yamaha 40hp misfire

[Bosunsmate]

Hello
Im looking over a 40hp yamaha 3cyl 2stroke for a friend which powers their boston whaler. They have not used the motor for about a year and recently took her out- she had hardly any power and would cut out. I ran it on a hose and noticed that it had a regular significant miss at all rpms.
She sits by the beach unused for most of the year albeit a bit covered up.
There is a good spark on all 3cyl so i dissembled the carbs to check the main jets, which were clear. I also cleaned the fuel bowls and it has fresh fuel. The fuel pump and hand primer seem fine. The carb bowls always seem full when i open them up to drain them and pumping the primer during operation does not alleviate the misfiring.

Ive checked the resistance values of the triggers and stator which are fine and im awaiting a timing light to check for a signature of an ignition fault during the miss. Im really hoping its not the cdi as it seems there is no cheaper aftermarket version of these.
I have read that on these yamahas fuel is mostly the problem with them when a problem occurs. I was wondering if the other small holes in the carb if blocked could cause this when it actually happens at all rpms (high, medium and at idle)? Since the main jets are clear and there is fuel in all the carb-bowls then surely no other carb channel is needed at high rpm or am i missing a fundamental here?
thanks

 

[jerryjerry05]

Re: yamaha 40hp misfire

Compression test?
CDI electronics has aftermarket???
They also have test procedures.
Outboard ignition . com has test procedures too and has parts.
ANY????? Dirt in the carbs at ALL???
Check the fuel pump diaphram.

 

[99yam40]

Re: yamaha 40hp misfire

As always test compression, spark, and timing to start out with.
Then on to the fuel .
Fuel pump leaking into pulse port will cause #3 to not fire properly so check that to start out with.

Best to post full model # so everyone knows what motor you are dealing with.

Here is some troubleshooting guides for the older Yamaha ignitions, but it is best to pick up a service manual for the motor you are working on. Yamaha starts on page 97 I believe.
CDI Electronics Practical Outboard Ignition Troubleshooting

 

[Bosunsmate]

Re: yamaha 40hp misfire

Its a 1990 yamaha 40HEO 6H4S, The model# is seen only (i think) in aus/nz so it doesnt seem to come up on part diagrams but it is seemingly the same configuration to the others listed as 1990.
Ive just bought a compression tester today so i will post readings when i get it back the few hundred kilometres to where the boat is soon.
There was some yellow stuff in the carbs which i think is called gum which i removed.

I was just wondering can a compression problem cause a miss (if so how?) or is that just part of a useful state of the motor check up?
Also how do i do this? "Fuel pump leaking into pulse port will cause #3 to not fire properly so check that to start out with."
Do i remove the fuel pump to do this? Im not familiar with what a pulse port is either.
Great to hear there are aftermarket cdi's

Thanks both of you for your advisories.

 

[99yam40]

Re: yamaha 40hp misfire

pulse port is where the crank case delivers the pressure/vacuum pulse to the fuel pump diaphragm.
Remove the bolts holding pump on motor without disconnecting the hoses, pump the primer bulb and see if fuel leaks out of pulse port on pump.

Or you can take pump apart and inspect diaphram for pin holes and tears.
proper compression, good strong spark, and timing(spark at correct time) and the proper fuel/air mix is required in each cylinder for that cylinder to fire properly.

Cleaning carbs properly usually needs for carbs to come apart completely and all jets and passages cleaned out with cleaner and compressed air to blow the out. Sometimes it may take ultrasonic bath cleaning.
Then follow the service manual steps on link and sync after reinstalling to make sure carbs and linkages are all set properly

 

[Bosunsmate]

Re: yamaha 40hp misfire

Ok thanks, il post later with what i find, unfortunately it has no trailer so this is all done in the cobbles on the beach

 

[Bosunsmate]

Re: yamaha 40hp misfire

Update.
I tried my timing light on it and there was no change in the strobe when it misfired so i went on to check the compression. My gauge didnt want to work as i had a cross thread in the connector so i didnt get a reading.
However i did the thumb test and got virtually no pressure on my thumb on all 3 cylinders. I replaced the sparkplugs and tried turning the flywheel by hand and it turned really easily. Ive a 50hp and it would never turn even a quarter as easy as that.
I ran the motor, it doesnt accerelate, even just on muffs so has no power but the head got really hot within a minute on idle, even with the telltale going.
Im wondering if a blown head gasket sounds like a possible cause here? I cant imagine any other reason for (seeminlgy but unofficial)low compression on all three cylinders. Apparently the overheat alarm went off just before she lost power, there was no mechanical destructive noise at the time.
I suspose it is possibly was all caused by a blocked thermostat, the water is still coming out the telltale all right but perhaps its not circulating internally

 

[99yam40]

Re: yamaha 40hp misfire

running lean fuel/air mix in cylinders due to plugging jet/passages can cause damage to pistons and cylinder walls from getting too hot . lean burn is a hot burn. bad Stat and/or plugged water passages can cause over heat and damage also.

A motor without compression is a waste of time trying to fix other things
blown Head gasket or cracked head are a possibility for low compression along with rings not sealing

Are you sure there was oil in the system, but lack of oil usually will get bearings 1st

Did you test all plug wires with timing light while looking for miss?

 

[Bosunsmate]

Re: yamaha 40hp misfire

Yep definitely oil in there, the mixture on the plugs was oily and when i took off the carbs the oil feed lines had oil on their outlets.
Tested all three plugs and no miss with timing light.
Is there any way to check if a lean burn caused this and is still going on causing the overheating? (ive sprayed fuel directly into the carbs with no improvement)
Should i confirm low compression or is being able to spin the flywheel with a couple of fingers under the flywheel guard really easily not how it should be like at all? I can pretty much only just notice a compression stroke happening slightly in only one cylinder while i spin it with all plugs in. Thats done when shes being warmed up too
thanks

 

[Bosunsmate]

Re: yamaha 40hp misfire

Its a bit of a harsh thing to do getting you guys to diagnose with no compression readings so i will go again and get them and post them as ive fixed the compression tester. I will try and whip off the LU too

 

[99yam40]

Re: yamaha 40hp misfire

Hard to believe a motor with no compression would even run.
Yes take reading and post what you get

 

[Bosunsmate]

Re: yamaha 40hp misfire

Hard to believe a motor with no compression would even run.
Yes take reading and post what you get

You are quite right 99yam40 seems my compression testing technique is worse than the compression tester i bought. I checked the tester on my ride-on lawnmower and got the reading back in spec. I then went to check it on the outboard but it just gave up again, so ive given up on this so called tester and have taken it to bits.

I decided to take the motor home as the boat has no trailer. Ive removed the LU, i like the look of the gearbox shift lever sealing cap. The impeller needs replacing but it isnt that used and the thermostat was dirty but not clogged and it opens well when heated in a saucepan. Im wondering if their hose is not powerful enough as its run on a header tank, but thats an investigation for another time.
I couldnt get readings with the compression tester but since ive got it home and taken the flywheel cover off to spin it manually it does not seem too bad. Il try and get hold of another compression tester but i think it can be ruled out at this stage as the reason for the miss.
To support that big call of mine (considering my stuff ups on this) is that i removed the compression tester from the top two plugs and it was wet yet when removed from the bottom cylinder it was dry.
I retried it and it was repeatable. Ive cleaned the bottom carb. Im thinking it may be the inlet reeds or the bottom crank seal.
I have really made a meal of this one.
I suppose i should crank it and see if there is some manifold vacuum on the bottom cylinder. Anything else i should be looking at for this bottom cylinder problem?
Ive put some ring freer spray into the cylinders just in case..
I cant explain their warning sound when they took it out still though.
Thanks again

 

[Bosunsmate]

Re: yamaha 40hp misfire

Ive taken the carbs off and noticed when i turned the flywheel id get a burst of air back out the bottom manifold. Ive added a video to it below.
Is that normal or are the bottom reeds not sealing or do they need a fast rpm to seal properly?
Its only happening on the bottom cylinder (the one that the plug was keeping dry on)
Sorry for the filming quality, i cant excuse myself by saying i did that while on the water
Im sure its happening on the exhaust stroke on the bottom cylinder but i will double check-
Yes it is, it happens on the exhaust stroke of the bottom cylinder

 

[99yam40]

Re: yamaha 40hp misfire

Go ahead and pull and inspect the reeds while you have it down that far already, that could keep fuel/oil mix from being drawn into the crankcase and oiling everything and stop that one cylinder from firing properly.

But it would not keep compression from forming.
You still need to get a good compression check of all cylinders.
Kind of strange the tester works on other motors but not that one.

 

[Bosunsmate]

Re: yamaha 40hp misfire

il do that this morning and post what i see.
The tester didnt work again on the lawnmower when i got it back home, i was very tempted to bash it instead of dismantling it. Ive discarded it and will borrow one hopefully later this weekend.
cheers

 

[Bosunsmate]

Re: yamaha 40hp misfire

Well that wasnt very constructive but i did learn a further thing about how this motor is designed.
I took the reeds off and ( although i should of noticed it earlier) I noticed that the bottom reed cover is constructed differently to the others. There is a small hole which goes through the plate cover and is matched on the carb by the small black gasket ringed hole (seen in photo). It seems its a pulse port for something and was where the exhaust pressure comes out on the exhaust stroke. i swapped the top and bottom inlet reed unit anyway but its still the same, so the reeds are operating correctly it seems.

I looked into the crankcase. Everything looked ok . There is though a slight difference in colour between the crankshaft counterweights. Im not sure if that means anything so ive posted a picture, the one that is more shiny is the one from the bottom cylinder. (photo 4) although its hard to see in the photo.
I will now dismantle and clean the bottom carb again

 

[Bosunsmate]

Re: yamaha 40hp misfire

Update.
Carbs are clean,
Ive put it back together.
Im thinking i will restart it and check the bottom carb is getting vacuum.
I noticed that i get spark from the grounded sparks while ive being turning it, so can i start it by rope starting or jumping the starter or do i have to go and get the controls off the boat? (YES i found out you can and i just killed it by blocking the carbs to choke it.
Il need a way to kill it so does anyone know the killwire colour/location? (Killwire is one of the top pins in the connector)
I should be able to manual choke her with some cardboard or something ( i hooked up a jumper wire at the connector to the blue choke wire to run the choke)

 

[Bosunsmate]

Re: yamaha 40hp misfire

Update
I restarted her. She has vacuum in all carbs and the fuel primer stays firm. I checked after it misfired and all three carbs have a lot of fuel in them.
I did a compression check with a proper tester and they were all good- in order-
102psi
100psi
99psi
I restarted it and pulled in turn a plug wire out and grounded it. Still a misfire happened with each plug out
I took gearbox off and ran it without water for a few seconds and the misfire still happened so i can assume its not caused by water intrusion. Im back to thinking its the powerpack.
Can a timing light confirm that? Any other suggestions as i dont think ive any
Il check the fuel is ok by running it in another motor but it was fresh only a couple of weeks ago

 

[Bosunsmate]

Re: yamaha 40hp misfire

Ive just bought a tested second hand cdi for it so i will report back if that fixes it. If it doesnt i will just resell it. It was going to cost me $100 at a dealer to test it so i thought id take the risk in that i have tested everything except the cdi and the flywheel magnets, which to me only leaves those two by way of elimination.

Im picking that when its malfunctioning (which it does intermittantly) its sending a firing signal at a weird time probably a very advanced spark as it almost stops the motor in its track during the misfire so im glad it hasnt done much damage (that i can observe at least.)
I could test with the timing light at night but i think il just hang on and wait until i change over the cdi to keep my self entertained wondering whether the rest of my testing was right

 

[Bosunsmate]

Re: yamaha 40hp misfire

Ive just bought a tested second hand cdi for it so i will report back if that fixes it. If it doesnt i will just resell it. It was going to cost me $100 at a dealer to test it so i thought id take the risk in that i have tested everything except the cdi and the flywheel magnets, so it only to me leaves those two by way of elimination. To me i dont think that it would be the flywheel magnets as its intermittent.

Im picking that when its malfunctioning (which it does intermittantly) its sending a firing signal at a weird time probably very advanced as it almost stops the motor in its track during the misfire so im glad it hasnt done much damage (that i can observe at least.)
I could test with the timing light at night but i think il just hang on and wait until i change over the cdi so i can keep myself entertained wondering whether the rest of my testing was right