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Author Topic:   Yamaha F20 four stroke rough mid range

tjxtreme posted 09-24-2012 07:26 PM ET (US)
I took my carbed four stroke Yamaha 20 hp out for the first time in four or so months yesterday. It ran fine at first, then seemed to misfire in mid rpms. I could get past it and get on plane and it ran fine. It idled smoothly, and revved smoothly in neutral.
I will go through the fuel system next, but pulled the plugs first. I will replace them today, but could someone help me read these? The bottom one was very slightly wet with what smelled like gas, and also a little grey on top. I take this to mean that the plug went bad and therefore there was excess gas that never burnt.
plugs
http://i1022.photobucket.com/albums/af349/tjxtreme/Repair/photo19.jpg
As an aside, the spark plug hex size is 18mm. It is very narrow in there by the head, and the deep 18mm I'd bought didn't fit. Fortunately I had access to a machine shop and was able to lathe down the socket just enough to fit (after reading about this online). Also, apparently some socket brands are thin enough to fit, but many don't.
customized 18mm socket
http://i1022.photobucket.com/albums/af349/tjxtreme/Repair/photo20.jpg

Thanks!

weekendwarrior posted 09-25-2012 03:28 PM ET (US)
That bottom plug looks too white to me. That usually means it's running too lean on that cylinder, not getting enough gas. My guess is the carburetor for that cylinder is clogged up.
tjxtreme posted 09-25-2012 08:27 PM ET (US)
Thanks for the reply. I could be wrong, but I think it only has one carburetor. Also because the plug is wet with gas I don't understand why it would be lean.
weekendwarrior posted 09-26-2012 11:32 AM ET (US)
I understand your point, but if it were getting too much gas I would expect it to be black and sooty. Gas on the plug doesn't necessarily mean too much gas, it just means that all of the gas isn't burning. If it has only one carburetor then that does change things. Try new plugs before spending too much time on other things.
tjxtreme posted 09-26-2012 12:49 PM ET (US)
Ok, thanks... I will take it out soon to test it under load. Got the new plugs in, gapped and torqued to spec.
I agree- keep it simple and get more complex only if needed. The only other thing I did was to inspect and clean the fuel filter.
pcrussell50 posted 09-26-2012 02:44 PM ET (US)
The trouble with reading plugs is, it is most valid for the conditions prevailing the instant before you shut it off and pull the plugs. For example, if you go out and run hard, then idle back through the no-wake zone for five minutes, you will only get a picture of the mixture situation during the idle back.
As far as the one carburetor situation goes, it is still plenty possible to have uneven mixture distribution if one of the intake runners is longer or of a different shape than the other.
-Peter
tedious posted 09-26-2012 05:47 PM ET (US)
Carbed motor, 4 month layoff, running rough = crud in the carb(s). Time to rebuild it / them.
Tim
tjxtreme posted 10-01-2012 12:07 PM ET (US)
A bad spark plug seems to be the problem. I finally had a chance to take it out yesterday and it ran wonderfully at all RPMs under load.
I forgot to note earlier that I use ethanol-free gas with stabilizer.

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Author	Topic: Yamaha F20 four stroke rough mid range
tjxtreme	posted 09-24-2012 07:26 PM ET (US) I took my carbed four stroke Yamaha 20 hp out for the first time in four or so months yesterday. It ran fine at first, then seemed to misfire in mid rpms. I could get past it and get on plane and it ran fine. It idled smoothly, and revved smoothly in neutral. I will go through the fuel system next, but pulled the plugs first. I will replace them today, but could someone help me read these? The bottom one was very slightly wet with what smelled like gas, and also a little grey on top. I take this to mean that the plug went bad and therefore there was excess gas that never burnt. plugs http://i1022.photobucket.com/albums/af349/tjxtreme/Repair/photo19.jpg As an aside, the spark plug hex size is 18mm. It is very narrow in there by the head, and the deep 18mm I'd bought didn't fit. Fortunately I had access to a machine shop and was able to lathe down the socket just enough to fit (after reading about this online). Also, apparently some socket brands are thin enough to fit, but many don't. customized 18mm socket http://i1022.photobucket.com/albums/af349/tjxtreme/Repair/photo20.jpg Thanks!
weekendwarrior	posted 09-25-2012 03:28 PM ET (US) That bottom plug looks too white to me. That usually means it's running too lean on that cylinder, not getting enough gas. My guess is the carburetor for that cylinder is clogged up.
tjxtreme	posted 09-25-2012 08:27 PM ET (US) Thanks for the reply. I could be wrong, but I think it only has one carburetor. Also because the plug is wet with gas I don't understand why it would be lean.
weekendwarrior	posted 09-26-2012 11:32 AM ET (US) I understand your point, but if it were getting too much gas I would expect it to be black and sooty. Gas on the plug doesn't necessarily mean too much gas, it just means that all of the gas isn't burning. If it has only one carburetor then that does change things. Try new plugs before spending too much time on other things.
tjxtreme	posted 09-26-2012 12:49 PM ET (US) Ok, thanks... I will take it out soon to test it under load. Got the new plugs in, gapped and torqued to spec. I agree- keep it simple and get more complex only if needed. The only other thing I did was to inspect and clean the fuel filter.
pcrussell50	posted 09-26-2012 02:44 PM ET (US) The trouble with reading plugs is, it is most valid for the conditions prevailing the instant before you shut it off and pull the plugs. For example, if you go out and run hard, then idle back through the no-wake zone for five minutes, you will only get a picture of the mixture situation during the idle back. As far as the one carburetor situation goes, it is still plenty possible to have uneven mixture distribution if one of the intake runners is longer or of a different shape than the other. -Peter
tedious	posted 09-26-2012 05:47 PM ET (US) Carbed motor, 4 month layoff, running rough = crud in the carb(s). Time to rebuild it / them. Tim
tjxtreme	posted 10-01-2012 12:07 PM ET (US) A bad spark plug seems to be the problem. I finally had a chance to take it out yesterday and it ran wonderfully at all RPMs under load. I forgot to note earlier that I use ethanol-free gas with stabilizer.