Yamaha Air to Fuel Ratio Question.

[sashko1]

Hi,

I have a 2005 Yamaha 4HP (4-cycle) outboard (F4A) motor. That I recently reclaimed and proved to be alive and kicking.

I ran this motor yesterday evening in a barrel with a good water level and the tell-tale was producing a good stream of water coming out.

Anyway, after some time playing with the motor at idle speed I did 2 or 3 WOTs for about 2-3 second time intervals. Then I slowed it down all the way to IDLE speed (in gear) and noticed that the motor started to die and finally after 4-5 bursts of brummm - brummm it finally died. Right away I shifted to Neutral with throttle at "START" pull it and it fired right up without any issues. I repeated the scenario, 2-3 short WOTs and lowest RPM while in gear -- and she does it again - broommmm..brummm and died.

Then I did the same thing but once I reached the lowest RPM switched to Neutral but still the issue was still there and she died again!!!

Then I really wanted to see what was going on internally and while it was "wrooommm ..wroooING" I unclipped both cowling latches and ...... once I lightly raised the upper cowling off the motor she fired right back to good Idle RPMs!!!!

I repeated this procedure several times and it was consistent - she revives as soon as the cowling is off!

......

Now, all the tubing/ hoses and wiring is nice and per original placement within the motor's enclosure, the cowling itself is plastic - nothing should be shorting, I think (strongly believe) that none of the air / fuel hoses get jammed while the cowling is on. What am I dealing here with?????? The only conclusion I could come up with is that the air gets too hot inside the enclosed space of the motor thus making the AIR to FUEL ratio very in-proportional.

Please help me out with my motor's mysterious behavior?

Thank you in advance,
Kuy.

 

[robert graham]

Re: Yamaha Air to Fuel Ratio Question.

The cowling has an air inlet somewhere near the top....maybe it's blocked with something, spiderwebs, wasp nest, etc.?...check it out. I recall a guy on here who had one of those dandy, tailored, fabric outboard motor covers(to keep sun/rainoff), and his motor wouldn't keep running, sorta similar to yours....figured it out to remove that cover during operation!

 

[robert graham]

Re: Yamaha Air to Fuel Ratio Question.

One other idea for you...are you sure you have the fuel cap vent open?...loosen the cap a bit and try it! Good Luck!

 

[sashko1]

Re: Yamaha Air to Fuel Ratio Question.

The air inlet is clear - I can see my fingertips as I slide my fingers into the inlets from inside of the cowling - I'm 100% sure about those being clear from any obstruction.

Fuel vent was opened. If it hadn't been then it wouldn't have make a difference with cowling on / off scenario. Agree?

 

[robert graham]

Re: Yamaha Air to Fuel Ratio Question.

Like you say, "mysterious behavior"....I'm confident it gets hot inside the cowling, but once the motor starts, it would very quickly suck all the hot air out...In a barrel and on the back of the boat under load are going to be quite different...my best guess is that you have a partially plugged carburetor jet that is allowing the motor to run seeming well in the barrel, but will likely bog down badly when on a boat and under load. Those carb jets are very tiny and can be plugged by the least bit of water/debris/whatever. You may try draining your carb float chamber onto a clean rag for an idea of what may be in there. Most carbs have a small screw/bolt drain plug near the bottom/side of carb float chamber. Good Luck!

 

[sashko1]

Re: Yamaha Air to Fuel Ratio Question.

This is a brand new ordered from Yamaha dealer carburetor.

I ran those tests with the motor under load (aka prop spinning in a barrel full of water).

 

[sashko1]

Re: Yamaha Air to Fuel Ratio Question.

....I'm confident it gets hot inside the cowling, but once the motor starts, it would very quickly suck all the hot air out...

What would suck the hot air out? The flywheel is closed out by the pull mechanism. I'm confused...

 

[robert graham]

Re: Yamaha Air to Fuel Ratio Question.

The motor running would suck any hot air out of cowling...straight into the carb, through the motor and out the exhaust. Motors use a lot of air! Just curious why motor has a "brand new carb"?...was it installed and adjusted by a Yamaha Technician?...What was wrong with original carb?....

 

[yamatech43]

Re: Yamaha Air to Fuel Ratio Question.

Funny how it takes a few posts to get to important info like a new carb......

 

[sashko1]

Re: Yamaha Air to Fuel Ratio Question.

Yamaha technician........ huh.....

No, it was installed by myself.

Why new? Because the original owner was an idiot and never took care of this motor. The carb was left full of gas for about 3 years. All internals of the carburetor have merely been destroyed. He (the original owner) then gave the motor to a "handy man" for fixing. The "handy man" couldn't get the needle valve out (it was stuck closed) and decided to drill a hole right next to it to make way for gas (!!!!!!!)

So, now, some time later the owner sells this motor on CraigsList and I decide to buy it.

Important, I have resurrected more than 30 "dead" outboards and I know what I'm dealing with (most of the time).

I bought a brand spanking new carb from Yamaha. Had it installed by myself. (Yes, I did remove the secret plug for slow speed needle, blah, blah...)

Then, also, I took the whole power unit apart (leak-down test was bad) to discover that both upper compression rings were severely stuck. I bought a new set and had it also installed myself. Yes, the rings were installed so that the neighboring ones do not occupy the same quadrant (I went by the service manual). Ordered all brand new gaskets. Torqued per specs.

What else ....?

Compression is just below 100 psi (very close to 93.87 psi ;-)

The motor went through the first 5 hours of the break-in period.

Now when I do those 10 second WOTs I do see the issue that we try to discuss.

Whew! Anything else? Oh, lower unit oil got changed as well (maybe that would be "funny" to find out at this point as well????)

.....

Anyway, I don't think I'm getting anywhere with the guesses and the "funny". I need someone that has some real experience with this motor and could tell me "do this, check that, make sure this and that". Fake "yamahatechs" are no good as I figured out long time ago - they just know green and no real knowledge and/or quality behind their work! That's why I do all this stuff myself. I don't trust any self-proclaimed professionals and wannabes.

Warm regards!

 

[Silvertip]

Re: Yamaha Air to Fuel Ratio Question.

One issue not discussed here is when an engine runs fine with the cover off but stalls with it on, can be due to an exhaust leak at the gasket between the power head and the lower unit. If that gasket is leaking then exhaust is entering the enclosed engine cover and it enters the carbs which causes the engine to stall.

 

[sashko1]

Re: Yamaha Air to Fuel Ratio Question.

Wow, Silvertip, now what you suggest makes sense!!!

I think I know what it is....!!!! Tell me if it's possible.

The gasket you are mentioning is a brand new Yamaha gasket. Here's the deal... My test barrel is originally some fluid container / barrel with the closed lid design. I made a cut-out on one side that is just enough for an outboard's lower unit to sneak inside the barrel and into the water.

So, how about this - it's truly so that initially the motor runs perfect when it's still not warmed up enough. But when it get's to temperature (I usually wait for about 5 mins during my break-in period) that when the discussed issue arises. I think that the exhaus gasses build up in the test barrel and escape upward! And then through the lower cowling openings (between the body of the power unit and lower cowling itself) and into the enclosed space of the motor! That's where the carb's air intake is pointed - downwards, right in the path of the exhaust gas escape path!!!) But once I remove the cowling, things improve since the gas is not that concentrated anymore....

What do you think of this theory???

Thank you.

 

[robert graham]

Re: Yamaha Air to Fuel Ratio Question.

That sounds like a possible answer to the mystery!...maybe to prove it, a strong fresh air fan blowing on the motor while it's running in the barrel to blow away exhaust fumes?....Silvertip may be the guy!

 

[sashko1]

Re: Yamaha Air to Fuel Ratio Question.

Sounds promising! I will try it out when I get home from work tonight. I think I might use a vacuum cleaner's air exhaust port as "fresh" air source or maybe a fan.

I will update the forum with my findings.

Thank you again!

 

[sashko1]

Re: Yamaha Air to Fuel Ratio Question.

Okay, update.

Got home yesterday. After a welcome-home-kiss-to-my-wife (this is a must, because she now says that I'm cheating on her with the motors) I went straight to my garage. Put the motor in the same barrel, fired her up, waited till she warmed up. Then I called my wife to help me with the experiment (sounded intriguing!)

Well, she was the vacuum cleaner operator - manager over the On/Off button. I had the vac hose at the air outlet hole (blowing port) of the vacuum cleaner. On the motor I had the other end of the hose held at the air intake on the top of the cowling.

So, after I would rev the motor several times at WOT and then put it immediately into IDLE the motor (without the vac running) would begin to hesitate, right at that time I would tell my On/Off button manager to turn it on and ....
... The motor would revive back to normal!

Ta- Da!!!

Silvertip, you are da man! Thank you very much!

I will confirm the "real-life" operation maybe tonight with the motor on the boat and on the lake. And update this post.

Thank you all for support!