TOPIC: Evinrude Lark 40 - Low Compression

Hello Braintrust- I've had the 1957 Erie with a 1960 Evinrude Lark 40 for 6-9 months,... and yet to put it in the water. I did change the plugs and impeller already, so I'm good there. In the water tank run it wouldn't idle well, but I could bump it in gear and run it up fine. I took it to a local shade-tree guy with a good reputation and skillset.

He said one cylinder is down at 60psi, rather than the nominal 120, but even operable 80 psi. When asked about a price to put it right he said it would be $800-$1,000. Sounding a bit steep to me, I think I want to try to attack pulling the head and cylinder to change out the piston(s) as required. I'm a previous life I was a fighter jet mechanic, and also dirt bike guy. Any reason I shouldn't take on the task of the piston/ring change? Parts look like they are available on;
Any advice eagerly requested. marineengine.com

Thx in advance - Tim

It's possible you have a blown head gasket. But with 60 psi, more likely is a scored cylinder or ring problems. Usually you'll get a very low value or Zero psi with a blown head gasket.

I'd recommend pulling the cylinder head for inspection. You can pull the head without removing the entire block (powerhead).

If you see anything wrong with the sealing surfaces of the head, you may have found the problem. The head can be resurfaced flat again.

If there's a problem with the block sealing surface, it's usually terminal. Not uncommon to see the cast aluminum block separate from the steel cylinder sleeve. The aluminum overlaps the top of the sleeve and if it's cracked or chunks missing, the block's a scrapper.

If the cylinder is scored or block bad, it's usually cheaper to just find either a good used replacement block or complete powerhead, either from Craigslist or eBay.

A scored cylinder requires an oversize piston, I see they're available at Marineengine.com but I don't see any rings. Not sure on the availability of those. Plus you'll have machine shop charges to bore-out the cylinder. So figure you'll have close to $200 in piston, rings, bore job by itself.

Here's a freshwater-used 40hp powerhead with almost 100 psi compression in both cylinders for $164 shipped on eBay, you couldn't rebuild one for that price:



Anyway, pull the head and see what you've got, then we can help you proceed from there. Hopefully your motor is freshwater-run, or you're in for a fight if a Salty Dog. Lots of heat on the head bolts, from the side of the head, will be required to release the bolts.

Either a propane or MAPP torch, the MAPP burns hotter and helps get the bolts busted loose more effectively.

A caveat on replacement powerheads, there are (2) styles of 40hp blocks, the one you see on eBay for the "Super Quiet" motors, with a rounded powerhead base, and another style which uses a more rectangular powerhead gasket.

Early crankshafts were smaller in diameter and had flywheel sized to match, later powerheads had a larger-diameter crankshaft at the flywheel area, with larger I.D. flywheel.

That eBay powerhead isn't a '59, BTW, it's a 1960 same as yours. It does have the early-style, small-diameter crankshaft and rounded base, which is what your 40hp should have as well.

HTH, let us know what you find........ed

Time to work on it is my bigger issue. Scouring CL I'm finding it is cheaper to buy a whole boat with "running" engine, than to find a engine by itself. So if I go the route of a newer engine I'm seeing for instance a 50 hp Yamaha 4-stroke with controls for $1,500. Question is though since my old MFG was rated up to 40hp, whats the worst that could happen. Are they rated that way because of overall weight, transom strength, or what?