yamaha 40hp misfire

[99yam40]

Re: yamaha 40hp misfire

Kind of sounds like the miss fire you are speaking of is what we call a lean sneeze.
This is caused from too lean of a fuel/air mix due to blockage, miss adjustment, or vacuum leak.
It ends up burning in the crank case and like you said causes motor to jerk violently

Need to make sure all carb passages are cleaned properly and then do the link and sync in service manual to adjust carbs and check timing

 

[Bosunsmate]

Re: yamaha 40hp misfire

Yep it definitely was having violent engine stopping jerks, it wasnt nice to see it happen to her. It did keep happening after i pulled out each spark plug boot in turn so i see what you mean about how a crankcase pre-ignition could cause it.
It did have vacuum in all three carbs and i have cleaned out all three.
Il pull the carbs again, ive definitely done the top passages pristine so will concentrate again on the lower pathways.
Is there anywhere else i can look for a vacuum leak on it? It seems too powerful a vacuum to suggest a suddenly failed lower or upper crank seal. It will suck a dollar bill right down all three carb bores.

Meanwhile for backup the cdi starts its journey here from near Nova Scotia.

 

[Yesterday]

Re: yamaha 40hp misfire

Try the crank seals. Some say ether sprayed around them while it's running will cause it to rev up, but I didnt experience with my sled engine that DID have bad crank seals.

Intake seals/gaskets around the carbs??

Near Nova Scotia you say? Thanks close to me! In fact we live IS very near N.S.

 

[Bosunsmate]

Re: yamaha 40hp misfire

Yep thanks that will be next if this powerpack doesnt fix it. I was really hoping for a month off from lifting a powerhead though!
It does have good vacuum in the three carbs, but i suppose that it could still be leaking enough to run it lean for a misfire. The bottom crank counterweight did look a slightly but also noticeable different colour in some areas compared to the other two.
Il reinspect the sparks again after next test.
The carb seals seemed sound.

Yep the powerpack is coming from near your way, always nice being able to buy somewhere else that isnt in our peak season. At last waypoint it was processed through a sort facility in Chicago but that was about 72 hours ago so hopefully its made its way through the nose into a 747-400f but i suppose its probably currently trucking or on the train heading west.
Just not sure what priority means for international priority on USPS

 

[Yesterday]

Re: yamaha 40hp misfire

Chicago?! Holy! It must be headed your way via the West coast, else it's some 2500 Kms in the wrong direction.

 

[Bosunsmate]

Re: yamaha 40hp misfire

Yep i suppose so. It would of being great to of thought it was coming by schooner around cape of good hope instead!

 

[Yesterday]

Re: yamaha 40hp misfire

LOL are you getting all sentimental on us now? :joyous:

 

[Capt Ken]

Re: yamaha 40hp misfire

Pretty good chance that it isn't going to fit. Parts for the U.S. market seldom fit those for the European market.

 

[Bosunsmate]

Re: yamaha 40hp misfire

Pretty good chance that it isn't going to fit. Parts for the U.S. market seldom fit those for the European market.

That would be a real kick in the goolies.
It looks exactly the same and has exactly the same wiring as the one on there current;y. Its probably an australian model as thats our closer main market than europe.
I cant find the part list for an heo but the price for a new one in the dealers here matches the price for a new one on US models...

 

[Bosunsmate]

Re: yamaha 40hp misfire

Hi cdi arrived, it was exactly the same as current one.
Motor had exact same problem
So you mentioned lean misfire 99Yam so...
What to check next bottom seals?
Video to follow
I did notice it ran a bit better from cold with the old cdi in too

 

[Bosunsmate]

Re: yamaha 40hp misfire

 

[Bosunsmate]

Re: yamaha 40hp misfire

i just rung the yamaha mechanic and he said do the carbs again so for the third time off they come and i will double check the bottom jets especially

 

[99yam40]

Re: yamaha 40hp misfire

a complete strip down and clean with a sonic bath may be what you need,
Just make sure you link and sync after reinstalling carbs

 

[Bosunsmate]

Re: yamaha 40hp misfire

Well i couldnt get any carb cleaner til monday but i took the carbs off and put some thin wires through the passages and blew it with CRC with the long thin nozzel pipe on the aerosol outlet- CRCs probably very similar to WD40- a couple of bits of gunk came out the top of the carb housing- it was a pleasant surprise. i did it to all of them and reinstalled. It was a bit better but still misfired.

I took them off again and cleaned them out some more including holding a thumb over some jet and holes outlets to increase the pressure through them while i sprayed. I did this through every single hole i could find. I then reinstalled.
And no backfire! I ran it for twenty odd minutes and no problems, i couldnt run it for longer as im in a drought here and im on tank water.

Hey thanks so much, this is a post of a stubborn/ haphazard man who should of done it right the first time so rub it in only if you really need too!
I was wondering though the idle/mixture screw on these, how far should they be backed out, i really dont want to run lean. Ive heard about one turn? Doesnt seem like much
Ive never worked on a yamaha before
Thanks again, one happy man and his friends motor


Photo is of one of the suspected jet and channel, i suspect its an idle circuit or something as the main jet is the one left in there, this is the one normally obscured under the black cap

 

[Bosunsmate]

Re: yamaha 40hp misfire

Her after cleaning, much different to the video of her in post 31

 

[99yam40]

Re: yamaha 40hp misfire

Yes that was the pilot/idle jet under the rubber cap
Which carb did it come out of?

you need to find a service manual for that 1990 motor to get the proper specs.
If you do not have a manual you probably are not following the link and sync procedures after reinstalling the carbs

My 1999 spec for pilot screws is 1 1/2 +- 1/4 But have no idea what your 1990 motor should be

 

[Bosunsmate]

Re: yamaha 40hp misfire

It was in the bottom carb. It came out of the hole in the bottom middle of the photo, you can just see it sitting on the right of the hole still, its not the hole on the short brass shaft. Apologies its a bit blurry but the gunk is in the photo too.

The yamaha mechanic said it should be around a one turn backout which seems really light to me. Il recheck with them today and about lync and sync or look at getting a manual.
I was wondering though is it richer when screwed further in or out?
Also are there any suggestions i can give to these people to minimise the chance of this happening again. It already has two filters, one on the motor and a big inline one.
I will get them to run it dry when finished but i think draining the carbs would be too much for them. Do fuel stabilisers or something akin to that work to stop gumming?

 

[Bosunsmate]

Re: yamaha 40hp misfire

I really need to sort out how you write on digital photos with arrows etc
NB i think the top carb had a slight bit of gunking too

 

[99yam40]

Re: yamaha 40hp misfire

I never have been a fan of running a motor with multiple carbs until it dies.
Some will run low on fuel before the others and that means you are running some cylinders too lean and no lube.

And you can never suck all the fuel out of the bowls, so the remaining evaporates sooner and leaves residue in the bottom of bowls.
seemsThe bottom carb is the last one to run out of fuel so it usually has more fuel left in it to leave residue when fuel evaporates. And then there is the fuel in the lines,pump, primer,etc that can and will dry out and leave stuff to hit the carbs

I like stabilizing the fuel and leaving carbs and system full. Just run it often.

 

[Bosunsmate]

Re: yamaha 40hp misfire

Ok, i definitely wont run it dry then, I dont want any lean burning happening again. Il get them to get a fuel stabiliser and run it every few weeks at least.
Cheers Rear Admiral