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Please Help!!! 1991 Mercruiser 7.4 Bluewater not running on all cylinders

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BryanZ

New member
I need someone with Mercruiser 7.4 experience. Port engine not running on all cylinders. Here is the history. Had a frozen valve on cylinder 4 had the head machined and new valve guides installed. Reinstalled the head 2,4,6,8 side. Not my first rodeo with this, my boat has had reversion problems in the past. Adjusted valve as per merc manual, Mark IV big block. New manifolds, wires, rotor, cap, plugs oil change, the usual tune up rebuild. Compression test 120 on all 8 cylinders. She started right up after rebuild. Here is were my problem is and im not sure where to go from here. With a timing light she wont even run unless im at 16 Degrees BeforeTDC. In order to make power, When i turn the distributor counter clockwise she runs better moving the time mark well past the markers up toward the water pump but still doesnt run on all cylinders. I check TDC and it lines up on the balancer and piston 1 is all the way up so i assume it is correct. You can pull the wires on cylinder 2 and 5 there is no difference while running. (again i have 120 compression across the board) I have checked my wires 50 times and im getting spark.
I dont know what else to do at this point. I cant get the timing correct and she isn't running on all the cylinders.

HELP HELP HELP!!!!
 
Manual says 180 psi gen IV 454

Typical pressure should be 150 psi on a used engine

SO 120 is low.


When you say you pull # 2 and #5 wires off and no change, have you confirmed spark AT THE SPARK PLUG END OF THE WIRE for both?


Do you see a lot of fumes/steam/smoke coming from valve cover "PCR/Vent holes?

Reversion I believe you mean water intrusion when shutting down?

Reversion is non marine terms typically occurs with high lift cams
 
kghost said:
Manual says 180 psi gen IV 454

Typical pressure should be 150 psi on a used engine

SO 120 is low.


When you say you pull # 2 and #5 wires off and no change, have you confirmed spark AT THE SPARK PLUG END OF THE WIRE for both?


Do you see a lot of fumes/steam/smoke coming from valve cover "PCR/Vent holes?

Reversion I believe you mean water intrusion when shutting down?

Reversion is non marine terms typically occurs with high lift cams
Click to expand...


Reversion is the right term Jack... The 7.4 is basically the worst marine engine ever built. The sound wave pulses in the exhaust cause reversion while the engine is running.

For an inboard install there is a formula to figure out where the resonator is installed in the exhaust hose to prevent this..... Anyone ever wonder why there were two hose clamps in the middle of their exhaust hose on a 7.4 inboard? They were holding the resonator in place.

2X on those compression numbers.

This is important! How did you set the rocker arms.... How much did you tighten the rocker nut after zero lash?
 
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