outboard backfire/fireing help please

shetland536

shetland536

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hi
yamaha 20hp twin cyl autolube . think its 1996-7.
i have a problem with constant backfiring.ticks over pretty well but gets really bad the more revs you use(every 3 or 4 seconds.
so far i have...
replaced the fuel,fuel line and primer bulb,checked lines to the carb.
i have a new diaphram and gasket kit on the fuel inlet on the side of the carb.
carbs have been stripped and cleaned.
checked earths.
when i run the motor and pull off either one of the ht leads it runs smooth.
when they are both on it runs rough again.
my next stage is going to be replace a coil at a time and then the cdi.
is there any bench tests i can do with the coils or cdi before i spend anymore money??
or any other ideas to what is causing this problem would be great.
thanks
lee
 
When you say back firing, spitting back through the carbs? That's known as a "lean sneeze" and is caused by a weak mixture. Usually due to dirty carbs (did you poke a soft wire through all the jets etc.) or to the mixture adjustment, if there is one, being wrong.

Any problems with fuel pumps and connections can usually be diagnosed by pumping the priming bulb.

By shorting one plug at a time you should have been able to pinpoint any problem with a plug or HT coil. Puzzled by it running better on one cylinder than two though.

CD unit, don't know, doubt it.

Just randomly replacing parts is not the way to go IMHO. You might just as well pay a mechanic to sort it as fork out for a whole shed load of parts you don't need.

I'd have another go at the carbs, if it were mine, before doing anything else. Not using stale fuel are you?

Try putting your question on the Yamaha board at iBoats but fully ID the model . You can confirm the year HERE
 
backfireing, chuffing smoke out the prop with a pop.i have not done any adjustments to any mixture screws,i have put every thing back as it was as it was running fine up till now.
i have been out about 4 times in the last 2 months so the fuel should be ok unless it was dodgy from the pump.took the jets out and they are clear or are there some i have missed,i do recal seeing some very fine holes which i gave a good blow,maybe i`ll go and do the wire thing now.
as for the serial number,there is not one which is proving to be a pain in the ....
i`ll go and pull em apart again now.
thanks.
 
Just some further thoughts, still on the lines that it's fuel related.
If it were fuel starvation due to blocked tank vent or fuel pick up from the tank the priming bulb would tend to be sucked flat by the pump.

If it is just a mixture problem due to what ever reason it would run OK if choked, at least if it has a strangler type choke. Maybe also learn something by operating a manual or electric pump as well if that's what it has.
 
the vent is open on the tank and i have tried with the cap off too.i have cut an angle at the end of the pick up pipe just incase it was directly on the bottom of the tank.
i have a manual choke,it stalls on tick over if i have it on but have not tried it with the throttle opened up a bit.
no pump as such ,just some matchbox sized thing with diaphrams and gaskets which i have replaced.
i have had the carbs apart and prodded holes with some wire and double checked the bowl jets.
i`ll go and chuck it back together now and try with the choke on.
 
It was running fine, then you did some maintenance, then it started playing up?
or
It was running fine, then it started playing up, then you replaced a few bits and pieces?

The chronology isn't clear, to me at least /forums/images/graemlins/frown.gif, in the posts so far.
 
sorry.
it was running fine up until the last outing on saturday,just leaving the harbour and opened the throttle up and the engine started to pop and shake,it would only run smoothly on very low revs,almost tick over speed. it ran like that for 2 hours to get back to the slipway.
i have got it all back together and its the same. whrn i pull the choke on it will stall at any revs.
 
Have you tried changing the spark plugs.........one of them may be breaking down or 'whiskered'?
Just had another thought, is the exhaust clear........if it's partially blocked somehow, that would choke the motor on higher revs?
 
I don't know the engine at all but FWIW a few thoughts...........

1. A shot in the dark. Reed valve on the way out? I suspect VicS will tell you whether or not that is a possibility. He'll certainly be able to say if it's worth following up.

2. It sounds like it may be worth getting hold of a manual and testing the readings on the electronics. Certainly cheaper than replacing bits until something works. I've had a coil go; it was fine one moment, faulty the next. £90 to replace but using the manual helped me to diagnose it.

3. It may be worth getting the flywheel off and making sure that nothing has working loose on the ignition system.

4. Check the vacuum pipes (if it has them) for the advance/retard mechanism.

I can understand you preferring to do it yourself than getting a mechanic involved!

Sorry. Wild guesses!
 
Sounds like a pressure issue if not a fuel one, split hose Intake or exhaust,

Just a thought - With your new plugs - did they foul up quickly afterwards? - that's probably the key, if they're not fouled or dried, it's probably a post ignition problem, if they're re-fouling (excessively) quickly then it's a fuel problem...
 
i have just let the mixture screw out 1 turn on both carbs and its running better but not great.
i`ve told the neighbours i`m not starting it again today cos it must be driving them mad,i`ve been running it on and of since 9am.
i`ll look into getting a manual,my suspicions are its the coil with an intermittent fault.
thank you all and keep it coming if you have any more ideas.
anybody got a spare coil i can try!?
 
i have checked the lines,but for the price of new i had might as well replace.
i`ll go and check the plugs now and see what they are like,i have not looked at them since i put them in.
 
have you tried pumping the priming bulb while it's running to ensure that it's getting enough fuel? (Sorry VicS, just spotted you'd mentioned this one already)
I'm probably showing my age here....does it have points or is it electronic ignition? I suspect it'll be the latter, if it isn't then check that the points aren't burnt and it would be a good idea to change the condenser.
It's got some strange symptoms that's for sure.......the one that's throwing me is the fact that it runs better on one cylinder than two..............you haven't got the HT leads transposed.....it shouldn't run if you have but we're clutching at straws now..
 
i`ve got a new priming bulb on today and have tried pumping it while its running,does not make any difference.
i has a cdi thing so i dont think it has points
the lengths of the leads suggests they are correct.
maybe it was a flook that it ran so well with one or the other disconnected,i`ll try it again tomorrow to make sure and i`ll change the leads over if i can or will that throw out the firing order?
i have had the air filter off most off today but it does not seem to make any difference.
 
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