3.9 Efi to carbs, how to?

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V8demon

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Location
Wiltshire
As many may have seen I’ve been having a tough old time finding an intermittent cut out issue on my 95 3.9 Disco.
I’m now thinking just put carbs on and be done with it as the only thing I haven’t changed is the TPS and there obsolete.

My questions are...

How do I go about this from an electrical point of view? What needs to stay connected and what doesn’t?

Am I going to run into issues without doing major electrical work?

Fitting the carbs and setting up isn’t really a difficult task for me but I don’t want to get half way into fitting and realise it’s not possible because of something else and cause myself more issues.

Edit: I’m assuming a 3.5 carb inlet manifold will fit 3.9 heads but carbs will need bigger jets?

Thanks in advance folks.
 
If it's an automatic gearbox you will need to sort the linkage for the kickdown cable. Advice from Ashcroft is that you must not run the ZF with the kickdown cable disconnected as this will result in damage to the box.
 
Ratae said:
If it's an automatic gearbox you will need to sort the linkage for the kickdown cable. Advice from Ashcroft is that you must not run the ZF with the kickdown cable disconnected as this will result in damage to the box.
Click to expand...

Glad you mentioned that, I was just thinking about the exact same thing.
 
Why would you want carbs? Don't give up on it. The EFI is not complicated. Search this section for possible checks and tests I promise at one time or another I have posted how to investigate pretty much the whole system. You need to remember misfires can be ignition or efi, sometimes deciding which can be a challenge.

Have you done any substitution testing? e.g. ecu, air flow meter, coil, ignition amplifier. Have you looked left of field, such as battery, alternator and earthing. Is the ignition timing correct? is the vac advance working or could it be your timing is wandering due to a stretched timing chain? Is the plenum clean and throttle disc correctly set? You mention TPS - don't go off on one until you have checked it. What is your base voltage on closed throttle. If it is too low the ecu doesn't read it and then doesn't switch on the idle stabilisation control.

I know the frustrations. It took me 2 years to get my Range Rover running sweet and since then it has been truly used and abused, it's been in deep water, deep mud, deep snow and deep s**t and never missed a beat and (crosses fingers) is still going.

Also don't forget if you swap to carbs you're going to have fun getting it through an emissions test speaking of which, could your running problems be due to failing 02 sensors? You'll need to swap your fuel pump as well for carbs, they won't take the pressure from the efi pump. Not sure how you would then wire the pump and the inertia switch as the pump circuit is part of the efi system.
 
Well the fuel pressure sensor came in the post today and I fitted it then went into town to pick up some antifreeze, it’s about. 15-18 mile round trip, I had no issues at all, maybe, just maybe I’ve cured the cutting out issue.

Just a recap on what I’ve replaced so far...

Every ignition component (apart from the dizzy itself) every sensor (apart from TPS)
Replaced fuel pump and fuel pump relay
Cleaned all electrical connections relating to ignition and fuel system.
Still waiting on an Efi relay to come in the post.

You mention vacuum advance, I did think about this earlier on today, correct me if I’m wrong but when the engine is idling if I pull the vacuum pipe (dizzy to plenum) shouldn’t the idle fall or raise? Mine does nothing!
 
Vac advance doesn't do much at idle but if the vac advance unit is shot you essentially get an inlet air leak which can upset the way the engine idles. Also, a duff vac unit will mean throttle response is slow because you done get the right amount of advance to pick up engine speed off idle, you can to some extent get around this by putting more static advance on but then you can upset high speed running because there is then too much advance although the Rover v8 is quite tolerant and will run with advance of 12 degrees and more.
You need to pull the vac pipe off the inlet and suck on it. If the vac unit is good you should feel the vacuum because your tongue should stick to the end of the pipe not sure if that makes sense but that's what it is. Next pop the dizzy cap off and do the same again and watch the inside of the distributor, you should see the base plate move. If it doesn't it means the thing is stuck and needs taking apart, cleaning and re-assembling. The dizzy is easy enough to take apart but be careful not to damage or lose anything especially the springs for the advance weights.
 
V8demon said:
You mention vacuum advance, I did think about this earlier on today, correct me if I’m wrong but when the engine is idling if I pull the vacuum pipe (dizzy to plenum) shouldn’t the idle fall or raise? Mine does nothing!
Click to expand...

There will be quite a bit of vac advance at idle in my experience, if you take the pipe off the revs should drop and if you then suck on the pipe the revs should rise. The vacuum advance unit gives a lot of advance and depending on the one fitted, one of my minis gave over 15 degrees extra advance on a light throttle (max vacuum)
(Leave vac pipe off when setting ignition timing)
 
Yes, that test method makes sense, thanks, I’ll try that out today. I may try that before looking at the TPS, no need to tweak the TPS if the issue is somewhere else.

Thanks to all for advice so far, I do feel like I’m getting somewhere, slowly!
 
Just a thought, if you get to the point of removing the dizzy (or even if you can make out the manuf. code/serial # with it still in place) check it with the original spec. Owners have been know to swap this component in the belief that 'one size fits all' which although basically true (post '76 I believe) the advance curves are different. I once had an early EFI that was fitted with a ROW carb unit.
 
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