3.5 hp coil - Pt No. F6T 405

[francismurphy]

I have an ancient 3.5 hp Yamaha which turns over but produces no spark. I suspect the coil has expired in the 15 years since the engine was last fired and am trying to locate a replacement.
I understand that the coil is probably obsolete and wondered if anyone knew of an alternative replacement that might be available.
I believe that this coil was used on a number of Yamaha and Mariner engines and also possibly on an old Nissan.
Any and all help much appreciated.
Thanks
Francis

[ribvanw11]

Francis,

This machine has two coils - the one you reference is the 'Ignition Coil Assembly'. The ignition coil is inside the flywheel. Before you blame the external coil (unless it is open circuit) have you pulled the flywheel off, reshaped the contact points and cleaned them of any contaminants and also cleaned up the other electrical connections attached to the points assembly?

I have 4 of these engines and the problem with 3 of them has been the contact points.

I must dash to the pub now but tomorrow I can post a few more details of my experiences.

Regards
Bob

[ribvanw11]

A little more:-
Note: The parts list refers to
Item 13-4 - ‘624-81311-20; COIL, Ignition’ which is the primary winding inside the flywheel. Removal requires pulling of the flywheel.
Item 13-26 - ‘624-82310-20; IGNITION COIL ASS'Y’ which is the HT winding and is mounted on the engine side and can be removed independently.

Continuity test on the primary and secondary sides of the HT coil:-

Continuity test can be made by measuring the resistance on the primary and secondary sides of the ignition HT coil using a standard ohmmeter.
a) Primary side test
The ignition coil is considered to be in good condition if the readings are within the following range:- 0.95Ω ± 15% at 20°C. [Allow for the short circuit resistance indicated when the ohmmeter probes are shorted together].

b) Secondary side test
The ignition coil is considered to be in good condition if the readings are within the following range:- 5kΩ ± 15% at 20°C. [Note that, if fitted, a noise suppressor cap fitted to the spark plug it will increase the resistance measured - mine doubled].

Try to get the resistance across the closed points to 0.1Ω. Mine were heavily pitted so I used a file (against all advice) to start with followed by a nail file. I cleaned the surfaces with switch cleaner and cardboard.

Cheers
Bob

[francismurphy]

Bob,

Firstly many thanks indeed for the advice and apologies for not being back sooner to say thanks.........have been on the road for a few days.

I will get at the engine tomorrow and will ask in advance that you forgive me for what I am sure will be a succession of questions that I am fairly sure that I should know the answers to!!

Regards,

Francis

[ribvanw11]

Francis,
No problem.
(Only posting this so that I receive an email if you post again - I didn't have my settings correct for receiving notice of replies!)
Cheers

[francismurphy]

Bob,

Thanks again for the help.........it's up and running!!! Like a dog.......but running nevertheless!!

To explain, it starts on choke and will run but only at low revs. Taking the choke off causes the revs to rise but 30 seconds later, the engine splutters and dies! There appears to be a strong spark at the plug so I think that the problem is probably plug gap and mixture setting.

The carb has been cleaned twice now and "appears" to be working as it should but I am completely ignorant as to where the mixture screw should be set. Any thoughts? Also, the engine has a new plug to replace the one that looks like it may be as old as the engine. Can you tell me what the gap should be?

Or .......have I got it all wrong and should I be looking somewhere else entirely?

All assistance hugely welcomed.

Regards

Francis

[ribvanw11]

Francis,
That is good news but that is just 'Carb Step1'.
The spark gap should be 0.5 to 0.6 mm and the points gap 0.3 to 0.4mm
The mixture screw should be wound out about 1 1/2 turns and then adjusted (see below)

The following is my edit of a very useful web article - I have customised it to my needs . I also have coloured diagrams to support the words but they are huge.
My first thought would be to check the bypass holes and the idle hole are clear (even though you have cleaned twice). Use carb cleaner blown into the idle air inlet.

I'll try to find a way of reducing the size and posting my diagrams.
Good luck
Bob

Quote
Mikuni Carb Tuning – edited from Rick Sieman: SuperHunky.com

WARM VERSUS COLD
When a tuned engine is warmed up, it usually re-starts easily. However, when it's cold, the fuel drawn into the engine tends to remain in liquid form. To create a combustible mixture in the cylinder, some of the fuel has to evaporate. The vapour mixes with the air and when the ratios are correct, it can be ignited with ignition spark. If there is not enough fuel, or too much, you can pull until you're blue in the face, and the motor may not start. When only a little bit of the fuel is vaporizing, you have to put in a great, excessive amount of liquid fuel just to get enough of it vaporised to start the engine. That's called choking the engine.

CHOKING
The Mikuni provides starting mixture enrichment. A lever on the carb body provides a special starting circuit built into the main body. Here's how it works:
The rotating engine creates a vacuum, which draws air through an inlet at the front of the carb. This air flows through a passage and passes across a dedicated fuel jet leading from the fuel bowl. The mixture of air and fuel flows through Passage B towards the outlet of the carb at a point behind the throttle valve at the manifold.
However, if you leave the choke on too long, you might end up with a fouled spark plug.

DON'T OPEN THAT THROTTLE WHEN USING THE CHOKE!
What makes high vacuum behind the throttle valve is the cranking of the engine with the THROTTLE CLOSED. Leave the throttle completely closed until the engine starts. If you insist on opening the throttle, the vacuum behind it disappears and the starting carb will not work. Most of the hard starting of Mikuni carbs is caused by this syndrome.

AFTER THE ENGINE HAS STARTED
Once the engine starts, it now has to idle including anytime the throttle is closed (or nearly closed). There must be sufficient vacuum to continuously pull idle mixture out through a small hole, which is just behind the throttle valve. There are also two holes to deliver additional idle mixture alongside the edge of the throttle valve. (This is often called a bypass orifice). As the throttle valve is opened, the alongside holes also start discharging mixture.

The idle circuit air intake is in the mouth of the carb. From there a jet (the pilot air jet) in the air passage adjusts the airflow and a dedicated jet (the pilot jet) then draws fuel from the float bowl. The mixture flows to the holes in the main bore of the carb - mentioned above. The flow of fuel into the pilot outlet is regulated by the pilot screw.

IDLE MIXTURE ADJUSTMENTS:
A throttle stop screw which goes into the side of the carb beside the throttle valve limits how much the valve can close. The valve should never fully close, (so some air ducts around it and flows into the engine when you think the throttle is closed). Adjust the throttle stop screw for the idle speed you want and the pilot screw for mixture strength at that idle speed.

The two controls interact and a good adjustment procedure is as follows:
a) Turn the idle speed up a little higher than you want it to be, using the throttle stop screw. Now turn the pilot screw inwards until the engine runs poorly or about to quit running.
b) Start backing it out and count half-turns of the screw as you do it. When it runs poorly, stop backing it out.
c) Count the number of half-turns you made while backing the screw out from run-poorly to run-poorly. Set the pilot screw halfway between these extremes or at a nearby setting where the engine runs fastest.
d) Back out the throttle stop screw to reduce the idle speed to what you want. Then re-check the pilot screw adjustment and reset to the place where the engine idles fastest or smoothest.

OVERALL PERFORMANCE
The idle system is part of the carb's entire operating system. By opening the throttle valve, the main system is supposed to take over and the idle system is supposed to decrease delivery of mixture. The transition between these two systems is critical to good performance when you first open the throttle, and on up to about Ľ open.

Some mechanics set the idle as described and then test throttle response by whacking open the throttle. They try slightly different pilot screw settings to see what gives the best throttle response off idle.

[ribvanw11]

Try these. (Sterter=Choke), (Slow=Idle)


Carb Choke Circuit.jpg

Carb Main Circuit.jpg

Carb Slow Circuit.jpg

Yellow is air
Blue is fuel
Green is mixture

[davidmacl140]

I have recently acquired an old yamaha F6T 405

I was wondering if someone could tell me where I could buy a new prop and pull cord flywheel for this model?

[vigilbeam]

There is a place over in Holland that have provided parts for my little Yammy. O recommend there service. Try Gerard & Tiny Klijzing at Klijzing Yamaha Buitenboordmotoren en Buitenboordmotor Onderdelen| or email tiny@buitenboord.com
They deliver real fast too.
Good luck
Dave

[vigilbeam]

Little 3.5 just won't keep running

Hi Guys,
I acquired this little Yammy 3.5 air cooled outboard which hadn't been run for a while.
It would start and die within seconds.
I removed the carb, cleaned it and no difference. Spark was good so i replaced the carb gasket as "sucking too much air" was the only suspect left.
After loads of pulls and a few adjustments with choke etc, it fired and ran perfectly.
I subsequently mounted it on the Transom, fuelled up and set off. Great for about 45 mins and then it died. It wouldn't start except on the choke and this is now where I'm at. It will idle on the choke but as soon as I open the throttle, dies instantly. Took me ages to get back idling on the choke!!.
The plug is clean. Is there anywhere else that it could suddenly start sucking air from?
Any help or advise would be greatly appreciated.
Thanks for reading

[westkeasman69]

My 3.5 hp ac Yamaha Mariner

Hello, I was wondering if anyone knows where I can get any parts for this motor. At the moment, I am desperately hunting a Prop shaft seal. Even if anyone knows of another seal that might work. The motor I have runs really well and it would be a shame to scrap it for a shaft seal. I would love to find the fiber gaskets as well but, would settle for making those myself, if I could get the prop shaft seal. Also does anyone know where you can get a diagram of the motor. showing all the parts. I would really love to do this motor up if possible. Any help of any kind would be appreciated, thanks in advance.

[almetelo]

Google this parts # 93103-14050 and confirm if is this what you are looking for.

If not obsolet an Yamaha dealer must supply it for you or go to you local bearings shop with the old one and maybe they can help you.

Good luck

[Barra Jack]

3.5 Yamaha hard to start (but runs OK)

Hi Folks,

My first post on this forum, good work, lots of useful info.
My little motor (just given to me recently) doesn't start easily. Good spark, and I think it is a fuel problem, I have replaced all the fuel lines. If I dip the plug in petrol and put it back in place the motor starts fine and then will run, however if she stops she won't start. Should I blow anything out with compressed air or have you some other suggestions that may help?

Keep up the good work,

Jack

[boscoe99]

Start a new thread. Post the complete model of the motor. The serial number will be even better.