Carb Settings

[peterjfox69]

Just had my carbs rebuilt and refitted with no issues.
For some reason they were then adjusted ( not by me) and were running a tad rich.
Measurements taken at local MOT station.
Without going in to to much detail does anyone know how many turns from full rich setting with new jets etc the carbs need to be set?
Local garage has adjusted carbs but now keeps cutting out with rough idle.

[sujitroy]

I recall 3.5 turns from end to end. the problem which I found is it comes off one end and you need to open up the top and push the needle back on the screw

[peterjfox69]

I recall 3.5 turns from end to end. the problem which I found is it comes off one end and you need to open up the top and push the needle back on the screw

Hi Sujit, what I was asking was how many turns from full rich would be an ideal setting with newly rebuilt carbs be? For some reason mine have gone AWOL. They are currently about 3/4 turn from full rich setting.
When the carbs were rebuilt they had set the C/0 to 3.25 which I was happy with. Ran them for two weeks and went back for a check etc. They then changed it to 4.33 after saying they were running bit lean. I noticed increased fuel consumption and sooty plugs so asked them to adjust back to original setting. Their emissions Guage seemed to start playing up as it was reading 2.99 but at my local MOT test station was at 6.0!
MOT station adjusted to 3.99 (3/4 turn from full rich setting) but now running rough.
I'm totally at a loss and rather peeved so any advice would be great.

[Goldstar]

It’s about 2 turns Peter, but I would try leaning both off half a turn at a time until you think it’s right and I probably wouldn’t go more than 1 full turn lean. Stroms should run fine at 4% tbh

[Lingen]

If the small sprung star washer at the bottom of the oil reservoir is working correctly, the needle should never become detached from the adjusting screw.
Check that the float levels are correct before attempting to set the air/fuel mixture.. Make sure that the throttle openings on each carburettor are synchronised. (1.5 turns up from shut to start with).
The correct way to get the basic needle setting is to unscrew it down, till its brass holder is flush with the base of the piston. Then turn the adjusting key clockwise 1.5 turns.
This should get the engine running reasonably well. Final mixture setting can be done pretty accurately with the "Piston lifting" method, or with a CO meter if one is available.
If your carbs. have the temperature compensating valves fitted, cleaning and re-setting them can make all the difference to getting a steady CO reading, and a nice low idle speed.
Mike.

[peterjfox69]

If the small sprung star washer at the bottom of the oil reservoir is working correctly, the needle should never become detached from the adjusting screw.
Check that the float levels are correct before attempting to set the air/fuel mixture.. Make sure that the throttle openings on each carburettor are synchronised. (1.5 turns up from shut to start with).
The correct way to get the basic needle setting is to unscrew it down, till its brass holder is flush with the base of the piston. Then turn the adjusting key clockwise 1.5 turns.
This should get the engine running reasonably well. Final mixture setting can be done pretty accurately with the "Piston lifting" method, or with a CO meter if one is available.
If your carbs. have the temperature compensating valves fitted, cleaning and re-setting them can make all the difference to getting a steady CO reading, and a nice low idle speed.
Mike.

Hi Mike,
I presume you mean idle screw when you say throttle openings?

[Lingen]

Yes Peter. The spring loaded ones.
Mike.

[peterjfox69]

Yes Peter. The spring loaded ones.
Mike.

Thanks Mike

[peterjfox69]

If the small sprung star washer at the bottom of the oil reservoir is working correctly, the needle should never become detached from the adjusting screw.
Check that the float levels are correct before attempting to set the air/fuel mixture.. Make sure that the throttle openings on each carburettor are synchronised. (1.5 turns up from shut to start with).
The correct way to get the basic needle setting is to unscrew it down, till its brass holder is flush with the base of the piston. Then turn the adjusting key clockwise 1.5 turns.
This should get the engine running reasonably well. Final mixture setting can be done pretty accurately with the "Piston lifting" method, or with a CO meter if one is available.
If your carbs. have the temperature compensating valves fitted, cleaning and re-setting them can make all the difference to getting a steady CO reading, and a nice low idle speed.
Mike.

Out of interest, what happens if the incorrect amount of oil is in the dash pots?

[Lingen]

Not a lot Peter, but it could overflow the top of the damper tube on its way down, and affect the speed at which it travels-I am told.

[peterjfox69]

Thanks for all your help, much appreciated

[Lingen]

Peter.
I have just read my reply again and realise how badly it was explained.
During acceleration, a vacuum is created above the diaphragm around the piston that holds the tapered metering needle, causing the piston to rise and let in more fuel. This upward movement is restricted by the oil, temporarily causing an even richer air/fuel mixture than normal when accelerating hard. Fixed jet carbs have a small pump connected to the throttle linkage to create the same effect.
The little piston sitting in the oil only restricts the oil flow through it in the upward direction, so the piston can quickly drop back when the throttle is closing.
Too little, or no oil will weaken the mixture and reduce acceleration. Too much just makes a mess and may shorten the life of the diaphragm if the oil gets to it.
Simples!
Mike.

[peterjfox69]

Thanks for the clarity Mike