HIF6 carbs needle advice, please


Active Member
first of all a real peaceful 2017 for all..
I think, I have to re-jet :confused: my HIF6 carbs, when I have finished the work on my engine.
It´s the standard 3.5 ltr engine and I have changed now the following components :
SD1 heads, ports enlarged, standard valves. Will skim them by 0,5-0,6 mm, so CR will be around 9.8.
Camshaft will be from the 3.9.
Inlet manifold ports enlarged, airfilter adapters for the carbs ported.
Will replace air filters with K+N (after MOT/TÜV...) and use them with the elbows.
Does anyone have an advice for the main jets ?

unstable load

Well-Known Member
I don't see any need to rejet. You haven't changed the displacement of the engine, so it will still be needing the same amount of air/fuel mix, except it will be a more efficient delivery with the heads, porting etc.
Get it running and drive it a bit to see how it is, and then, if you want to tune it, by all means rejet it, but I don't see it being needed simply for different heads.


Active Member
Hmh, thank´s for it, but I´m pretty sure, the airflow will be different (I wrote standard valves, but meant the bigger standard valves from the post P6 heads). The amount of aluminium I got out of manifold, heads, carb adapters was impressive.
The V8 Owners Forum - :: View topic - SU Needles
Some more discussions about SU tuning. Good thing is, you learn a lot of this stuff.
Idea was to re-jet, when the carbs are open now, but it´s not a must, it can be done also by starting with standard and then see, what the plugs will look. Interesting thing, even with the original heads and a worn-out standard 3.5 cam, valves and plugs looked very bright o_O

clive P62

Active Member
Will probably run ok with any of the HIF type needles, If it was me being as you've invested a lot of time and money I would have it Rolling Road turned by a company who knows SU s
That way your getting the best out of it.
I remember reading a article in David Visards book on mini tuning, he had a race spec mini Rolling tuned, didn't sound any different after but managed to get 40 BHP extra as a result.


Well-Known Member
Or how about this on a Rover V8 with twin SUs (albeit an SD1)... 100bhp at the wheels to 165bhp. This is why you should put your tuning plans in action in one go and then get it set up. Literally anything you change will affect the SUs.

You might be able get 130+ at the wheels with this setup and the auto box on a P6. That would indicate 170-180 at the flywheel.


Well-Known Member
You would expect in the interests of economy that you'll be lean on a standard set-up. Improving airflow even with a filter will make that worse. There's a third factor too, modern fuel is formulated differently and older engines tend to need to run a touch rich anyway. If you tune a RV8 on carbs using only the optimum idle CO numbers on a gas analyser it will run horribly. I can get mine down to 2.5% but it runs best at 3.5-4%

Rolling roads don't/can't lie.

clive P62

Active Member
I have messed about with C/O adjustments at work on our gas analyser ( remember your only adjusting mixture at idle ) about 3 to 4.5 % is about right as long as everything not badly worn.
You can also set mixture as well by setting the H/C to as low as you can get
And ignoring C/O levels ( as long as your car is before 1975 for our MOT requirements)
Set your mixture too weak increases H/C as it cannot burn efficiently or setting it too rich does the same to the H/Cs


Active Member
Thanks guys. Yes, you´re right, I should bring it on a Rolling Road, when I´m finished. It´s the land of Solex, Zenith and Bosch, but I´ve found an adress not too far away and they ask for EUR 75,00 for one hour and that seems to be a very fair price. Balancing the carbs, timing etc etc I have to do myself, before this happens, that´s clear.


Well-Known Member
€75/hour is cheap! However lack of SU experience would bother me a bit. Please share your before/after dyno readings. Hope you are as lucky as that SD1 driver. There's nearly always power to be had even without any mods. There was a thread where a 2000TC got from memory around 115bhp. That's got to be over 130bhp before the gearbox which a impressive for a late 50s 2 litre engine. In fact it's better than most 3 litre 6s of the time albeit nowhere near as smooth and tractable.

As a benchmark most standard 3500 autos give 95-105bhp at the wheels, a 3500S manual around 120-125. So quite an additional loss for the auto, although around 6bhp of the difference is reputed to be down to the better flowing exhaust.

Another non-engine thing you might not have considered is a tweaking of your transmission control cable (I refuse to call it a "kick-down") so your gearbox co-operates with your tuned engine. There's nothing more frustrating when you only have 3 gears than a transmission that either won't shift unless you drive it really hard or shifts abruptly as soon as you tickle the throttle.


Active Member
You know, the people around Stuttgart search years to save even small money :D
Yes, the kickdown-cable I´ve "met" already. It was broken and connected with 2 or 3 wires left only. Have a new one in now. I know the procedure from my former W108. When I got the car, the automatic was very disappointing. It took some time in the Mercedes Forum to learn about the importance of the kickdown cable. Some adjustments and the car was another one. A classic car without a forum support will not make us happy.
Just my tuppence...

3.9L engine, Crower stage 2 cam, standard 3.9L heads, HIF6s. Original needles were BBVs. Not good. "Any of the HIF type needles" doesn't always work.

Much messing about with timing and needles, settling for the moment on BACs. It's MUCH snappier. Will drive awhile and look at the plugs to see if too rich. Cam begs for about 14 deg initial timing.



Active Member
I had a lot of trouble trying to set up my 2200SC with the HIF . It had 73K miles on the clock Had a word with SU Service, when still in Birmingham ( Late 70's) and was advised to change the jet for a new ones as the biased needle wears a minute scallop on the edge of the jet making it run significantly rich at idle. If you then set the mixture correct at idle it is running weak at anything above. Needle apparently wears very little as its rubbing over the whole length. New jet and it then ran beautifully