We are changing mine from EFI to carb to gain driveability and pick up the power we're losing by using the stock EFI manifold setup. We mad 501 hp on the dyno and ran out of manifold at 5200 rpm. Our driveability problem has to do with getting the fuel map correct at idle. When the engine was on the dyno, we had to send the ecu back and forth several times over several days to get the program close. We have it pretty well dialed up top, but the idle is just too rich and wants to load up. My engine guy did pick up some software, but without having it back on the dyno, we can't get id dialed. With the carb, we should be able to get it where it needs to be.

Overall, I think you can get EFI closer to dead on, but you have to have the right manifold setup, and you have to have it in the dyno making run after run to get it just right. That can turn into a pretty big financial investment for a 500 hp engine.

Guys I would rather run the EFI for sure.
I am just done putting money into this old MEFI-1 system.
If/when I go back to EFI, I'll use my newer MEFI-3 computer and harness.
I'll also have an O2 sensor bung added to the risers to double check the mixture.

I can't even imagine trying to calibrate any efi system by mail!!! Seems a lot of people around here try, and almost all turn into a nightmare. If you're going to try to modify an EFI engine, you HAVE to have the ability to monitor and change the calibration at the boat. Imagine trying to tune a carb through the mail everytime you needed to change a jet????

I've been hearing good things about this new MEFI TUNE system. I'm anxious to try it out. Some systems are easy to tune and some are clumsy, but ALL are easier to tune than a carb. The only reason people have trouble tuning an EFI is because they try to play around with too many variables at once or they just don't understand how an IC engine works. Of course this assumes the system is functioning correctly, and that's a BIG if when we're talking about electrons :)

You also have to have the right manifold to start with. Trying to make over 500hp with a manifold and throttle that are only made for 450hp max is always going to be an exercise in frustration.

I have in my stockpile of parts an AZ Speed intake, throttle body, camshaft, and other parts from a 500EFI. It has been my plan for a while to rebuild my 502MagMPI into a 500EFI clone. Getting the proper calibration for that combo should not be a problem. We’ll see if I have the time and $$ this winter. Right now, I’d like to just get the thing running OK to finish out the last 6 weeks or so.

I don't know iif it is to any help but how can a MEFI-1 get stuck in knock retard mode? If I recall correctly knock retard is controlled by the knockmodule which in a knock event delays the ignition trigger signal from the distributor IC to the ECU.
The ECU doesn't have a clue if there is knocking going on or not. That is how simple this generation of MEFI ecu's are.

My MEFI-1 ECU stopped communicating with the IC module there for the IC module takes over the timing control. The two fault codes are continuous knock and IC communication failure. What ever you want to call it the timing was retarded and the fuel mixture was way too rich…

He is not doing a custom calibration, he has a stock eng with a stock ecm cal.

efi vs carb
 

i have been also thinking about a change from efi to carb.what i want to know is how much fuel comsumption do you give up w/the carb?

Pat,
Any chance you would let me test your MEFI1 ECM (no charge)? I have a test bench for MEFI 1 and 2 controllers, with knock module and everything. I am very curious about how/why yours failed.

Dave

Sure Dave.
PM me your address, and I'll send it to ya.