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turbo'd twin choke (progressive) weber carb
FASTdan - 9/9/11 at 06:21 PM

My xr2 turbo technics runs a 32/34dft carb (std early xr2). It's the progressive type whereby the second choke is cam activated from about 50% throttle position onwards. For years it has suffered from a lean condition on part throttle (say 25% - 50%, mildly loaded cruising 2000-3000rpm. This occurs before the second choke kicks in.

Afr on light throttle cruise is a healthy 13.5-14. But as soon as you load it up it wheezes away with Afr going up to 14.5-15 exactly at the time I need it to richen. All is well tho if you open up the 2nd choke, Afr comes down to a healthy 11-12 and the car takes off.

I've tried millions of jet combos, but if I enlarge the first main jet I end up with Afr of 11 on light cruise (sooty plugs n bad mpg). But the condition does improve.

Just wondered if anyone has experience of these un-documented progressive carbs. I've never fully understood the function of the second idle jet? Also I always thought the idle circuit did the fueling until 2000rpm, but given that the main jet seems to drastically change the low down fueling I can't see that it does.

r1_pete - 9/9/11 at 06:50 PM

Give Eike Wellhausen at Chatsworth Motor Spares in Chesterfield a call, he's amongst the top Weber men in the UK...

imp paul - 9/9/11 at 07:07 PM

as above he is very good

BaileyPerformance - 9/9/11 at 08:02 PM

quote:
Originally posted by FASTdan
My xr2 turbo technics runs a 32/34dft carb (std early xr2). It's the progressive type whereby the second choke is cam activated from about 50% throttle position onwards. For years it has suffered from a lean condition on part throttle (say 25% - 50%, mildly loaded cruising 2000-3000rpm. This occurs before the second choke kicks in.

Afr on light throttle cruise is a healthy 13.5-14. But as soon as you load it up it wheezes away with Afr going up to 14.5-15 exactly at the time I need it to richen. All is well tho if you open up the 2nd choke, Afr comes down to a healthy 11-12 and the car takes off.

I've tried millions of jet combos, but if I enlarge the first main jet I end up with Afr of 11 on light cruise (sooty plugs n bad mpg). But the condition does improve.

Just wondered if anyone has experience of these un-documented progressive carbs. I've never fully understood the function of the second idle jet? Also I always thought the idle circuit did the fueling until 2000rpm, but given that the main jet seems to drastically change the low down fueling I can't see that it does.


Hi,

Sounds like the main jets are spot on, but the primary choke air corrector is too big, making the air corrector smaller will richen up the mixture at the top range of the first choke. I normally solder up the standard jet and re-drill smaller, that way you can start small, chk AFR, drill bigger if required. You will never completely sort the problem as the carb was not designed to accommodate air density changes under boost, but you can tune it out 90%. (carbs meter fuel on air flow, so when you compress air it gets denser, leaning out the mixture)

Let me know how you get on!

FASTdan - 10/9/11 at 08:10 AM

quote:
Originally posted by BaileyPerformance
quote:
Originally posted by FASTdan
My xr2 turbo technics runs a 32/34dft carb (std early xr2). It's the progressive type whereby the second choke is cam activated from about 50% throttle position onwards. For years it has suffered from a lean condition on part throttle (say 25% - 50%, mildly loaded cruising 2000-3000rpm. This occurs before the second choke kicks in.

Afr on light throttle cruise is a healthy 13.5-14. But as soon as you load it up it wheezes away with Afr going up to 14.5-15 exactly at the time I need it to richen. All is well tho if you open up the 2nd choke, Afr comes down to a healthy 11-12 and the car takes off.

I've tried millions of jet combos, but if I enlarge the first main jet I end up with Afr of 11 on light cruise (sooty plugs n bad mpg). But the condition does improve.

Just wondered if anyone has experience of these un-documented progressive carbs. I've never fully understood the function of the second idle jet? Also I always thought the idle circuit did the fueling until 2000rpm, but given that the main jet seems to drastically change the low down fueling I can't see that it does.


Hi,

Sounds like the main jets are spot on, but the primary choke air corrector is too big, making the air corrector smaller will richen up the mixture at the top range of the first choke. I normally solder up the standard jet and re-drill smaller, that way you can start small, chk AFR, drill bigger if required. You will never completely sort the problem as the carb was not designed to accommodate air density changes under boost, but you can tune it out 90%. (carbs meter fuel on air flow, so when you compress air it gets denser, leaning out the mixture)

Let me know how you get on!



Sounds logical, recall years ago soldering them up drilling etc but I think I may have been on the second choke when tuning for full boost. I've just got it back on the road after a few years off so its all a bit hazy. I'll start there, drill one nice and small (what happens if you block it completely?) n see how it goes. I remember changing emulsion tubes to little effect (holes only high up in them etc to try keep rich as the float drops, but this is complete guess work).

Anyone shed any light on the idle jets? I'd guess the primary will do very low down light load fueling? But where does the second one come in?

Does Eike mind people calling for advice? Always feel a bit cheeky calling people for free advice. But I will do anything to shed some light on this.

Further info on car for reference, its a t25 on a 1600cvh (approx comp 8.8:1), currently running 13psi. Full boost achieved at 3000rpm. It's a little rocket but this flat spot really ruins the drive.

It's not helped by the crude vac advance/retard unit on the dizzy (from a saab 900T), would really benefit from map based megajolt. Or just ditch the carb and dizzy and go efi, but that's expensive n kinda spoils the whole original tt thing.

BaileyPerformance - 10/9/11 at 08:53 AM

quote:
Originally posted by FASTdan
quote:
Originally posted by BaileyPerformance
quote:
Originally posted by FASTdan
My xr2 turbo technics runs a 32/34dft carb (std early xr2). It's the progressive type whereby the second choke is cam activated from about 50% throttle position onwards. For years it has suffered from a lean condition on part throttle (say 25% - 50%, mildly loaded cruising 2000-3000rpm. This occurs before the second choke kicks in.

Afr on light throttle cruise is a healthy 13.5-14. But as soon as you load it up it wheezes away with Afr going up to 14.5-15 exactly at the time I need it to richen. All is well tho if you open up the 2nd choke, Afr comes down to a healthy 11-12 and the car takes off.

I've tried millions of jet combos, but if I enlarge the first main jet I end up with Afr of 11 on light cruise (sooty plugs n bad mpg). But the condition does improve.

Just wondered if anyone has experience of these un-documented progressive carbs. I've never fully understood the function of the second idle jet? Also I always thought the idle circuit did the fueling until 2000rpm, but given that the main jet seems to drastically change the low down fueling I can't see that it does.


Hi,

Sounds like the main jets are spot on, but the primary choke air corrector is too big, making the air corrector smaller will richen up the mixture at the top range of the first choke. I normally solder up the standard jet and re-drill smaller, that way you can start small, chk AFR, drill bigger if required. You will never completely sort the problem as the carb was not designed to accommodate air density changes under boost, but you can tune it out 90%. (carbs meter fuel on air flow, so when you compress air it gets denser, leaning out the mixture)

Let me know how you get on!



Sounds logical, recall years ago soldering them up drilling etc but I think I may have been on the second choke when tuning for full boost. I've just got it back on the road after a few years off so its all a bit hazy. I'll start there, drill one nice and small (what happens if you block it completely?) n see how it goes. I remember changing emulsion tubes to little effect (holes only high up in them etc to try keep rich as the float drops, but this is complete guess work).

Anyone shed any light on the idle jets? I'd guess the primary will do very low down light load fueling? But where does the second one come in?

Does Eike mind people calling for advice? Always feel a bit cheeky calling people for free advice. But I will do anything to shed some light on this.

Further info on car for reference, its a t25 on a 1600cvh (approx comp 8.8:1), currently running 13psi. Full boost achieved at 3000rpm. It's a little rocket but this flat spot really ruins the drive.

It's not helped by the crude vac advance/retard unit on the dizzy (from a saab 900T), would really benefit from map based megajolt. Or just ditch the carb and dizzy and go efi, but that's expensive n kinda spoils the whole original tt thing.


Hi, dont close the air corrector completely, this will cause it to run VERY rich, the air corrector is normally at least twice the size of the main jet, small adjustments will make a big difference on AFRs - but only as the airflow thou the primary choke reaches the top end.

The idle jet on pri choke does just idle, 1000rpm max, it works along side the idle screw.

The idle jet on the sec choke helps during transition between pri and sec chokes, you should not need to touch this as your lean problem sounds like its in the pri choke only.

The boost retard system used should work fine, you would gain power if you went to EFI or just megajolt as the advance curve required for a CVH turbo is not straight forward - it will require re-advance at the top end of the rpm range (5000 to 6500) to overcome cam and head shortcomings, but plenty of retard at peak torque to prevent detonation. (4000rpm)

The boost retard unit just retards the timing across the board under boost, so is not ideal.

let me know how you get on.

ss1turbo - 10/9/11 at 11:48 AM

Killed one carb, never resolved it apart from buying a twin downdraught Dellorto, so was going injection, then got carried away....but need to revisit this at some point! Having the twin progressive isn't the best way IMHO; only the Lanica Delta and I think one of the Renaults (R11?) used a progressive carb; everything else turbo + carb uses either a single (R5, Montego) or one-per-cylinder setups (Esprit, Alfrasud). When the second choke opens, you get a bit of imbalance in pressure which doesn't help with the float levels which also knocks the fuelling out (which is why I found it tricky to get from partial to full throttle - either full from below the boost threshold, or very slowly open the throttle otherwise I got terrible stuttering..)

FASTdan - 10/9/11 at 06:17 PM

G

quote:
Originally posted by ss1turbo
Killed one carb, never resolved it apart from buying a twin downdraught Dellorto, so was going injection, then got carried away....but need to revisit this at some point! Having the twin progressive isn't the best way IMHO; only the Lanica Delta and I think one of the Renaults (R11?) used a progressive carb; everything else turbo + carb uses either a single (R5, Montego) or one-per-cylinder setups (Esprit, Alfrasud). When the second choke opens, you get a bit of imbalance in pressure which doesn't help with the float levels which also knocks the fuelling out (which is why I found it tricky to get from partial to full throttle - either full from below the boost threshold, or very slowly open the throttle otherwise I got terrible stuttering..)


What car was that then? Had considered using a single dncf (I actually have one) but never got round to it, only thing is I like the economy I can get from the progressive, all the other guys who've gone to an r5 carb don't get the mpg I do.

I do get a harsh shove in the back if I open the 2nd choke too quick or at the wrong time. Sounds like that could be helped by tweaking the 2nd idle jet? Also thinking o modding the linkage between the two so you can tune the point at which the second comes in.

Went from a 150 pri air corrector to 100 this morn, totally eliminated the flat spot (12.5-13 Afr) but richened light throttle too much, seeing 11.0-12 at 2000rpm in 3rd. Might try a 120, or reduce fuel jet slightly.

BaileyPerformance - 10/9/11 at 06:44 PM

quote:
Originally posted by FASTdan
G
quote:
Originally posted by ss1turbo
Killed one carb, never resolved it apart from buying a twin downdraught Dellorto, so was going injection, then got carried away....but need to revisit this at some point! Having the twin progressive isn't the best way IMHO; only the Lanica Delta and I think one of the Renaults (R11?) used a progressive carb; everything else turbo + carb uses either a single (R5, Montego) or one-per-cylinder setups (Esprit, Alfrasud). When the second choke opens, you get a bit of imbalance in pressure which doesn't help with the float levels which also knocks the fuelling out (which is why I found it tricky to get from partial to full throttle - either full from below the boost threshold, or very slowly open the throttle otherwise I got terrible stuttering..)


What car was that then? Had considered using a single dncf (I actually have one) but never got round to it, only thing is I like the economy I can get from the progressive, all the other guys who've gone to an r5 carb don't get the mpg I do.

I do get a harsh shove in the back if I open the 2nd choke too quick or at the wrong time. Sounds like that could be helped by tweaking the 2nd idle jet? Also thinking o modding the linkage between the two so you can tune the point at which the second comes in.

Went from a 150 pri air corrector to 100 this morn, totally eliminated the flat spot (12.5-13 Afr) but richened light throttle too much, seeing 11.0-12 at 2000rpm in 3rd. Might try a 120, or reduce fuel jet slightly.


Hi,
Your on the right track, aim for 13.5AFR first choke 3 or 4 PSI of boost, if too rich at light throttle then reduce the main jet.

But as i said b4 you will never completely sort the problem, as the turbo starts to make boost the air density goes up (more oxygen) so the AFR goes lean, slightly less throttle (less boost) and it will be rich. Another problem is air temps - On an EFI system the air temp sensor is used to trim the required fuel. (hot air = less oxygen = less fuel required)

If you are thinking of swapping the carb then the best one to go for is the metro turbo SU, but you MUST use the little manifold that bolts onto the inlet of the carb, the clever folks a British Leyland (RIP) fixed the lean/rich issue by putting a small restriction inside the carb manifold, this causes more of a pressure drop across the carb making boost calibration easier.

ss1turbo - 10/9/11 at 09:29 PM

This was on a Scimitar SS1 1600, which used a slight variation on the XR3 kit using the 32DFT and running at 8-9psi boost. Biggest problem was getting it sealed as rubberised gaskets were no longer available, not helped by the carb body and top being warped through overtightening (before I had it).

Best success I had was with a HIF44 from a Maestro turbo but I used an RS Turbo manifold which wasn't best for mixture distribution...

Aside from an aborted attempt with the KE-Jetronic system from an Escort RST (not sure the 15ft of inlet piping helped), the next stage was to be a Dellorto DRLA, turbo prepped, which I have.

I think the sudden airflow change when the second choke opens does upset the chamber level (being boost referenced) as I had another TT converted SS1 which still had the "stutter", but was much better, probably as the carb hadn't been touched apart from a TT dealer (the original owner, as it happened).

FASTdan - 12/9/11 at 07:34 AM

quote:
Originally posted by BaileyPerformance

Hi,
Your on the right track, aim for 13.5AFR first choke 3 or 4 PSI of boost, if too rich at light throttle then reduce the main jet.

But as i said b4 you will never completely sort the problem, as the turbo starts to make boost the air density goes up (more oxygen) so the AFR goes lean, slightly less throttle (less boost) and it will be rich. Another problem is air temps - On an EFI system the air temp sensor is used to trim the required fuel. (hot air = less oxygen = less fuel required)

If you are thinking of swapping the carb then the best one to go for is the metro turbo SU, but you MUST use the little manifold that bolts onto the inlet of the carb, the clever folks a British Leyland (RIP) fixed the lean/rich issue by putting a small restriction inside the carb manifold, this causes more of a pressure drop across the carb making boost calibration easier.


Reduced the pri jet to a 120 from 125 and am now seeing 12-13 AFR light throttle cruise, 13.5ish with 3-4psi boost mildly loaded which feels fine with no hesitation. So definitely getting there wouldn't mind a bit leaner on cruise so might go down another fuel jet size and see if I can get away with it. As you say though its always going to be a compromise. Feels a million times better for round town driving.

A couple of the lads on the XRtwo forum run the MG SU carbs though most seem to have opted for the R5 item. The Lancia delta ran the 32/34DMTL which I also have kicking about, but seems like an almost identical carb to the DFT. I think for the time being I'll be sticking with the current set up as dont have the time to invest in a conversion - quite happy with how it's driving now (just needs a bit more boost :cool

Thanks for the help guys

ss1turbo - 12/9/11 at 07:42 AM

Curious to know how those guys are fitting the SU...

I spent ages looking for the Delta carb, getting nowhere (mainly because you could, at the time, get a rebuild kit for it). R5 one is a bit small for even the quoted output on mine (140bhp) although I do have one knocking around somewhere. Will probably end up using the DRLA but having a play with the throttle linkage to get it a bit more progressive.

FASTdan - 12/9/11 at 09:45 AM

quote:
Originally posted by ss1turbo
Curious to know how those guys are fitting the SU...

I spent ages looking for the Delta carb, getting nowhere (mainly because you could, at the time, get a rebuild kit for it). R5 one is a bit small for even the quoted output on mine (140bhp) although I do have one knocking around somewhere. Will probably end up using the DRLA but having a play with the throttle linkage to get it a bit more progressive.


The delta carb was the same as used on mk3 XR3's - which is where mine came from (albeit possibly modified for turbo use). The R5 carbs do cause a large pressure drop due to their small size but they have been proven way over 200bhp. Thats another benefit of the twin choke, overall venturi size.

I'll see if I can find the post where the guys using an SU carb in an XR2.

One concern I have is that I might be bore washing at 12-13 AFR on cruise?

FASTdan - 15/9/11 at 06:22 AM

Just another quick question on this, at what AFR does bore wash become a problem?

I've read below 10 is a serious problem, this sound about right? Will a cruise AFR of 12.5-13 be causing any bore wash? Obviously cruise speeds are where the average road car spends most of its time so I want to be sure its not a problem.....

BaileyPerformance - 15/9/11 at 08:26 PM

quote:
Originally posted by FASTdan
Just another quick question on this, at what AFR does bore wash become a problem?

I've read below 10 is a serious problem, this sound about right? Will a cruise AFR of 12.5-13 be causing any bore wash? Obviously cruise speeds are where the average road car spends most of its time so I want to be sure its not a problem.....


Sounds like you've cracked it!

12.5 AFR is fine for cruse, but will effect MPG (nothing major) 11AFR is about a rich as you can go without engine damage at light loads, full throttle you can get away with 10AFR but MPG and power will be crap.

You need to see 11 to 12 at full boost, the extra fuel cools the combustion to help prevent detonation.

R5 carbs are really crap (as are the cars!), i have had a couple on the dyno, difficult to get the fueling right.

The MG metro turbo SU is the way to go, good enough for way over 200BHP. The metro SU is better than the Montego SU as it has no electronic idle or fuel control stuck on the side.

FASTdan - 16/9/11 at 07:10 AM

quote:
Originally posted by BaileyPerformance
quote:
Originally posted by FASTdan
Just another quick question on this, at what AFR does bore wash become a problem?

I've read below 10 is a serious problem, this sound about right? Will a cruise AFR of 12.5-13 be causing any bore wash? Obviously cruise speeds are where the average road car spends most of its time so I want to be sure its not a problem.....


Sounds like you've cracked it!

12.5 AFR is fine for cruse, but will effect MPG (nothing major) 11AFR is about a rich as you can go without engine damage at light loads, full throttle you can get away with 10AFR but MPG and power will be crap.

You need to see 11 to 12 at full boost, the extra fuel cools the combustion to help prevent detonation.

R5 carbs are really crap (as are the cars!), i have had a couple on the dyno, difficult to get the fueling right.

The MG metro turbo SU is the way to go, good enough for way over 200BHP. The metro SU is better than the Montego SU as it has no electronic idle or fuel control stuck on the side.


Yes its definitely getting there, thanks a lot for your advice you've certainly explained a few things I was never sure of - you can spend all day swapping stuff out and trying to quantify the changes, but when all you have is a live on the road AFR reading its not always easy to determine what improvements each change makes.

It currently dips into the 10's on full boost at around 4500rpm and you can actually feel the power drop off, but by 6k its back up to 11.8ish. This is another anomoly I never eliminated, it ALWAYS richened up at those revs on full boost. I wonder however in that case if its jetted too rich, but the needle isn't flowing enough fuel (I 'think' im running a 200 at the mo) so the leaning back out could be attributed to that.

BaileyPerformance - 16/9/11 at 01:29 PM

quote:
Originally posted by FASTdan
quote:
Originally posted by BaileyPerformance
quote:
Originally posted by FASTdan
Just another quick question on this, at what AFR does bore wash become a problem?

I've read below 10 is a serious problem, this sound about right? Will a cruise AFR of 12.5-13 be causing any bore wash? Obviously cruise speeds are where the average road car spends most of its time so I want to be sure its not a problem.....


Sounds like you've cracked it!

12.5 AFR is fine for cruse, but will effect MPG (nothing major) 11AFR is about a rich as you can go without engine damage at light loads, full throttle you can get away with 10AFR but MPG and power will be crap.

You need to see 11 to 12 at full boost, the extra fuel cools the combustion to help prevent detonation.

R5 carbs are really crap (as are the cars!), i have had a couple on the dyno, difficult to get the fueling right.

The MG metro turbo SU is the way to go, good enough for way over 200BHP. The metro SU is better than the Montego SU as it has no electronic idle or fuel control stuck on the side.


Yes its definitely getting there, thanks a lot for your advice you've certainly explained a few things I was never sure of - you can spend all day swapping stuff out and trying to quantify the changes, but when all you have is a live on the road AFR reading its not always easy to determine what improvements each change makes.

It currently dips into the 10's on full boost at around 4500rpm and you can actually feel the power drop off, but by 6k its back up to 11.8ish. This is another anomoly I never eliminated, it ALWAYS richened up at those revs on full boost. I wonder however in that case if its jetted too rich, but the needle isn't flowing enough fuel (I 'think' im running a 200 at the mo) so the leaning back out could be attributed to that.


No problem mate! glad i can help.

As you say could be a fuel delivery problem, i assume you have the boost sensitive fuel pressure regulator and electric pump on your car? You'll prob find the fuel pressure is dropping (or not rising enough with boost) at the high rev end but is ok at 4000-4500RPM. So, in actual fact the second choke is calibrated too rich but leans out due to fuel starvation.

Can you rig up a fuel pressure gauge in the car and take for a drive? Baseline should be 3 psi, then 3psi plus whatever boost you are running. (3 + 13 = 16psi)

I see you make manifolds? i have alot of people asking me about bike throttles on car engines.

FASTdan - 16/9/11 at 01:52 PM

quote:
Originally posted by BaileyPerformance
No problem mate! glad i can help.

As you say could be a fuel delivery problem, i assume you have the boost sensitive fuel pressure regulator and electric pump on your car? You'll prob find the fuel pressure is dropping (or not rising enough with boost) at the high rev end but is ok at 4000-4500RPM. So, in actual fact the second choke is calibrated too rich but leans out due to fuel starvation.

Can you rig up a fuel pressure gauge in the car and take for a drive? Baseline should be 3 psi, then 3psi plus whatever boost you are running. (3 + 13 = 16psi)

I see you make manifolds? i have alot of people asking me about bike throttles on car engines.



It is an electric pump (a later spec renault 5 item infact! As Turbo Technics specced originally) and a boost sensitive regulator (which I believe is a metro turbo item, again TT specced). Back in the day I rigged up a fuel pressure gauge and stuck it under the windscreen wiper, it did as you say and iirc was 1:1 ratio against boost, starting at a base of 4psi. However that was when running circa 7-9psi, not sure I ever checked it when I lowered the comp and raised the boost. I still have the gauge so will set it back up. As you say that reg might have a limit, wouldnt have thought the pump was at its limit as im sure the R5 boys run daft boost.

Yes I make manifolds, if you have any requirements drop me a PM or email.