I may consider upgrading from my Holly 4 barrel to TBI. As far as I know, I will need the complete system that includes the engine control module. Is there any difference between a ECM for a big block V8 or small block? It seems Summit Racing may have what I need, but the complete package is a bit out of my price range. I have noticed there are used ECMs on ebay at a fraction of the price, but mostly for small block mercruiser engines. Can anyone make a suggestion?

Its not the ECM is how it has been programed. You should not install one programed for even a 7.4 on a 502, or the other way without knowing the program or checking AFR's. Don't install one which was programmed on a vehicle, its not safe to be used on a boat

In short do your self a favor and stick with your carb

I'm not a fan of ebay parts,.. cheap for a reason because they didn't work for person selling them?

Greetings! I'm glad I asked because I really don't know that much about EFI. Last year I rebuilt the holly and the last time I fired up the engine, it was running rough and wouldn't idle bellow 1100. I have many carb rebuilds under my belt usually with good results. I handled this one like I was doing brain surgery. The engine had been sitting for 2 years. Is it possible the lifters need to pump up? I pre oiled the engine by spinning the pump with a priming shaft. Either the rebuild went bad, or I have a vacuum leak somewhere. I would only consider going with TBI if I find out the carb is shot. Thank's for the input!
I beg to differ where ebay is concerned. I have been dealing with ebay sellers for quite some time and I can honestly say I've never been screwed. I have purchased new mercruiser parts for much less than the dealers are asking. I will admit I would be Leary of electronics, but most dealers have a very high satisfaction rating!

High idle is an indicator of a vacuum leak for EFI motors. But first make sure the throttle rigging is allowing the blade to close all the way.