CD’ Strombergs What I have learnt.
Thought I would share what I have learnt about CD’s over the years and still learning as there is not alot of info out there on CD’s although SU operation is similar. This is not gospel just my own experiences, yours may differ. I have also included my experiences with different needles and modifying them.This may help anyone wanting to get more from these carbs or to trouble shoot any issues. Hope it helps but just be aware of the dangers in modifying needles.
The CD Stromberg is a simple carburettor and unfortunately is a massive compromise in its mixture operation because you cant change mixture at W.O.T ( wide open throttle ) without affecting cruise mixture and the light acceleration mix which is why there and hundreds of needle profiles available. One other problem the Stromberg has is that is doesn’t have an accelerator pump to enrich the mixture while accelerating, it relies on oil in the dash pot to slow the rate at which the air valve rises causing a richer mixture. This is fine for normal driving but for W.O.T you loose performance as the air valve takes so long to open up it works like a second butterfly restricting the air / fuel mix to the engine. This is why Holden said NO OIL to be used. On a dyno my engine produced 20 ftlbs more torque without oil; but this caused hesitation under light acceleration due to leaning out. So do you want drive ability or power ??? Holden went some way to fix this with the 6D needle as it runs a very rich profile at low RPM to compensate for this leaning on acceleration. The damper oil enrichment system is nowhere near as effective as a power valve set up in say a Holly; while the oil does slow the rise rate of the air valve creating a richer mix its not rich enough at light throttle and leaning the carb to get good AFR at lower RPM you will also be leaning out the already lean acceleration mix this enrichment is way too much if you slam the boot in from low rpm,
So use heavier oil; yes that will improve the acceleration under a light to medium throttle application while sacrificing W.O.T response, again compromise !! . Were not the only ones, those chaps with their TR6’s have the same issues. I verified this recently on a twin CD175 TR6. The American TR6’s ran fuel injection to be able to pass emissions in California as the CD’s would not pass as they had to run on the rich side to help with this lean acceleration.
So what do you do. If you’re not comfortable in playing with different needles stick with the 6D and set them as Holden recommended or ¼ turn leaner. Just remember leaning may cause you to get hesitation / surging at light throttle applications. Running the 6D any richer will cause engine to stumble off the lights and cause cam jerk at low RPM in top gear plus poor economy. Again you need to compromise or your gunna end up with a cracker of a headache.
Oil or not to oil. Hmmmm if your driving like you stole it then take the oil out cause who cares about light acceleration but for general cruising I would use oil. Easy; if your getting surging use oil. Just a bit hard to do this at the lights, hehehe
What oil? S.U and Zenith specify 20 weight oil, I use 0W20. Lighter oil will help the air valve rise quicker but this can also cause hesitation.
Stromberg and S.U’s are good carbs and the variable venturi idea does have a big advantage over Webbers at anything under W.O.T such as low RPM throttle response and economy but getting the mix right is a compromise and not to knock the Webber as they are a great popular carb, there are a couple of group NC guys who are always up the front running 2 inch S.U’s; any one had Webbers bog down out of a corner at mid range RPM’s, variable venturi carbs don’t seem to have this problem.
So affecting factors to consider if your gunna play; needle profile, spring rate, damper oil and of course jet height setting, remembering each will effect the other and effect top end performance and drive ability at the same time AND open up Pandora’s box and not in a good way. I blame these carbs for going grey and sleepless nights.
Books and parts; Haynes produced a book many years ago called Stromberg CD carburettors owners workshop manual. Very helpfull.
SU Midel in Sydney carry a lots of parts but not all, eg throttle shaft spindles.
Stay up floats or Burlen in the UK have everything for these carbs and have been very helpful, easy to order part using Paypal and not bad freight rates. Burlen.co.uk
Stromberg needle summary
6D needles ( std XU1 Needles) work very well up top but are way rich down low and in cruise, this is because the needles are profiled to be used with out oil in dash pot I.E air valve damper. This is great on track or high power driving but no good under general driving unless you lean them out a bit.
6C Greatly improved general drive ability being a lot leaner than 6D down low but the 6C leans out above 3200 RPM aprox 120 Kph in cruise and hesitation on light acceleration at around 3000RPM.The 6C is way too rich at W.O.T ( wide open throttle ) A plated set of 6C needles were made and worked very well at W.O.T with improved AFR ( Air fuel ratio ) ( blue std springs )
6AC needles ( with blue springs. More on springs latter )were way too lean for general driving and suffered hesitation on acceleration even with dash pot oil used but were very good at W.O.T. 6AC was used with red damper springs, set at 2.5 turns and found to be very good, responsive and smooth but way too rich at W.O.T. A plated set of 6AC was made, this improved W.O.T but are now a little lean at 2.5 turns, better at 3 Will rectify this.
6E ( LC XU1 ) needles were tried also using red springs and oil and found way too lean, leaner than 6AC although the Stromberg chart says they should be a little richer than 6AC from station 1-7. The 6E was by far the best needle with “red springs at W.O.T. Incidentally the 6E profile from station 8-12 is very very close to my custom plated 6C
AFR readings in cruise top gear reference only all with std blue damper springs
RPM 6D 6C 6AC 6E
2000 11.7 12.9 12.8 13.9
2500 12.1 13.1 13.9 13.6
3000 12.8 13.7 14.9 15.9
3500 13.6 15.2 15.7 +16
NOTE 6D was set at 2.25 turns; 6C, 6AC, 6E needles needed to be set at some 4-5 turns to get them to run properly. What the above shows is where the jet was set to get a compromise of AFR between normal driving and power. Obviously you can play and make the rich 6D leaner and the others richer but you lose top end.
AFR 12.5 – 12.9 is a general setting at W.O.T for most power and an engine with a performance cam is generally around the 13 to 13.5 mark in cruise. There are varying opinions on this depending on who you talk to.
14.7:1 is the stoichiometric value where all the oxygen and fuel is all burnt, lower number means a richer mix while a higher number is a leaner mix.
A richer mix doesn’t mean pouring out smoke like a diesel it just means richer than 14.7 eg 13:1 is not a rich mix for cruising around with a performance engine.
Modern cars with ECU’s have no problem in getting all the figures right but with CD’s I have found they like a richer mix down low to obtain a reasonable mix under acceleration. You can get in the 14’s with CD’s for cruising but 1, this is lean for a cammed up engine, 2 it will not accelerate properly and 3 good luck trying to find a needle profile to give you everything.
Now I mentioned red springs; The heavier the spring the lower the air valve height will be at any given RPM and hence an overall richer mix but springs have different compression rates similar to suspension springs. Zenth have 3 damper springs, light, medium and heavy. Light has no paint, Blue is medium and red is heavy. The CD2’S for the XU1 had Blue springs from factory but don’t panic if yours don’t have any paint left as most don’t. I used the red springs with 6AC needles at 2.5 turns, this helped the lean out at 3000 RPM in cruise and also helped the leaning on acceleration and was probably the best combination for general driving I found but lost top end due to the rise rate of the damper; bugger. You can waste so much time and money trying to find the sweet spot as I have tried to do only to find you cant have it all with these carbs or SU’s for that matter. While were mentioning air valve rise rate, back in the 80’s I changed the diaphragms and first came across this hesitation under acceleration; Carby man said its must be something I had done. Many years later I found out the newer diaphragms are more supple than a NOS set I found. The NOS diaphragms fixed the issue but didn’t last and soon cracked so I would not be surprised if others have experienced. A more supple diaphragm will allow the air valve to raise quicker causing a lean mix.
Needle profile is another thing. Some are great at low RPM but way off the mark up top and vice versa but these needle may work perfect in the Humber super snipe or some other British jalopy. Different cams and timing, gear ratios, compression exhaust all play apart, the XU1 202 is really a mild engine which the 3 CD 175’s proved adequate but as soon as you modify heads, cam and exhaust the CD 175 are too small. I once took a video of the CD 175 piston raising at W.O.T of my 202 engine which has an XU1 cam and a better head only to find the poor old CD was tapped out as far as CFM goes at 5000RPM so revving any harder with these carbs was pointless. Power gains could still be made as fitting this better head with higher comp and bigger valves change the mixture up top so now the 6D needles had to be replaced and that was why I purchased an O2 analyser to work out what I needed to do as far as needle profile was concerned, now I suffer from anxiety attacks and headaches chasing the perfect tune; stupid brain. I have had success with changing needle profiles by electroplating at various needle stations but its not easy. The CD needle are measured in 1/8th of an inch stations, 13 in all and are measured in thou. The mixture Jet that the needle goes into has an opening of 90 thou so the needle diameter will be less than this and tapper off towards the tip of the needle. To give you an idea of how sensitive the measurements of the needle are I will give an example. Note the following is what I have found with my gear but its very close. At a particular needle station it might measure say 650 ( the Zenith list shows it written this way ) that is 65.0 thousandths of an inch in diameter so by changing this figure by 0.3 of a thou to 653 or 647 will change the ARF by approximately 0.4 so if the AFR was at 12.5 for example, now you will have either 12.9 or 12.1, either of these extremities are close to if not outside the max power window. 0.3 thou is also very close to half a turn of the mixture screw; Yes we are talking 0.1 of a thou so think very carefully be for you get out your emery paper to richer up all 3 needles accurately to 0.1 of a thou across the stations and carry plenty of spare ones for when you stuff up as i have done. Note: 0.3 of a thou at ½ turn of the mixture jet is not how much it raises or lowers the jet but equates to the approximate change in needle diameter for this new jet position the jet will shift something like 15thou in height for half a turn. To try to achieve the perfect AFR at all rev ranges at W.O.T with the CD I believe is not possible as the air valve with or without oil will raise at different rate depending on engine load I.E what gear your in. But you can get a compromise with many many hours work, Is it worth it, probably not. Will you get frustrated and a massive head pain.
If your on the dyno with CD’s and they do a 4th gear pull from 3k RPM to 5k RPM don’t be surprised if they say its way too rich, thats what happens with these carbs, but if you could simulate a ¼ mile run on a dyno with all the gear changes you would find 1st gear from idle to 5k at W.O.T would be rich then lean out up in the Higher RPM then 2nd, 3rd and top from say 4K RPM to 5.5K RPM would be ok; its just the way they are. So be aware of 4th gear pulls on dynos with CD’s I would expect similar with SU’s ?
Time for a coldy then lay down.
Cheers, Geoff.
Hope this helps and not confuses you all. Remember it just my findings over the years
Edited by 73TORANA!, 27 March 2019 - 03:39 PM.
