Yamaha outboard F2.5 hp issues

[scud]

My Yamaha F2.5 has definitely some issues. It has been my spare lawnmower for years after I bought it as a 2nd hand and I never really put it to test.

Now has become time for it to play first line but it didn't deliver!

Starts immediately but without choke (if I pull the choke dies)

Doesn't revv up beyond a minimum of RPMs (dies if I shift in gear)

If I close the air vent of the tank or the fuel tap then it becomes better and revvs up but then dies for fuel starvation.

A lot of oily black gunk is expelled from the exhaust
The spark plug (new) is covered by a sleek layer of black oily crud.

In my mind everything pointed to a rich mixture but I took apart the carburetor, blowed all the jets, cleaned and with no avail.

Tested the cylinder compression (cold or at low temp) and it resulted about 80psi.

I moved on on disassembling the carburetor again and I found that the screw that regulates (by the book) the mixture composiition at low revvs had a damaged micro O-ring that fell apart.

COuld be that the reason of my predicament? hard for me to believe and even harder to find a replacement here in order to test the theory.

What about a leaking head seal? but I can't see water in the oil at all.

What about worn out piston rings? but 80psi cold is not bad (vs. 100 at running temperature)

What about valves? but as I said it is easy to fire it up and that to me doesn't squares up with the theory of damaged valves.

suggestions very appreciated! Thanks folks

[tkeithlu]

"In my mind everything pointed to a rich mixture"


Yep, and it could be the mixture screw. Your gas is OK and you are getting enough of it. Your air appears to be OK. You have a spark. 100 PSI isn't great, but it will do. The mixture is so rich that it starts cold without the choke and is too rich to run as it warms up. You have not put a wrong jet in the carb. If the head gasket was leaking you would see puffs out the side, and the compression would be worse.

The o-ring you need may be too small for the usual cheap set such as this one

https://www.amazon.com/Assortment-In...2&sr=8-54&th=1

but it's still worth having your own set for most situations. You should be able to buy a carb rebuild kit, or get the o-ring from a Yamaha dealer or perhaps an ordinary auto supply store. Start there. Good luck with it.

[scud]

Actually I am measuring 80 PSI cold while theoretically after warm up I am supposed to measure 100psi on a new engine. But I read cases of 60PSI on outboards that required 100 and still run well.

Unfortunately hte O-ring is 2.20mm inner diameter and .40mm thick. Measues taken with a hand caliper gauge on what is left of the broken O-ring. So much for micro-metric precision!

I'm searching and annoying all the local workshops and dealers but seems the mold of the old mechanics has been lost forever and the few remained have retired or died.

The dealers have no apparent interest at selling a small O-ring (selling price about 5 bucks) that they ought to order to the mothercompany.

I'll keep on searching and rummaging. Whole carburetor is sold at 20 bucks on ALiexpress (vs 170 bucks of Yamaha) and I may go for it instead of persevering in my quest.

I just wanted someone to look at my same leads and come up with his own angle (matching mine or suggesting alternative scenarios).

Thanks for this one.

[tkeithlu]

"Whole carburetor is sold at 20 bucks on ALiexpress"

I was tempted to suggest that route, but I didn't know the availability for that engine. I'll bet that is the core reason you are having trouble repairing your carb- replacing it is cheaper than the cost of opening it up for repair. Go for it.

[Madehn]

Replace the carb-- the bit about the mix being too rich is dead on. Not sure where you are but there are any number of sites that can order the o-rings - just google Yamaha outboard parts and they will come up- going the whole carb route bay be easier and simpler for the extra $15 bucks it is going to cost you plus you will then have a spare float and needle valve from the old carb.

[MultiCountryDon]

Or find an o-ring with same cross section but larger diam.

Make a 45 degree cut.
Make a matching cut that will reduce the diam to the o-ring needing replacement.
Crazy glue the 2 ends together.
Install.

Works quite well in hydralic presses at 2500 psi or more.

Don

[MultiCountryDon]

Or get an o-ring with the same cross section but larger diam.

Make a cut at 45 degrees.
Make a matching 45 degree cut to result in the same diam. as the o-ring being replaces.
Crazy glue the 2 ends together.
Install

Works on hydralic presses with 2500 psi or more.

Don

[Jammer]

Originally Posted by scud

In my mind everything pointed to a rich mixture but I took apart the carburetor, blowed all the jets, cleaned and with no avail.

I have an F8. Typically the Yamaha carbs have a very small bypass air port that, if clogged, will result in an overrich mixture as you describe.


You have to remove all the cover plates and screws, and blow out or rod out (with a tiny wire) the little passages, and maybe look for the bypass air passage specifically to fix it. It took me 3 tries and I like to think that I'm good at this sort of thing.

Tested the cylinder compression (cold or at low temp) and it resulted about 80psi.

[...]
What about a leaking head seal? but I can't see water in the oil at all.

What about worn out piston rings? but 80psi cold is not bad (vs. 100 at running temperature)

What about valves? but as I said it is easy to fire it up and that to me doesn't squares up with the theory of damaged valves.

It is rare that 2.5 HP motors get enough hours for these sorts of things to be a problem. Nobody runs a 2.5 HP all day, every day, the way some resort/rental operations will do with, say, a 9.9.


<<<o-rings>>>


Mcmaster.com has every o-ring imaginable and will ship to most destinations worldwide. You do have to know what size you want, though there are online guides to help with that, and you can measure what you have with a caliper and get close enough.