58' Merc 78A compression question and no-spark?

[ErrorS]

I did a compression test on my 78A.. I've not been able to get any spark and figured I'd just not mess with it, I was sure such an old motor would have deeper issues.

I rewired it per some wiring diagram I found, but later fuond out the diagram had been switched with the non-A, so I did it all wrong. wired it back but still can't get any spark

Anyways, I figured I'd do a compression test, see low compression and just dump the motor or sell it as scrap.. but here are the results

Cyl 1: 112PSI
Cyl 2: 110PSI
Cyl 3: 108PSI
Cyl 4: 115PSI
Cyl 5: 115PSI
Cyl 6: 110PSI

That's good compression, isnt it? I thought it was, especially for a motor from the 50s.

I have little to no experience with 2-stroke engines and can't figure this one out. I get voltage (12v when the key is turned) to the distributor, it cranks fine, but there is no spark on any of the wires.

Stupid question, does the top cover have to be on it to get spark? I know there are magnets and rectifiers and other stuff I have no clue about at work here.. there's a piece of metal on the top cover that spins with the pull cord to start the engine. I dunno, I honestly have no idea what I'm doing here. I can work on a car engine with no problems but I'm thinking (and hoping) the differences between a 2-stroke and 4-stroke are what's causing my issues.

I can't afford a distributor cap but I sanded down the rotor/distributor cap contacts.. I get good current from the distributor cap to plugs and from the ignition coils to distributor,

 

[Willyclay]

Re: 58' Merc 78A compression question and no-spark?

Your compression numbers are great! If you don't have a factory manual, get one before trying to solve the ignition problem. I'm not an expert but have been down that same path with a Mark 75A. Good luck!

 

[ErrorS]

Re: 58' Merc 78A compression question and no-spark?

Yeah, I'm pretty happy about the compression.. just sad that I can't afford to work on the motor. I'm halfway through another project...

I found a service manual but it's $75. I'm going to try changing spark plug wires (they're rough) and figure out how points work (oldest engine I work on is a 78, electronic ignition).. someone told me it's probably as simple as cleaning the points.

Would the operating manual help me any? I know how the controls work and everything, I have a good wiring diagram.. but even little things like finding the water inlet has been a chore.

I've got a lot of problems with the motor.. the wiring is just a mess, it looks like it sat outside in the rain for a while. The carbs leak but they look easy enough to rebuild without a kit.

 

[Willyclay]

Re: 58' Merc 78A compression question and no-spark?

I was hoping some of the forum "experts" would post some suggestions for you. My Mercury manual went with the Mark 75A, so I don't have any reference material. Maybe you can find one in a public library and photocopy what you need. My motor had been worked on by a young fellow who did NOT have a manual and was pretty screwed up. I replaced the plugs and plug wires plus rebuilt the fuel pump and carbs. The biggest challenge I had was making a degree wheel to set the timing. The carbs are pretty straight forward and should not be a problem for you to clean and rebuild. The link and sync will be a different story without the manual. Good lucK!

 

[Laddies]

Re: 58' Merck 78A compression question and no-spark?

Re: 58' Merck 78A compression question and no-spark?

Errors, the reason your not getting much advise is exactly what Willy told you. To repairer a direct reversing engine it impossible with of a manual I have repaired them for 40 years and when ever one comes around and needs ign. repair the first thing you need is a manual, the manuals expensive, the parts are expensive, you will need a couple special tools for the job, without all of these things in place your wasting your time. I know this is not what you want to hear but it's the truth. I love those old engines but the were very difficult to repair, to give you a idea the flat rate for a tuneup was 6 hours and there were no short cuts.

 

[ErrorS]

Re: 58' Merck 78A compression question and no-spark?

Re: 58' Merck 78A compression question and no-spark?

Alright, maybe I'll spring for a manual..
I wish I could find someone that would be willing to buy it, at least for enough it might cost me to pick up another (lesser) motor but I doubt anyone will be interested with it in the condition it is in.

I did find that wiring diagram.. but I definitely see what you guys mean, the distributor is odd, the two starter solenoids are odd (i've done something where it wont spin in reverse now).. the motor is older than anything I've ever worked on, things like mechanical advance, points, etc are all brand new to me.

you all think I'd have much luck in it's current condition putting it on eBay? It's been spraypainted black, but overall it's not in horrible condition, the lower unit looks really good (free of chips/cracks, prop looks good, etc). I'd imagine the compression would make it worth it for someone to buy but I'm not sure.