Yamaha 15F 2-stroke outboard hard to start

[Munoz]

I have an older, very worn Yamaha 15F that has always been hard to start when cold (when warm it normally starts first pull). Sometimes when it is cold it will start if I give it a hard enough pull, bit often I have to squirt a bit of carb-cleaner into the carb to get it to start, which isn't ideal.

I've cleaned the carb and the motor runs fine once it's going (although the carb is very worn and I suspect it's running rich). I've also compression tested it and the compression is low (85 and 90 psi).


Any suggestions on making it easier to start? Rebuild or replace the carb? Or maybe it needs new rings?

Cheers

[Scubaseas]

Spray WD 40 or an oil into it. Carb cleaner washes off the oil you need to lubricate the rings. You might have a valve problem. This is a four stroke yes? If your rings are worn that bad It would probably use oil. does it? Usually smokes a bit too.

What model and year? A lot of the Yamahas only sell the entire head and some of these are no longer available.

A cylinder leak down test would tell you if the problem is rings or valves

If it's a two stroke it could be rings or pistons (worn lands). F15 is a 4 stroke

There should be a tag on the mount with the model, year and serial number.

[SailingHarmonie]

If the seats and needles in the carb are worn, they should be renewed no matter what else is going on. On a 2-stroke the most common thing I do is clean the low speed jets in the carb, so it might not be a bad idea to have a spare you can bolt on in a hurry, so maybe you want a new carb AND rebuild the old one?

I’d bring the carb up to snuff, see if it performs well, because that is relatively easy and cheap before ripping open the power head.

Have you tried a squirt of oil in the cylinder and seeing if that improves compression? That’s a pretty standard test for poorly sealing rings…

[Munoz]

Originally Posted by Scubaseas

Spray WD 40 or an oil into it. Carb cleaner washes off the oil you need to lubricate the rings. You might have a valve problem. This is a four stroke yes? If your rings are worn that bad It would probably use oil. does it? Usually smokes a bit too.

What model and year? A lot of the Yamahas only sell the entire head and some of these are no longer available.

A cylinder leak down test would tell you if the problem is rings or valves

If it's a two stroke it could be rings or pistons (worn lands). F15 is a 4 stroke

There should be a tag on the mount with the model, year and serial number.

The F15 is indeed a 4 stroke, but the motor in question is a 15F (two stroke) sometimes known as the "leisure" model to differentiate it from the 15hp Enduro (e15d?)
Model number is 682, but parts starting with 63 seem to fit as well.

Thanks for the tip RE WD40

[Munoz]

Originally Posted by SailingHarmonie

If the seats and needles in the carb are worn, they should be renewed no matter what else is going on. On a 2-stroke the most common thing I do is clean the low speed jets in the carb, so it might not be a bad idea to have a spare you can bolt on in a hurry, so maybe you want a new carb AND rebuild the old one?

I’d bring the carb up to snuff, see if it performs well, because that is relatively easy and cheap before ripping open the power head.

Have you tried a squirt of oil in the cylinder and seeing if that improves compression? That’s a pretty standard test for poorly sealing rings…

Thanks for the tips,

I will start by replacing the carb and see how that goes
I had read that these motors tend to run rich when the carb jet gets worn,
But I don't recall finding a replaceable main jet when I cleaned the carb, and the carb rebuild kits don't seem to contain one either, just a low speed/idle jet that you can screw in or out to adjust the mixture, is this the one you are saying often caused problems?

[Scubaseas]

Did you buy the motor in the States or Caribbean/someplace else? Just curious. Not seeing this model on Boats.net parts diagrams but they only list North American market Yamahas.

No idea how accurate your compression gauge is but 85psi is lower than what one would like to see for good cold starting. 110 to 130psi would start a lot better. The two cylinders should also be within about 10% of each other and checked with the throttle wide open. But it's also possible your gauge is not accurate. We had 6 compression testers in my past life and they rarely all agreed.

The off idle jets are machined into the venturi. The main jet maybe also be non-replaceable and done by size of hole drilled into the casting. Look for small slits in the bore just above the throttle plate for the off idle fuel and air and idle air mix circuits. The idle fuel is regulated by the needle screw. Also make sure to clean out the air idle jet which will be a hole on the face of the carb or further up the venturi from the throttle plate. No question a properly working carburetor will make starting easier at any temperature. Conversely a perfectly working carburetor won't help a low compression hard start issue. But sure, try cleaning and or replacing it and see if it helps.

Before you spring for a new carb it may be worthwhile to look carefully down the spark plug hole and see if there are any visible scratch/scoring marks on the bores. If there are you may want to reconsider an carb work.

You might also want to check the spark while you're at it. You need three things to make a motor work. Fuel, spark and mechanical. Might be worthwhile to pop the flywheel off and check the clearance on the magneto to pick-up and make sure there is no rust or crud inside the flywheel stuck to the magnet

If you do indeed have low compression you might try spraying seafoam into the cylinders and let it sit overnight. Put the engine with the plugs facing up. Take the plugs out, put a piston at the top of the stroke. Add seafoam and let soak for a couple of hours or overnight. Flip the engine over to drain. Repeat for the other cylinder. Then flip the motor over, hand crank over the flywheel to drain out the seafoam. Then put the motor upright and with the plugs out crank over with the pull cord a few times. Spray some WD or oil into the cylinders, put plugs back in. Start and run (there will be a large amount of smoke when you do this) the motor for at least 20 minutes at various speeds. Accelerating hard from idle to WOT will be helpful as will running at speeds about 5 minutes. Like 2000 rpm for 5 minutes. 3000 for 5 minutes, wot for a minute or two, idle for a minute or two, etc. The above would help if your rings are stuck with carbon or sludge.