put it in service mode with the scan tool rpms come up set base timing with timing light at 8 degrees select service mode off

Don't get wrapped up the the timing jumping all over the place at idle. The Merc ignition is setup to perform that way in the idle regime (idle spark stabilization). Get your base timing set and then leave it, which it sounds like you've done.

If you are reverting with a stock cam then you have a significant valve timing event issue.

thanks trash I feel the cam is degreed in properly and I pulled the plenum off and found oil in the intake runners so I mighty vac the intake runners again and they are not holding vacuum now. going to pull the intake this weekend and take a close look at things pain in the ass

Vacuum leaks will cause those motors to idle high or weird. If you can see what the IAC is doing that may help in diagnosing issues.

Did you reuse the lifter valley tray ? If using non OEM Merc intake gaskets sometimes you have to trim them a touch where the tabs to the valley tray should sit against the bare heads.

Just throwing it out there if you have an intake leak.

232 did you ever resolve the low manifold vacuum issue with your 454? I have an almost identical problem with my 502 MAG MPI after installing a new long block(everything but the intake and exhaust manifolds, and fuel pump and distributor) . If you have I would like to hear how you finally resolved it. I have been fighting this for two seasons.

232 has only posted 32 times. send him a PM. he will get it if he still uses the same email. what cam specs your new long block? 502 HO?

Thanks for the response dereknkathy. As you suggested I sent 232 a PM. As for your question about the 502-- It is the standard GM marine 502 425 HP roller rocker crate engine so I assume it is an ICL of 109 and LSA of 115. It is supposed to have the MERC 850478-431 grind but I don't want to tear it down to check it.

ok, anyway it next to impossible to mess with cam timing without at least loosening the oil pan. is there room for a degree wheel in front of your balancer? you can download a small one, print it out and stick it on cardboard. or are you gonna accept that GM got the cam timed right and move on to vacuum leaks and timing and such? I have heard that the 425 hp engine has a 502 HO cam. but I have also heard it has a sub-.500 lift lo-po truckish cam. this and a degree wheel and you can at least see what cam is set to. https://www.harborfreight.com/catalo...dial+indicator the one with the vice grip mount.

Did you guys find out what the problem was with the low vacuum? I have the same issue with a newly built 502 with only 7 inches of mercury, have done leak down test, smoke test, compression test, the only thing I can think of is valve timing.