Mercruiser 502 mpi mag rich idle low vacuum new rebuild
 

I'm at wits end, just installed a rebuilt bored .30 over new pistons everything balanced pressure tested and professionally assembled engine. Compression tested at 150 psi, injectors rebuilt all new sensors except TPS and IAT sensor. The engine sounds great above idle with the exception of a light lifter tick. At idle it has an extremely rich fuel mixture with a very loopy idle for a stock 502. Seems to have very low intake vacuum at idle map sensor voltage is at 3.6 all other sensors look fine injector pulse width is at 3.9ms at idle. I tried changed MAP sensor with no positive results. I went as far as using the old map sensor plugged into the harness with a vacuum pump and drew the voltage down to 1.6 with smoothed out idle and reduced injector pulse width to 2.1ms. I then capped all vacuum ports to the intake manifold to make sure there were no intake leaks, Base timing was set at 8 degrees BTC with a roll back snap on timing light with the ecm in base timing moded.Also I checked the thottle body to make sure that the cable was not holding the butterfly open. :cartman:

Is it possible the timing chain set was not installed correctly?? Maybe off a tooth??

I spoke with the engine builder this morning and he is 100% sure the timing is correct. I got a new manual vacuum gauge today and it is reading on 7 in and what I am reading it should be somewhere around 14 in.

I also swapped the throttle body with the good running port engine and acts the same.

7 "Hg vacuum at idle :

Very large camshaft
or
valves or pistons not sealing
or
worn cam (is it a flat tappet or HR) - if FT I would check to see if worn.
or
ignition timing is fubar

Not a large cam shaft....stock 502 420hp
New engine so I hope it isn't a sealing problem....
Machine shop said the cam was in good shape
ignition timing is 8 degrees before top dead center base

New engine, old cam??

Yes

Was this your original block and heads? If so what was the cause for a rebuild if any?

That is why I always use a degree wheel when installing cam's to make sure its 100% correct. In my younger days I installed a cam one tooth off. Low vacuum and fell on it's face trying to get on plane. Sounded great on the trailer....