Nearly There - SU Needle Question

[Bainzy]

I've been tuning the carb needles this week in combination with the LC-1 wideband, and am getting really close to having it in perfect tune. None of the profiles people usually recommend for filters+exhaust seem to work with this car, maybe it's the 88c thermostat or the Megajolt, or the fact that the K&N's are deeper than most or because of the way the Bell 4/2/1 manifold exits the head so nicely. They all end up requiring a really rich idle otherwise they'll be too lean everywhere else.

Anyway! After getting fed up with AAR's I tried AAA, better but still way off looking at the wideband. So I set to work modifying them using Des Hamill's guide in his book on SU Carburettors - using calibrated wooden dowels inside the dashpots, but using the wideband to check each needle position was around 14.7.

This worked perfectly for everywhere up to about needle position 4, at which the car was reaching 5000rpm (it idles just between positions 1 and 2). Positions 5 to 16 therefore can't be measured in this way (though it won't reach 16...), but it's a bit tricky figuring out which needle station the car is at for a given scenario.

I've got the foot-to-the-floor area (top end) pretty close by making the upper half of the needles like AAM's (which are quite rich), but there's still a lean spot somewhere so perhaps someone might be able to give guidance on where to look. It basically exists when you're travelling at around 40-50mph in OD top, and quickly stab your accelerator pedal down further - after a second or two or if you apply more throttle the carbs will catch up and give the correct AFR.

Should I be looking at needle stations around 9-11? Or higher? I tried taking a little off 7 and 8 and couldn't see any measurable improvement to this flat spot, obviously don't want to take too much off any one spot.

[Nick Jones]

Sounds like good work being done so far.  I admire your patience!

What you describe sounds more like "accelerator pump" action that needs tuning.  You will have noticed SUs don't have accelerator pumps though..... this is is what the dampers in the dashpots are for.  They hold the the pistons down on snap throttle opening which increases the vacuum in the venturi, pulling more fuel through and enriching the mixture.  This can be tuned to an extent by varying the viscocity of the oil in the dashpots.  Thinner oil allows the piston to rise faster and reduces the accelerator pump effect.

Cheers

Nick

[Bainzy]

Thanks Nick. I would've put it down to that, though every other range when driving doesn't really suffer from this issue. You can accelerate in most scenarios and find the AFR drops nicely into the mid 12's, then returning to 14-15 if you hold that throttle position.

It's not just accelerating that brings it on, I just used that as a badly worded example of where it can be reproduced. It will also resurface when coming onto a dual carriageway, steadily driving until you get to about 40mph+ when it will go back to normal ranges.

It's annoying how I've got so close yet can't seem to find this lean area yet, and know I could find it instantly on a rolling road but don't really see the point in paying for a session when I'm going EFI soon and also changing the cam + head work, since the settings will just end up being wrong again.

[Deleted User]

1381 wrote:

Thanks Nick. I would've put it down to that, though every other range when driving doesn't really suffer from this issue. You can accelerate in most scenarios and find the AFR drops nicely into the mid 12's, then returning to 14-15 if you hold that throttle position.

It's not just accelerating that brings it on, I just used that as a badly worded example of where it can be reproduced. It will also resurface when coming onto a dual carriageway, steadily driving until you get to about 40mph+ when it will go back to normal ranges.

It's annoying how I've got so close yet can't seem to find this lean area yet, and know I could find it instantly on a rolling road but don't really see the point in paying for a session when I'm going EFI soon and also changing the cam + head work, since the settings will just end up being wrong again.

Just leave it as it is and go and have a pint then ;)

[nang]

For what it's worth, My Mk4 with K & N's is running AAD's . Not checked on rolling road or anything but seems good to me.
Tony.

[piman]

Hello Bainzy,

"They all end up requiring a really rich idle otherwise they'll be too lean everywhere else."

That's because they are lean elsewhere, the idle setting is the start and you should work from that basis. You'll have noticed that the idles dimension are virtually identical on all needles. (Of the same group) Maybe a lighter spring would help?
I wonder how difficult it would be to connect fuel gauge senders to the pistons with a couple of gauges to measure piston lift while running? (Maybe better to use a multimeter as the car gauges seem heavily damped)

Alec

[Nick Jones]

Can remember what cam you are running, but if this always occurs at aroud the same (lowish) rpm, then could be that you are having some kind of reversion/fuel stand-off?  Not likely to be the case except if you have a fairly wild cam.  Bell-mouths/rampipes might help or just move the problem to a different rpm.....

Nick

[Bainzy]

I like that idea Alec, I also thought about using a miniature spy camera feed in there but it would probably be cheaper to pay for a rolling road than to rig all that up properly.

Nick, only Triumph's super hot 18-58 standard 1500 camshaft  ;D

Eventually to be replaced with a TR5 type grind, if I ever finish porting the high compression cylinder head I plan on matching it with...