Loss of spark on two cylinders (mercury thunderbolt 50hp )

JasonMay

Cadet
Joined
Jul 19, 2017
Messages
12
Hello. I recently purchased a boat (from someone I know) with a 77 mercury thunderbolt 50hp (serial number 4360108). The motor had not been ran for a few years, but it did run great.

The motor starts and idles, but will kill if I don't have the high speed idler lever up. It will run in reverse and rev up. It will run in forward, but it does not rev up at all. I initially thought it was a fuel problem or carb related and looked up basic troubleshooting. Being very new to boating and mechanics, it took me a couple days to complete. It wasn't until further research I checked for spark.

I have spark on the bottom two cylinders, but not the top two. I tried disconnecting the mercury tilt switch, but no change.

Again, I'm new to mechanics and would like a "for dummies" type of reply, if possible.

Thanks for any basic troubleshooting help you can offer.
 

Mercurylips

Petty Officer 2nd Class
Joined
Sep 23, 2015
Messages
189
Jason- I can give you tons of advice and the first is, get a good compression test to see if your motor is economically feasible to recondition. Then get a service Manual (preferably oem) I have the same motor and started by changing out the obvious things. That series of Mercury motors have terrible wiring ( insulation on the wires crumbles off) so a new wiring harness and new stator were in order. The carburetor problems and the stator were my thorns but yours sounds a little different. Perhaps a switch box is bad or two coils went out at the same time.(unlikely). I am not good with electrical tests but that is where you need help with your problem. I am currently waiting to lake test my lower unit. It was getting water in it and I think I found where that was coming from.
 

JasonMay

Cadet
Joined
Jul 19, 2017
Messages
12
Thank you. I'll try looking up those things. I'm sure I'll need some tools I don't have to get this done.
If compression is low, is it a lost cause? I think I read that you want them to be over 100.
 

Mercurylips

Petty Officer 2nd Class
Joined
Sep 23, 2015
Messages
189
That tester should be an "autozone" borrow. The Harbor Freight testers seem to be inaccurate. As far as a decision to scrap your motor......... That will be painful depending on how much time and money you have invested. The only reason I put up with this old motor is due to the fact that I don't have that much time left on the planet and I don't want to saddle my wife with $15,000 problem to sell if I pass. BTW, mine tested out to 124 and 125 cold on all 4. It then ran like a new one after finding the strangest problem yet and fixing that.( main fuel nozzle vibrated loose and ended up in the float bowl) I can only surmise that the road to the lake is too rough and in need of repair.(not to mention that I may not have tightened things up when I rebuilt the carbs.)
 
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