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1994 90hp 3cyl 2S runs great up to 5200rpm but bogs after that with throttle plate WOT

2

20hpmercury

New member
Rebuilt a number of Mercury's, but this one has me baffled.

Got a 1994 90hp 3cyl 2S that the previous owner rebuilt the powerhead.

I replaced stator, trigger, voltage reg, switchbox, coils and plugs and plug wires with CDI

rebuilt all 3 carbs per OEM manual, main jets .062/.064/.062

compression is 129/130/126 cold with throttle plates WOT

rebuilt fuel pump, new hoses, swapped fuel supply twice, no change

adjusted idle mix screws, no change

ck'd TDC of #1 piston with dial indicator and it matches the indicator

DVA +coils at idle is 180

oil injection is disconnected

pulled reed plates off, reeds look ok and hole to fuel pump is open

set timing to 20deg BTDC at 3000rpm in fwd gear

tried 19p and 21p props, no change

If I set the throttle cable for the plates to be WOT, it will bog about 5200rpm, if I set the throttle cable for the plates 3/4 open, it all runs great and maxes out at 5200rpm

Starving for fuel or ignition??

Any suggestions would be much appreciated.

Much Thanks.
 
Last edited:
faztbullet said:
Fuel.....
Click to expand...


Much thanks and had not thought of that. Running 91 no ethanol with 50/1 oil. Might try some 93 with ethanol, the best around here and see what happens. Hey tried everything else, lol's! also I think you can add octane booster, but do not know if that stuff works or not, nothing ventured nothing gained.
 
First thing I would do is to hook up a temporary fuel pressure gage at the carbs and see what pressure is being maintained.

I've had brand new fuel pump check valves screw up on me. Put the old ones back in and the motor ran fine! My point is that NEW parts are not always GOOD parts, so relying on them can trick you.

Jeff
 
fastjeff said:
First thing I would do is to hook up a temporary fuel pressure gage at the carbs and see what pressure is being maintained.

I've had brand new fuel pump check valves screw up on me. Put the old ones back in and the motor ran fine! My point is that NEW parts are not always GOOD parts, so relying on them can trick you.

Jeff
Click to expand...


Thanks for all of the suggestions, will try different fuel and install a fuel pressure gauge. I have rebuilt and installed 2 different fuel pumps with Quicksilver rebuild kits, you would think that would not be the problem, but maybe so. Thanks again and Merry Christmas and Happy New Year!
 
OB fuel pumps are simple devices that work well--except when they don't; and they don't for mystifying reasons at times. I've had brand new check valves screw up on me, yet the motor worked fine AFTER I put the old ones back in. Go figure... And how did I determine this? By using a fuel pressure gage at the carbs.

Jeff
 
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