Needles and mixture - 3.9 l

gbvona

Member
Long story short, I currently have a Range Rover 3.9l engine (9.25 CR, Crower Stage 2 cam) in my 1971 NADA P6. I am using two SU HIF6's for carbs. I made a few earlier postings documenting adventures with tuning.

The leap forward that I report here comes after acquisition of an Innovate fuel/air mixture meter. This is a gizmo that lets you hang a real Bosch O2 sensor on the tail pipe and drive around watching the mixture. Tech is wonderful.

Originally the carbs came with BBV needles. which are a standard 3.5l needle. Performance of the engine was anemic at best, presumably reflecting the draconian detuning done by the factory to meet emissions specs. I came upon a pair of BAC needles, which were also used with the 3.5l engine in earlier editions. This made a world of difference in idle, acceleration and general running. My worry was that the BAC needles were too rich, especially at the high end, which is warned of in several write-ups. As a precaution I got the Innovate meter and hooked it up.

My initial setup with the BAC needles was a bit lean at idle since I was worried about the richness at the other end. The meter revealed this concern was not warranted. At the outset, idle read about 14 and driving around 50 mph (2250 rpm) on the flat yielded about 16. After fiddling with the mixture screw I got idle down to about 12.5 and driving at speed down to 14. Further richening of the mixture dropped the idle even lower (11.4) but driving at speed remained stubbornly around 14, going to 13 (briefly) only with heavy acceleration. The performance of the car was pretty good, but the readings while driving still seemed a bit high for a somewhat tall cam. I am looking for a steady driving mixture around 13-13.5.

My thinking is that the mixture at idle is VERY sensitive to jet height, since the proportional changes of the opening at the jet are large for small changes in jet height. At speed, a difference of a turn of the adjusting screw is proportionally less effective in modulating the mixture, since the effective diameter of the needle is much smaller when the pots are raised. Bottom line, it would appear that the only way to get a richer mixture at speed is to go to a needle that is richer yet. I would note that by observing the rise of the carb pots when accelerating the engine (whilst stationary, needless to say) I doubt very much that the needles are going much beyond stages 7 or 8 while driving around. This does not really get to the excessively rich regions of the BAC needles.

So more needle fiddling seems in order. I plan on trying both the BCW and the BBW next. Stay tuned.

gbvona
 
Very interesting. The more I learn, the more I yearn for the wonderfully simple but very effective hotwire injection.

All the needles seem pretty much the same up until 7 and 8 and I often wondered how that could possibly be.
 
I would note that by observing the rise of the carb pots when accelerating the engine (whilst stationary, needless to say) I doubt very much that the needles are going much beyond stages 7 or 8 while driving around. This does not really get to the excessively rich regions of the BAC needles.

Your mistake is that revving an engine like this, there is almost no load, hence there is no need for the carb pistons to rise fully. The spring in the bell chamber of the car is designed to let the piston reach its maximum height when the throttle is wide open and the engine at max bhp rpm.
 
I thought about an Innovate system but they are quite expensive so I opted for a rolling road session and would do so again. The benefit of trying different settings under load conditions far outweighed trying to get it right at home. The session was 100 quid and I had a couple of hours and around 5 power runs, nailed the fuelling on the second set of needles = BAW in 2" HS8 on a 4.6.
 
I appreciate your strategy of using rolling roads. The practical challenge is that I live in Tennessee -- we have no shortage of rolling roads (think NASCAR wannabes), but no one here has the slightest notion of what an SU is. If it ain't a Holley, it ain't a carbureter. There is no easy way to source needles, have to get them one at a time via mail order, so I need to navigate my way through the jungle with a map, hence the Innovate. The BAW is on my list, it is richer still than the BBW in the midrange but a bit leaner up at the tip.
 
Access to rollers is the major factor, if you have that then air / fuel ratio is the same whatever the carb. Burlen fuels do sell a needle book with all the needles listed with all the graduation measurements.
Same here with needle purchasing, all by mail and guesswork.
You could get close then book a roller session and take a cordless drill with you and some emery paper, 'custom grind' some needles on site, then when set either leave them in or measure up and order the closest later.

Try
Carburetor Visual Library - JOE CURTO, INC.
 
Thanks for the reply. Yep, got the SU needle book. I was in England in September (Goodwood, Yeah!!) and tried to get needles from SU directly--they were at both the Beaulieu Autojumble and Goodwood -- but they did not have any of the ones I wanted to try in stock. I just ordered pairs of BAW and BBW from Joe Curto, been dealing with him for many years. I think I am getting close. The Innovate logs both air/fuel mixture and rpm under all driving conditions, so I think I can get close on the needles without a rolling road. However, once that is done I still need to finesse the timing, for which a rolling road is essential.
 
After considerable fiddling, I am reasonably happy with the BBW needles given my configuration (3.9l with stage 2 cam). It runs at around 14 air/fuel ratio under normal cruise.

gbvona
 
Good to hear some feedback on this.
It would be very useful to have a sticky or library post under carbs/ fuel that people could post their engine spec and resulting needle selection, that way folks who are modding can get an idea of a base setting.
 
Yes - i can say for definite if you have 10.5:1 hs6s, electronic ignition and high flow air filters in an sd1 housing then KP needles are bob on.
 
Interesting. I've got a "level 2" cam from Crower and I'll have large valve heads and free flow filters. What comes after KP?
 
One thing's for sure, the standard KO is very marginal and almost any improvement pushes it over the edge, at least with modern 98 unleaded.
 
The only needle i have listed which is richer accross all stations is RC which is very rich...

The HS6 carbs dont seem to list many needles at the top end... I guess because the 13/4 carbs are slightly undersized and you'd probably spec 2" carbs if you were going to have 2, or 4 1.5 carbs.

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That profile is strange being more or less exactly linear - doesn't look too good to me being rich where you sit cruising but not offering much more higher up where you want it

I think HS8s are pretty much essential on a 4.6 but I'm thinking on the 3.5 you are going to get poor performance at low RPM and a drop in torque. Like I say, the more you read, the more appealing even a fairly simple EFi system becomes, especially on an auto where you don't want to compromise that torque.
 
Personally I am going EZ-EFI on an Offenhauser intake with my S build, self learning and far superior to carbs. I install these systems on many of the Jensen Interceptors that I build, and I must say that the improvements on cold start, hot start, economy, power and general driveability are astounding. You even get a handheld that allows you to play with different states of tune if you like fiddling with things. Absolute Godsend if you are fortunate enough to live in the US because here, the shipping and UK Government slice hurts but, hey ho the overall package is a worthy investment IMO
 
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Personally I am going EZ-EFI on an Offenhauser intake with my S build, self learning and far superior to carbs. I install these systems on many of the Jensen Interceptors that I build, and I must say that the improvements on cold start, hot start, economy, power and general driveability are astounding. You even get a handheld that allows you to play with different states of tune if you like fiddling with things. Absolute Godsend if you are fortunate enough to live in the US because here, the shipping and UK Government slice hurts but, hey ho the overall package is a worthy investment IMO

Which Offenhauser intake are you using? The one that would have originally taken 2 x Rochester GC 2-barrels ( like 1956 Pontiac carbs for eg ) I see Holley make a 2GC throttle body now

Did you use a fuel return line to the tank?

best

G
 
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