Yamaha F4A Idle Adjust

[Arida]

My Yamaha F4A 4 stroke Outboard runs fine at speed but tends to idle a bit slow and can die at idle. Instead of just twiddling as I have (woth other engines) in the past, i thought I would download the service manual and adjust idle speed using the prescribed method.

Attached is the relevant section. It seems to have suffered from poor translation. Can anyone interpret please?

Does the pilot screw go 1 7/8 turns in, or is it screwed fully home and then backed off by 1 7/8 turns? Understand after this that it may acceptably be adjusted by +/- 1/2 turn, but its the starting point I am unsure of.

Many Thanks,

Rob

 

[VicS]

My Yamaha F4A 4 stroke Outboard runs fine at speed but tends to idle a bit slow and can die at idle. Instead of just twiddling as I have (woth other engines) in the past, i thought I would download the service manual and adjust idle speed using the prescribed method.

Attached is the relevant section. It seems to have suffered from poor translation. Can anyone interpret please?

Does the pilot screw go 1 7/8 turns in, or is it screwed fully home and then backed off by 1 7/8 turns? Understand after this that it may acceptably be adjusted by +/- 1/2 turn, but its the starting point I am unsure of.

Many Thanks,

Rob

Clear enough to me, but then I suppose I knew what sort of procedure to expect.

Screw it right in until it just gently seats Then back out by 1⁷/₈turns

Id suggest then screwing it in a tiny bit at a time, keeping the revs to an absolute minimum, until it falters. Then back out again until it runs sweetly. Finally adjust the revs to the correct or a acceptable speed

 

[Arida]

Ok, thanks Vics. Will try that.

 

[Arida]

Finally got down to the boat and spent some time playing with the engine.

First surprise was that my engine is like the "USA version" from the manual - it has no pilot adjust screw (presumably just a fixed jet), only an idle speed adjustment. Presumably to stop people messing up the emissions....

Anyway, the problem I have is that the engine runs fine with the choke fully out - starts first pull, excellent telltail and seems to work pretty well under load. Perhaps slightly rough idling, but not terrible. However, as soon as the choke is pushed in even half way, it dies completely. This is the case even when it has been running for 20 minutes or so and is fully warmed up.

By increasing the idle speed, it can be made to idle (very roughly) with the choke half way in, but seems to "choke itself" and die as soon as the revs are increased. And even with the highest idle setting it will not run at all with the choke fully in.

On my Kingfisher, the engine sits in a well. I have heard of some outboards being choked by their own exhaust fumes when run like this, so I took it off and ran on the tender - behaved exactly the same, needed choke fully out to run. In this state it seemed pretty good - certainly pushed the tender along quicker than my old 2 stroke (also Yamaha 4HP), though the state of the prop on the latter means this is not really a fair fight....

Obviously needs sorting. Any thoughts on where to start? Is it likely to be a pilot jet blockage or is it more likely to be air intake related? Should I try a spray of carb cleaner?

(Engine is 2011 build, new to me this season. Looks totally new, both inside and out (or did before I lugged it on and off the boat a couple of times) so I cannot believe it has had a hard life.)

Regards,

Rob

 

[Arida]

Just an update: had a bit of time today, so ran the engine up at home - same problem. I guess running ok with choke but not without it implies fuel starvation.

Have had the carb apart and cleaned out jets etc. To be honest it all looked almost new - just a tiny amount of residue in the bottom of the bowl. Float, needle valve, butterfly valves etc. all seemed ok, though I will admit it is the first carb I've ever had apart.

Other possibility might be the fuel filter. I did try to buy a replacement, but inadvertently ended up wit the "strainer" filter from the bottom of the internal tank, rather than the main in line fuel filter. I have the correct part on order, but in the mean time I bypassed the filter with a piece of tubing. This seemed to have no effect - engine ran just the same. Possible there was air getting in as the bodged arrangement did not have a great seal - I think this is clutching at straws, but will know for sure in a couple of days when the correct part arrives.

What is left which would explain my problem? All I can really think of is the fuel pump. Not sure if sensible to try disassembling this...

Regards,

Rob

 

[VicS]

Just an update: had a bit of time today, so ran the engine up at home - same problem. I guess running ok with choke but not without it implies fuel starvation.

Have had the carb apart and cleaned out jets etc. To be honest it all looked almost new - just a tiny amount of residue in the bottom of the bowl. Float, needle valve, butterfly valves etc. all seemed ok, though I will admit it is the first carb I've ever had apart.

Other possibility might be the fuel filter. I did try to buy a replacement, but inadvertently ended up wit the "strainer" filter from the bottom of the internal tank, rather than the main in line fuel filter. I have the correct part on order, but in the mean time I bypassed the filter with a piece of tubing. This seemed to have no effect - engine ran just the same. Possible there was air getting in as the bodged arrangement did not have a great seal - I think this is clutching at straws, but will know for sure in a couple of days when the correct part arrives.

What is left which would explain my problem? All I can really think of is the fuel pump. Not sure if sensible to try disassembling this...

Regards,

Rob

Your symptoms are still those of a blocked carb jet !

You can prove whether or not the fuel pump is faulty by using the fuel line bulb. If the pump is faulty , or if air is being sucked in, you will be able to get it running normally by repeatedly squeezing the bulb.. ie by taking over from the pump

 

[Lakesailor]

Have had the carb apart and cleaned out jets etc. To be honest it all looked almost new - just a tiny amount of residue in the bottom of the bowl. Float, needle valve, butterfly valves etc. all seemed ok, though I will admit it is the first carb I've ever had apart.

You really need an airline to blow out the jets. The symptoms are clear as day. The idle (slow running) jet is blocked or restricted. Possibly even the main jet.
You haven't said if it runs OK at wider throttle openings.
The gunge that blocks them is often all but invisible.

A blocked fuel filter would give problems when trying to run at higher throttle openings.

 

[Arida]

Thanks Lakesailor/Vics

With the choke fully out it does run ok at wider throttle openings and even made a reasonable account of itself under load on the tender. With the choke half way on it will still run at moderate revs, but runs very roughly and eventually dies as the throttle is opened. With the choke fully in it will not run for any length of time at any throttle setting.

I do have access to an airline at work so could clean this way if necessary.

Regards,

Rob

 

[VicS]

Thanks Lakesailor/Vics

With the choke fully out it does run ok at wider throttle openings and even made a reasonable account of itself under load on the tender. With the choke half way on it will still run at moderate revs, but runs very roughly and eventually dies as the throttle is opened. With the choke fully in it will not run for any length of time at any throttle setting.

I do have access to an airline at work so could clean this way if necessary.

Regards,

Rob

I suspect then something other than the obvious.

Nevertheless a thorough strip down and clean, ensuring all jets and passages are completely clear, is the first step I am sure.

Check any joint washers, O rings etc, although I don't see any on the diagram, Check for cracks or other defects in the carb body. Check the joints between the cab, manifold and engine

Check the float and needle valve operation.

 

[Arida]

Success!

Carefully cleaned out the carb jets and passages. Reluctant to admit given Lakesailor's comments, but did not use an air line but a thin strand of copper wire from electrical cable - significantly narrower than the jets and hopefully soft enough not to enlarge anything.

Re-assembled and tried to start - nothing happened. After ten minutes of head scratching, I remembered the kill cord....

Engine now starts easily and happily idles at very low revs with the choke in (may have set a bit too low actually, but don't have a tacho. Easy to adjust later anyway). Will rev freely and seems to go to rather higher max revs than it would before. Put into gear at idle and no "hiccup" at all. Not tested properly under load at higher revs, as my test rig is not really man enough, but very confident to be honest.

Fuel filter not yet arrived, but can't imagine it is needed.

Many thanks both.

Regards,

Rob

 

[VicS]

Thanks for the feed back.

I use a copper wire... not having an air line.

I lost count how many times I cleaned one before I got it to run properly after it had got some water in there and left for a good few months. A piece of fuse wire through the slow running jet sorted it eventually. Doubtful if an air line would have blown that one clean anyway