1991 V12 NO Start and exhaust backfire

Thought I’d start a new topic on this one. I did read a bunch of old posts but didn’t come across any addressing this issue, yet.

Quick rundown how I got to this point.

I bought this car not running and bad melted ignition wiring. I have sorted the wiring and got spark and fuel last week. I put everything in place, and it started right up. there are a couple vacuum leaks I didn’t hook up yet just plugged on B side. It was running great on A side, but B side was missing on at least 2 cylinders. It ran for about 20 minutes and stumbled and died. I figured out that it ran out of fuel (it was old fuel and I had put in Seafoam. I was happy to find out it was less than expected. I then put 5 gallons of 90 no ethanol fuel in it. When I tried to restart it now has a gunshot backfire. Very loud twice so I haven’t tried again. Since I re-checked all the wires that are correct. Still have platinum plugs that it came with. Just cleaned. Distributer cap has no evidence of a crack or moisture. and Fuel injectors all fire with ignition on and slight turn of the throttle. I am looking for any thoughts on next step to diagnose. I did pull oil cap and cam chain is spinning and seems tight. Odometer says 29000 which I am hoping is correct but not verified yet.
My initial thought is to pull the top off again and do compression test and see what is happening on the B side.

I did purchase new DENSO plugs but wanted a recommendation on where to get cap and wires. seen them on Ebay but not sure if that’s the quality parts I need for this project.

Thank you.


Forgot to mention, I did verify coil A and B wiring went to the correct coils, which are both replaced with new.

If you sure that spark is good then it’s a fueling issue.
Did you change the fuel filter?
How is your pump?
Did you ever look inside the fuel tanks? There are filters in the pick up tube that can get blocked if the tank is full of crud.
Check also if the FPRs are leaking from their vacuum ports.

The injectors seem to have a good spray and good fuel pressure from my first checks but haven’t checked since I ran it out of fuel.
but I will replace fuel filter now that mention it.

I just noticed that the vacuum line going to the B side fuel regulator had a brittle hose that cracked off at some point. What would that do? no vacuum to it!!

Increase fuel pressure at all times.

Probably not to the extent that the engine wouldn’t start…although probably enough to make it run a bit rich


Last night I routed the vacuum to left side Fuel pressure and still have back fire.
My cap and rotor are what I suspect at the moment. I pulled them off last night and both look new. I read about the cheap aftermarket ones that fail. Mine both say Italy on the rotor and cap so maybe they a decent quality. I found a Marelli from Welsh for about $200 for everything new. Does anyone have a used one that that in good I can buy for testing while I get it running, before I lay out the cash for the good ones? I still plan to pull the top apart this weekend and do compression on all cylinders. I also bought one of the small cameras a few years ago and will try that too since I never used it yet to view in the cylinders.

Thank you