Motor Heads

J/24 class rules require a 14 kg (30.9 lbs) minimum outboard (Section 3.8.8). I have an older 80s vintage Mercury 4 hp, 2 cylinder, long shaft (aka "gnat") that weighs 19 kg (42 lbs). It usually runs smooth and is quiet. However, lately I've had to always play the choke and throttle to simply keep it running.

It's a real pain to do all that juggling while trying to steer. I've tried fixing the carburator several times but I can't get the gas entering the bowl to shut-off. Eventually the Merc carb overflows with fuel and chokes itself. The needle valve shows no signs of wear but is probably hardened from age so it no longer seats properly to shut-off the fuel. I'll have to have my Dad give me a hand with when I get around to ordering replacement parts.

Since the Mercury gnat is "too heavy," I've been looking for a Nissan or Tohatsu 3.5 hp long shaft outboard for the past couple of months. These 2 stroke 3.5 hp motors just meet the minimum weight, while the new 4 stroke motors are heavier given the same power rating.

There doesn't seem to be many used long shaft engines out there now that 2 strokes are banned and those that I did run across were on the coasts. There are still a fair number of short shafts around. I finally ran across a Nissan long shaft on eBay in which the seller couldn't get it to run. I figured worst cast I was buying a long shaft conversion which was worth the $142 I paid for the engine.

Winter is a great time to work on projects that don't require a large heated garage or involve epoxy and paint. My oldest son helped me out on this project. The spark was good and there seemed to compression for such a small cylinder so our first task was to look into the carburator. Cleaning out a few fine bits of grit and reassemblying this very simple carb, my son helped put reassemble everything. I figured we had it licked.

Nearly pulling my arm out of joint from yanking so many times and my son laughing deliriously from the sideline, I gave up. If it hadn't been for few plumes of smoke and a couple "pops," I would have given up sooner. It was funny looking back and I'm glad my son helps me laugh at myself.

After buying a compression tester, my son and I were back at it. The Nissan only had 60 lbs of pressure which explained why the motor would try to fire occasionally but never start. So what was causing the lack of compression?


With my guidance, my son took off the four bolts to remove the head. The head gasket appeared to be leaking from corrosion "growing" between the water jacket in the head and the gasket probably due to not flushing out the salt water from the engine using a pail of fresh water like the manual explains.

As we were cleaning out the corrosion with a putty knife, all of a sudden a corner of the head popped off in my hand. The head was cracked under all that corrosion. The rest of the block looked fine. There were no scored walls on the piston and the rings were likely making a good seal provided the head could be replaced.

Doug Russell Marine has great prices on replacement outboard parts. Tohatsu rebadges their 3.5 hp outboards for Nissan, Mercury, and Johnson! The parts from a Johnson J3RTLSSR appeared to be the correct parts and are lower cost than Mercury. It was going to run me $65 for a new head and gasket. Not knowing if the rings were good or not, we may as well replace them too.

I ended up buying a Mercury 3.3 hp donor for $100 in order to simply replace the powerhead. The compression on this Mercury 3.3 was 120 lbs. Much better!

On a gorgeous 45° 31-Jan day, my son and I removed the mere six bolts, replaced the powerheads, and reassembled. Ready to give the engine a test again, it was now my son's turn to give to give the rope a yank. It was my turn to laugh. Two pulls later and the engine started right up! This Nissan 3.5 is certainly noisier than my old Mercury gnat, but my son and I think of it as our own little Harley, shaking handle and all! We both had huge smiles!

Once the weather gets warmer, my son and I will have to repaint the engine to give it a brand new look for Maybe (and Liz).