Yamaha 50 only 4200 RPMs at WOT less if Loaded

[Malone111]

I have a 92 50HP 2 Cycle 3 Cylinder Yamaha on a 16ft bass boat that will only get 4200 RPMs at WOT with 1 in the boat. With 2 and gear runs at 3800 RPM. Also it is a little tough to get on plane. I feel like it has a fuel restriction but... It will pick up a bit when you hold the choke. Has the original Yamaha aluminum tank and seems to build up vapor pressure in it. The tank cap appears like it might have a vent that is suppose to open when you turn it but we just purchased it so not sure.

I have:

Changed spark plugs. Old ones didn't look fouled though.

Taken apart all three carbs and cleaned them.

Cleaned cylinders and carbs with Seafoam.

Cleaned fuel filter.

Has new fuel lines.


Thanks

 

[rodbolt]

Re: Yamaha 50 only 4200 RPMs at WOT less if Loaded

hitting the choke?
just your immagination running away with you
that motor has no choke,its a primestart model and fuel enrichment is controled by the electrothermal valve which is controled by lighting coil output voltage.
key switch has nothing to do with it.
has it ever turned 5300 to 5500 RPM that you have seen ?

 

[Malone111]

Re: Yamaha 50 only 4200 RPMs at WOT less if Loaded

It is a 92 50 TLHQ and it has a choke on the engine and a electric choke on the shift/throttle assembly. The motor does pick up a little when I hit the electric choke. I just got the boat so not sure about the top end RPM. I have owned a newer 75hp and would guess it would top out somewhere between 5250 and 5750 RPMs.

Yesterday I removed all fuel lines and inspected them for leaks and clogs which looked to be in good shape and clear. However I am not convinced it tank (that I can tell) not having a vent doesn't restrict the gas flow which may be why the choke thing causes it to pick up by making it suck in more gas temporarily. But not sure that that would decrease WOT by 1000 RPMs. Any thoughts or advice is greatly appreciated.

 

[mphelle8vld]

Re: Yamaha 50 only 4200 RPMs at WOT less if Loaded

Since the motor is new to you (and us), is the primer bulb on the fuel line staying hard at WOT, what are your compression numbers, what prop are you running (if not sure post any numbers stamped on it)

 

[rodbolt]

Re: Yamaha 50 only 4200 RPMs at WOT less if Loaded

sorry I read it as 02.
a 92 like yours should have a choke assy, if by hitting the choke it picks up it indicates a clogged main jet. there is enough fuel in the bowl to manually enrich the air fuel mixture.
if the tank is pressurizing its not venting which after a few minutes could cause a vacum in the tank and loss of fuel pressure.
could be a waek or damaged pump diaphram or the oring on the cowl connector is swollen and restricting fuel.
when you tap the choke all your doing is manually forcing the fuel mix rich.
have you tested the compression?

 

[Malone111]

Re: Yamaha 50 only 4200 RPMs at WOT less if Loaded

Compression test today was cylinder 1-55, 2-60,3-50. Prop is 13-0 is the only #s I could find. I took apart all 3 carbs and cleaned and cleaned jets with a needle #2 carb was the only one clogged. Is there a main jet in addition to these? Also yesterday I took apart the fuel pump and the diaphragms had no holes and I could blow though them with no restriction (mystery to me how that works). Where is the cowl connector o-ring? Will check it tomorrow. Also I tried to run it with the cap off the tank at home so wasn't able to do WOT but it didn't idle any better as it seems to want to die after 10 sec without a little tap of the electric choke. Thanks for the input! Bought this boat for my old man and he itch'n to fish.

 

[mphelle8vld]

Re: Yamaha 50 only 4200 RPMs at WOT less if Loaded

I've got the same motor and the compression readings should be around 125psi, might want to borrow another gauge and re-check, it would be amazing that it could push your boat around at 4200rpm if your readings are correct.

On a side note as we're trying to get your motor up and running, my dad passed two years ago and few enjoyed fishing more than he, if you're anywhere near St Louis, I'd be happy to get you guys out on the water.

 

[Malone111]

Re: Yamaha 50 only 4200 RPMs at WOT less if Loaded

Mphelle8vld, thanks for the kind words. I rechecked the compression doing a wet test as I forgot about this earlier seeing as I haven't done this in a while. The tester did not seem that accurate as the psi readings came back across the board so I will be retesting it with a friends on Monday. However cylinders 1&2 came back 90 PSI + every time and was quite a bit higher several times, but cylinder 3 came back at about 60 each time. The boat has been sitting for 1 1/2 - 2 years. Thinking Yamaha Ring Clean might help some but I hear it could be either intake valve, exhaust value or maybe the more likely culprit worn rings in cylinder 3. Only reason I say that is that it appears the seals for intake and exhaust look like there 1 seal for all 3 cylinders, but ... that is just my guess. See diagrams below if they help. Thanks again!

See intake / exhaust diagram http://www.boats.net/parts/search/Yamaha/Outboard/1992/50TLHQ/REPAIR KIT 1/parts.html

See piston rings http://www.boats.net/parts/search/Yamaha/Outboard/1992/50TLHQ/CRANKSHAFT PISTON/parts.html

 

[mphelle8vld]

Re: Yamaha 50 only 4200 RPMs at WOT less if Loaded

Looks like compression is going to be the main issue so another test with a good gauge will help alot. I would do this test "dry". I use ring-free as a prevention but don't know if it has the ability to actually free up a stuck ring, but I'm sure someone will address this.

The 2-stroke doesn't actually have valves, it has ports in the walls of the cylinders that are opened and closed by the action of the piston. Unless someone can verify that ring-free could free up stuck rings, and the compression readings don't radically change, I think the next step would be removing the cylinder head to inspect the cylinder walls for scoring, pretty common with these motors due to overheating or lack of lube. Let us know what you find out.

 

[99yam40]

Re: Yamaha 50 only 4200 RPMs at WOT less if Loaded

a motor sitting for that long may have a problem with rust in the cylinders causing low compression.
A bore scope of the cylinders or pulling the head to inspect is in order