Carb needle sizes

Post by **biker_bits** » Sat Sep 15, 2007 1:32 am

For 2 years that I had the old 1098 I could not set the carbs properly - mixture all over the place, revs either too high or too low, etc. It is a standard, later type carb, from what I can tell.
Took the needle out:
Marking I could make out: AN. Is this the right one for the standard fit?

I've never had all these problems with the 948 with the earlier carb on it!

As the carb is now certinaly clean, needle centered and free to move, oil topped up in the damper, etc., I start to suspect that somene has fitted a different (richer) needle.

To get a resonable idle mixture (no fuel pooring out of the exaust, plugs going thick black in seconds), I have to set carb mixture nut nearly all the way up (to lean).
In that setup acceleration is very poor, almost like on a freezing cold engine or with ignition timing way out, idle is erratic when hot.

I've tried richer setting (around 1.5 turns out on the nut) and it goes like a rocket! But the exaust is very smoky and smelly with unburnt fuel at idle. On the hot engine that is.

Hence I'm wondering about the size/profile of the needle fitted.

Any ideas?

Post by **Packedup** » Sat Sep 15, 2007 3:49 pm

IIRC AN is the right needle.

Have you checked the dashpot oil is the correct type (I know some people use watery stuff like 3 in 1, which has never seemed to work for me). And is the spring the correct one, and still as springy as it should be?

Post by **biker_bits** » Sat Sep 15, 2007 5:01 pm

Dashpot oil is 10w40 engine oil. I also tried 20w50 - no real difference.
The spring is still springy too - I have not checked it for the correct free length though. Is there a measurement given anywhere for that?

Post by **MoggyTech** » Sat Sep 15, 2007 5:35 pm

The AN needle is the correct standard size.

Have you checked that the needle shoulder is FLUSH with the bottom of the carb piston. I've seen a few that have been pushed all the way into the piston when fitted, and the mixture goes way too rich. Also check the grub screw that holds the needle in place is not slack.

Post by **biker_bits** » Sat Sep 15, 2007 7:12 pm

I did, in the early hours of this morning. Removed the needle and re-set it in against a vernier caliper edge. So certainly fitted OK and the screw is tight.
Car is still the same though.

Post by **MoggyTech** » Sat Sep 15, 2007 7:39 pm

biker_bits wrote:I did, in the early hours of this morning. Removed the needle and re-set it in against a vernier caliper edge. So certainly fitted OK and the screw is tight.
Car is still the same though.

I would do a full engine diagnostic to narrow this down.

I realise you may have tried the following, but bear with me.

1: Full Ignition Service. Plugs, Points, rotor arm, dizzy cap, condensor

2: Strobe check timing, and make sure mechanical advance and vacuum advance are both working

3: Compression Test

4: Check for air leaks on inlet manifold/carb, and also the crankcase ventilation system.

5: If possible, get an exhaust gas test done, preferably 3 gas test.

If all that looks good, then it does point to the carb, which I take it is an HS2. Very simple carb with only a few possible faults.

1: Float Level and needle valve seating
2: Piston sticking
3: Jet not centered, or bent jet needle
4: Worn Butterfly spindle bushes (which causes weak mixture) but usually only after very high mileage on the HS2

If you are anywhere near Livingston, PM me as I would be happy to do the exhaust gas test for you.

And if all else fails, threaten car with a BIG mallet.

Post by **bmcecosse** » Sat Sep 15, 2007 7:49 pm

Almost certainly the float chamber is flooding - does the it sit dead upright when bolted to the manifold ? If not - it will never be right. The chamber angle is adjsutable - undo the bolt on opp side and the chamber comes away revealing an angled setting block. Various blocks are available (from Burlen etc) - but you can just modify the one you have until the float is vertical. Other possibility is that the level is just too high - either set too high, or needle valve leaking. MOSS now sell a 'Grose' needle valve for the SU which is much better - but if you buy this make sure you set the float level correctly on installation - because they are slighlty longer than the original valves.

Post by **biker_bits** » Sat Sep 15, 2007 10:25 pm

Float chamber is dead upright.

Checked needle valve with 1 lung power - it does close fully. By the way, it is a plastic type, so not sure if this is the same size as the original brass one would have been.

The float fitted is like here:http://www.burlen.co.uk/partDetail.aspx?partID=3274
How do you adjust that?! There are not tabs to bend or anything - just the plastic float and the pin that retains it.

Seem to recollect the original float was found filled with petrol and the needle valve was totally gone (guts fell out!), so the current needle valve/float kit was bought a couple of years ago from ESM, so must have been matched up for the correct carb.... I hope.

Then the car was off the road for welding for 6 months or so, and never really got used till last summer, when it became obvious that the fuel prob is still there.

As I say the car had the same issue with the original (broken) set, hence, having replaced the obvious bits, I was quite looking forward to the problem going away.

Post by **eastona** » Sat Sep 15, 2007 10:54 pm

try the jet. Once they wear, it's difficult to get consistent mixture. because of the wear, it's difficult to get the mixture lean enough at idle, so if the idle is set good, the car is very lean when you open the throttle.

I've had V similar problems with my MG, took me ages work out what it was and my guess this is the same, it's the jet.

Andrew

Post by **biker_bits** » Sun Sep 16, 2007 12:03 am

I might have cracked it with your help!

Found slide was not dropping all the way down even with the damper rod out. So, with a torch and a couple of spanners re-centered it. Now drops down with a lovely 'clonk'. Re-tuned the carb best possible. Quick thrash around the car park - seems to accelerate a lot better!
Will find out for sure on the way home at 7am.
Let you all now then!

Post by **MoggyTech** » Sun Sep 16, 2007 1:02 am

biker_bits wrote:I might have cracked it with your help!

Found slide was not dropping all the way down even with the damper rod out. So, with a torch and a couple of spanners re-centered it. Now drops down with a lovely 'clonk'. Re-tuned the carb best possible. Quick thrash around the car park - seems to accelerate a lot better!
Will find out for sure on the way home at 7am.
Let you all now then!

Yup sticking carb piston will do it. Might be worth polishing the piston housing body with some metal polish, and replace the big spring. BUT, if you've fixed it leave it alone

Post by **Alec** » Sun Sep 16, 2007 9:16 pm

Hello MT,

"Might be worth polishing the piston housing body with some metal polish,"

Please do not do this, only use a solvent cleaner on the interior of the dashpot and piston; use any abrasive often enough and you'll have to replace them both.

Incidentally, with mixture problems, at idle, the needle does not matter as there is little or no difference in the idle dimension of all S.U. needles of that group, i.e. 0.90", 0.100" or 0.125" jets\needles. In other words, if you can't get the idle mixture correct, don't blame the needle, something else is wrong.

Alec

Post by **biker_bits** » Mon Sep 17, 2007 8:08 am

Well the run in yesterday morning at least proved that the setup up is more stable now!
All I need to do is 'tweak it up' to reasonable settigns and keep an put for the problem returning.
Thank you for your help!

Post by **chickenjohn** » Mon Sep 17, 2007 9:17 am

If the piston was sticking in the housing- yes this would give the symptoms described, and yes good idea to clean the inside of the carb. Use solvent (thinners/parrafin/petrol) or WD40 seems to be good at remoing the black crud that builds up inside carbs.

Post by **bmcecosse** » Mon Sep 17, 2007 9:31 am

Carb cleaner from Poundland is good - it's almost pure Toluene!

Post by **chickenjohn** » Mon Sep 17, 2007 10:07 am

Pure Toluene?? Could use that as thinners or even in the petrol tank- a £ for how much?

Post by **bmcecosse** » Mon Sep 17, 2007 10:13 am

Not a lot - spray can - but it works, and for a lot more than just carb cleaning - excellent general de-greaser. Highly flammable of course!