Yamaha 8hp 4-stroke runs fine up to fast idle, dies out mid-range and faster

ncpi

Seaman
Joined
Jun 23, 2012
Messages
62
I have a 2005 Yamaha 8HP 4-Stroke. It acted up on me yesterday and I had to return about 5 miles to the ramp at fast idle speed (just past the start icon on the handle). If I tried to run faster, it would die out. It did run pretty smooth at idle/fast idle and would start just fine. Started and stopped several times, even let it cool off for about an hour one time (just in case it was overheating), didn't make a difference hot or cold...same problem.

Background: ran fine earlier this year, this was about the 5th time I had it on the water in the last month. Has factory fuel filter and a water separator filter, both changed last fall and look clean. Only filled up with 1 gal of fuel added to existing fuel in 6 gal tank yesterday. Always use blue stabil added to fuel. Checked plugs and both are a perfect tan color.

Any suggestions? Any common problems with this engine?

Does this carb have a high speed jet? While it seems unlikely with 2 filters, maybe it got plugged? Any good (effective) way to clean without pulling the carb?

Thanks, Bill
 

robert graham

Admiral
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Apr 16, 2009
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6,908
Re: Yamaha 8hp 4-stroke runs fine up to fast idle, dies out mid-range and faster

Sounds like a carb jet plugged with water/debris/dirt/whatever. Maybe drain the carb onto a clean rag and look for any foreign material in float chamber...but you're still going to need to remove carb and clean those jets and passages. I use Stabil and Ringfree and Fuel/Water separator, but I still drain my carbs several times per year and especially for any lengthy lay-ups of several weeks or months....it's about the only way to avoid plugged carb jets....Good Luck!
 

ncpi

Seaman
Joined
Jun 23, 2012
Messages
62
Re: Yamaha 8hp 4-stroke runs fine up to fast idle, dies out mid-range and faster

Sounds like a carb jet plugged with water/debris/dirt/whatever. Maybe drain the carb onto a clean rag and look for any foreign material in float chamber...but you're still going to need to remove carb and clean those jets and passages. I use Stabil and Ringfree and Fuel/Water separator, but I still drain my carbs several times per year and especially for any lengthy lay-ups of several weeks or months....it's about the only way to avoid plugged carb jets....Good Luck!

Thanks for confirming. Guess I need to pull that carb.
 

seamanben

Seaman Apprentice
Joined
Mar 12, 2007
Messages
31
Re: Yamaha 8hp 4-stroke runs fine up to fast idle, dies out mid-range and faster

Thanks for confirming. Guess I need to pull that carb.

Well I just came across your post. Hopefully it's working fine now. But if not, I had the same symptoms with my 8hp 4-stroke yamaha and the shop said it was the "trigger". They replaced the trigger and it works fine. I'm not even sure what the trigger is... but you might look into that.
 
Joined
Sep 25, 2010
Messages
6
Re: Yamaha 8hp 4-stroke runs fine up to fast idle, dies out mid-range and faster

Cleaning the carb is a good idea. Just did mine on my T8. Use CRC carb cleaner and compressed air only. Don't use any brushes to clean the jets or passages. Very soft metal and they are easy to scratch. If that does not do the trick then check the trigger.
 

ncpi

Seaman
Joined
Jun 23, 2012
Messages
62
Re: Yamaha 8hp 4-stroke runs fine up to fast idle, dies out mid-range and faster

Thanks. Cleaned the carb, still need to run on the water. Can you explain to me the trigger; what it is, where it is located and how to test it?
 

raczekp1

Lieutenant
Joined
Mar 30, 2010
Messages
1,327
Re: Yamaha 8hp 4-stroke runs fine up to fast idle, dies out mid-range and faster

stop switchi stis"tigger"
to disconect switch folowe to cableb going from swich to electrical terminal, and disconect one of this cables
 

ncpi

Seaman
Joined
Jun 23, 2012
Messages
62
Re: Yamaha 8hp 4-stroke runs fine up to fast idle, dies out mid-range and faster

Are you saying the "Stop switch" is the "Trigger"? The "Stop Switch" wouldn't really make sense for my problem would it? The engine was running fine at fast idle (for a good hour) and consistently didn't run well above that speed. It seems like the stop switch would either fail or be intermittent...nothing about this problem is intermittent.
 
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