cooling on mark 78

[gwmark78]

I have a 1958 Mark 78 Merc we have had in the family since new. It was not running when I started this project I have got it running now, but have some concerns with lack of water coming out the exaust, since this is the only place I can find where the water would exit. There is no small hole on the side as on the newer models, so I assume all water taken in should come out the exaust. I have taken off the cowl while running and put my hand on the sides of the powerhead and it feels cool. The waterpump appears flexable and ok. I did replace one of the valves which was split and cracked. Should this motor shoot out a good flow of water out of the exaust?

 

[emckelvy]

Re: cooling on mark 78

You probably won't get a lot out of there, except at speed. Most of the water is going to discharge down the center section.<br /><br />If you've felt the exhaust cover and it's cool or just barely warm at best, you're getting sufficient cooling to the block.<br /><br />If I recall, these old blocks had a screw-in plug at the top of the block in the back, you could always remove that, install a fitting and hose out to your own telltale. Just use an exit fitting off a newer motor, or devise your own.<br /><br />BTW if you're running it out of the water on a flusher, you'll probably never see any water come out of the exhaust relief. You should get a stream of water out the exhaust area in the lower unit, and you'll know that it's circulating thru the motor by the fact that it'll be warmer than the water going in.<br /><br />HTH and have fun with the Dockbuster!...........ed

 

[gwmark78]

Re: cooling on mark 78

Thanks for the reply Ed, but I have a couple more questions/concerns hopefully you could answer. After tearing around the lake for about 20 minutes, I noticed a little bit of steam coming off the casting where the exaust is. now I know with the exaust being there it should be warm, but I do not remember that in the past that it had steam (not alot) rolling off the area just above the waterline. Also, when I'm reverse dock busting, I did have quite a bit more water coming out the exaust. By the way, in my teenage years, we did bust a few docks and shake up a few people!

 

[emckelvy]

Re: cooling on mark 78

Hard to say, probably depends on the lake temperature, too, if it's warm this time of year the motor will of course run a bit hotter. No 'stats on this one Fer Sur!<br /><br />I'd still go by the old 'feel' method, if the exhaust plate isn't getting real warm you've got more than enough water going thru the motor. The exhaust will heat that plate up astronomically if there's not a lot of water flowing thru it.<br /><br />If you can lay your hand on the motor for a few seconds, and it feels quite hot but not burning your hand off, that's about 140 (which this motor should never see). Comfortable to lay hand on for extended periods, probably 120 or less. Can't even touch for a second, 180 or better and you'll be seeing lots of steam at those temps!<br /><br />Or, go the Scientific route and pick up a cheap infrared thermometer from Rad. Shack to get a precise read on engine temps.<br /><br />On the reverse thing, you've got more backpressure against the exhaust when going in reverse; this would force more exhaust, and with it more water, out the exhaust relief in the center section.<br /><br />One thing you may want to check if you don't have a Warm Fuzzy about how much water circulation you've got, is to remove the cyl block water jacket cover (back of the block, over the spark plug holes). Check for buildup of silt/trash; sometimes it's halfway up the block. You can even cut your own gasket if you can't find one anywhere, just pick up some bulk gasket paper from the auto parts store. If the bolts don't want to cooperate, this may not be a great idea unless you feel there's really a problem. You'd have to pull the powerhead to get any reluctant lower bolts or if one breaks.<br /><br />I've got a decent old MK 78 laying in the garage, waiting for a full restoration when I retire. Years ago I picked up an old 700E Dockbuster for $25. Just had to clean the points and fix a busted pump housing with JB Weld! (things were so much simpler then!!!). I ran like a jewel, and I drilled ventilation holes in the prop so exhaust would spin up the motor better for skiing. It was a real Screamer on a 13-1/2" Sea King runabout and we slalomed all summer with it! Definietly a low-budget operation!!!!!<br /><br />These motors look great on a classic 'glass or wood boat, that's what I figure I'll do with mine after she's restored to her former glory.<br /><br />Good Luck with yours and enjoy........ed

 

[gwmark78]

Re: cooling on mark 78

Thanks again Ed for your quick reply. This motor is 0n the original Alumacraft hull which was purchased new from the dealer. I can still take most newer I/o boats out of the hole. It tops out about 45mph. My teenage kids cannot seem to appreciate this classic and would rather take our newerI/O boat out but I just love it! Thanks <br />Gary

 

[gwmark78]

Re: cooling on mark 78

Hi Ed,<br />I'm back again with one more question. I bought a vintage Airguide tach to install on this motor which I thought should be easy to hook up-now I'm not quite so sure. The Airguide has the jumpers which I believe are already connected for an alternator system and it has one black wire and one yellow coming out the back. Would I be correct in assuming the black to ground and yellow to the negative side of one coil? I do not want to screw anything up here so any advice?<br />Thanks, Gary

 

[emckelvy]

Re: cooling on mark 78

Well, if you think it's made for an alternator motor then you'd connect the signal lead (the yellow wire no doubt) to one of the rectifier leads coming off the stator (yellow wires in a green or black sheath probably), and the other one would have to be grounded.<br /><br />If it's set up for an automotive coil-type ign, it'd be like you say, one to neg on coil and one to ground.<br /><br />BTW a car tach hooked up to one coil and set on 6 cylinders will also work.<br /><br />My buddy used a V8 car tach on his old 90 HP Selectric V4 Evinrude and it worked great. I had a '64 Merc 1000 on a '69 14' Fiberform, he had a '76 Fiberform (same hull on the bottom but fancier top half), they were very close in speed but the 100 Merc would pull him on the top end. He siezed it up one time racing down the lake with it tilted out too far. Unlike the old Mercs, it lived for many years after that. Tough old motors, they sure had a lot stronger rings than the Mercs but I still like the old Towers of Power better.<br /><br />I still have the old 700E kicking around in pieces (sold the boat but several years later the motor ended up back with me!), maybe when I retire I'll rebuild it too. Got plenty of projects to keep busy, that's fer sur!!!<br /><br />Good luck with the tach and happy boating.........ed