76 Mercury 850 runs good on muffs but wont accelerat much past idle in water

kevmerc850

Recruit
Joined
May 27, 2021
Messages
4
im at wits end iv been through this engine from cowl to prop and still same result . the crazy thing is it ran like a brand new engine in march the next trip 2 weeks later couldnt even get out of the no wake zone. any suggestions would be amazing thank you in advance
 

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
38,975
They all sound good ( bark and roar ) on a hose.-----In the water open the low speed mixture needles 1/8th turn at a time .----Bogging might go away.
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,837
Get yourself an aerosol can of Sea Foam...at any auto parts store. Get a helper to run the helm. Remove the engine cowl and try the hole shot (pedal to the metal) without spraying into the carbs. Get a data point. Spray a squirt into each carb and immediately hit it again. If it roars at you, you have a fuel problem, usually caused by the low speed jets being turned in too far.

But on the water your engine needs fuel to burn to develop the HP that it doesn't need on muffs in the driveway. Fuel means clean fuel delivery lines, fuel pump diaphragm that works properly, inline filters that aren't clogged with old fuel fungus stuff, carbs clean and all that. Sometimes grasping the squeeze bulb and giving it a squirt will help to determine fuel supply problems...associated with the above areas. Bulb should be soft when running, not hard ( carbs not allowing fuel in...float level), or sucked in flat...blockage between tank and bulb.

On later engines the serv. manual says open slightly (some say a screwdriver blade width, others 1/16", others 1/8") all simultaneously, until the engine will take the hole shot without stumbling.....course trim and boat loading and all kinds of other "stuff" could be your problem......but maybe not.
 
Last edited:

kevmerc850

Recruit
Joined
May 27, 2021
Messages
4
Get yourself an aerosol can of Sea Foam...at any auto parts store. Get a helper to run the helm. Remove the engine cowl and try the hole shot (pedal to the metal) without spraying into the carbs. Get a data point. Spray a squirt into each carb and immediately hit it again. If it roars at you, you have a fuel problem, usually caused by the low speed jets being turned in too far.

But on the water your engine needs fuel to burn to develop the HP that it doesn't need on muffs in the driveway. Fuel means clean fuel delivery lines, fuel pump diaphragm that works properly, inline filters that aren't clogged with old fuel fungus stuff, carbs clean and all that. Sometimes grasping the squeeze bulb and giving it a squirt will help to determine fuel supply problems...associated with the above areas. Bulb should be soft when running, not hard ( carbs not allowing fuel in...float level), or sucked in flat...blockage between tank and bulb.

On later engines the serv. manual says open slightly (some say a screwdriver blade width, others 1/16", others 1/8") all simultaneously, until the engine will take the hole shot without stumbling.....course trim and boat loading and all kinds of other "stuff" could be your problem......but mayb
Get yourself an aerosol can of Sea Foam...at any auto parts store. Get a helper to run the helm. Remove the engine cowl and try the hole shot (pedal to the metal) without spraying into the carbs. Get a data point. Spray a squirt into each carb and immediately hit it again. If it roars at you, you have a fuel problem, usually caused by the low speed jets being turned in too far.

But on the water your engine needs fuel to burn to develop the HP that it doesn't need on muffs in the driveway. Fuel means clean fuel delivery lines, fuel pump diaphragm that works properly, inline filters that aren't clogged with old fuel fungus stuff, carbs clean and all that. Sometimes grasping the squeeze bulb and giving it a squirt will help to determine fuel supply problems...associated with the above areas. Bulb should be soft when running, not hard ( carbs not allowing fuel in...float level), or sucked in flat...blockage between tank and bulb.

On later engines the serv. manual says open slightly (some say a screwdriver blade width, others 1/16", others 1/8") all simultaneously, until the engine will take the hole shot without stumbling.....course trim and boat loading and all kinds of other "stuff" could be your problem......but maybe not.
Thank you so much for the suggestion it seems to me that the bulb stays medium not soft the tank is a new built in aluminum port side fill the other thing that seemed strange to me is the control handle has to be pushed what seems like hyper past what you would think WOT would be im gonna try your suggestions and will post back on the results again thank you for your reply.
 

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
28,420
That seafoam trick is unnecessary. If your carbs are clean, and all cylinders are firing it is likely that they are set too lean to allow acceleration. I do not know how seafoam will diagnose or fix your problem.

Put the boat in the water, with fresh 50::1 fuel in the tank. Start and warm the motor. Put her in gear and try to accelerate. If she stumbles or stalls, open the top carb idle mixture screw 1/8 turn and try to accelerate again. If she still stumbles, open the bottom carb idle mixture screw 1/8 turn and try again. Repeat sequence until motor accelerates smoothly and idle is reasonable.
 

kevmerc850

Recruit
Joined
May 27, 2021
Messages
4
That seafoam trick is unnecessary. If your carbs are clean, and all cylinders are firing it is likely that they are set too lean to allow acceleration. I do not know how seafoam will diagnose or fix your problem.

Put the boat in the water, with fresh 50::1 fuel in the tank. Start and warm the motor. Put her in gear and try to accelerate. If she stumbles or stalls, open the top carb idle mixture screw 1/8 turn and try to accelerate again. If she still stumbles, open the bottom carb idle mixture screw 1/8 turn and try again. Repeat sequence until motor accelerates smoothly and idle is reasonable.
Thank you so much for responding im at a point where i will try any suggestions as i said the tank is brand new lines, bulb, connectors, fuel, pump i cleaned the carbs and replaced gaskets i didnt , however replace mixture needles or screws i thought i had adjusted the carbs as much as i could but its apparent that i didnt do it correctly i will give an update ...... all suggestions accepted. again thank you
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,837
That seafoam trick is unnecessary. If your carbs are clean, and all cylinders are firing it is likely that they are set too lean to allow acceleration. I do not know how seafoam will diagnose or fix your problem.

Put the boat in the water, with fresh 50::1 fuel in the tank. Start and warm the motor. Put her in gear and try to accelerate. If she stumbles or stalls, open the top carb idle mixture screw 1/8 turn and try to accelerate again. If she still stumbles, open the bottom carb idle mixture screw 1/8 turn and try again. Repeat sequence until motor accelerates smoothly and idle is reasonable.
Engine will run on SF, big guns on here years ago attested to that. Idea in hitting the venturis with something combustible is that if the engine needs something combustible to put out the HP then SF will provide it and you will know you are lacking adequate fuel.....so you chase a fuel problem...like low speed jet in the carbs is too tight.
 

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
28,420
Mark, if you tune those carbs for smooth idle like everyone does, or leave them at initial settings, they will be too lean to allow acceleration. it is an idiosyncrasy of the side bowl carbs on Merc motors of that vintage.

I really doubt he changed the idle jets and they are too "tight". If you really want to add additional fuel to test your theory, simply hit the choke button just as you accelerate. That will add fuel.
 

kevmerc850

Recruit
Joined
May 27, 2021
Messages
4
Mark, if you tune those carbs for smooth idle like everyone does, or leave them at initial settings, they will be too lean to allow acceleration. it is an idiosyncrasy of the side bowl carbs on Merc motors of that vintage.

I really doubt he changed the idle jets and they are too "tight". If you really want to add additional fuel to test your theory, simply hit the choke button just as you accelerate. That will add fuel.
Its been like monsoon weather here in Ga so i havent had the boat bact to the lake to try either of the tricks yall suggested. How likely is it that it could have somethnng to do with the reed valves?
 
Top