7.4LX MPI to Carb
 

I have a 7.4LX MPI and want to change over to a carb set up. I have read the previous posts on this matter and have decided to do it anyway. I was told that I could use a Elderbrock Perfomer RPM intake and carb, and was also told to use a square port intake and gaskets on my oval port heads. Has anyone heard of this before, will it work? I was also told that my orginal dist. would work??? I was planning on using a MSD 6m-2L ign. box and new dist. Does anyone have any other ideas on upgrading the 7.4 without breaking the bank? Thanks in advance....

What do you think you are going to gain?????? A 1mph gain on the top end and 10% reduction in fuel economy are the most likely results.

You are going to regret changing unless your MPI setup is messed up. Why spend $600-800 to go back to old school tech?? I still run MPI on my boat and it is nowhere near stock. I can't imagine f*cking around with a carb again!

My MPI in screwed up, the high pressure pump is bad and needs replaced. With the key on, engine off, the knock sensor is reading a knock, I can watch the scan tool say yes, no, yes, no, yes, yes, no.... Before the fuel pump went bad, the engine would start some of the time and just start and then stall the other half of the time. I looked up the history on this motor on MercNet and it has had nothing but issues since day one. When it was under warranty the ecm was changed twice, all the sensors were replaced, and even the wiring harness was repalced. Mercury finally took the motor back and gave the orginal owner a new motor. Some how it ended up in my boat.....AGHHH... Thats why I want to get rid of the MPI and go back to old fashioned carb. I have tried everything I can think of to fix it, but if Merc couldnt fix it, then carbs it is. So with that being said, does any one have any suggestions on what to do to bring this thing to life?

I used an Edelbrock Performer intake on my 1989 7.4L and it ran great. The Edelbrock 750 carb is a good value as well and should work right out of the box
Don't use a rec port intake on a oval head; that makes no sense. Use an oval port intake on an oval port head.
I'd buy a "ready to run" ignition distributor like DUI or MSD. The MSD box will be a waste on a low output 454.

Why didn't you say so!

That also changes my perspective.

Performer RPM intake--use the correct one for your heads.
Holley 750 mech secondary carb
Merc TB IV distributor and 24* module.

You probably won't see any significant performance gains unless you upgrade the cam and exhaust.

That's the plan I would suggest. But I'll add for you to check your engine hatch clearence to make sure the Performer RPM will fit. It should, but it's a quick check you should do before buying parts.

Also, if the engine is out of the boat, then I agree with Griff it would be the perfect time to change the cam.

Thanks for all your suggestions. Can I get away with changing the cam without doing anything to the heads? I already plan to change the exhaust. The motor is in the boat but I have no problem pulling it. Also should I find a thunderbolt IV dist or use a MSD? Thanks again.

Justin

Make sure your valve springs can handle (i.e. not bind) the added lift from the bigger cam. Also, if using stamped rockers you should put new ones on if you are changing the cam since their wear will be for the old, shorter lift cam.

You won't see a performance difference between tbIV or MSD so go with what is cheapest.