Lost please help! 77 Mercury Blackmax 175hp bogging down on wot...

pcrowley1

Cadet
Joined
Apr 25, 2012
Messages
13
Not sure what else to try here??? Motor bogs down and stalls under full load. Things replaced/checked/repaired. Carbs rebuilt and new jets, new fuel pump, new power pack, 6 new coils, new plugs, compression @ 117-120 all cylinders, timing checked on the water is good, new fuel bulb, fresh gas, crank shaft resealed and no signs of any damage, lower unit replaced. This engine has been completely torn apart and no apparent reason why its doing what its doing. Engine starts and idles fine and runs great on a hose but as soon as its in the water and throttled it stalls out. Carb cleaner in the middle carb will get it over the hump but all the carbs have been removed, cleaned, new jets and rebuild kits 3 times now. Floats are fine too. Shop mechanics are completely baffled as well. Any thoughts/insight would be greatly appreciated! One other thing, the engine did overheat a cpl years ago as the water pump went out but upon inspection everything other than gaskets were fine. No scored cylinders either. HELP PLEASE!
 

Dukedog

Captain
Joined
Oct 6, 2009
Messages
3,235
Don't see anything about fuel pump or stator? Did ya engauge tha choke/enrichner when it started tha "bog"? Pretty dangerous squirting anything except a fuel/oil mixture in a runnin' 2 stroke motor. It will displace tha real mix and tend ta dry things (bearings) out.
 

enginepower

Petty Officer 1st Class
Joined
Jul 5, 2014
Messages
260
Like duke said, not a good idea to spray carb cleaner into engine as it has no lubricant but since you already did and noticed increase in response, you know you have a fuel problem. Does the bulb pump up and get hard? Does it relax after engine runs for a little bit? The check valve inside bulb could be bad. Also check your tank vent system for restriction. You can also try to get somebody to pump bulb while you are trying to throttle it to see if that helps. If it does, your fuel pump isn't doing its job even tho you said you replaced it, maybe its not getting a pulse signal.
 

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
27,036
Gee, Sounds like the idle mixture is too lean. Start richening it a bit (1/8 turn at a time) with the mid carb and see if it gets better. If you have no idle mixture adjustments, then the carbs may still be dirty or the floats misadjusted.
 

Faztbullet

Supreme Mariner
Joined
Mar 2, 2008
Messages
15,589
These carbs are fixed jets, nothing adjustable but linkage. Also engine should have butterfly choke and no priner unless converted. What was the bore size as I have seen worn out motors with decent compression, The old style 2L blocks are bad about this as way block was designed, I have seen these .010 oversize at top of cylinder and still have 110psi compression but just wont go. That block is different than a 150 block as exhaust chest and openings are larger("D") shaped.You can also open up roller to throttle arn distance to about 1/4 as sometimes extra timing before fuel will help..
 

DorkyDan

Recruit
Joined
Sep 1, 2014
Messages
3
I had a similar problem with my Johnson 235 HP. Turned out to be the Black/Yellow Stripe wire was shorting somehow in the Control Box. I installed a new wire all the way from the Power Packs to the front ... and added a separate kill switch (since there was no telling where in the control box the short was occurring).
 
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