Merc V6 135 too much fuel & bogging down

[UKMike]

Hi all,

I wonder if someone can advise, over the last couple of months my motor has not been running right, it's getting far to much fuel, even to the degree where when I switch it off and tilt the engine all the way up I get fuel running out of the air box. When the boat is on WOT it runs great but up to about 3500 revs it bogs down. I know it's fuel as it smells fuel coming out of the exhaust. I also know it is firing on all cylinders as I have strobe lighted all 6 leads and it's not running abnormally on idle.

2 years ago I replaced the floats and the jet and shutoff needle and I have reset the floats 2 times now trying to resolve this issue but it doesn't seem to have resolved it. I also changed the fuel pump diaphragm last year and the year before so I can't see it's a fuel pump issue. I also replaced all the jets 2 years ago with a full carb rebuild. The engine is really smoky on idle and I have deduced that this is a result of the additional fuel as the premix pump has also been replaced trying to resolve this issue. I have put in new boyesen reeds to help with the back pressure but again still no change.

I'm running out of ideas so if anyone could add other ideas of what it might be or how I can resolve the issue I would be really grateful as I've just about replaced everything on this engine but I still can't stop it flooding .... Grrrrr

Best
Mike
 

[UKMike]

To add to this, if you look at the reed block and lower end you can see that the top cylinder seems to be the one getting excessive fuel, it is bleeding down to the second set of carbs, this is evident via the black oil stains in the bleed return channels. Or am I wrong in my opinions?

Thanks for any help.

Best
Mike 

[Plugcheck]

I have had a similiar issue with my rides in the past, and nearly every time it is related to fuel bowl level.  Aftermarket kits that I had purchased, forgot from where, didn't last a year and leaked past the needle.  What I ended up doing was making a fixture to hold carb, feed it with a facet pump at 3psi, and check/adjust fuel levels.  I've had newly rebuilt carbs leak, not all kits are the same.  Just a side note, a fixed fuel/oil mix 50/1 I've noticed does smoke a bit at idle.  The VRO's, are not fixed in ratio, it changes the mix throughout RPM range. 

[UKMike]

Thanks Michael,

I was thinking it can't be anything other than the shutoff valve in one or more carbs, definitely the top for sure looking at the reed plate signs but I can't believe when paying so much for the carb kits that they could not work as they should. I have even reset the floats to hang 1 mm rather than the 1.6 mm to shut off the fuel earlier but it's still flooding, can you think of anything else that could cause this issue? I was wondering if the fuel pump was over pressurised and forcing more fuel in but I would guess that if it was over pressurised it would be less likely to overfill the bowls, and maybe if it's under pressurised it could be weeping more fuel passed the shutoff valve... It's so annoying so any advice or thoughts appreciated.

I wonder if I could build a similar device or maybe use a syringe to pump fluid into the bowl with the top off so I can see if the floats do shut off before the fluid reaches the top... mmmmm

[UKMike]

Another thought, but I don't know if it could possibly make a difference but what if I have not put item 12 on one of the jets? Would that cause the issue because the jet would be lower in the unit and further away from the float tab?

[Plugcheck]

Absolutely part #12 is critical.  It sets the seat height for the needle valve.  Most carb kits, at least for automotive stuff, come with several washers to set the seat height.  In most cases there is the measured height upside down(needle closed) and the droop height of the floats.  Fuel levels in the bowl are critical to the air)fuel ratio.  High, you'll run fat, low, you'll run lean on main jet. 

[UKMike]

Thanks Michael,

I think I need to get another set of jets, the floats are all new also, although the old ones were original and all working well. Do you think I should go back to stock parts where possible, even if it means putting in the old floats, which are brown in colour, all the new aftermarket ones seem to be white.

Thanks again for your thoughts.

Mike.

[UKMike]

Quite a difference between the Mercury original part on the left against the aftermarket part on the right.... length and quality are not the same...

[Plugcheck]

I'm certainly a fan of fuel injection, but I cut teeth on carburators in the 70-80's.  Even factory carb kits require  a bit of massaging.  Probably why I'm a fan of Weber and Dellortos, every metering orifice can be changed.

[Rich_V174SS]

Use OEM carb kits where possible. I've seen differences in aftermarket kits, some were useless. Float shut-off heights are critical and if the parts list shows the washer is required between the seat and carb body then is needs to be installed. I set the floats so that when the carb is held upside down the float is level/parallel with the bowl flange of the carb body.

[UKMike]

Thanks both,

I'm going to try and get some OEM replacements and do exactly as you say. I know the measurement should be 1.6 mm with the floats hanging upside down but as I said I have already set them at 1 mm and still got the issue, must be the shutoff valves, so a replacement and setting them level must surely sort the issue out.

Thanks again for the advice and thoughts.

Best
Michael.