Carb Messing-up err I mean Tuning Update

[McMuttley]

So..

• Changed the Coil - new go faster lucas one
• Changed the leads - go very faster Magnecor ones
• Cleaned out the floats (only the one for 3/4 had a bit a sediment - 1/2 was clear) foat height slightly lower on 1/2 but very similar
• No sediment in fuel pump bowl (didn't get as far as pump filter)
• Checked manifold and carbs golted so no air leaks there
• Cleaned float and chamber - nowt rough - needles (BDL) and dampers straight
• Set jets to 9 flats down (midway between the two suggestions of 6 and 12)
• Re-assembled dashpots - with new red springs (much longer than irregular old ones - see pic) - they dropped OK without and then with damper oil
• Balanced carbs with tickover at circa 850
• set both throttles so that they moved together (probable source of previous report of slower 1/2 - it was engaging later)
• Didn't bother with choke for now as pulling the choke seem to do nowt to revs

with air filters still off ?....

 

• Adjusted mixture using Gunson
• On 1/2 - having set a start point of -9 flats down - it was rich, so turned anticlockwise until it started to turn blue - however, this took all available 15 turns, i.e., the jet is as high as it can possibly be ! Surely this is not right?
• On 3/4 I had to raise by at least 10 flats to get it to turn blueish

Pics below are first 1/2 - up to top, second 2/4 not far off

 

Can I run the car with he jets this high or will the piston start to bash them.

 

Should I have checked the mixture with the air filter on? Is this why they are showing so lean? (they were soaking in petrol at the time)

 

Is it a case of re-setting the jets with the air filters on and doing the Gunson again, or do I have ridiculously high jets ?

 

Jet for 3/4 (rear) this high (to very top) to turn Gunson blue

 

 

Jet nearly as high on 1/2 (about 3or 4 flats from top)

 

 

 

Old irregular springs (middle) new much longer springs outer

[RobH]

Are the needles fitted properly in the pistons Austin? The shoulder on the needle should be level with the bottom face of the piston.

 

Rob

[peejay4A]

Maybe a deviation here but aren't BDL needles Spring biased? So you have later HS6 carbs. Not that it should matter.

[Peter Cobbold]

Austin,

No you can't run jets that high.The pisiton will block the fuel flow and mess up the depression, badly. They should be set a few thou below the bridge.

I think a pair of new needles might be needed, a previous owver may have had the emory paper on them.

 

If the pistons are still not lifting equally then sort that problem first by doing the test in the video PeteW posted .**

New springs may not have cured it. The carbalancer cannot measure piston lift, it only measures throttle opening.

It is quite possible despite equal carbalancer air flows to have the lift of a faulty piston lag a good one. And that in turns messes up fuel flow as the needle is in the wrong position.

 

Peter

 

** Here it is: the drop test is done without springs and without dampers

https://youtu.be/IfU47Oqq9wA

Edited November 15, 2015 by Peter Cobbold

[Lebro]

Are they the right needles (SM) & are the jets the correct 100 thou diameter.

 

The jets should definitely not be anyway near that high,

 

Was it ticking over nicely before you started winding the jets up?, & how was the tickover afterwards? did revs increase or decrease ?

 

I would try adjusting the mixture using the "raise the piston a tad using the pin" method.

Have you tried driving it in your "raised jet position" I would expect it to be undrivably weak.

 

Bob.

[McMuttley]

Hi,

 

BDL are indeed spring loaded.

 

It did tickover ok (9 flats down) (maybe a tad lumpy) before tweaking with the gunson.

 

What would a safe minimum number of flats down from the 'level with top' be to test drive?

 

I tried the manual lift method, but very difficult to tell, revs rose slightly and stayed whilst i held the piston up a tad.

 

Night had closed in so unable to take for a test drive

 

I suspect they need to go off for a rebuild, TRE have suggested a guy.

 

At least whilst they are away I can sort that exhasut out !

[Peter Cobbold]

The diameter of the old spring wire is visibly smaller - I think - than the new. That will make big difference to downforce. The pistons will now be lifting more than before, so the old needles are bound to run rich.

Compare old spring with new using Lebro's trick of two on one rod.

Peter

 

 

Doubling wire diameter reduces deflection by 16times. Deflection follows fourth power of wire diameter:

http://www.mitcalc.com/doc/springs/help/en/springs.htm

[McMuttley]

This is the needle - looked straight ?

 

 

Using Bob's compression comparison, the old spring compressed first ?

[peejay4A]

Compared to the BAE needles in my HS6s the BDL is much richer at WOT.

[Peter Cobbold]

 

 

Using Bob's compression comparison, the old spring compressed first ?

An old piston spring and new one on the same rod, then push on the ends. If they are equal they will shorten the same.

Edited November 15, 2015 by Peter Cobbold

[John L]

Do you have a tag on either of the float chambers screws, it will be a clue to what car they are from. It will give a part number of the pair of carbs, like AUD 607F and AUD 607R.

I could then then tell what parts are needed.

From my SU manual the HS carbs from a TR2500 TC the needle should be BDB, and the BDL are very much richer.

The main jet should be 0.100" you may have 0.090" which would let a lot more fuel thru.

Have you actually checked the fuel pressure at the carbs?

Selecting needles is easy with this program http://www.mintylamb.co.uk/suneedle/

It looks also that the springs you got are for a ball type damper. What part number did you order, Is there a colour code on the spring?

John

[McMuttley]

Hi John, no tags, the carb body flange to the air filter has '5N2' stamped in it, if that is of any use?

 

I just bought 'red' springs for HS6 (from rimmer) as they were average and my existing pair were odd in length - so thought it might be away of addressing irregular float mvmt.

 

My ability to test fuel pressure is limited to taking the HT lead out of the coil, disconnecting the fuel pipe to the float, turning over and noting a good 'splurge' of fuel going into the awaiting bottle.

[Lebro]

 

The main jet should be 0.100" you may have 0.090" which would let a lot more fuel thru.

 

 

John

a .090" jet would allow LESS fuel through, & the needle may well jam into it.

 

Bob.

[RobH]

The SM standard needle and the BDL are similar up to station 5 so that shouldn't have any bearing on the jet-setting problem and neither should the piston springs. I think it is probable that since the car was running acceptably until the breakdown, the jet size is right and the fuel pressure cannot be excessive. (I don't believe the mechanical pump can fail in such a way that the pressure is increased.)

 

If you are sure that the float chamber needle valves are closing properly so that fuel level in the jet is OK, I think your problem must be that either the jet or needle (or both) are worn sufficiently to prevent correct idle mixture setting or the needle is mounted too far into the piston so it does not fully enter the jet at idle.

[BlueTR3A-5EKT]

a .090" jet would allow LESS fuel through, & the needle may well jam into it.

 

Bob.

 

 

So do we think that it may be fitted with .125 jets then?

Winding them up as far as possible being an attempt to lean off the mixture, and unseating the piston in the process.

 

Measure the jet hole if you can. You are looking for .100" if you want to follow the orig TR spec.

 

Peter W

[Peter Cobbold]

Igntion timing set badly retarded could cause hot stalling and failure to restart.

Its not on the list at the start of this thread.

Peter

[Lebro]

MM.

I would suggest going back a few steps.

remove the dashpot covers, pistons & springs.

engage full choke to get the jets as low as possible

prime the fuel pump till no more fuel is flowing into the chambers

with a torch, peer down into the jet, & observe the. fuel height. (I use a digital calliper to measure)

it should be approx. 10mm down from the bridge, if it is much higher than this i.e. overflowing the bridge or near then you have a float related problem either valve not seating properly or too much pump pressure etc.

 

If all that is OK, try re-assembling, but with the old springs.

If your new springs are stronger than they should be, then that would hold the piston down more that correct springs, & this increases vacuum, & so will pull up more fuel.

 

There has to be something fundamentally wrong at the moment, "servicing" the carbs is not IMO the answer, because when carbs wear - butterfly spindles etc that lets more air in, not fuel, the problem has to be:

 

Piston height (due to springs, or sticking, or air leak around piston)

wrong / worn needles

wrong / worn jets

fuel height much too high.

 

Good luck & stick with it !

 

Bob.

[oldtuckunder]

Forgive me for throwing in my Tupence worth, but have sort of been following MM's multiple threads started since the Motorway breakdown so its a bit hard to work out what has been replaced and in what order to resolve the original problem, and I'm not quite sure if the current problem is a new one as result of carb tinkering after the original problem was solved i.e. Why it stopped, would start and run for a few mins and then stop, or if that problem was never resolved and the problems are getting worse as we fall down a technical hall of mirrors. The original problem almost certainly wasn't anything to do with needles, jets or springs, the car had been running OK and then it wasn't, apart from a needle dropping out that isnt a symptom of a gradually wearing needle, jet or spring.

 

I'm hoping that the original problem was resolved, and that this is just a follow on problem in cleaning resetting the carbs, otherwise you almost have to go back to basics and check reset everything from scratch timing, ignition etc because we don't know if the current problem is cause or effect.

 

However I have one tiny suggestion thats at least worth checking as something appears to have fundamentally changed. I'm no expert on SU's but am very interested in CD carb operation and on a long winding trawl following one of Peters links I did stumble across a piece about the SU's and the air bleds for the dampers, which was that there are two type of air bleed for the SU damper, one is a simple hole drilled in the cap and the other is an air hole drilled from the neck of the damper airway into the dome (through that little angled casting piece next to the neck of the dome) The comment was that one is required, either will work fine but that two is a definate No No and getting engines to run with two can be a nightmare.

 

Which remined me of a comment in one of MM's threads about identifying the carbs in question, where someone pointed out that the carbs had been fitted with the earlier H brass damper caps with a hole in, and from looking at the picture I can see see that the carbs also have the casting for the other hole. Now its possible that that internal hole isn't drilled (in which case I've waste a bunch of words) but it did just cross my mind that as MM has now had the carbs apart and done a thorough clean, that if there are duplicate air bleeds and previously one or more had been clogged with years of **** and now their not...................

[Peter Cobbold]

Bob

+1.

That air leak through two holes would cause the piston to run lower than it should for the air flow. That gives more depression and at low lifts more venturi 'suction'. Despite the lower position of the needle I could see the mixture becoming richer..... or leaner - it is more complicated than it looks at first sight.

But two holes are bad, and easy for Austin to check.

Peter

Edited November 16, 2015 by Peter Cobbold

[McMuttley]

Gents

 

Have just been out to check the air bleeds (Wkg from home today !) - there is only a hole in the damper lid - none in either dashpot lid - phew

 

I have been through all of the various threads as I suspect my less than systematic ADHD fuelled approach to analysis is not ideal.

 

Going back to the original motorway issue, we weren't able to repeat the scenario. I cleaned the carbs and changed the coil and it ran in the driveway (but for fear of calling out the RAC again - we didn't drive it - so don't know if its a heat issue (I have two spare condensers ready). I have also changed the HT leads.

 

After warming up, the colour tune showed it running rich, thusI took to the carbs to try and reduce the mixture. This seems to have revealed that for whatever reason, it is running very very rich.

 

My motorway issues may have been a weak spark compounded by a very rich mixture.

 

Since I bought the car it has always run too rich - this is the first time that I have tinkered with the carbs.

 

I have been back through all of the various threads.

 

For now, I have put the old springs back in and set the jet to 9 turns below the starting point (seems to be no point going lower when we know it is rich) - I haven't run it yet, but we know it will be rich by around the equivalent of 9-16 flats.

 

I don't think that I can tell whether the jets or needle are worn - having never seen another to compare !

 

The float level issue is one that needs investigation. But with non-adjustable floats - not sure what I can do here?

 

Not sure how I test if the float needles are moving when the lid is in situ, but they were clean, no grit (again the gaps between lids and float are generous)

 

I suspect there is an issue with my choke, as when I pull the choke cable, the jet doesn't drop - is that what is being suggested should happen ?

 

I have peered down the tiny jet hole, but cannot see enough. I might try to see if i can check by say inserting something stiff but not too hard down there to test when it gets wet.

 

I will go through the MG Mans piston drop test (maybe over a bench not the floor !)

 

Then risk to the road !

 

Otherwise, having identified insufficient talent to sort it out, my xmas present will be a carb re-build (rather than service) Presumably i can specify appropriate jets/needles for the fast road build state of the engine and floats that are adjustable ?

 

Thanks as always.

[Peter Cobbold]

Austin I'd buy a couple of standard needles and refit the red springs. That way you know both are pukka. We dony know anything about the old springs or needles.

A fast road engine will use more mixture ( maybe a bit less at low rpm) but the standard SU should cope OK for initial tuning.

Drop test next ? ( over a cushion)

Peter

[Kiwifrog]

Gents

 

Have just been out to check the air bleeds (Wkg from home today !) - there is only a hole in the damper lid - none in either dashpot lid - phew

 

I have been through all of the various threads as I suspect my less than systematic ADHD fuelled approach to analysis is not ideal.

 

Going back to the original motorway issue, we weren't able to repeat the scenario. I cleaned the carbs and changed the coil and it ran in the driveway (but for fear of calling out the RAC again - we didn't drive it - so don't know if its a heat issue (I have two spare condensers ready). I have also changed the HT leads.

 

After warming up, the colour tune showed it running rich, thusI took to the carbs to try and reduce the mixture. This seems to have revealed that for whatever reason, it is running very very rich.

 

My motorway issues may have been a weak spark compounded by a very rich mixture.

 

Since I bought the car it has always run too rich - this is the first time that I have tinkered with the carbs.

 

I have been back through all of the various threads.

 

For now, I have put the old springs back in and set the jet to 9 turns below the starting point (seems to be no point going lower when we know it is rich) - I haven't run it yet, but we know it will be rich by around the equivalent of 9-16 flats.

 

I don't think that I can tell whether the jets or needle are worn - having never seen another to compare !

 

The float level issue is one that needs investigation. But with non-adjustable floats - not sure what I can do here?

 

Not sure how I test if the float needles are moving when the lid is in situ, but they were clean, no grit (again the gaps between lids and float are generous)

 

I suspect there is an issue with my choke, as when I pull the choke cable, the jet doesn't drop - is that what is being suggested should happen ?

 

I have peered down the tiny jet hole, but cannot see enough. I might try to see if i can check by say inserting something stiff but not too hard down there to test when it gets wet.

 

I will go through the MG Mans piston drop test (maybe over a bench not the floor !)

 

Then risk to the road !

 

Otherwise, having identified insufficient talent to sort it out, my xmas present will be a carb re-build (rather than service) Presumably i can specify appropriate jets/needles for the fast road build state of the engine and floats that are adjustable ?

 

Thanks as always.

 

 

Throw away the colour tunes and use the damper lift mezthod as outlined in the SU tuning manual. When happy with mixture take it for a drive and chop the plugs at regular intivervals after different driving styles (thrash, gentle stroll and long high speed run) to help fine tune mixture.

 

I generally follow Stuarts rule of starting with the jet at the bridge and than wind down 2-1/2 turns as a starting point. Also check the dampers fall and lift at the same rate.

 

Do your carbs have waxstats ??

 

 

Alan

Edited November 16, 2015 by Kiwifrog

[Alec Pringle]

Austin,

 

do you have the pukka SU and Burlen manuals and parts books ?

 

If not, you're welcome to borrow mine if you wish - might help !

 

Can be in the post if you need them . . . . . .

 

Cheers

 

Alec

[McMuttley]

Thanks Alec, I have the little blue boook and the Haynes manual is on its way via Ebay. Talent is somewhat lacking, but I am advised Andrew Turner has plenty.

 

Just need the biatch to be able to splutter to Greenwich and back for a gathering at Cutty Sark on 25 Nov, then carbs off for rebuild, which will leave a nice hole to access the exhaust !

[McMuttley]

ps I dont seem to be able to identify a std code for a 'spring' needle ?

Edited November 17, 2015 by McMuttley