C115hp Yamaha....stumped!!!

[nahanni]

Hello, a brief point form description of the problem.<br /><br />At idle, bottom left cylinder very weak.<br /><br />1. Good spark, even switched coils....no difference.<br />2. Compression 128lbs. consistent on all 4.<br />3. Removed and checked and cleaned carb, looks great.<br />4. Plug gets soaked. (Tried 3 new plugs in that cylinder)<br /> How I know it is weak is that it is the only cylinder that when idling, can remove spark plug cap and no difference in RPM. This situation started after idling engine in a lake for approx. 1 hour.<br />Engine bogs out, or dies if pushed over 4000-4200RPM. Sometimes it will surge in RPM but goes back to not taking full throttle. Will idle and run midrange all day.<br />Please help, any suggestions???

 

[rodbolt]

Re: C115hp Yamaha....stumped!!!

have you checked the fuel pump diaphram?<br /> they go bad and do bad things.<br />good luck and keep posting

 

[vinney]

Re: C115hp Yamaha....stumped!!!

Check your reed valves. Use a timming light with the air box off the front of the carbs. Watch for fuel spitting out of the carbs. If you have a bad reed valve it will be spitting fuel out of the front of the carb. Look at the hole you think isnt running.

 

[nahanni]

Re: C115hp Yamaha....stumped!!!

Hi Guys,<br />Couldn't wait, after reading your post, went out and checked the reeds and fuel pump. Everything looks like new. Pump works fine as it allows air through one way only and clean as new on the inside. Checked reeds, they are all sealing fine, no chips or cracks. Wonder if it is a CDI issue....even though there is fire, wonder if it is at the wrong time?

 

[vinney]

Re: C115hp Yamaha....stumped!!!

Use a timming light on each wire and watch the light or index the flywheel to each cylinder and watch the fire. They make a rpm prob you can attach to each ignition wire and read the rpms of each coil wire. If you think its ignition use a new set of spark plugs and run the engine and read the spark plugs if you have one bad hole you will see it on that plug.

 

[nahanni]

Re: C115hp Yamaha....stumped!!!

Thanks vinney, I'll give er a try. A buddy of mine has one of those RPM probes. Is there a chance that the cylinder may need to be decarbonized......I have heard that too much carbon can sometimes cause this condition?????? It did idle for an hour and then this problem immediately occurred.

 

[nahanni]

Re: C115hp Yamaha....stumped!!!

Me again.....used the RPM probe and all plug wires are signaling proper RPM's. Sprayed a can of POWER TUNE through the carbs and engine seemed to work better. Took full RPM and idled fine in lake. What is confusing is when I got back home, tried pulling the plug wire on #4 again, and the same thing, very little change. Yet, engine seems to be working fine in lake.????<br />p.s. Checked plugs after run and they are all burning consistent. Why does #4 not change RPM at idle when plug wire is pulled?

 

[vinney]

Re: C115hp Yamaha....stumped!!!

I'm not a big fan of pulling ignition wires off. If the engine turns the full rpms. at wot then its making power.

 

[nahanni]

Re: C115hp Yamaha....stumped!!!

I am thinking it may be a carb synchronization issue....gonna try that.