Ask David and Farrar: Technical Questions and Advice About Carbureting a DeLorean

[NightFlyer]

Well... actually, in 2007, my car was running open loop on thanksgiving, and I was driving home when the car stalled. I got out to look at the engine and could see something glowing under the car. It was my catalytic converter glowing red. I remember this vividly because neither my mother or my girlfriend at the time were happy about the possibility of us missing thanksgiving dinner due to, (as it was commonly referred to at the time), "that f&*$in' DeLorean. (it wasn't that reliable back then)

Turned out I was running lean due to plugged injectors, melted the catalyst, and all but completely plugged up the cat. I was able to limp the car home, driving a few minutes, then stopping to let it cool. Later on I pulled the exhaust off the car and completely hollowed out the plugged catalytic converter. I still always ran open loop after that, but always made a point to set the mixture by using the voltage from the O2 sensor, instead of just what sounded good.

I know this is just one instance but it's still one

Ah, but you attribute the issue to other components not operating as efficiently as they should (namely the injectors) and an incorrect CO adjustment, and not solely to the fact that you were running open loop, right? My original statement was premised on everything else in the system operating normally.

While a carb (or at least the kind that most owners are running) doesn't allow for analysis of exhaust and on-the-fly adjustments being made to the AFR by a supplemental system (such as our lambda), they're also a lot less complex and don't rely heavily upon on the optimal operation of other components in the system, as the k-jet mechanical fuel injection system does.

[Josh]

So then all the owners who are running their lambda systems open loop with k-jet are in essence killing their catalytic converters, right?

And yet, I've never heard of any of those owner's failing an emissions test or having a back-pressure issue due to a plugged up catalyst.

Not to mention that several owners who have converted over to carbs are still running their stock exhaust, and to my knowledge anyway, not a single one has reported any catalyst related problems/issues.

The theory appears to be that precise mixture control is necessary for the catalyst to survive, however, the practical experience of such is that perhaps the catalyst is a bit more robust than the theory suggests - would that be a fair assessment?

Open loop does not take input from the O2 sensor to correct the air fuel mixture. So the car can only run on the pre-determined fuel map. So in some cases the AFRs can go high or low. High would cause the engine to run lean giving you poor performance, high combustion temperatures (not good for pistons and valves), and effectively killing your cat.

No carb setup Ive seen on a delorean (or most cars for that matter) integrate any sort of O2 based mixture correction

[NightFlyer]

Open loop does not take input from the O2 sensor to correct the air fuel mixture. So the car can only run on the pre-determined fuel map. So in some cases the AFRs can go high or low. High would cause the engine to run lean giving you poor performance, high combustion temperatures (not good for pistons and valves), and effectively killing your cat.

Or in the case of a mechanical system, as good as the rest of the system has been adjusted and is operating.

[dustybarn]

No carb setup Ive seen on a delorean (or most cars for that matter) integrate any sort of O2 based mixture correction

In the '80s there were quite a few carbureted cars that had three-way catalysts and lambda control. The sole reason for the oxygen sensor is to keep the catalyst happy. A three-way catalyst needs to have a feedstream that oscillates about stoichiometry in order to simultaneously oxidize CO and hydrocarbons, AND reduce oxides of nitrogen. Without a 3-way catalyst there is little reason to have an O2 sensor. Peculiarly, the engine-out emissions are poor at stoichiometry, but the resulting improvement in catalyst efficiency more than makes up for it.

That being said, a wide-range oxygen sensor can be a valuable tuning tool, even with carbs. On my car I have a Renault Alpine GTA Atmo carbureted intake system, but I nevertheless replaced the stock sensor with a Bosch wide range one, with the A/F ratio displayed on a dashboard instrument. It was invaluable in helping adjust the idle mixture and re-jet the carbs.

[DCUK Martin]

In addition to the other great replies, look out for delorean with bulges in the centre of the rear fascia. This is a sign of a dead cat where the guts have accumulated in the muffler and caused heat build-up. Many stock cats are now long since dead with the owners oblivious.

[Nicholas R]

Ah, but you attribute the issue to other components not operating as efficiently as they should (namely the injectors) and an incorrect CO adjustment, and not solely to the fact that you were running open loop, right? My original statement was premised on everything else in the system operating normally.

While a carb (or at least the kind that most owners are running) doesn't allow for analysis of exhaust and on-the-fly adjustments being made to the AFR by a supplemental system (such as our lambda), they're also a lot less complex and don't rely heavily upon on the optimal operation of other components in the system, as the k-jet mechanical fuel injection system does.

I agree that I do attribute the root of the lean condition to plugged injectors, however; it does show that catalytic converters are not infallible. Even if it had not been plugged injectors, the car could have just as easily had a miss adjusted fuel mixture. Lean mixture is just as easy to have on a carb as it is to have on an open loop K-Jet. Technically, I think it is less likely to have a lean condition on a properly setup closed loop system.

Also, the main reason I posted is because you said you had never heard of a car with back pressure issues due to a failed catalyst; now you have

As a side note, I just want to say that having said all of this, I am still a fan of running open loop over closed loop on DeLoreans. Just make sure to properly set the mixture. Even after burning up my cat, I always ran my PRV open loop; especially after putting the UK SS exhaust on. UK SS Exhaust with open loop mixture adjusted via Cliffs exhaust gas analyzer was when I put down my best ever PRV dyno run; 99.7hp!!

In addition to the other great replies, look out for delorean with bulges in the centre of the rear fascia. This is a sign of a dead cat where the guts have accumulated in the muffler and caused heat build-up. Many stock cats are now long since dead with the owners oblivious.

This was actually my first thought in this discussion. After 30 years (often with years and years of sitting dormant) how many original catalytic converters are even still functioning?

[RoyHinkley]

He is running a different engine. I was researching the carb topic and found a post in which he detailed the different engine mounts. From what I read the engine started its life as a carburated Peugeot barge powertrain. I don't know whether the compression is the same however.

Sent from my LGL55C using Tapatalk 2

I heard Bills PRV was designed to operate a bilge on a barge.

[SProfita]

IMO the carb conversion is a genius alternative to fuel delivery on a DMC...im sure others have thought about it, and maybe there are possible plans in the works. Here in my town aircraft capitol of the world we have tons of machine shops that could possibly CAD desighn/reverse engineer a peugot mani to make in bulk. While it may increase the price, could be made in bulk kits and sold. But who knows if its worth it for a vehicle in which 8,000 cars exist... maybe the "new" models will be carbed... 

[Soundkillr]

Please avoid double posting. This is the exact post you have in another thread.

[The Aviator]

is farrer still around? i haven't talked to him in years