Adjusting air/fuel 85 hp

[Matthew A.]

Hi everyone,

Wow! been awhile since I've visited this forum. Greetings all.

1986/88 Force 85 hp.
I am having a bit of difficulty in adjusting my motors air/fuel mix. It would seem that when I attempt to find the point where the motor is running too rich before setting the mixture halfway between too rich and too lean, the air/fuel screws on all three carbs can be backed out until no longer threaded [or damn close] before the motor shows signs [running rough/low idle] it's about to die. The idle speed is set to 650 to 750 rpms at 0 degrees advance.

All carbs adjusted 1/16 to 1/8 turns each while engine running
Lean condition reached @ or near 1/4 turns out from seated [all carbs]
Choke butterfly's fully open on all three carbs.
Air/fuel mix screws initially set @ 1 1/2 turns from seated
Warm engine
Fresh fuel/oil
New plugs
Sea level @ 75 degress F. and 30 percent humid.
25 hours on full rebuild
Throttle butterfly's in sync for all carbs.
Any idea's what's goin on [meaning...how come air/fuel screws can be backed out so far without much affect on run condition of engine.]

 

[pnwboat]

Re: Adjusting air/fuel 85 hp

Sounds like the motor is running too lean period. I might be wrong but to start out I'd check the carbs first. Floats may be set too low. If there is any foreign matter in the float bowls, a thorough cleaning... maybe a re-build is in order. Next, fuel pump diaphram (not pumping enough fuel) or fuel line/fuel pump filter screen obstruction.


Ed

 

[Frank Acampora]

Re: Adjusting air/fuel 85 hp

Your low speed circuit is not getting any/or enough gasoline. Clean the carbs! Completely disassemble them and run a pipe cleaner soaked with WD40 through the thin brass idle tube. Be sure to squirt WD40 through all passages with the needles totally removed. WD will remove varnish almost immediately and will not harm the sealing compound used on the welsh plug.

 

[Matthew A.]

Re: Adjusting air/fuel 85 hp

Hiya Frank....nice to see ya.

One of the things I did while undergoing my recent rebuild was to completely rebuild the carbs. Replaced just about everything there is to replace as far as carb inerds. Float adjustment height might not be quite correct since one of the three carbs is of a different year motor with different spec listed for float height. So I spec'd the all the same regardless.

Despite going thru everything, electrical and fuel system, paying special attention to sync'ing the throttle valves, timing, voltage, rebuilt fuel pump, replacing all three of it's check valves. new fuel filter canister, new fuel hose and clamps,......the motor isn't getting upto the same WOT rpms as it used to before the rebuild. 4350 rpm at 24 + mph untrimmed and 4500 rpm at 26 + plus mph. boat trimmed.

The motor has around 30 hours since the rebuild. Might this be a reason for the motor not revving as high as I would prefer?

Plus the we're getting really shi##y fuel mileage. Averaging 2.7 mpg....even in relatively flat seas. Normally we'd get around 3.2 on rough seas and 3.8 or so on calm seas.

 

[Frank Acampora]

Re: Adjusting air/fuel 85 hp

Hi Matt! You know, I say it time and time again but nobody listens. These carbs are jetted and air-bled for expected full throttle airflow through the venturi.

A little lesson so you understand: A venturi is a narrowing in a pipe or cylinder. Think of it as a circular airplane wing. According to Bernoulli's principal, air flowing through a venturi must travel faster than the air in the other sections of pipe and so has a lower pressure than the surrounding air. Thus relatively speaking, a partial vacuum is formed and fuel is drawn up the dip tube and atomized in the airstream. Also according to the principal, the faster the air flows, the greater the pressure drop. Thus: as the throttle is opened wider, the venturi will draw more fuel than extra the air requires and the engine will run richer. That is why there is an air bleed circuit in the carb to keep fuel draw constantly at the correct ratio. (different than the low speed air bleed.) On some carbs, looking into the throat, you will see it right on the side of the venturi. In others, it is a small drilled hole somewhere near the venturi. There is also an air bleed on the carb casting exterior but this is to maintain atmospheric pressure on the fuel in the bowl to avoid fluctuating fuel draw. The venturi air bleed admits air to the fuel at or near the dip tube; it does not bleed air from the venturi.

The low speed circuit operates differently than the venturi and in these carbs in some respects is completely independent. In other respects, the two are related as you will read further on.

The jet only limits the total amount of fuel passed to the venturi and low speed circuit at full throttle. The air bleed circuits (there may be one or more)regulate the amount of fuel passed by the low speed circuit AND venturi at partial throttle. The low speed circuit is always at manifold vacuum pressure at all throttle positions and therefore always draws fuel. That is why leaning the low speed needle at idle will run the engine lean at full throttle and melt pistons.

It is Ok to interchange carbs from one engine to another, however, the interchanged carb(s) must be adjusted to the specs of the engine from which they came. For example: a Chrysler 75 has jets about .066 and the 120 with WB carbs has jets about .086. If you are using carbs off a Chrysler 120 on a three cylinder engine--for whatever reason- Then the jets must remain the jets installed in the 120, not the jet size for the 3 cylinder. Same thing with float height adjustment.

Your logic in adjusting the different carb float height was flawed. The float height is set to account for the fuel draw characteristics of the venturi. The float height regulates the height of the fuel in the bowl and thus the distance it must be drawn up into the venturi. The float's purpose is to keep fuel level constant to avoid fluctuations in fuel draw. The further fuel must be drawn, the greater the atmospheric pressure on it and the less fuel will be delivered to the venturi. As anyone who has run an engine dry will tell you though, It doesn't seem to make much difference until the fuel bowl is almost empty. We are not talking about a great change in distance.

One float out of adjustment should not cause the amount of problems you are reporting.

Let me think about this for a while and then get back.

 

[Matthew A.]

Re: Adjusting air/fuel 85 hp

Hi Frank,

Thank you for going into detail about the carb circuit. I did as you suggested and tore apart the carbs and cleaned them. I readjusted the float height for the one carb. I also removed the reed valve assembly since one of the reed pedals for no. 1 wasn't seated fully flat against the reed cage. With the motor turned off and due to the motion of the swell while at rest, the rise and fall of the water entering the exhaust chamber would cause the reed to vibrate just enough to sound like a wood instrument. Replaced the old one with a new one.
You're gonna laugh at this one...well maybe.

Anyhow, while reinstalling the intake/carb manifold to the block...I couldn't help but notice the three EXTRA!! bolt holes on the centerline of the manifold directly behind each float bowl. You know...the one's that are supposed to have bolts in them...What a dumb ***....WOOOOPS!
Gee,...I wonder if that could of had something to do the air/fuel adjustments seemingly unresponsive and the lack of power at top end?

Lucky that those shiny new pistons aren't shiny new paper weights.

Got the boat on the water today for a quick test and WOT rpms 5250 to 5400depending on trim. Throttle response like never before.

While I've got you here Frank...I need to add some oil to the power trim motor. I fail to recall...do so at half tilt, full tilt or lowered all the way?