Slow acceleration bog and Low WOT RPM's 120 2.5L Mercruiser

[VT Mercruiser]

120 2.5L 1985

Symptom: Motor almost dies on slow acceleration between 1200-1500 Rpm, then will pick up and run great up to 3200-3300 rpm. Idles fine and runs fine up to failure rpm. Pump the throttle and it shoots out of the water and planes almost instantly. Everybody complains about the "merc bog" on hard accelleration, this boat is opposite... ?!

What has been done: Replaced Plugs, Wires, Cap, rotor, Cleaned distro flyweights and springs, Rebuilt carburetor and set float height and drop to merc specs. Cleaned spark arrestor, replaced both fuel filters. Checked timing base at idle and advance under load at "wot". Fuel flow and pressure is good from pump. Fresh fuel and additive. Replaced all bellows, manifold and riser gaskets. Boat barely ran when purchased so we are slowly improving, just not there yet.

According to specs boat should achieve 4200-4500 rpm at WOT. Boat is a 1985 Regal 185 with a 15x15 aluminum prop in good condition. From everything I have read that small pitch prop I should be revving out.

On the drawing board, compression test, gravity fuel to carb?

Anybody have any ideas?

 

[Bondo]

Re: Slow acceleration bog and Low WOT RPM's 120 2.5L Mercruiser

120 2.5L 1985

Symptom: Motor almost dies on slow acceleration between 1200-1500 Rpm, then will pick up and run great up to 3200-3300 rpm. Idles fine and runs fine up to failure rpm. Pump the throttle and it shoots out of the water and planes almost instantly. Everybody complains about the "merc bog" on hard accelleration, this boat is opposite... ?!

What has been done: Replaced Plugs, Wires, Cap, rotor, Cleaned distro flyweights and springs, Rebuilt carburetor and set float height and drop to merc specs. Cleaned spark arrestor, replaced both fuel filters. Checked timing base at idle and advance under load at "wot". Fuel flow and pressure is good from pump. Fresh fuel and additive. Replaced all bellows, manifold and riser gaskets. Boat barely ran when purchased so we are slowly improving, just not there yet.

According to specs boat should achieve 4200-4500 rpm at WOT. Boat is a 1985 Regal 185 with a 15x15 aluminum prop in good condition. From everything I have read that small pitch prop I should be revving out.

On the drawing board, compression test, gravity fuel to carb?

Anybody have any ideas?

Ayuh,... Welcome Aboard,... How much Crud did ya find in the carb,..?? filters,..??

Ya might wanta verify the the drive is a 1.98:1 ratio,....
'n do a compression/ leak-down test

 

[thumpar]

Re: Slow acceleration bog and Low WOT RPM's 120 2.5L Mercruiser

It sounds like the accelerator pump is not working and some other issues. Yes, compression check for sure. That prop is little on pitch. I spin a 19 pitch at 4500 RPM with my 120.

 

[VT Mercruiser]

Re: Slow acceleration bog and Low WOT RPM's 120 2.5L Mercruiser

Compression and leakdown test finished last night.

125 on the low cyl and 140 on the high which I am more than alright with.
No audible noise from the carb or the exhaust on any cylinder and they all
held for more than 2 minutes during the leakdown test with shop air at 110
psi.

How do I verify the drive is a stock 1.98 to 1?

The screen in the carb had quite a bit of varnish, the pump filter looked ok. The carb was ok except the accel pump screen and venturi cluster were both crudded up. I soaked overnight and used solvent, shop air, and carb clean after the soaking. passages are all clear. Accel pump is functioning near perfect by visual inspection and the boat has a pretty good hole shot in the water.

 

[Bondo]

Re: Slow acceleration bog and Low WOT RPM's 120 2.5L Mercruiser

Compression and leakdown test finished last night.

125 on the low cyl and 140 on the high which I am more than alright with.
No audible noise from the carb or the exhaust on any cylinder and they all
held for more than 2 minutes during the leakdown test with shop air at 110
psi.

How do I verify the drive is a stock 1.98 to 1?

The screen in the carb had quite a bit of varnish, the pump filter looked ok. The carb was ok except the accel pump screen and venturi cluster were both crudded up. I soaked overnight and used solvent, shop air, and carb clean after the soaking. passages are all clear. Accel pump is functioning near perfect by visual inspection and the boat has a pretty good hole shot in the water.

Ayuh,.... It may,... or may not be stamped, portside, upper, just aft of the vent plug....

Or,...
Rotate the motor 2 turns, 'n the prop will turn once...