20hp mercury parts interchange 1975 to 1977-78 is it possible?

[ONERCBOATER]

1st thank you for your time;

I have a 20hp short shaft serial #42927## I am fairly certain it is a 1975 It needs to be awakened from a 9yr slumber ran well but unneeded and cast away thoughtlessly by bro in law, but the wiring on her has cracking insulation, I found a possible parts engine that is serial # 47098## and I believe it is a 1977-1978 engine long shaft.

the parts engine theoretically had its throttle gears stripped and the tiller lost then sat for last 5 years or so. What I intend to do is use the 1975 engine for parts if needed and focus on the 1977-78 engine as it has the long shaft i need anyhow.

I already have the 1975 and would be paying $50 plus $50 gas to go get the 1977-78 engine. didnt want to spend $100+ if the parts wouldnt interchange, and the drive is 6hrs round trip so i cant just walk over and look at it.

I realize that both will require carb work likely a cleaning and rebuild. And am perfectly comfortable doing the work on em.

What do you guys think?
or should I just hunt down a running engine for more $

Sean

 

[jbjennings]

Re: 20hp mercury parts interchange 1975 to 1977-78 is it possible?

Some of the parts will interchange, some won't. You can compare part numbers on the online parts catalogs with diagrams at crowleymarine.com

It is likely that the '78 model will need rewiring as well. They may have slightly different ignitions, I'm not sure. The good thing about the '78 is that it will have a stainless drive shaft and might actually still hold a seal, or at least not be pitted so that you can put the new seal a little higher or lower on the shaft to get out of the groove the old seal cut in the shaft.
Honestly, '70s mercs can be money pits on electrical parts, but they are smooth running, powerful motors when fixed right. If you like mercury 20horse motors, the '78 would probably be a pretty good choice to try to get running good. I'd look at the parts diagrams at crowley's and see how much certain parts were, and if they were available, like triggers, switchboxes, etc.
JMO,
JBJ

 

[ONERCBOATER]

Re: 20hp mercury parts interchange 1975 to 1977-78 is it possible?

well got the motor, it is in very good shape overall, i found the throttle gears off of the 1975 fit just fine, having a bit of a hard time with the linkage that goes from the stator to the throttle, specifically the cam seems no matter which way i place it it isnt in the right spot. Anyone know a link where there is a picture of this area of engine?

Anyhow havent done compression check yet, but I am sure she has more then enough to fire and run (judging by turnign flywheel by hand I know it is a reckneck check but how else to get an idea of compression with engine partially disassembled.)

carb is all nice and clean, ready to install, well once i get this linkage figured out, then I will have to figure out how to connect the throttle and shift cables after that.

Sean

 

[ONERCBOATER]

Re: 20hp mercury parts interchange 1975 to 1977-78 is it possible?

ok solved the throttle timing issue
I tell ya, mercs are very different animals than johnyrudes.

Sean

 

[Gibbles]

Re: 20hp mercury parts interchange 1975 to 1977-78 is it possible?

I was in the same boat

I have a 77 or 78 and picked up a 75 parts engine for cheap.

In the end I cleaned/rebuilt the carb and was able to get the engine running

The drive shaft was rusted pretty solid but after several hours of work I broke it free and replaced the impeller, now I have two running 20hp mercs.

Electrical is close, but not quite the same, most of the wiring is the same, and only real difference I saw on mine was the switch box, but they look interchangeable.

Only thing it's missing now is the front cover face plate for the engine cover.

Once spring sets in I need to get it tuned and running well, then I plan on linking my two engines together side by side (my boat is underpowered with just a single).

I'm sure I'll run into all sorts of problems with the setup, but I have two control boxes now and they seem to be set to change the handle side and able to bolt together with a help of a spacer.

The only real issue these things seem to have is the fuel inlet, it's plastic and cracks easly.

My orignal engine had a slightly cracked inlet and it would cause fuel issues, not bad I just had to prime ever hour or so, the "parts" engine was totally cracked and would not suck any fuel, and it was very hard to prime enough to start.

After much searching I finally found a local shop that has a supply of old merc parts, he had a metal inlet that should last forever.
If you need the Part number Message me and I'll find the package.

These are really good little engines.

My throttle linkages also look backwards from my newer model, I'm not sure if it was just installed wrong.. but it's another thing I need to look into when it warms up.

 

[ONERCBOATER]

Re: 20hp mercury parts interchange 1975 to 1977-78 is it possible?

Glad to hear things worked out for you, so far my experiences have been as expected given the fact that going in I knew the new 1978 motor was missing some parts, what was described missing was the gears for the base of the control linkage where it makes a 90* direction change from the tiller to the carb and stator. What i found missing and not included with the parts was everything from the gears up to the stator and to the carb, fortunately I had all needed parts for this area on the 1975 donor engine. When it is time to tear into the engine more in the future I will replace the lower cowl as the guys "son" used a hole saw and broke pieces off with pliers to make access to a bolt rather then do a further tear down in trying to remove the broken (now missing) gears. needless to say that part is ugly but just cosmetic, I intend to use an epoxy paste some glass roving and fill the hole until I get to a point where i feel like removing the power head from the exhaust housing for some reason, until that point no one but us and myself need know cause it will not show once primed and painted.

I am anticipating firing her up this evening, having replaced the fuel lines and intending to purchase a short length of stainless all-thread this afternoon to make the final portion of the linkage from the main linkage to the stator as this part is different between the engines.

it is solid state ignition, and I am not sure exactly how to initially set the timing, so I am going to aim for slightly retarded feeling this will be safer for initial start up, then I will apply a timing light and see if i can adjust it to the recommendation that is on the flywheel. additional info or suggestions on this would be great. I haven't read up on the sync quite yet but assume that the timing should go through its initial advance prior to the throttle opening then the timing advance should complete its arc. I am having issues finding obvious marks to go by, but honestly I havent looked that hard yet.

The compression on the newer engine turned by hand feels like that of a new engine, I hope after I get her running she performs as new

Sean

P.s. if anyone thinks i am missing something please let me know, I am a johnyrude guy and this is my 1st real work on a merc, they are a lil different to say the least.

 

[mercury713]

Re: 20hp mercury parts interchange 1975 to 1977-78 is it possible?

I just fixed a 79 20hp stripped linkage. Need to take the whole bottom end off to get at a bolt.

Its just the tip of the gear that strips because(in my case) they were loose.

To fix.

Just over gear theres a washer with a pin holding it. get small grade wire and wrap a turn or two between the washer and pin. This pushed both gears together with no problems.

The gear teeth are about 1/4" long and 1/8th was worn off because loose.

 

[ONERCBOATER]

Re: 20hp mercury parts interchange 1975 to 1977-78 is it possible?

well figured I would post an update, maybe it will help someone in the future.
I took the old wiring harness out of the engine and after cutting back into the rubber boot found good healthy wire and jacket. I then soldered in splices to allow me to rebuild the harness with all new wire, I then placed the new harness on the engine. Then I repaired one wire on the CDI unit by cutting it putting two different size heat shrink tubes on it, soldering the cut ends together and extending the heat shrink to recover the bare wire (about 4") then applied a small dab of epoxy to fix the wire and heat shrink to the CDI so it would not be pulled free by accident.
then came the alternator, the windings on it looked great but the wires leading to and from it were trashed, so off they came, soldered in new wires on either end of it, applied heatshrink to prevent shorts and coated with epoxy as the manufacturer had done so in the past. I returned this to the engine as well.
Upon hooking up the external wiring harness and putting the engine in a test tank (50gal drum) I gave her a crank and everything worked but she didn't fire, so I made some adjustments to the timing and tried again, she ran halfheartedly but was clearly firing on both cylinders. Off the carb came for the second time, this time a very complete and careful cleaning, complete disassembly and 3hrs in the ultra sonic cleaner for all metal parts with dawn dish soap and very hot water to help out, withing 25min the bath turned dark and had to be replaced lol, go figure. The net result is a brand new looking carb that functions much better, I can now get the full RPM range with the exception of idle, thinking I will get a fuel pump rebuild kit and see if that helps with the idle after i do a link and synch, per the directions it needs to be run at full throttle to do the maximum advance setting then adjusted for the low speed part and then the starting part adjusted, so it sounds like it will have to be done at the lake as I don't have the ability to keep water in the 55gal drum at full speed.
Anyhow I would call this portion a success.

next will be hooking up the remote throttle and the remote shifting and fabricating the parts needed as I am failing to find them online.

Sean