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6hp yamaha 4 stroke missfiring/shaking at higher rpm

M

Marmotic

New member
Model: Yamaha F6CMHS

I bought a used yamaha 6hp outboard for my tender.
It works great until 50% throttle, but when i go to 60% it doesnt sound as good anymore. (ut still runs okay-ish)
Above 60% the engine sometimes cuts out and appruptly shakes until i remove some throttle. (it seems like its missfiring)

This happens when i drive the boat AND when i test it in neutral! Exactly above half throttle, the missfired and shaking starts happening.

I changed the oil to new yamalube marine spec oil. this didnt help.

I would be super grateful if any of you could give me some tips. I do not have the money to bring it to a dealer...

Cheers
Marmo
 
i noticed the fuel i used from the old owners external tank is a bit old (over 1 year at least)
I emptied the old fuel in the engine and filled it with new fuel and ran it for 15 minutes in idle and neutral. but it doesnt look like it improved much.

Over 50% throttle the engine feel like its in the rev limiter. still shaking and sounding not harmonic at all
 
Hi racerone

I dont have a testing device. Is there any way i can test it or should i just replace the spark plug?

Im in Switzerland and the parts avaliablity is a joke.

right now i have a ngk spark plug, is there any better one?

I already ordered a new fuel filter from china, not sure if that helps since i can still blow trough the old one. but i figured i replaceit since the outboard is already 3years old and it ran on older fuel
 
Make a spark tester out of a few scraps, like this. You can use any method to hold the wires, hot glue gun, a short bit of wood or metal with a screw through it etc.
Like their motorcycles, Japanese motors usually use NGK. They are fine.
 

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Is it something similar to this?

an extended wire with a gap in the middle where you can see the spark travelling towards the spark plug?

Otherwise i dont really understand the contraption in the pic from you Edchris. heh :oops:
 
That video is way to much work, might as well spend $10 & buy one. I thought my photo would be self explanatory…
Stick the end with the bolt into your sparkplug boot- the bolt is only there to hold the wire inside the boot, you could also just bend the copper wire into an appropriate sized loop.
Put the other wire to ground-I generally hook it into the sparkplug hole.
Adjust the two short wires to a suitable gap.
Pull starter cord- if a bright blue spark jumps the gap, your spark is good.
 
I took it to a marine engine shop since im a newbie

Apparently everything is alright with the engine. The excessive shaking seems to be the rev limiter according to the shop.

I was just perpelxed because this rev limiter in neutral already happens at 50-60% throttle!

I just got my inflatable takacat 280L with 2 persons and barely any gear up to 7.5mph (gps). I got this speed at 50-60% throttle If i increased the throttle to more i would get no more speed and just engine shaking.

If this really is the rev limiter, how can i get any more speed from the engine? The shop man advised a prop change.

Im a bit dissapointed, i wanted to hit at least 10mph with this engine and 2 people in the boat :( Many other seem to get way more speed than 10mph as well in much bigger boats.

What i havent tested yet is the boat in actual water with new high quality petrol in the system. the weather is pretty bad now so i cant test it sadyl.
 
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