Rough idling / stalling 175hp Yamaha saltwater series

[SunnyCoastrunner]

I have a 175hp Yamaha saltwater series and when it's in the water it runs rough at idle at about 500rpm and sometimes stalls but runs sweet at anything above idle. Then when I take it home and put it on the earmuffs it idles perfect at 700/800rpm.
This has been going on a while and I have completely emptied fuel tank and cleaned, taken carbies apart and cleaned jets bowls etc after dolphin marine reckon they cleaned carbies $340 which is crap cause I found deposits in fuel bowls that I had to scrub out with toothbrush and carbie cleaner!
I also replaced all spark plugs.
Is there anyone out there who can help me solve this problem any ideas would be great thanks!

[ballsie]

Is this 500 rpm in gear holding the boat on trailer or in neutral or completely off the trailer

[Jarrah Jack]

Get a compression check. If its on the low side then the extra back pressure from being in the water can cause what you've described.

[captain rednut]

GET THE CARBIES DONE AGAIN. and then it will be sweet.

[Fed]

500 RPM could be a bit slow have you checked the specs?

[SunnyCoastrunner]

In neutral it won't stall but in gear at idle it stalls

[SunnyCoastrunner]

500 RPM could be a bit slow have you checked the specs?
It used to idle in gear at 700rpm so that's what I'm aiming for

[SunnyCoastrunner]

Get a compression check. If its on the low side then the extra back pressure from being in the water can cause what you've described.
Smart thinking. I will buy compression tester and check that all 6 cylinders are the same unless you know what pressure they should be

[SunnyCoastrunner]

GET THE CARBIES DONE AGAIN. and then it will be sweet.
Thanks I might test compression and if that's ok I will strip carbies and clean again as it's quite easy to do not much to them

[Jarrah Jack]

Smart thinking. I will buy compression tester and check that all 6 cylinders are the same unless you know what pressure they should be

Don't know what they should be but I do know that they should be within 10% or so of each other.

My new boat had the same problem recently and the cause was found to be the old merc fuel lines which were desintergrating inside causing a partial fuel block.

[Noelm]

Buy a genuine manual for your motor if you don't have one, Yamaha carbies are NOT easy to clean properly, they have itsy bitsy passages that need to be mechanicly cleaned, blowing with air, and washing will not clean then properly, also buy kits for them, and replace all the bits supplied and to the specs in the manual, your idle is too low, and could be a simple setup and adjustment, that will be in your manual.

[ballsie]

Had a similar problem with my 150 saltwater. Got mick at Coorparoo marine to clean out gunk and reset carbies. They blocked so much that fuel was leaking out of the seals. I think the manual says revs should be 700 to 800 rpms on these models

[fish50]

Sounds very similar to an issue i had with my 130 saltwater series - in my case it was pin holes in fuel tank so constant water getting in fuel - I spent a fortune thinking it was the motor, new tank and runs perfect again, sounds like you have checked the fuel tank issue already though.

[SunnyCoastrunner]

Don't know what they should be but I do know that they should be within 10% or so of each other.

My new boat had the same problem recently and the cause was found to be the old merc fuel lines which were desintergrating inside causing a partial fuel block.

Yeh I have heard of that happening! Are the fuel lines your talking of grey colour and quite stiff ? Unlike rubber automotive fuel line that is black and very flexible