tegwin
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| posted on 9/9/09 at 09:15 PM |
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Su Carb questions
Do we have any SU carb gurus in the house?
I am struggling to get my TVR to run properly. The only way to make it driveable is to run the carbs very rich and about 12 turns appart on the
mixture. (they should both be identical!)
I am trying to work out why this might be....
One thing that is worrying me is, the float chambers on the side of the carb... how critical is it that they are upright relative to the road?
How does one ascertain if the needles and jets one is using are the correct ones for the engine?
What fuel pressure is normally required for such carbs? One line of thought suggests that I might be blowing too much fuel at the carbs??!
Im really getting fed up with these now, they are a mystery :s
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owelly
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| posted on 9/9/09 at 09:20 PM |
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Have you checked for air leaks?
http://www.ppcmag.co.uk
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Xtreme Kermit
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| posted on 9/9/09 at 09:22 PM |
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It's a long time since I touched SUs, but I would think the top float chambers should be near horizontal.
Check the float needles for wear, that caused me a problem once.
Check the needles are:
a) identical part numbers
b) straight
c) mounted and seated correctly in the piston
Other than that, check the oil level in the dampers is the same.
Not sure about fuel pressure, but I would imagine it would be similar to webers 2-3psi.
I never really had much trouble apart from that lot... 
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tegwin
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| posted on 9/9/09 at 09:22 PM |
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quote:
Originally posted by owelly
Have you checked for air leaks?
Where? On the intake manifold? I had done to the best of my abilities... What method is best?
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mark chandler
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| posted on 9/9/09 at 09:32 PM |
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How old are they, seperate fuel bowls is knocking on a bit, even for SU's.
Anyway these old chaps need a bit of care with set up, the needle must drop in the centre of the hole, the pistons should drop evenly with a gentle
tap as they land.
The mixture is adjusted below with the big brass nut, this tube also slides to choke the engine, if the tube is slightly bent then you will get all
sorts of problems.
As for needles etc, the ratings are all over the place, the only thing you can do is google for a listing.
If they are proper to the engine then a good clean should resolve most issues.
Regards Mark
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MikeRJ
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| posted on 9/9/09 at 09:39 PM |
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SU's are just about the most simple carbs ever made, I can tell you didn't own an old mini as your first car
If you have to screw the main jets well down to get it to run that would suggest an air leak, either between carb and manifold, or manifold and head,
or maybe even a disconnected vacuum pipe or a vacuum take off that needs to be blanked.
Make sure needles have the same code on them; presumably you have the later HS carbs with the sprung biased needle (i.e. is it wobbly in the end of
the piston?). These can cause main jet wear as they get older, so check the main jet is round rather than oval.
Pistons must be absolutely free to slide within the dashpots, no sticking at all. This means piston and dashpot must be spotless.
Check the big spring is the same on each carb, they should have a colour painted on them, but it probably won't be visible by now. Free length
and wire diameter should be the same though...and a quick test with some weights should show if spring rate is roughly the same.
Old SU can also suffer from wear around the throttle spindles, just like most carbs, and this makes getting a steady, repeatable idle almost
impossible.
Obviously the dampers must be free within the pistons as well, and both topped up with oil. 20W50 should be fine,
What engine does your TVR use? It should be possible to find the correct needle code via a hunt on the web. Somewhere I have a book of SU needles
and applications but I haven't seen it in a while! Will have a quick look in a sec.
One thing I forgot...make sure the choke mechanism is working and correctly adjusted. This simply lowers the main jet to get a richer mixture.
[Edited on 9/9/09 by MikeRJ]
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owelly
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| posted on 9/9/09 at 09:45 PM |
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To check for air leaks on the inlet manifold, squirt some WD40 around the joints with the engine running. If there's an air leak, the engine
will rev up a bit as it sucks the WD40 in.
http://www.ppcmag.co.uk
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austin man
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| posted on 9/9/09 at 10:05 PM |
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Big problem with SU has historically been bent needles , lack of oil in the daspots and damage to the piston. May also be a build up of petrol residue
if they have been stood for a long time
Life is like a bowl of fruit, funny how all the weird looking ones are left alone
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miikae
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| posted on 9/9/09 at 10:22 PM |
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As already mentioned , check that the needles and the jets they slide into are identical , there are many different needle profiles/taper
progressions , also that the needles are centralised and not bent or fouling the sides of the jet as this can cause needle wear and uneven running due
to the needle drag.
The springs need to be the same colour and length, oil needs to be the same weight , i used redex in mine if i remember rightly.
SU did a book with all the available needle sizes, I dont know if you can still get them but i have one somewhere.
Float needle valves can be a pain as i have had them sticking in the past , if in doubt replace them , float chamber top needs to be horizontal or
very near to, float to needle valve gaps set the same also within the limits for you carb.
I never had fuel pressure problems but 2 to 3psi sounds about right to me.
Which type of SU carbs have you got.
Mike
I must learn to not eat supper between starting post and finishing it .
Are they SU's or dual Stromberg CD-w carburetors as fitted to some models.
[Edited on 9-9-2009 by miikae]
If it can be done it i will be done .
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02GF74
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| posted on 10/9/09 at 12:03 AM |
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^^^ unlikley to be Strombergs as they did not have separate fuel chambers.
the chambers should be horizontal and this is achieved using a slotted grommet - these are colour coded.
with the older brass needle valves, these can be restored to some extend if not too worn using some fine wet n dry; likewise the seat - I've
done it. the later needles - rebber tipped = viton - will need to be replaced.
check the float level but usually if te needle valve is not sealing, fuel will be leaving the carb via an overflow
nothing more to add..
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tegwin
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| posted on 10/9/09 at 08:16 AM |
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They are HS carbs...
The HD carbs used slotted grommets... the HS carbs appear to use different castings to adjust the angle of the fuel float bowl.
The engine is a TR6 2.5L straight six. It would have had Strombergs origionally, but someone has upgraded with these SU items at some point in the
past.
When I restored the car I stripped and cleaned the carbs and they are spotless inside and all parts seem to work and fit correctly.
Which is why I cant figure out why it doesnt want to work properly.
I guess its possible that the float levels are not even ...that would explain why one is much richer than the other. Possibly..
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John P
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| posted on 10/9/09 at 08:48 AM |
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Might be worth having a look through the technical section at http://www.sucarb.co.uk/Technical.aspx
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AndyGT
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| posted on 10/9/09 at 10:19 AM |
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Has the rubber diaphragms not perished with age etc? I had this problem after trying to get a chevette started after being stood for 15 years. Found
a new diaphragm and this solved it all, car started and ran as good as chevettes can be expected...
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mookaloid
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| posted on 10/9/09 at 10:22 AM |
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quote:
Originally posted by AndyGT
Has the rubber diaphragms not perished with age etc? I had this problem after trying to get a chevette started after being stood for 15 years. Found
a new diaphragm and this solved it all, car started and ran as good as chevettes can be expected...
They were Stromberg carbs the SU's don't use the rubber diaphragms
"That thing you're thinking - it wont be that."
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miikae
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| posted on 10/9/09 at 10:34 AM |
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quote:
Originally posted by mookaloid
They were Stromberg carbs the SU's don't use the rubber diaphragms
Some SU's did have diaphrams as i remember well sorting out the carbs on a MG Magnette which had a holed diaphram and was a real pain.
Added it was the HD's that had a jet diaphram.
Mike
[Edited on 10-9-2009 by miikae]
If it can be done it i will be done .
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tegwin
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| posted on 10/9/09 at 11:08 AM |
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The hs6 doesnt have a diaphragm...
It did have wax temp compensators, but I removed those as recomended by the experts
Do we think its possible that timing/dwell angle would effect the running of the engine like this?
How does one set dwell angle?
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MikeRJ
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| posted on 10/9/09 at 04:23 PM |
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If it is a points system then dwell is set by the points gap.
You problem seems to be exactly the same as this guys, and the
problem was down to the non-horizontal float bowls as the wrong parts were used. Definitely worth checking the part numbers of your float bowls
againt the ones mentioned in that thread.
Also worth getting a burr on a drill into that manifold if yours is as bad as the one in that pictures!
[Edited on 10/9/09 by MikeRJ]
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rusty nuts
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| posted on 10/9/09 at 06:40 PM |
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Check for correct tappet clearances as tight tappets will cause all sorts of problems. Worn distributors can cause erratic running. Only after
everything else is set up 100% adjust the carbs
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tegwin
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| posted on 10/9/09 at 07:20 PM |
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The issue im having with adjusting the rockers, is I have no idea what the heck they should be set to... I have no idea atall what the cam is, and
therefore, no idea what they should be set to..
Its really case of starting with all of the variables as unknowns... and not knowing where to start...
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rusty nuts
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| posted on 11/9/09 at 07:12 PM |
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Been a long time since I worked on a Triumph straight six but IIRC inlet and exhaust clearance should be 0.010" (ten thou) Sure someone will be
along to correct me .
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02GF74
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| posted on 11/9/09 at 07:34 PM |
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quote:
Originally posted by miikae
Some SU's did have diaphrams as i remember well sorting out the carbs on a MG Magnette which had a holed diaphram and was a real pain.
twin HD4 - from diagram the diaphragm was part of jet assembly - I think different to the stromberg diaphragm that separated the vauum chamber from
something.
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02GF74
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| posted on 11/9/09 at 07:36 PM |
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quote:
Originally posted by rusty nuts
Been a long time since I worked on a Triumph straight six but IIRC inlet and exhaust clearance should be 0.010" (ten thou) Sure someone will be
along to correct me .
good memory as shown here
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