8hp mariner/yamaha misfire spluttering

deeperblue11

hi all.
i have a 8hp mariner/yamaha engine. the year is 1988 i think. the problem is that it starts to miss fire/splutter when it gets warm!
when i start it first i can travel for about 5 minutes on full throttle before it starts to splutter. if i ease of the throttle it stops for a few minutes then starts again. if i ease off the throttle another bit it stops again for a another minute and starts again.

i find that eventually i have the engine down to less than half, maybe quarter throttle before it stops all together.

if i turn the engine off for half an hour then start it again it runs on full throttle for a while before starting to miss fire again and so on.......!!!
can anyone help??????

fergal.b

The first thing to check is that the air vent on your fuel tank is open, next I would be looking for a leak in the fuel line or primer bulb where air might be getting sucked in after the bowl on the carb is empty.
It sounds very much like a fuel starvation problem.


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deeperblue11

fergal.b wrote: »

The first thing to check is that the air vent on your fuel tank is open, next I would be looking for a leak in the fuel line or primer bulb where air might be getting sucked in after the bowl on the carb is empty.
It sounds very much like a fuel starvation problem.


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i have checked the line and the vent is open. the problem only seems to happen when its warm?

fergal.b

Ok next make sure that the choke is not sticking and you have a good flow of water flow from the pee hole if they are all good I would be looking at rebuilding the carb and the little fuel primer pump diaphragm.

pedroeibar1

When did you last change the spark plugs?

deeperblue11

pedroeibar1 wrote: »

When did you last change the spark plugs?

thanks for your reply. new plugs at the start of the season.

deeperblue11

fergal.b wrote: »

Ok next make sure that the choke is not sticking and you have a good flow of water flow from the pee hole if they are all good I would be looking at rebuilding the carb and the little fuel primer pump diaphragm.

the choke is fine and good flow of water. i took out the carb and gave it a good cleaning. i did not blow out the jets but a friend of mine is going to do that for me. also the diaphragm didnt look great. not perfectly flat. i have one lined up on ebay. when i get it il get the jets blown also. i thought i would have it done by now.

if the diaphragm was gone or a jet blocked would the engine not splutter all the time??? it only seems to do it when it heats up!!!??

fergal.b

It may not be when it heats up but when it burns up the fuel in the carb bowl then the diaphragm will come into play. I'd say your jets are fine as it runs well for the first few minutes.




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deeperblue11

fergal.b wrote: »

It may not be when it heats up but when it burns up the fuel in the carb bowl then the diaphragm will come into play. I'd say your jets are fine as it runs well for the first few minutes.




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yeah it runs perfect for five minutes or more on full throttle then the miss starts.
if i turn the throttle down slightly it stops for a minute then starts again. i then turn the throttle down a little more and it stops again for a minute. this keeps happening until i eventually have the throttle down to quarter rev!! then it runs fine, and slow!!

its the first time the engine ever gave any trouble. usually it goes like a bee.

i know a lad who had the same problem with his and it turned out to be the head gasket!!

fergal.b

You can do a simple compression test to check if the head gasket is gone or have a look at the spark plugs to see if one is wet and clean looking, do you think it's getting hotter than normal or can you see steam coming out with the exhaust water.




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deeperblue11

fergal.b wrote: »

You can do a simple compression test to check if the head gasket is gone or have a look at the spark plugs to see if one is wet and clean looking, do you think it's getting hotter than normal or can you see steam coming out with the exhaust water.




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i dont think its getting hotter than normal, the water that comes out the pee hole is only luke warm after a while. and i havnt noticed any steam.

how can i do a compression test??? and what will a wet clean plug mean??? (wet with oil or water???)

breghall

does this issue only happen when you go to ease off on the throttle?? it sounds by the original post that it happens after 5 mins at full throttle... does it always happen after it's been running never after 30 secs or a min after start up?

fergal.b

If there is water getting into a cylinder it will clean the carbon off the electrodes on the plug. To do a compression test you need one of these you can get them in any motor factors, just unscrew the plugs and screw it into the hole and turn over the engine then do the other hole, if there is more than 10% between each chamber you have a leaking gasket or worn piston rings the bigger the difference would lean more towards the gasket. But as you said the engine runs fine for a while so if there was water in one chamber you would hear that the engine was running on one cylinder.

deeperblue11

breghall wrote: »

does this issue only happen when you go to ease off on the throttle?? it sounds by the original post that it happens after 5 mins at full throttle... does it always happen after it's been running never after 30 secs or a min after start up?

it only happens after 5 or 6 minutes of full throttle. not at startup or a minute after start up. when i ease of full throttle a little it stops for a minute the starts again and i ease off a little again and it stops for another minute and starts to miss fire again and i ease off a little again. this goes onn until i have the throttle down to about a quarter!! then it goes fine until i wind it up a little.

deeperblue11

fergal.b wrote: »

If there is water getting into a cylinder it will clean the carbon off the electrodes on the plug. To do a compression test you need one of these you can get them in any motor factors, just unscrew the plugs and screw it into the hole and turn over the engine then do the other hole, if there is more than 10% between each chamber you have a leaking gasket or worn piston rings the bigger the difference would lean more towards the gasket. But as you said the engine runs fine for a while so if there was water in one chamber you would hear that the engine was running on one cylinder.

i dont thing there is water getting onto the plugs because when i took them out to clean them they were a little brownish! burnt oil prehaps?? i cleaned them with a wire brush and wd40.

deeperblue11

does the engine have to be running to do the compression test or do i just pull the cord to turn it over.

fergal.b

deeperblue11 wrote: »

does the engine have to be running to do the compression test or do i just pull the cord to turn it over.

Just pull the cord a few times, you might be able to get a lone of a tester from your local mechanic just check that the screw in bit is for small plugs they normally come with both sizes when you buy the kit.




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deeperblue11

just looking at some on ebay. might get one, handy to have.
thanks for your help. il let you know how its going after the diaphragm goes in. also il let you know results of compression test.

fergal.b

deeperblue11 wrote: »

just looking at some on ebay. might get one, handy to have.
thanks for your help. il let you know how its going after the diaphragm goes in. also il let you know results of compression test.

They should be about 130 psi to be good.
Best of luck with it.



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breghall

i don't think it's a compression thing as it runs well on start up, if comp was bad it'd be playing up from the start, it wouldn't idle in gear well or at all, it'd be hard to restart with water in cylinders....

Matt123

Hi deeperblue11. Have u done what Fergal said? Did u get this fixed?

deeperblue11

yes got it fixed, cdi was gone. picked up one on ebay for 50 euro including postage. all's well now thanks.