I have a new-2-me '79 Evinrude 15 hp. Checking it out on muffs, she was tempermental in
starting at first, traced that to the carb roller not contacting the timing plate cam early enough,
so throttle valve wasn't open unless it was in gear (or you twisted VERY hard on the handgrip-
must have been what the PO was doing when we started it at his place. Adjusted that, now it
starts on 1st or 2nd pull. Doesn't idle great yet, however. Pull start compression readings of 80 lbs
on both cylinders +- 2 psi, not great, but probably OK.
Seemed to run pretty hot, and I had to put a lot of pressure on the muffs to get the pee-hole
going and couldn't see much water coming out the upper leg ports. So I pulled the head and
Bypass cover to check if there were signs of severe overheat damage. Both bores look excellent,
no scoring, diameter is 2.188" right on the money, can still see some crosshatching on the bores
here and there, can't see any scoring on the pistons through the intake ports, very little
carbon on pistons or head. Did notice the head had been hot enough to melt the plastic coil
mounting bosses a little...
But, question 1) How much piston/bore clearance is acceptable? The Service Manual I bought
specs 0.0025-0.0038". I'd guess there's at least 0.005" clearance, maybe a little more.
Is this acceptable? I don't want to replace pistons if not needed.
On the cooling side, the water passages in both lower leg and powerhead are very clean, no
sign of crud or debris, just a little smut on the passage walls. I'm going through it replacing
all the maintenance items- water pump, thermostat, etc to try and solve the heating problem.
But I noticed what I'd call crevice corrosion on the driveshaft at the various seal contact
positions. Pretty bad on the upper waterpump seal, a ring of pits with the largest probably
0.025" long by 0.015" wide, by 0.010" deep. Most of them aren't that big, maybe 0.005"
dia x 0.005" deep. Lesser amounts on the lower waterpump seal, and very light pitting on the
gearcase seal lands.
So, question 2) Is this pitting a big problem, and is there a way to fix it? Seems to me those big
pits would tear up seals pretty fast, but a new driveshaft is about $212, and buying a used one
of e-bay would be a pig-in-a-poke as to whether it would be any better than this one.
Continuing with analysis, the pump impeller was not worn at all, but had a definite set to the
blades. And the lower plate had pretty good wear grooves. I'm guessing that's why it was
overheating. I am going to put low hose pressure on the water tube up into the powerhead
tomorrow, just to try and check the passages are clear.
I also noticed the fuel pump has a crack- if I pull the inlet hose away from the block, there's
a crack/leak right at the base of the inlet stub on the top of the pump. So got a new fuel
pump, and I'll replace the two fuel hoses while I'm at it. That may be part of the rough idle
problem, don't have experience to tell. After I get the powerhead gasket set, thermostat,
and lower gearcase driveshaft seals, I'll button her back up and see how she runs. Even if
she idles a little rough, I'll take her out to the lake for a real test- everything up till now has
been on muffs.
Thanks in advance for all help,
starting at first, traced that to the carb roller not contacting the timing plate cam early enough,
so throttle valve wasn't open unless it was in gear (or you twisted VERY hard on the handgrip-
must have been what the PO was doing when we started it at his place. Adjusted that, now it
starts on 1st or 2nd pull. Doesn't idle great yet, however. Pull start compression readings of 80 lbs
on both cylinders +- 2 psi, not great, but probably OK.
Seemed to run pretty hot, and I had to put a lot of pressure on the muffs to get the pee-hole
going and couldn't see much water coming out the upper leg ports. So I pulled the head and
Bypass cover to check if there were signs of severe overheat damage. Both bores look excellent,
no scoring, diameter is 2.188" right on the money, can still see some crosshatching on the bores
here and there, can't see any scoring on the pistons through the intake ports, very little
carbon on pistons or head. Did notice the head had been hot enough to melt the plastic coil
mounting bosses a little...
But, question 1) How much piston/bore clearance is acceptable? The Service Manual I bought
specs 0.0025-0.0038". I'd guess there's at least 0.005" clearance, maybe a little more.
Is this acceptable? I don't want to replace pistons if not needed.
On the cooling side, the water passages in both lower leg and powerhead are very clean, no
sign of crud or debris, just a little smut on the passage walls. I'm going through it replacing
all the maintenance items- water pump, thermostat, etc to try and solve the heating problem.
But I noticed what I'd call crevice corrosion on the driveshaft at the various seal contact
positions. Pretty bad on the upper waterpump seal, a ring of pits with the largest probably
0.025" long by 0.015" wide, by 0.010" deep. Most of them aren't that big, maybe 0.005"
dia x 0.005" deep. Lesser amounts on the lower waterpump seal, and very light pitting on the
gearcase seal lands.
So, question 2) Is this pitting a big problem, and is there a way to fix it? Seems to me those big
pits would tear up seals pretty fast, but a new driveshaft is about $212, and buying a used one
of e-bay would be a pig-in-a-poke as to whether it would be any better than this one.
Continuing with analysis, the pump impeller was not worn at all, but had a definite set to the
blades. And the lower plate had pretty good wear grooves. I'm guessing that's why it was
overheating. I am going to put low hose pressure on the water tube up into the powerhead
tomorrow, just to try and check the passages are clear.
I also noticed the fuel pump has a crack- if I pull the inlet hose away from the block, there's
a crack/leak right at the base of the inlet stub on the top of the pump. So got a new fuel
pump, and I'll replace the two fuel hoses while I'm at it. That may be part of the rough idle
problem, don't have experience to tell. After I get the powerhead gasket set, thermostat,
and lower gearcase driveshaft seals, I'll button her back up and see how she runs. Even if
she idles a little rough, I'll take her out to the lake for a real test- everything up till now has
been on muffs.
Thanks in advance for all help,