Any Dellorto carb experts?
Discussion
I have a North American spec 1984 Lotus Turbo Esprit that I'd like a second opinion on (as much as possible) with regard to the tuning of the Dellorto carbs.
I have actually removed all the US emissions equipment as an FYI.
I've fully rebuilt the carbs and its currently fitted with size 56 idle jets. I have experimented with other jets, 52, 54, 55, and 58 and the 56 seem to suit it.
I've balanced them with a 4 way manometer and I have tuned them referencing the Des Hammill book. I also have a Gunson Gas Analyser to measure CO levels.
Starting from cold requires no choke. I give the accel pedal several pumps, but I find it starts with the pedal almost held wide open until it fires then idles fine and I can drive it away.
Starting once the engine has warmed up is usually a flick of the key and no gas required.
A problem with these cars is often a stumble from idle to progression around 2500-3000 rpm when driving. My car is generally OK with this, but I get an occasional stumble.
It can pop and 'fluffle' on the over-run and sometimes when I turn it off, after 2-3 seconds, it will sort of backfire like unburnt fuel is igniting.
I am confident the timing is good and I have a 123 Ignition fitted that has been tuned by an ex Lotus engineer. Fuel pressure regulator is working correctly.
Attached is a photo of a spark plug. I haven't seen a plug with a burn pattern like this before, what does it indicate?
The fuel consumption seems high in the low twenties and I don't seem to be getting the range that other owners get.
So, my initial question is, is it running too rich or lean with the symptoms I've described?
The part I struggle with is ensuring you've got the idle mixture right for each barrel. The idle jet mixture screws are currently set around 4 full turns out, so in the right range for these as far as I understand.
I have actually removed all the US emissions equipment as an FYI.
I've fully rebuilt the carbs and its currently fitted with size 56 idle jets. I have experimented with other jets, 52, 54, 55, and 58 and the 56 seem to suit it.
I've balanced them with a 4 way manometer and I have tuned them referencing the Des Hammill book. I also have a Gunson Gas Analyser to measure CO levels.
Starting from cold requires no choke. I give the accel pedal several pumps, but I find it starts with the pedal almost held wide open until it fires then idles fine and I can drive it away.
Starting once the engine has warmed up is usually a flick of the key and no gas required.
A problem with these cars is often a stumble from idle to progression around 2500-3000 rpm when driving. My car is generally OK with this, but I get an occasional stumble.
It can pop and 'fluffle' on the over-run and sometimes when I turn it off, after 2-3 seconds, it will sort of backfire like unburnt fuel is igniting.
I am confident the timing is good and I have a 123 Ignition fitted that has been tuned by an ex Lotus engineer. Fuel pressure regulator is working correctly.
Attached is a photo of a spark plug. I haven't seen a plug with a burn pattern like this before, what does it indicate?
The fuel consumption seems high in the low twenties and I don't seem to be getting the range that other owners get.
So, my initial question is, is it running too rich or lean with the symptoms I've described?
The part I struggle with is ensuring you've got the idle mixture right for each barrel. The idle jet mixture screws are currently set around 4 full turns out, so in the right range for these as far as I understand.
Its years since I worked on Webbers & Dell'orto's. Sounds like its massively rich. The cold start underlines that, it should need coaxing/fast idle for at least 20-45 seconds when started from cold.
Also that's the the backfire issue, that is unburnt fuel in the exhaust manifold igniting. Those plugs look a bit carboned up as well.
Start from basics, normally float levels & go from there. Weaken off the idle mixture screws, normally around 2-2 1/2 from my experience.
Note plug colours are only relevant on new ones.
Also that's the the backfire issue, that is unburnt fuel in the exhaust manifold igniting. Those plugs look a bit carboned up as well.
Start from basics, normally float levels & go from there. Weaken off the idle mixture screws, normally around 2-2 1/2 from my experience.
Note plug colours are only relevant on new ones.
It's 100% normal for D'Ellortos to need priming with a couple of prods of the throttle BUT if it's a cold start you should need the choke as well, just to get it fired, then hold some revs with the throttle and back off the choke asap.
You've got the engine set too rich, as confirmed by that plug and your mpg/range.
From a cold start you shouldn't need the throttle depressed. Once primed it should start on the choke. Holding the throttle down has the opposite effect to the choke, again suggesting rich mixture.
Holding the throttle down and cranking is actually the hot start procedure (no choke) if a hot engine refuses to start.
You've got the engine set too rich, as confirmed by that plug and your mpg/range.
From a cold start you shouldn't need the throttle depressed. Once primed it should start on the choke. Holding the throttle down has the opposite effect to the choke, again suggesting rich mixture.
Holding the throttle down and cranking is actually the hot start procedure (no choke) if a hot engine refuses to start.
All Dells will start from cold without choke, it's a pretty standard way of doing a cold start - pumping or opening the throttle activates the accelerator pump jet and squirts a jet of neat fuel into the throats.
OP, I played around for years with twin 40 Dell on my Lotus TC, rebuilding, re jetting, playing with float heights and chokes and what you describe sounds par for the course and is probably as good as you can get without going to FI. I think it was Dave Walker who called carbs a 'box full of compromises'. You might wish to check float heights too as they do have an effect.
Going down a choke size eliminated the transition stumble for me and improved driveability, but it lost out on the top end so I went back to larger chokes. I gather you are using a Morgan Carbtune, which is an excellent bit of kit?
What did have a big effect for me was playing with the plugs and going hotter. The plug brand also made a significant difference and NGK's, once fouled, were pretty much knackered and unrecoverable. I found Champions worked best, nr 6 I think?
There's a guy on your side of the pond (surname Engels) who was the guru and used to supply modified emulsion tubes and other things for Dells which were supposed to eliminate some of these issues. He ran a wesbite called 'Side Draft Central' if it's still active.
OP, I played around for years with twin 40 Dell on my Lotus TC, rebuilding, re jetting, playing with float heights and chokes and what you describe sounds par for the course and is probably as good as you can get without going to FI. I think it was Dave Walker who called carbs a 'box full of compromises'. You might wish to check float heights too as they do have an effect.
Going down a choke size eliminated the transition stumble for me and improved driveability, but it lost out on the top end so I went back to larger chokes. I gather you are using a Morgan Carbtune, which is an excellent bit of kit?
What did have a big effect for me was playing with the plugs and going hotter. The plug brand also made a significant difference and NGK's, once fouled, were pretty much knackered and unrecoverable. I found Champions worked best, nr 6 I think?
There's a guy on your side of the pond (surname Engels) who was the guru and used to supply modified emulsion tubes and other things for Dells which were supposed to eliminate some of these issues. He ran a wesbite called 'Side Draft Central' if it's still active.
Which would make you wonder why they bothered to fit a choke...
"Solving the hot starting problems associated with turbocharged engines and carburettors isn't easy. Full manual choke is needed from cold and even then the Lotus isn't eager to roar into life."
http://www.lotusespritworld.com/ERoadtests/AutoCar...
"Solving the hot starting problems associated with turbocharged engines and carburettors isn't easy. Full manual choke is needed from cold and even then the Lotus isn't eager to roar into life."
http://www.lotusespritworld.com/ERoadtests/AutoCar...
Hi,
I think it's also worth considering what the turbo is bringing to the equation and the general health of the engine. The turbo may create crankcase blow by, and the air can stall at certain speeds if the chokes aren't right. So crankcase ventilation needs to be right. Remember that the carbs rely on a pressure differential internally to work correctly, the turbo fundmentally impacts on that. Also the fuel pressure regulator needs to be compensating for the effect the turbo is having on the fueling, specifically with carbs.
The D'ellortos do cope better than Webers with turbocharging. Not saying that these are your issues, but food for thought.
I think it's also worth considering what the turbo is bringing to the equation and the general health of the engine. The turbo may create crankcase blow by, and the air can stall at certain speeds if the chokes aren't right. So crankcase ventilation needs to be right. Remember that the carbs rely on a pressure differential internally to work correctly, the turbo fundmentally impacts on that. Also the fuel pressure regulator needs to be compensating for the effect the turbo is having on the fueling, specifically with carbs.
The D'ellortos do cope better than Webers with turbocharging. Not saying that these are your issues, but food for thought.
geeman237 said:
I also have a Gunson Gas Analyser to measure CO levels.
Gunsons do not offer an actual CO meter. They measure CO2 and guess CO from that, largely pointless.Either get a proper gas analyser, or much easier, just fit a wideband to the vehicle, ideally a setup you can log, although even just a gauge visual reference is fine
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