SU Carb Needles

[SteveJP]

I have been trying to tune my carbs since fitting an extractor manifold and sports exhaust and cannot seem to get a lean enough mixture. It also has pancake filters (not K&N just cheapo speedograph ones) I have checked the needles and they are ABT - should I be using a leaner profile?

I have followed James' (mintylamb) SU carb tuning advice to the letter, whilst the car runs nicely, it is still running rich, all manifold bolts have been checked for tightness - they were slightly loose at one point but this has now been rectified.

Thoughts please?

Thanks

Steve

[Deleted User]

Rolling road. The only advice I can give. At least its not running really lean.

[TimW]

Rolling road!!!!!! :)

[Freek76]

Manifold and pancake "filters" richen the mixture.
You should be able to weaken the mixture no matter what modifications though.

How can you tell it's too lean?

Just a few things that spring to mind:
Float height set too high, leaking float valve, fuel pressure too high, vacuum leak.

I'd check those things first plus plugging the engine breathers and the vacuum pipe to the distributor.

Then it's either rolling road or diy with a wideband lambda sensor.

[James]

If your air inlet mods have worked and everything else is the same then your mixture should be too lean - otherwise it's more restrictive.

Usually when you put on K&Ns etc you have to get a richer needle to get more fuel to match the increase of air.

[Richard B]

I'd suspect that it's running rich because you have those 'crap' air filters.


Try putting the OE airbox and filters on and compare....   :-/

[Freek76]

Freek76 wrote:

Manifold and pancake "filters" richen the mixture.

Sorry, James is right, those mods weaken the mixture.

Agreed, get rid of those filters, they don't filter very well at all and actually reduce flow.

[SteveJP]

Thanks for all your replies folks - I traced the problem which appears to have been my choice of oil for the dashpots. I have always used 3 in 1 until now with no problems, but after reading more on the subject of carb tuning it was clear that this is too thin taking into account the new manifold, changing it to 20/50 has solved the main problem of too rich a mixture and I have just this afternoon managed to get a good balance of mixture and performance. I also run BPR6ES plugs with standard points ignition - tried Aldon last year but went back to points for peace of mind on 10CR.

One day I WILL change those crappy pancakes for K&N!

[piman]

Hello 76Spit,

you are mistaken in the function of the damper oil, it doesn't affect the mixture. It's function is to stop the piston rising too quickly when the throttle is opened suddenly which would cause a momentary weakening of the mixture. In fact 20\50 will give a richer mixture than 3 in 1 when accelerating suddenly.
How are you judging the mixture strength?

Alec

[shedmonkey]

Very good point pie man and its got me thinking about my hot start problem which came about following a needle change- could it be that the damper oil is to thin and the car is overfueling and flooding on a hot start?-in which case what sort of damper oil should I  try its 20-50 at present,and or should I  put a harder damper spring in? -sorry high jacked the school boy questions but its sort of relevant!? cheers

[piman]

Hello AJP,

I wouldn't think it would have any effect at cranking speed? Perhaps the mixture is a bit weak at idle?
In case it is rich, open the throttle and see if it makes any difference?

Alec
P.S. I'm not a baker :-)

[shedmonkey]

fair point ...I shall tinker on

The sensible thing to do would be to put the old needles back in and see if the hot start problem goes away

PS couldn't resit the baker pun!

[piman]

Hello AJP,

needle selection is largely irrelevant at idle as virtually all (of the same group) have the same idle setting, it's only as you go up the steps do the diameters change.

Just try unscrewing two or three flats as a trial to see if it alters the hot starting problem. It could also be fuel vaporisation, so check your fuel line's run in case they are too close to hot bits of the engine?

Alec

[Pete Lewis]

the dashpot has to damp the rapid rise of the air piston to give you a momentary rich mix for acceleration or you get flat spot, heaven knows what viscosity 3in1 has when its hot  presumme similar to cats pee.
as the pancake filters give less restriction(well supposed to) you have to fit a richer needle as the depression under the the air piston and over the jet is lower so less fuel gets sucked out into the stream.  a heavier damper of thicker spring would give some improvement maybe.

never ever use any R suffix (suppressed) plugs in these old ignition systems they dont have the HT voltage to overcome the restistive built in and fire correctly, stick to plain BPES 5 or 6  no R's
and I know some suppliers are providing the "R" versions as OK   they are not

Pete

[shedmonkey]

piman wrote:

Hello AJP,

needle selection is largely irrelevant at idle as virtually all (of the same group) have the same idle setting, it's only as you go up the steps do the diameters change.

Just try unscrewing two or three flats as a trial to see if it alters the hot starting problem. It could also be fuel vaporisation, so check your fuel line's run in case they are too close to hot bits of the engine?

Alec

will let you know how i get on- its a bugger as sometimes it will fire up first flick when hot

[cammmy]

ajp wrote:

will let you know how i get on- its a bugger as sometimes it will fire up first flick when hot

Would that be when it's fired up again after a short period rather than later? Would make the fuel vaporisation theory plausible?