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Carburettor issue

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Old Apr 24th, 2018, 17:59   #1
Uplander
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Smile Carburettor issue

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.
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Old Apr 24th, 2018, 18:02   #2
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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
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Old Apr 24th, 2018, 18:38   #3
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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.
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Old Apr 24th, 2018, 18:41   #4
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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!

Last edited by Ron Kwas; Apr 24th, 2018 at 18:45.
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Old Apr 24th, 2018, 19:27   #5
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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.
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Old Apr 25th, 2018, 00:34   #6
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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...

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Old Apr 25th, 2018, 08:47   #7
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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?
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Old Apr 25th, 2018, 09:41   #8
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Quote:
Originally Posted by Uplander View Post
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?
Disconnect the choke cables and check. Easy. Check needle re-centering.Easy make sure that you disconnect the carb linkage before tuning so that they are both working independently when yo make adjustments.
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Old Apr 25th, 2018, 12:35   #9
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Simon;

Don't just fill Dashpot reservoir...external surface of Dp tube should get a slight film of oil so that it is perfectly lubed within Dometube...

Choke doesn't necessarily need to be disconnected, but when in disengaged position, fast idle adjustment must not be affecting Throttle position...this in turn would raise DP...Choke linkage does not affect Throttle directly. Correction: ...Dashpot height directly !

...but the FIRST thing that needs to happen is DPs need to have free lubed/unjammed movement throughout their entire operating range when engine is OFF! See Link!

Good Hunting!

Last edited by Ron Kwas; Apr 25th, 2018 at 15:31. Reason: I meant to say Choke Link does not affect Dashpot height directly...it absolutely affects Throttle pos directly!
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Old Apr 25th, 2018, 13:06   #10
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"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."

100% agree with this. Domes and pistons are precision matched, On a pair they do get swapped now and again in error and this should be avoided. The close match allows the piston to be raised by vacuum and ANY increase the gap will reduce this ability. The grooves in the piston improve this seal. An attribute first shown in the design of Minié (inventor) ball bullets back in the US civil war era which had 2 or three grooves around the base to reduce the blow by when the charge was detonated. I'm sure you wanted to know that.......... lol
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