You may remember I have been having to feather the throttle to get my EFI 502 to start, once started, she is fine. (since new SM exhaust, plugs, cap, coil, all factory parts.)
Today, water test;
She ran great the first 20 minutes, up to 4500 RPM, then a loss of higher RPM, TACH IS ALL OVER THE PLACE, so we limped back at 2000 RPM.
She starts fine, but that is it.
HELP

I will our club mechanic look at it, ideas?

I think distributor and timing?
Bryan

Maybe water in fuel or carb. I would drain the carb an try again. Change the water seperator filter if you don't have use quichsilver dry fuel

LOL....hummm....it's a 502 EFI....check the basics...fuel, air, heat... at all cylinders...heat=spark and compression....then pin out the sensors....get a read on your TP at idle and wot.

Tach readings eratic indicates a problem with the distributor or wiring. Check to b sure no wires are chafed or cut, all connectors are tight. Check inside the distributor for corrosion, loose parts. If the tach is crazy, the problem is not fuel related.
Good luck!!!

uuh EFI "drain the carb" shame on me

All wires are brand new, new plugs, new cap and rotor (all merc stuff).
Prior to all this, she rocked for 2 seasons, 157 Hours.
Gas is all fresh, including seperator. All maitenance is done every 30 hours, very well maintained, I am very anal.
I think the cap and rotor is the culprit, we had to pry the old rotor off! Our Merc mechanic on the phone thought the IAC sensor was the reason for having to feather the gas to fire her.
Like I said, she ran great for 20 minutes, then the problem hit.
We should know soon, she is now clean and covered....
Oil Press perfect, temp perfect 175.
Thank God for warranties.
Bryan

Maybe your fuel pump doesn't work right.

Your problem is in the primary ignition somewhere. If you have fuel or secondary wiring (cap, rotor, plug wires...) the tach will remain stable. The pickup in the distributor could be the problem, but not likely.
The rotor commonly sticks unless you use anti seize on the shaft.

Lubed the new rotor, th eold was a bear to remove.
The old was factory installed, 2001 OM series 502 Mag.

BLOWN 1500, the tach is now way erratic, 1800 to 2400 bounce when etc...
Bryan

Check all your ground connections on the block! An erratic tach (and running bad) on a Merc EFI setup is a sure sign of a missing ground to the EFI system. There are several ground straps coming out of the harness that must be connected.