sonus
Europe
Posts: 1,386
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I have isntalled a 02-sensor bung in my exhaust to use for tuning my carburetors. The engine is a Daimler V8 that uses two SU HD6 carbs. I've bought an Innovate LM-2 to measure the AF-ratios..
Tonight I connected it all up, but I am slightly confused of the readings. Idling the LM-2 reads ~10.8. Using the choke drops this to ~9.8. However letting the engine run up to 2000rpm the AF hits ~14.5-15. Then when accelerating hard the AF ratio climbs to 16.5-17. Then the LM-2 reboots.
The lean conditions would explain why the engine gets rather hot. But it does not explain why the engine detonates throught the exhaust on the over run after having been been given some beans.
Could the detonation in the exhaust system be caused by incorrect ignition advance? I thought it might have been due to the engine was running rich, but the AF-reading suggest it does not run rich, but rather lean.
Any ideas?
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Current
1968 TVR VIXEN S1 V8 Prototype
2004 TVR T350C
2017 BMW 340i
Previous
BMW 325d E91LCI - sold
Alfa Romeo GTV - sold
Citroen AX GT - at the breakers
Ford Puma 1.7 - sold
Volvo V50 2.0d - sold
MGB GT - wrecked by fire
MG ZT 1.8T - sold
VW E-golf Electric - sold
Mini Countryman 1.6D -sold
Land Rover Discovery TD5 - sold
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Istr that a lean mix burns slower for some reason and promotes pops and bangs from the exhaust. My Dolly on twin HS4s certainly used to do it at AFR of 15+ stopped when I fattened it up.
Have you played with needles, spring weights and damper oil thickness at all?
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Darkspeed
Club Retro Rides Member
Posts: 4,711
Club RR Member Number: 39
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As a lean mixture takes longer to burn than a rich one if your ignition is not advanced enough then it will still be burning when the exhaust valve opens - Also if the mixture is not actually being burnt in the cylinder due to being too weak to ignite with a spark then it can be burnt in the exhaust when a still burning mixture enters the exhaust - POP. Do you have a timing curve?
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What oil are you using in the dashpots? Are the needles floating or fixed? Are the jets Waxstat ones? In my Minis,i always got rid of the waxstat jets for normal ones, used floating needles and 10/40 engine oil in the dashpots. This combo seemed to give the best results all around i found.
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'88 Cadillac Brougham hearse (white)
'91 Carlton GSi 24v
'72 Dodge dual cab pick up
'99 Mercedes S55 AMG
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slater
Club Retro Rides Member
Posts: 6,390
Club RR Member Number: 78
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It's exactly the lean conditions that make it pop and bang. That's why it happens when you shut off the throttle. The mixture suddenly goes extremely lean. Doesn't burn properly and ends up burning after its gone through the engine and into the exhaust.
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sonus
Europe
Posts: 1,386
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Thanks all!
I'll adjust it to be a bit richer, but what about the idle richness? I'm at AF ratio of 10.8 already at idle. Maybe the carbs where adjusted incorrectly? I'll try following the set up guide on burlen.co.uk for the HD6 carbs 👍🏻
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Current
1968 TVR VIXEN S1 V8 Prototype
2004 TVR T350C
2017 BMW 340i
Previous
BMW 325d E91LCI - sold
Alfa Romeo GTV - sold
Citroen AX GT - at the breakers
Ford Puma 1.7 - sold
Volvo V50 2.0d - sold
MGB GT - wrecked by fire
MG ZT 1.8T - sold
VW E-golf Electric - sold
Mini Countryman 1.6D -sold
Land Rover Discovery TD5 - sold
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Stick some heavier oil in the dash pots, should stop the lean spike at heavy load, heavier spring will also help. Also, how high is the fuel level in the jet? Float might be set too high, causing a richer mixture at idle.
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Darkspeed
Club Retro Rides Member
Posts: 4,711
Club RR Member Number: 39
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Idle mixture is set with the jet height - just wind it up a couple of flats !
Oil makes no difference to steady state running conditions so changing that will make no difference
Springs - Softer spring - makes it leaner all the way through the range - stiffer spring makes it richer all across the range - I think I have that right!
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Last Edit: Jun 3, 2017 17:58:46 GMT by Darkspeed
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Oil thickness in the damper changes the rate at which the piston can climb, does affect the mixture when changing from steady to what.
Thicker oil slows the movement of the piston, increasing the air velocity across the jet, but not increasing the air volume, fattening up the mixture for hard acceleration.
The spring is more there for steady cruising.
I had absolutely huge issues with idle fueling on my Triumph, I could get the AFR numbers to sit exactly where needed under all conditions apart from idle. Then when I changed the floats and needles, the idle improved massively. So I adjusted the level of fuel in the float chambers until I had a stable idle that I was happy with.
The height of the fuel in the float has a direct relationship to the height of fuel inside the jet.
This is all in my experience at least, others results may vary!
Apologies if I'm teaching anyone to suck eggs.
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Darkspeed
Club Retro Rides Member
Posts: 4,711
Club RR Member Number: 39
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Yep - oil is purely for the transient enrichment a few seconds of effect, but makes no changes to steady state fueling which appears to be the issue here. [pedant mode]the air volume must increase for the velocity to increase as you cannot have one without the other. (It's the port CSA that is not increased - well it is, but slower due to the damper [/pedantmode] The enrichment is due to the increase in depression above the jet pulling more fuel for a given air volume - When the volume stabilises at steady state the piston has risen (increase in port CSA) to the position dictated by the resistance of the spring, the velocity through the port, the needle position in the jet and the height of the fuel in the bowl/jet set the steady state fueling.
With the old carbs jet alignment is also critical and a PITA to get right and jet alignment tools are not as easy to get as they used to be.
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93fxdl
Posted a lot
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sonus
Europe
Posts: 1,386
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Okay, so I did some adjustments today.
Started by adjusting the idle to 7-800rpm. It should idle silky smooth at 450-500, but it won't idle cleanly that low. Probably due to the ignition system not being adjusted correctly? Then I enriched the carbs so that it dropped from AF 10.8 til AF 9.2 at idle. Then had a spirited drive and the bangs and crackles when on the overrun are certainly a lot less obvious now. It feels like it pulls a bit stronger aswell. AF at a steady 2000rpm with a light load is now AF12-13. At light load on a up hill climb at 3000rpm in second it now reads AF 14.2-14.8. At higher rpm it still goes lean, all the way til AF 16-17 at 5-6000rpm. So I will try a thicker damper oil. I am using a 'dash pot' oil bought at the local motor factors, but it doesn't state viscosity, so I will have to try a few different oil thicknesses.
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Current
1968 TVR VIXEN S1 V8 Prototype
2004 TVR T350C
2017 BMW 340i
Previous
BMW 325d E91LCI - sold
Alfa Romeo GTV - sold
Citroen AX GT - at the breakers
Ford Puma 1.7 - sold
Volvo V50 2.0d - sold
MGB GT - wrecked by fire
MG ZT 1.8T - sold
VW E-golf Electric - sold
Mini Countryman 1.6D -sold
Land Rover Discovery TD5 - sold
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The oil thickness purely effects the transition to what, it won't effect the top end at all. I purely mentioned it before as you said on hard acceleration it climbs to afr of 17.
To be that rich at idle and so lean at high revs, wrong needles.
Granted all my experience is in HS and not HD carbs, but I doubt they're that dismilar.
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sonus
Europe
Posts: 1,386
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The engine I have is out of a Daimler 250 V8 saloon which is much heavier than my car. Could it be that the needles for this car takes i t account that the engine would be working harder when accelerating? The lighter Daimler SP250/Dart has different needles. I might try those and see.
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Current
1968 TVR VIXEN S1 V8 Prototype
2004 TVR T350C
2017 BMW 340i
Previous
BMW 325d E91LCI - sold
Alfa Romeo GTV - sold
Citroen AX GT - at the breakers
Ford Puma 1.7 - sold
Volvo V50 2.0d - sold
MGB GT - wrecked by fire
MG ZT 1.8T - sold
VW E-golf Electric - sold
Mini Countryman 1.6D -sold
Land Rover Discovery TD5 - sold
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ChasR
RR Helper
motivation
Posts: 10,195
Club RR Member Number: 170
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This will sound silly but have you checked for air leaks? Most SUs I have seen have them, even 'rebuilt' ones. With the leaks it is guesswork setting them up and an AFR is useless in this case. I am thinking about leaks at the Spindles.
It can be done but it's not as easy as a set without air leaks. I know your readings are rich at idle but I do wonder if the extra air could be throwing the mixture off (SUs work off the airflow dragging in the fuel witha fixed velocity of air thus the moving damper).
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Last Edit: Jun 5, 2017 6:33:18 GMT by ChasR
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Would you have not just been better of tuning it normally with out sensors etc ? Seems to be over complicating things . Could you have not just bought a gunson gas analyser ?
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Can't use a gas analyser whilst driving along and they react too slowly.
its only one sensor he has.
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Of anything I'd have expected fuel demand in a heavier car to be higher at faster engine speeds. Something isn't right there.
Guessing non stock exhaust and high flow air filters have been fitted too...
What needles are fitted? Should have three letters stamped on the shank.
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sonus
Europe
Posts: 1,386
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This will sound silly but have you checked for air leaks? Most SUs I have seen have them, even 'rebuilt' ones. With the leaks it is guesswork setting them up and an AFR is useless in this case. It can be done but it's not as easy as a set without air leaks. I know your readings are rich at idle but I do wonder if the extra air could be throwing the mixture off (SUs work off the airflow dragging in the fuel witha fixed velocity of air thus the moving damper). I'll check for leaks tomorrow 👍🏻 |
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Current
1968 TVR VIXEN S1 V8 Prototype
2004 TVR T350C
2017 BMW 340i
Previous
BMW 325d E91LCI - sold
Alfa Romeo GTV - sold
Citroen AX GT - at the breakers
Ford Puma 1.7 - sold
Volvo V50 2.0d - sold
MGB GT - wrecked by fire
MG ZT 1.8T - sold
VW E-golf Electric - sold
Mini Countryman 1.6D -sold
Land Rover Discovery TD5 - sold
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sonus
Europe
Posts: 1,386
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Of anything I'd have expected fuel demand in a heavier car to be higher at faster engine speeds. Something isn't right there. Guessing non stock exhaust and high flow air filters have been fitted too... What needles are fitted? Should have three letters stamped on the shank. I'll try to find out tomorrow what needles are fitted. Burlen has the TZ: .100in Jet Needle listed for the Daimler 250 V8 saloon and TR (weak) and standard TS listed for the dart/SP250 |
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Current
1968 TVR VIXEN S1 V8 Prototype
2004 TVR T350C
2017 BMW 340i
Previous
BMW 325d E91LCI - sold
Alfa Romeo GTV - sold
Citroen AX GT - at the breakers
Ford Puma 1.7 - sold
Volvo V50 2.0d - sold
MGB GT - wrecked by fire
MG ZT 1.8T - sold
VW E-golf Electric - sold
Mini Countryman 1.6D -sold
Land Rover Discovery TD5 - sold
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