Carburettor issue

[Uplander]

Hi

Having removed the air filter on my '69 1800S and intent on balancing the carbs (something I hadn't done properly on a previous visit) I noticed the rear carb was hissing more than it should while idling. I've discovered that when I start the engine the piston raises very slightly, just enough to see a bit of the needle. When I turn the engine off the piston drops. To be honest I'm not sure if this is correct or not, but the front carb doesn't do it and on idle you can't see that needle at all. So I suppose the question is which one is wrong, why is it doing that and how should I fix it?
Many thanks

Regards
Simon

PS I'm not a total SU novice, but neither am I an expert so please speak slowly.

[classicswede]

First off is the dashpot oil full on both carbs? Step one in setting them.

Next is will the piston rise if pushed by hand. It should do, if not there is a problem

[Uplander]

Yes, dash pots always topped up.
Yes, the pistons on each can be raised by hand, but now noticed the rear doesn't always drop smoothly down like the front, sometimes sticks. But when it does go down it only goes to the slightly open position mentioned before.

[Ron Kwas]

Simon;

"hissing more than it should while idling" ? What do you make that statement based on?...SUs hiss because air is being sucked past that venturi restriction, so that's their absolutely natural mating call!...what you should see is both Dashpots being lifted similarly (but don't make yourself crazy trying to get the precisely equal!)...vacuum of engine running lifts them, so when engine is turned OFF, they should both drop completely down to bridge/venturi as vacuum goes away. If one does not fall completely, it either needs lube (dashpot oil) OR Metering Needle is bent/uncentered and making contact with Jet as Daspot falls, jamming it. This must be resolved! I'd open both to inspect and lube before mucking around trying to synchronize!

I understand you're not an SU novice, but info here should still be of interest and use: http://www.sw-em.com/SU%20Carb%20Service.htm

Good Hunting!

[green van man]

Clean the dashpot innards and piston with brasso, and I think you will solve the problem.

I would clean an su dashpot twice a year with a spot of brasso on a rag, let it dry to a haze and polish.
Setting mixture with a Gunston colourtune plug at tickover, two are easier but at the time I could only afford one.

Ballencing the carbs, I used a Gunston carb ballencer. Not having a musical ear I found the floating indicator in the airstream a far better method.

I did find that once multiple drivers used the car the SU s would go off song quickly, if only the one driver they did not need balencing for months.

I also used this method to balence twin Webber DCOE carbs with took some time with only the one colour tune plug as it needed moving between the 4 barrels and the carb ballencer had to be used on each barrel also not just the 2 carbs.

Paul.

[Ron Kwas]

Paul;

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

[Uplander]

Thanks for the replies.
I'll try some thorough cleaning of the dashpot and piston again and refill with oil.
That may well sort out the occasionally sticky piston. If not, would it be worth re-centring the needle?

As far as the slightly raised piston is concerned, I now wonder whether the choke cable needs adjusting on the rear carb. If the choke cable position means that the fast idle is still slightly 'in play' would that not have the effect of the piston raised higher and thus sitting differently from the other carb?

[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.