4hp Yamaha, need a little help.

[TJ D]

Hi guys,

My little 4 is giving me fits. It starts and runs ok, but if I turn off the choke and give it some gas, it gets starved for fuel and dies. A little choke on and it runs again, but I can't get full power out of it. Basically, it seems like the high-speed jet isn't working.

I've taken both jets out of the carb, and they're clean. Obviously I'm missing something here. What else should I be looking at?

Thanks much, TJ

[atoll]

Originally Posted by TJ D

Hi guys,

My little 4 is giving me fits. It starts and runs ok, but if I turn off the choke and give it some gas, it gets starved for fuel and dies. A little choke on and it runs again, but I can't get full power out of it. Basically, it seems like the high-speed jet isn't working.

I've taken both jets out of the carb, and they're clean. Obviously I'm missing something here. What else should I be looking at?

Thanks much, TJ

perforated fuel pump diaphram.
dirt or varnish in the bottom of the fuel tank.
float valve not opening properly.
breather closed on fuel tank.
idle adjustment closed,should be open about 11/2 turns

[CSY Man]

Had the same problem on a Yamaha 4hp, 4-stroke.
Soak the jet in brake cleaner or similar chemical.
Black goo will come out of it, blow it clean and the motor should be back to normal.
I suspect the problem was caused by ethanol fuel.

[TJ D]

Originally Posted by atoll

perforated fuel pump diaphram.
dirt or varnish in the bottom of the fuel tank.
float valve not opening properly.
breather closed on fuel tank.

Fuel pump diaphram- maybe, but why is it only when the choke's off that this is happening?

Fuel tank, no-it's all good. Again, it's only with the choke off.

Float valve, is this choke dependent?

Breather-open.

I think that it has to be something in the carb, no? The only time the engine is starved for fuel is when the choke is open. The things listed here don't seem to be related, I don't think.

[TJ D]

Originally Posted by CSY Man

Had the same problem on a Yamaha 4hp, 4-stroke.
Soak the jet in brake cleaner or similar chemical.
Black goo will come out of it, blow it clean and the motor should be back to normal.
I suspect the problem was caused by ethanol fuel.

Ok, I'll take the carb off and give that a try. The jet looks clean, but it's pretty small, perhaps I missed some goo.

Thanks, TJ

[atoll]

Originally Posted by TJ D

Fuel pump diaphram- maybe, but why is it only when the choke's off that this is happening?

Fuel tank, no-it's all good. Again, it's only with the choke off.

Float valve, is this choke dependent?

Breather-open.

I think that it has to be something in the carb, no? The only time the engine is starved for fuel is when the choke is open. The things listed here don't seem to be related, I don't think.

if it starts easily but won't run it is fuel starvation somewhere .
the choke increases manifold pressure so effectively sucks more fuel through.

if the fuel bowl is not filling fast enough engine will stall.

though it does sound like the carb still has a blockage.

[TJ D]

Ok, the carb is in pieces in the cockpit-- I'll let you guys know how I made out after I put it back together.
Thanks!

[Captsteve53]

Dont forget to check the fuel idle adjustment screw, screw it all the way in, then back out 1 1/2 turns, as previously posted, nearly all small motors i have worked on with these issues turned out to the adjustment (yes it does and can move under normal engine running) If you get it running just play with screw backing in and out a very amount until the engine sounds the happiest

Cheers Steve

[TJ D]

Ok, we've got a classic good news/bad news situation here...

The good-

The engine runs as normal once I go out and open it up.

The bad-

The bloody thing won't slow down. If I turn off the choke, it idles at about 5000 rpm...
The only way to drop the idle is to choke the darned thing, making lots of smoke and unburnt fuel in the process. The throttle cable is slack, the butterfly for the carb is all the way closed-the idle set screw is not even making contact.

It's basically at WOT all the time, no matter what I do with the throttle.

What the heck have I done??

[puffcard]

Pretty certain it's the jets. Get some very thin brass wire that they use for jewelry. They sell it at hobby stores. For my Honda I need two sizes. Pull the jet tubes out of the body and run the wire thru the very small holes in the jet. If you can pull the low speed/idle screw out do so, can shoot carb cleaner up in there and thru any ports you see. After you get it running, never leave gas in the carb, run or drain it out.

[TJ D]

Well, the jet cleaning project is definitely done. The engine is getting full fuel. All the time. No matter what I do to the throttle.

I'm going to go rip it apart again. I obviously didn't put something together right....

[Captsteve53]

Sounds like the main butterfly valve is wide open, when you have it back and hooked up (with out air cleaner if it has) look into the carb, the main butterfly valve should be in the closed position(ie : you should not be able to look into the reed valves), open the throttle and you see the valve start to open

This is with the engine OFF

[TJ D]

Nope, the butterfly valve is definitely fully closed and it's still idling at basically WOT. I just took the carb back off and confirmed all this. I've backed off the idle screw so that the butterfly is as closed as possible, and the thing's still racing along.

Maybe it's time to seek help... I just don't get it. This isn't rocket science. Well, it is, but the outboard isn't.

Thanks, TJ

[Captsteve53]

Wow, now thats con fusing !!, I would have said that the needle valve/float set up in side the carb wasn't there allowing free flow of fuel into the engine BUT if the butterfly is closed basically the fuel cant get into the engine!

Just check that the small spring that goes around the linkage arm for the butterfly is in the rightful place, when you open by hand (with the cable off) tit should snap shut (well most models any way)

The only other time i saw this was with a broken reed valve (inside the air chamber) thus was running lean,

I would take the carby off again and go through one more time and be sure all the parts are in right place,

I take it this issue is since you took it apart-so there's the issue

Cheers Steve

[TJ D]

Final update:

VICTORY IS MINE.

Nothing was outwardly wrong, but when I took the carb back off, I unscrewed the low speed/idle jet a bit, and that did the trick. It had been fully in previously, and I don't really see how this would solve things, but I'm not complaining!

The engine runs like the day it was new now.

Thanks for the assistance, gents. I didn't expect this to take all day, but the sweetest wins are the hardest earned, I guess.

TJ