as a trade-school text book called “Understanding the Outboard Motor”. I continue to study.

As mentioned in this thread, I now own 4 Merc outboards under 10 hp. All made in Canada. I’ll build 2 great motors for myself, learn a ton along the way, and amass a decent collection of spare used parts.

Cheers from BC, Canada
Keith

 

[Mc Tool]

Outboard HP ratings can be weird
I have a 35 hp honda that has 35 hp between 4600 and 5600rpm
The 45hp has 45hp between 5 and 6000rpm and there aint a foot/lb difference in max torque between them just the 45 hp is max at 1000rpm higher.
They are the same motor ( I have checked heaps of relevant part #'s ) they just measure the power at different revs.