sancarmella
Recruit
- Joined
- Aug 25, 2023
- Messages
- 1
I was given an old family boat which had a seized 3.7l mercruiser lx with a rochester 4bbl. The block and head were completely seized due coolant leaking from the exhaust riser. I bought another 3.7l mercruiser which had a 2bbl and I wanted to use the aluminum manifolds that came from the lx so that the 44bl could be used.
I stripped the new engine down to the block so that I could inspect the new block, I inspected the crank, connecting rods Pistons cylinders, cam shaft push rods, lifters, valves, basically everything and found no points of concern.
The parts I swapped from the lx model are as followed,
-aluminum exhaust and intake manifolds after having them ultrasonically cleaned
- removed the stator and replaced with alternator kit from lx
- swapped the flywheel and coupling assembly to match the drive on my alpha one drive
- starter and slave solenoid from lx
- attached the oil filter assembly to match the intake manifold
- switched to the closed cooling system from original engine
-switched coil, oil pressure sensor and low oil alarm sensor
-Rewired the engine with original harness as the old one had been wired to bypass master plug
New parts
-fuel pump fuel line and new carburator after finding gelled fuel through system
-fuel tank and lines cleaned out
-new fuel separator
- new carburator float height adjusted to spec and secondary metering rods changed, choke set primary air doors tested die function
- cast iron exhaust riser (original was cracked)
- all new gaskets
- new water pump seal and impeller installed
- new plugs and wires
- all fresh fluids/filter
While I was changing the stator the engine crank shaft cover/ pulley was put back in correct position against timing marks on the block at TDC on compression.
The engine was then timed to 4 degrees btdc at 750rpm, the advance was never checked aswell as the dwell. At the beginning of this process I was definitely a rookie so I did not know that could be checked.
The distributer was not changed before first start as I wanted to test it with the original first. The new engine still had the points and pertronics system but was left on.
I then was able to use the boat every weekend on a lake while making adjustments to the carb throttle, and mixture screw etc. Aswell as making small timing adjustments.
- After around 40 hours of actually running the engine it started to run poorly, it would start fine and throttle up correctly but as you slow down it begins to "backfire" as a
fuel air mixture will blowback out of the carb every few revolutions then engine bogs slowly and dies. it then had trouble
Starting and the "backfire" continues to happen. It can be started in this state by using the neutral throttle But was only done so it could be returned to me at the dock. Checked the points and rotor which looked fine. I turned the engine to TDC on compression. but did not have a timing light available so it was done by visually checking rotor position against points and timing marks. I adjusted the timing which seemed to be way to far advanced. Then ran correctly for an hour but the problem returned. I did not check to see if timing had slipped from where I had set it. So Assuming it was a distributer problem I decided I would switch them. After returning home I switched to the one from the old engine which is an electronic distributer. And timed the engine to 4° btdc at 750rpm with a photo tachometer. The choke was fully open when this was done.
The engine ran smooth so I increased thr neutral throttle to 2500 rpm. The harmonic balancer has timing marks of 0,5,10 btdc so I can only guess but it looked like it had advanced to around 30 degrees. When I try to slow the motor back down it does not Rev down. The throttle linkage does return to the closed position and it doesn't rev down below 2000 unless I hold the primary air doors closed or retard the distributer.
I have now replaced the cap the rotor and the whole distributer, the coil , tested the wires and new plugs. I am now thinking it could be a problem with the timing chain or potentially lifters but I am hoping it could be something else.
If the engine is tested in the water in gear it start fine runs well then slows down in gear correctly and shifts back to neutral then revs up in neutral.
I stripped the new engine down to the block so that I could inspect the new block, I inspected the crank, connecting rods Pistons cylinders, cam shaft push rods, lifters, valves, basically everything and found no points of concern.
The parts I swapped from the lx model are as followed,
-aluminum exhaust and intake manifolds after having them ultrasonically cleaned
- removed the stator and replaced with alternator kit from lx
- swapped the flywheel and coupling assembly to match the drive on my alpha one drive
- starter and slave solenoid from lx
- attached the oil filter assembly to match the intake manifold
- switched to the closed cooling system from original engine
-switched coil, oil pressure sensor and low oil alarm sensor
-Rewired the engine with original harness as the old one had been wired to bypass master plug
New parts
-fuel pump fuel line and new carburator after finding gelled fuel through system
-fuel tank and lines cleaned out
-new fuel separator
- new carburator float height adjusted to spec and secondary metering rods changed, choke set primary air doors tested die function
- cast iron exhaust riser (original was cracked)
- all new gaskets
- new water pump seal and impeller installed
- new plugs and wires
- all fresh fluids/filter
While I was changing the stator the engine crank shaft cover/ pulley was put back in correct position against timing marks on the block at TDC on compression.
The engine was then timed to 4 degrees btdc at 750rpm, the advance was never checked aswell as the dwell. At the beginning of this process I was definitely a rookie so I did not know that could be checked.
The distributer was not changed before first start as I wanted to test it with the original first. The new engine still had the points and pertronics system but was left on.
I then was able to use the boat every weekend on a lake while making adjustments to the carb throttle, and mixture screw etc. Aswell as making small timing adjustments.
- After around 40 hours of actually running the engine it started to run poorly, it would start fine and throttle up correctly but as you slow down it begins to "backfire" as a
fuel air mixture will blowback out of the carb every few revolutions then engine bogs slowly and dies. it then had trouble
Starting and the "backfire" continues to happen. It can be started in this state by using the neutral throttle But was only done so it could be returned to me at the dock. Checked the points and rotor which looked fine. I turned the engine to TDC on compression. but did not have a timing light available so it was done by visually checking rotor position against points and timing marks. I adjusted the timing which seemed to be way to far advanced. Then ran correctly for an hour but the problem returned. I did not check to see if timing had slipped from where I had set it. So Assuming it was a distributer problem I decided I would switch them. After returning home I switched to the one from the old engine which is an electronic distributer. And timed the engine to 4° btdc at 750rpm with a photo tachometer. The choke was fully open when this was done.
The engine ran smooth so I increased thr neutral throttle to 2500 rpm. The harmonic balancer has timing marks of 0,5,10 btdc so I can only guess but it looked like it had advanced to around 30 degrees. When I try to slow the motor back down it does not Rev down. The throttle linkage does return to the closed position and it doesn't rev down below 2000 unless I hold the primary air doors closed or retard the distributer.
I have now replaced the cap the rotor and the whole distributer, the coil , tested the wires and new plugs. I am now thinking it could be a problem with the timing chain or potentially lifters but I am hoping it could be something else.
If the engine is tested in the water in gear it start fine runs well then slows down in gear correctly and shifts back to neutral then revs up in neutral.