Yamaha f115 (0406) missing on one cylinder

[bigfish3]

Yamaha F115 seems to be missing on one cylinder at low/idle RPM.
This is a tiller steered model with no gauges so I can’t be sure of max RPM.
This just began after chasing a fuel problem, I fixed the original issue and the motor ran great for at least 20 hours before the miss began.

To date I have done the following:
Replaced VST filter
Replaced water separator filter
Replaced spark plugs
Using new tank with new fuel
I can see spark, but I cant really measure it in any way.
YDS software doesn't show anything helpful.


I have ordered new ignition coils but they wont be here for a few days at least. I am wondering what the next steps are or if I am even going in the proper direction?

[TownsendsFJR1300]

Can you re-check the spark plugs and see if one is burning differently (different tip color) from the others?

If you have a timing light, putting it over each wire while the engine is running should verify if your getting spark thru that particular wire..

A compression test wouldn't hurt either.

[99yam40]

I will second the compression test after a fuel problem, and the timing light for spark testing

[rodbolt17]

you will now own a spare ign coil.
its a waste spark and any coil issue will take out 2 cylinders.
my guess would be a clogged injector filter.

[bigfish3]

you will now own a spare ign coil.
its a waste spark and any coil issue will take out 2 cylinders.
my guess would be a clogged injector filter.

Injector filter? is that an integral part of each injector? or is it in line somewhere??

I do have a spare set of reconditioned injectors that I have not installed yet.

[greasyshaft]

When the original fuel problem was supposedly solved were the injectors cleaned?

[bigfish3]

I never cleaned the injectors, the VST filter was the culprit. New filters and fresh fuel solved the problem. Perhaps there was crud left in the fuel system and it finally made its way to the injector?

I do have a spare set of injectors. I will install them and go from there.

Thanks to all so far.

I will post a status soon...

[rodbolt17]

those filters will clog with motor brush dust just running normally.
if you ever cut one open it will scare you.
the fuel path is into the pump intake,through the rotor style pump,over the armature. over the brushs and commutator and out the outlet.
any brush dust goes to the rail.

[bigfish3]

Status Update:

I installed the spare injectors, while removing the old ones I noticed that one had a cracked O-ring and one had an O=ring that was completely broken.

I put it all back together and ran it on the garden hose, it seemed to improve but I was still not sure.

The following day I put it in the water to shake it down.

It fired right up and had sort of a high idle for a moment which seems pretty much normal based on past experience.

When it settled down I left the dock, a few minutes into it it stumbled for a moment, I gave it a little throttle and it recovered. I ran it a varying RPMs throughout the day and it performed flawlessly at all ranges even dead idle under load!

Rodbolt17 is dead on in his diagnosis!

I probably should have done the injectors when I did the VST filter and other filters.

I am back in business, thank you to all!

[bigfish3]

I do have a follow up question.

When you replace injectors does the computer need to learn how the new ones work?

In other words could the momentary stumble I experienced be because the engine didn't know how to gauge fuel flow with the new injectors?

I have started this engine now about 30 times since this job and run roughly 15 hours and not one single incident since.

[boscoe99]

I do have a follow up question.

When you replace injectors does the computer need to learn how the new ones work?

In other words could the momentary stumble I experienced be because the engine didn't know how to gauge fuel flow with the new injectors?

I have started this engine now about 30 times since this job and run roughly 15 hours and not one single incident since.

No. The ECU neither knows or cares if an injector is even installed. Or, is working.

[TownsendsFJR1300]

You might have had a little bit of air in the system initially. Once it was all cleared out, back to normal..