I have a brand new gm 502 long block that I just finished installing. When I go to set the base timing with the mercury tool the motor stalls. I can't restart the motor until I take it out of base timing mode and back into to normal mode. Any ideas? My other motor in the boat ramps up to 1300 rpms when base timing is selected.

you have to advance the timing a little more to keep it running..

Yes your timing has to be within a close range of specification to run correctly at start up. Also If your ECM is a MEFI 1 or 2 you have to manually advance the throttle between 1200 - 1600 RPM. The MEFI 3 does it automatically.

will add engine must be at operating temp plus you can use the Mercruiser special timing tool part # 91-805747A1 without the scan diagnostic tool to adjust your timing. If using the scan diagnostic tool it has to be set in the SERVICE MODE to be able to set the timing. As above posted depending on what MEFI the engine has to be manually set with engine throttle at 1800 rpms beforehand setting the timing..

You don't have enough initial timing in it. It's not advanced enough to stay running.
Eddie

Both motors have MEFI 1 ECM. #1 ramps up to 1300 rpm once base timing mode is turned on without toughing the throttles. Once I turned it to 8 deg BTDC it sits around 1600 rpm. #2 motor runs ok now, not as much power as #1 but runs smooth at low speeds, once I give it the gas it lacks torque. When I put it into base timing mode it dies. If I run #2 up to 1500 rpm without base timing mode selected it reads 8 deg BTDC. Doesn't this show that the timing is set far enough in advance for base timing to be set?

Once the engine is in base time mode, does this adjust the RPM's to the point where the zero degree marker can be used for timing with a conventional timing light? or do I need an adjustable timing light? I have a 1998 7.4 MPI and there is only a single marker on the timing chain cover, I believe this is the zero degree mark. Any input would be greatly appreciated!

That 8 degree reading through the scan tool means nothing, it is only what the computer thinks is going on assuming the base timing is set correctly. Read it with a light, a cheap non digital light off cyl number one.

Your engine should not be sitting at 8 degress at 1500 is base timing mode is off.

Any update on this?? I have a engine doing the exact same thing.