Carburettor issue

[Uplander]

Thanks all

[Ron Kwas]

Derek;

What "grooves in the piston" are you referring to? I presume you are referring to the ones around the Dashpot perimeter "Airseal" at Red? That area may (emphatically) not be abraded either, but it really wasn't the area I was referring to...I meant the area at what I call the Dashpot and Dome tubes (Green).

...the Red area is certainly a precision fit, but we know there is absolutely no contact here (and it is crucial never to remove any metal from either surface to increase that gap!), and this precision fit is only possible because of the precision (lubricated) fit at the Dashpot and Dome tubes (Green)...and this fit may also never be increased by removing metal with abrasives.

Interesting about the circumferential grooves helping with Airseal at red...thanks for that esoteric bit of info and history...(remind me never to engage you in a game of Trivial Pursuit!)...

Cheers

[green van man]

Originally Posted by Ron Kwas

Paul;
G
Brasso is a polish and therefore abrasive (if only mildly)...this means it removes material (if only minutely) which results in changing the precision clearance between Dashpot and Dometubes, and this is to be avoided...I recommend against abrasive of any kind...I do recommend cleaning and flushing (especially the Dashpot and Dome tubes) with a hydro-carbon solvent (spirits, carb-cleaner, etc.), wiping absolutely clean, and lubing pre-installation with ATF.

Colortune used to check and set mixture at idle is fine, but so are the Dashpot lifting pins already present on the carbs...but this is again just for idle...ideally, mixture (and exhaust emissions) want to be checked and adjusted for good tractability throughout the entire load range, where the carbs spend the most of their operating time, so a dyne or rolling road dyne should really be used for this.

Regarding balancing...I feel the Dashpots of an SU themselves are the perfect indicator of ingested air (because rising up directly proportional to the intake air is in-fact their one and only function!)...so I also feel that little else in the way of test equipment is necessary or required once the Dashpots have passed the 5-7 Sec drop test and are reinstalled...I adjust linkages and idle Adjust screw so the Dashpots show a similar (but not necessarily absolutely identical!) amount of rise at idle, and I'm happy with it and don't make myself unnecessarily crazy about it...

I have no explanation or comments about the different action of carbs from single to multiple drivers, but do find this rather strange...

Cheers

I only speak from experience and the many su carbs I cleaned with brasso never suffered ill efects, and certainly worked better after.
The mk2 colour tune could be used at revs above tickover which was useful for checking mixture as needle lifted.
Back in the day we were up for swapping needles to increase performance and it was the subject of much discussion at the local motor club.
I moved on to Webber's when I baught fords.

I offer advise based on experience, it's up to others if they want to follow it.
Twin su on 120s, with one driver they stayed in tune when the owner let others drive home from the pub they quickly went out of tune. He was a mate and I regularly got calls to call around as the car was not going as it should.

It's many years ago now that I worked on these cars but lessons learned in youth stay with one, I'm happy to tell how I overcame a problem, it's up to owners if they want to follow my example.
If memory serves the precision clearance between piston and pot was about .005 a quick lick with a bit of brasso would be unmeasurable with feeler gauges which was how we did it back in the days before carb cleaners.

Paul.

[swedishandgerman]

Hi Simon,

If the pistons are not 'behaving' the same in both carbs, then they are not balanced properly.

You're welcome at any mutually convenient time to trundle that car down here to Exeter and I'd be delighted to go through step by step how to get them running exactly the same.

You have to start by checking the float heights are the same in both chambers. There are many ways you can do this, some more precise than others. As well, you have to check the piston drops onto the bridge with a good 'clink'. If not, then you have to centralise the jet.

Next you HAVE TO disconnect the balancer bar, throttle shaft, whatever you want to call it to make sure the carbs are running idependently from each other for the next few adjustments. Then screw back the choke screws/fast running screws then adjust the idle screws so both carbs are running at the same speed: listen to the same hissing sound, use a Gunson Carbalancer, compare the heights of the pistons - whatever method you want to use, but I recommend using a little hand held air speed gauge (anemometer) available from Amazon for about a tenner. Then adjust the mixture: Colortune, CO probe up the exhaust, whatever method, but I do it by ear listening out for when the mixture is just too strong and it's lumpy, then screwing back in until it runs fast. After doing the mixture, you have to re-check the idle speed hasn't altered followed by adjusting the choke screws back up so that there is a tiny, but matching, gap between them and the cam thingamer for fast running.

Once both carbs are running identically, you then BALANCE them. This very non-technical bit gets left out so, SO frequently!

You have to set the carb levers to activate at precisely the same time by adjusting the balancer shaft so that the pivots touch the carb levers at the same position. This is really important. It opens the carbs at the same time and closes them at the same time. You can check it but watching the pistons bob up and down when you 'blip' the throttle. If one activates more eratically than the other, the carbs aren't balanced. You can also check fast air speed using the air gauge, but generally, if pistons are acting the same, that's pretty accurate.

As I say, jump in the old car and bring it down to Exeter and I'll show you how!

PS I don't see a problem with cleaning out the domes etc with Brasso, though never heard of that method before nor see the point of it. However, I would recommend severe caution of going anywhere near the needle or jet with Brasso. In fact, don't even touch the needle with a rag as the brass is so, so soft you can very easily wreck them.

[Uplander]

Thanks for taking the time to put all that down and the offer to help.
I have gone through the tuning routine before, but I didn't disconnect the throttle shaft so I plan to do it again.
I actually bought one of the anemometers you mention, on your recommendation in a previous post.
I think now that the immediate problem is that I'm unable to balance the carbs as the rear one is being interfered with by a poorly adjusted choke cable but i'll check that at the weekend. Once that's sorted I'll try another re-tune.
I know you're probably my nearest 1800 and I should make a visit sometime.

Simon

[swedishandgerman]

If there's one single guide that I recommend, it's this The SU Carburettor High-Performance Manual by Des Hammill:

https://www.waterstones.com/book/the.../9781787111684

Mummy bought it for my Christmas present!

There is a lot of information in there that you'll probably never need, but the tuning sequence is fantastic and makes sense. It's easy! I say, the stuff written in the old Haynes Manuals there is over complicated and misses important stuff out concentrating far too much on what the thing idles like rather than the most important part of balancing the airflow at just off idle.

[Uplander]

Just to update the thread, it wasn't the choke cable. The throttle linkage between the two carbs was incorrectly adjusted so it could have been that. After loosening them off and readjusting so they lifted the pistons simultaneously, the problem was solved.

[swedishandgerman]

It is so, so frequently that at fault!