Hey guys I got a friend with a 1997 fountain 27 fever with a 502 mag mpi. He just upgraded from stock manifolds to Gil headers. Since he's put the headers on he has been running lean. He got a tune from Arizona speed for headers and still running dangerously lean (plugs white). Was wondering what is making it lean even after the new tune from Arizona speed. He's checked everything in the fuel system.

Thanks

What are you using for plugs ? Some are real hard to read.

i worked on a 96 502mpi , from brand new that added gils after a few years otherwise stock motor that did not ‘go lean.’

Not doubting you, but wouldn’t vac hose being off regulator make engine rich all times except under load and then be in range?

You are totally right. More psi = richer. 3 hrs sleep yesterday and apparently some Sometimers. Lol. . Will edit.

What plugs are you trying to read and how old are they ?

Might be the tune, agree headers won't cause it to go lean
Without AFR readings would be good to see some data from the sensors.

Happen to know if the fuel pressure is at 37 psi for VST?

Is MAP (Pin-J1-9) voltage at 4.9V with ignition ON, and what is it running (around 1.5V)?

He is using NGK bpr6es. I believe these are the recommended plugs from Mercury.

After adding Gil's you didn't need a tune?

thats a pretty hot plug

Is he using ethanol free fuel? Stock tune is fine. Good luck!!!

Do these engines have aftermarket heads ?

The BPR6ES have a .750 reach and are gasketed, which many aftermarket heads use.

The factory 502mpi uses .460 reach plugs with tapered seat (no gasket)
Fsctory plug MR43T. Many people use R43TS. Which is projected tip version. Not available in Marine (M).

The Gills are far from a tuned header. Ther are not a header either. They are just slightly better than stock manifolds on a NA engine. Much less weight though. 80+lbs lighter.

Factory plugs are
AC - MR43T
Champion - RV15YC4
NGK - BR6FS