Here's a new one- starts easily with MMO in the cylinders, won't start otherwise.

[cdhickey]

As I mentioned in a previous post, I've been tinkering with an old spare parts engine in my garage as a way of dealing with boat withdrawal during quarantine. I don't have a starter on this one so I'm using a hand crank to start. At first I wasn't having much luck, but did manage to get it chugging a little at times, and finally managed to get it start after much cranking. I shut it down to adjust a few things, but couldn't get it started again after that. No big surprise, as it's a cobbled together engine of unknown pedigree. I was happy that it started even once!

So here's where things got strange- while trying to crank it again the next day, I wasn't having any luck getting it started and it was getting harder to crank. I assume that this was because hand cranking doesn't generate enough oil pressure to properly lubricate things. I added a little MMO to each cylinder to lube things up, and lo and behold the engine roared to life on the next crank. It was smoky, but it was running! I shut it down again after a minute or so. Tried to restart, but once again no luck. Added a bit more MMO, and she fired up easily.

I think this is telling me that I've got an issue on the fuel side, but I'm not sure exactly what. I had rebuilt the carb before installing it and I confirmed that the choke is closing fully. The fuel I was using was a bit old, but I replaced it with new fuel to no effect. After cranking for a bit the plugs are wet and smell like gas. I'm running from an outboard tank with good venting. I'm running out of ideas on what the issue could be on the fuel side.

On the electrical side, I've got spark but it's a bit weak and yellow. I'm assuming that if the spark is enough to run on MMO it's enough to run on gas, but I'm really just guessing.

Any thoughts on where to go with my troubleshooting?

[ronstory]

A nice fun project in the "shelter at home" realm.

Have you done a compression check? MMO squirted in to the cylinder will increase compression, but I'm not positive how much it will help with ignition.

Have you checked the voltage at the coil?

YMMV

[romantic comedy]

I have seen the oil in the cylinders trick. It seals the rings and you get better compression to get it started.

Time for a compression check.

[cdhickey]

I hadn't really thought about the compression angle in adding the oil to the cylinder. I haven't been able to d oa proper compression test without a starter installed, but in hand cranking it passes the thumb test.

[ronstory]

Thumb test is fine for finding the compression stroke, but it's really just about moving air out the cylinder vs. creating the needed compression for combustion.

Without a starter, getting a comparable compression number to the recommendations from MMI is really not very practical.

However, putting an ounce or so in each cylinder and letting it sit for a few days really won't hurt anything, and will likely help free-up any gunk on the rings. It won't help with a glazed cylinder, but A4 improvements can be a "game of inches".

Good luck!