late valve timing
 

I have a 1998 454 mpi mag motor fresh rebuilt and it only has 10 inches of vacuum at idle upon checking the cam timing with a degree wheel we found it to be sitting at 120 degrees icl that is with the timing gears dot to dot I found the spec of this cam on this sight and it shows it at 109 icl and122 ecl do I need to degree this cam to the 109 icl at top dead center ?

I'm not sure about the 454 "MAG", but the 454 MPI L29 Vortec based engine, in the marine and truck form, had the cam timing retarded from factory, in a manner of which you speak.

This is what I have in my notes for Merc Big Block Mag and Non Mag engines:

Mercruiser BBC Flat Tappet Cams

Standard Flat Tappet (oval heads)
Merc#431-4734 / GM#3904359

.004” 287*, 302
.006” 278*, 289
.050” 214*, 218*
.200” 111*, 119*
Valve lift with 1.7 .460” , .480”
ICL 110 ATDC
ECL 118 BTDC
114 LSA


Mag BBC 454/502 Flat Tappet (rect heads)

.004” 296*, 296*
.006” 286*, 286*
.050” 224*, 224*
.200” 130*, 130*
Valve lift with 1.7 .510”, .510”
LSA 115.5
Merc Part#431-9830 / GM#14096209
ICL 114 ATDC
ECL 117 BTDC
GM Part#10185060 (same as above but ground with 5 degrees advance)
ICL 109 ATDC
ECL 122 BTDC

Merc Std BB Roller
Merc# 431-850453T / GM#12552296
.004” 280,299
.006” 266*/284*
.050” 204*/209*
.200” 113*/114*
Lift .480”/. 483”
ICL 115.5*
ECL 120.5
LSA 118*

………………
Merc Mag Roller
Merc# 431-850478 / GM#12551622
.004 300*/ 300*
.006 288*/288*
.050” 225*/225*
.200” 125*/125*
Lift .483”/. 483”
ICL 109*
LSA 115.5*

What would you expect your vacuum to be at idle? Just curious.

Is the motor idling correctly? If the cam was way off it would be 'reading' odd parts of the BPM table assuming MAP was out to lunch.

have you tried using a different cylinder and checking,,almost sound as if the cam lobe/s are worn.

my merc manual says vacuum should be no less than 15 inches, map sensor is showing 2.56 volts at idle vacuum increases as rpms go up 20 inches at about 2000 rpm motor sounds a bit sloppy at idle and timing is jumping all over the place at idle but holds steady and climbs as rpms increase the engine is being ran on an engine stand with no load not to get side tracked but the motor pulls a lot of water back into the exhaust manifolds while at idle I have a set of dry headers on it for now until I get the vacuum where it needs to be yes the cam has been inspected and all cylinders have been checked with a degree wheel and dial indicator just to make sure the cam is ok the cam is the gm # 12551622 as sb has listed above the cam is now in at the 109 degrees icl and will be running it tomorrow see what happens

so what changed over time,,did you rebuild this w/ new timing chain and found it running badly?then checked cam timing?we need more of the story.

just got done running the motor picked up 2 inches of vacuum so now we are at 12 inches engine sounds better and base timing is not jumping around nearly as bad so these are my degree readings on how the cam is sitting in there now intake opens @ .006 35deg. btdc intake closes @ .006 70deg abdc exhaust opens @..006 87deg bbdc exhaust closes @ .006 20deg atdc 60+157/2= 108.5 icl 74+170/2= 122 excl so past history long story short exhaust manifolds started leaking and rusted the exhaust valve seats when doing a leak down check found multiple cylinders leaking and when it was pulled apart found the timing chain clocked one tooth advanced I assume with stock timing set it now has a cc double roller set wondering if a couple more degrees of advance is in order but not sure just to be clear this motor was not pulled apart before I got the boat I find it strange that the dots did not line up on the timing gears from the factory it seems to run about the same as before but I think there is something still not right based on the intake vacuum does not appear to have a vacuum leak I made a block off plate to put over the intake runners and mighty vac each intake runner to 20 inches of vacuum and all 8 held just fine any mercruiser techs out there please chime in

How are you setting base timing and to what setting are you putting it at ?