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7.5 S# 30221572 - need advice on changing the upper lower block gaskets

N

Nubby86

New member
Working on grandpa's low hour motor thats been sitting for 20 years in a heated garage and getting in going for the next generation.

Compression check was good both cyl strong.
Timing and timing light shows full adv.

I have replaced with OEM parts both ignition coils, plugs, wires, points, point insulators, ignition wires, metering 0 ohms now, Rebuilt carb and fuel pump OEM, fuel lines on motor new gas, gas line, bulb and plastic tank. Dialectric greas on all electrical parts. Motor starts second pull idles good, throttle response very snappy, motor is smooth up to about half throttle then falls flat will not spin up RPM.

I sprayed starter fluid around base of block on port side and got the motor to stumble. I need some help on understanding what all I will get into to dismantle the motor and separate those gaskets and replace them. When I'm that deep what else should I be changing out to keep the motor strong for another 20 years? I just purchased the online manual as my manual at home does not get that deep into disassembly.
 
Does spark jump a gap of 5/16" on both leads ?----There is no " head gasket " on it.-----There is no gasket between block and crankcase cover.-----Upper and lower crankshaft seals will need to be replaced.---Water pump impeller needs to be replaced.----Washer at the top of water tube might need to be replaced.----Replace lower crankshaft bearing.
 
I don't have my head fully into all the parts yet as I just discovered what I believe to be a bad gasket at the base of motor on the port side. I assume it is one of these parts I highlighted, 100% I could be wrong, I'm just learning my way through it now.

I am running surface gap plugs per the manual, I think this motor is unique for the surface gap because of the ignition as I understand it from you tube university. I purchased a hand tack I can mount to the throttle handle and a spark gap simulator to see if I can jump the wide gap. I have not confirmed that yet. Once I shot starter fluid around the base and heard the motor stumble I have thinking through the next step.




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Also failed to mention I am running the 50/1 red can of premix fuel and after I run the motor for 15-20 minutes I am getting oil residue dripping from the prop and down the side of the lower unit. it does not smell like lower unit fluid as I have already replaced the lower unit fluid.
 
racerone knows these a lot better than I do, listen to him first...

But those two things you mentioned might be one problem, not two. On a two stroke the crankcase is the intake side. Any air that gets in past a seal or a joint leans the whole motor out, and leaning out is exactly what falling flat at half throttle looks like. A lower crank seal does both jobs at once. It lets air in, and it lets crankcase oil out down into the driveshaft housing, which is where a black drip off the back of the prop comes from...

You said it doesn't smell like gear lube. That's the part that makes me think it's coming from up top and not out of the gearcase...

Before you split anything on it I'd pressure test it. Cap the intake and the exhaust, put a few psi into the crankcase and see whether it holds. Then spray soapy water at both crank seals and at the case joint and watch for bubbles. Costs you nothing and it tells you straight out whether tearing it down is even worth doing...

The starter fluid trick is a real find, but it only tells you air is getting in somewhere near where you sprayed. It doesn't tell you which joint. I did the same thing on an old one and swore up and down it was the base gasket. Turned out to be the seal up behind the flywheel the whole time, and I had the thing half apart before I found it...

If it does come apart, do both crank seals together. Do the water pump at the same time while the lower unit is already off, an impeller that sat 20 years is hard and glazed even if it looks okay. And I'd want to look at the reeds while it's open, they should sit flat on the plate with no daylight under them...

Does that drip start before the motor is fully warm, or only after it's been running a while? And is it thin and black, or thick and dark...

Thanks
 
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