Help! Mixture problems

Post by **RupertBrisley** » Tue May 12, 2009 4:12 pm

I have a standard 1960 saloon. It runs well, is smooth and quiet and idles beautifully. However it seems to be running a tad rich, I get a patch of soot on the floor below the exhaust after idling for a minute or so. The exhaust and plugs also look a little dark. I decided to tweak the mixture - simple enough according to the manual. So I find the jet adjusting nut (at the bottom of the carb directly beneath the needle/piston?) and try to adjust this up to weaken the mix. Strangely it is tight and appears to be wound fully home (the spring above the nut is compressd). It winds down easily so isn't jammed or siezed. I looked at the needle and this seems Ok but I have no idea if it is correct for the car/carb. It is an HS carb but the only number I could see on the carb manifold was stamped S7. Am I being silly or is something amiss? I don't want to leave things as they are in case the pistons coke up (or is that unlikely?).

Thanks

Post by **mike.perry** » Tue May 12, 2009 4:39 pm

The standard carb for your car should be an HS2 with an M needle.
At tickover the mixture nut should be wound down approx 2 turns or 12 flats. Mark one flat of the mixture nut with tipex or similar so that you can keep track. With the mixture nut in that position you should be able to adjust it up or down a couple of flats to obtain the smoothest tickover which should be near enough the correct setting.
If you are not getting the correct setting then there are some things to check
Is there wear in the carb. butterfly spindle?
Can you lift the piston and allow it to drop with an audible clunk?
Is there thin oil (3 in 1) in the dash pot?
Is the shoulder of the needle flush with the bottomof the piston?
You can check the needle code by unscrewing the clamp screw on the piston and carefully removing the needle. The letter should be stamped on the shank.
Is the air filter clean?
Is the choke fully in
Try those checks and see if there is any improvement.

Post by **RupertBrisley** » Tue May 12, 2009 5:13 pm

Sorry to sound stupid but how do I know if mine is an HS2? where will this be marked?

Post by **Declan_Burns** » Tue May 12, 2009 5:43 pm

Here's a photo of a HS2 which might help -(has a different spring/needle). Came off ebay but I don't know from what car.<br>

<br>

Post by **RupertBrisley** » Tue May 12, 2009 7:43 pm

This looks like mine I think (raining now so I'll check tomorrow) and earlier I took out the needle which appears in good condition and after some squinting does have an M stamp. So looks like an HS2 carb.

I tried pushing the piston lift up and although it was hard to tell where it touched the piston and even harder to judge a .8mm lift, I did get a noticeable increase in revs and pushing the pin further resulted in a near stall (Choke fully in, engine warm).

The needle collar is flush with the piston face.

I have now replaced the dashpot oil with 3 in 1 - I'll check tomorrow if I'm getting the clunk!

The air filter is new.

Question: How can I tell if there is wear in the butterfly spindle?

So as yet I'm none the wiser. Why is the jet adjusting nut wound in so far? If I back off to the factory recommended 12 flats as a start point then isn't that going to make matters worse but richening the mix dramatically?

Thanks for advice by the way.

Post by **MarkyB** » Tue May 12, 2009 8:05 pm

Make sure the shoulder of the needle is level with the bottom of the piston.
If it's too high the jet would have to be screwed in to compensate.

If the needle and jet are worn the mixture will be richer than optimum.

Post by **RupertBrisley** » Tue May 12, 2009 9:03 pm

Needle is level. But I think I will replace the needle and jet to be sure.

Post by **bmcecosse** » Tue May 12, 2009 10:14 pm

Most likely explanation is that the fuel level in the float chamber is too high - and flooding over when idling. Wear on throttle spindle would cause a weak mixture - not rich. To check fuel level - remove the bell - switch on fuel pump - look down at the jet to see if any fuel overflowing - and check if the pump keeps ticking ? The fuel should be visible about 1/4" beloe thye top of the jet - if too high you need to consider if the little valve in the float chamber has worn to a ridge, and is allowing fuel to pass through continuously - this would affect idle but would not matter much when running at revs since the fuel will then be consumed by the engine ! If all seems well - you can always set the needle just a tiny but proud of the base of the piston - but only by ~ 10 thou - no more! This will then allow you to wind the jet down slightly and give some room for adjustment!

Post by **RupertBrisley** » Wed May 13, 2009 12:48 pm

OK checked the clunk - all fine. 3 in 1 added.

BMCECOSSE thanks for your post - Dumb questions -

Remove bell - do you mean the pot on top of the carb that contains the oil?

And switch on the fuel pump - how do i do this? Simply switch on the ignition or start engine?

Should I also replace the float valve as a matter or course?

Presumably setting the needle proud is a last resort?

Post by **bmcecosse** » Wed May 13, 2009 2:19 pm

Yes bell = the whole assembly - with piston and needle. Yes - switch on ignition to run the pump - just for a minute or two - you obviously can't start up! Renew float valve if you like - but seems daft if nothing wrong with it. Needle proud by 10 thou or so will be fine - no harm in that!

Post by **Alec** » Wed May 13, 2009 6:28 pm

Hello Rupert,

another and very off the wall suggestion, if a jet from a larger carburettor is fitted then the jet size is too large. I think the HS2 range uses a 0.090" jet and of course the 0.090" needle. Larger HS series (I can't remember if the increase is with a HS4 or a HS6, incidentally S.U. carburettor are sized as 1" + model number in 1\8th", so HS 2 is 1" + 2 x 1\8ths = 1 1\4" and so on) use a 0.100", so ten thou larger. This may explain why you are so far out with adjustment, that is if the fuel level is correct. The latter fault is far more common.

As a guide, seeing as few people are able to actually measure a jet, the idle dimension, directly under the shoulder, is normally between 1 and 2 thou less than the jet size, so would have very little movement if you use the needle as a gauge.

Alec

Post by **bmcecosse** » Wed May 13, 2009 7:36 pm

HS4 uses 90 thou jet. Only goees to 100 thou with carbs larger than 1.5" ie HS6 etc and HIF 44.
100 thou jet with 90 thou needle would be amazingly rich! However - this is done when running strange alcohol type fuels !

Post by **Alec** » Thu May 14, 2009 7:16 pm

Hello BMCE,

as I said, an odd thing, but with the adjuster screwed up tight would any engine run at all? When you look at how far down the jet moves at full choke? I think it's worth checking?

Alec

Post by **bmcecosse** » Thu May 14, 2009 10:11 pm

I see what you mean Alec - but if fuel flooding over - it would probably run ok. Not very well - but ok!

Post by **mike.perry** » Thu May 14, 2009 10:23 pm

My old H1 carb on the Series MM ran with the mixture nut wound almost right up. The carb was well worn but it still worked. I have now fitted a new carb.

Post by **Alec** » Fri May 15, 2009 7:42 am

Hello Mike,

the beauty of S.U. carburettors is that there are really very limited areas of wear that affect running. The obvious is the throttle spindle, but really not much else in a properly assembled carburettor. (later sprung loaded needle types excepted.) The H series needed some attention to the jet seals from time to time.

Alec