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MOT fail for CO2 - adjusting a Stromberg 175 CDUS

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Old Aug 16th, 2018, 10:29   #11
Bob Meadows
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This link may help:~
https://www.howacarworks.com/fuel-sy...rg-carburettor

Bob
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Old Aug 16th, 2018, 13:27   #12
nu11eaf
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If I remember correctly (never had a carb engine since 1990) there should be a line on the inside but about half to 2/3 way up should do it. The carb will not run correctly if there is no or very little oil in the dashpot.

Last edited by nu11eaf; Aug 16th, 2018 at 13:30.
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Old Aug 16th, 2018, 13:32   #13
Old Bean
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Ok, went and tinkered with the spare carb in the garage. The diaphragm is complete with no splits, as is the float bowl gasket. The little silver valve thing on the side appears to work, in that I took it off and if I blow into it there’s no airflow but if I suck on the valve it opens and lets air through. I can see how the operation of that valve would be massively affected by the condition of the hose that connects onto it- the hose on this carb is perished at both ends. I tested the float and it holds air and doesn’t leak.

I have a tool that fits the mixture adjustment on the bottom, but I don’t have a tool that will fit the... not sure what it’s called, the float bowl inlet jet in the float bowl that keeps the float bowl full but not overfull. It’d have to be an extremely thin walled socket to remove it as it’s close to the edge of the floatbowl.

The main needle is clearly bent, so I’m curious how the car this came from ran...

I think I have enough spares to at least remove the current carb and look at it.

Cheers,

So- dashpot, how much oil?

And is there a guide online anywhere to tuning these carbs?

Is the needle really bent, or is it a spring loaded "biased" needle?

Best regards, Doug
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Old Aug 16th, 2018, 15:05   #14
Clan
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Quote:
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Ok, went and tinkered with the spare carb in the garage. The diaphragm is complete with no splits, as is the float bowl gasket. The little silver valve thing on the side appears to work, in that I took it off and if I blow into it there’s no airflow but if I suck on the valve it opens and lets air through. I can see how the operation of that valve would be massively affected by the condition of the hose that connects onto it- the hose on this carb is perished at both ends. I tested the float and it holds air and doesn’t leak.

I have a tool that fits the mixture adjustment on the bottom, but I don’t have a tool that will fit the... not sure what it’s called, the float bowl inlet jet in the float bowl that keeps the float bowl full but not overfull. It’d have to be an extremely thin walled socket to remove it as it’s close to the edge of the floatbowl.

The main needle is clearly bent, so I’m curious how the car this came from ran...

I think I have enough spares to at least remove the current carb and look at it.

Cheers,

So- dashpot, how much oil?

And is there a guide online anywhere to tuning these carbs?
the needle won't be bent , it is spring loaded to one side ,, use ATF oil for the dashpot or a very thin 3 in-one type oil , if you warm up the engine and screw the adjuster up a little you should feel the RMP go up and and the idle smooth out , but only turn 1 flat at a time . it is very sensitive . You might find the nut is "tight" which means someone has already screwed it up as weak as it will go ..
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Old Aug 16th, 2018, 15:44   #15
Derek UK
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Removing the air filter element sometimes helps. Just for the MOT, replace it afterwards. A clean up of the engine and surrounds will probably improve the mood of your MOT tester. At least it shows you're trying. Nicer for you to work on too.
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Old Aug 17th, 2018, 00:30   #16
Bugjam1999
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Some progress...

Took the carb off the car this evening, was easy enough to remove - the spare carb is seized to its inlet manifold, I was glad the one on the car just unbolted.

I took the carb to bits and used a lot of carb cleaner to get it as clean as possible- the diaphragm is in perfect condition, it might have been replaced recentlyish as it feels soft and supple. The needle has a tiny, just visible to the eye, mark down near the tip, a horizontal scratch around maybe 1/5 of the circumference. Hard to see, I can feel it with a finger nail though. I guess a replacement needle would be a good idea. The air valve on the side was seized and gummy, even after cleaning it with cleaner it didn’t operate like the one on the spare carb so I swapped them over.

The adjustment bolt on the bottom had another 1 and 1/4 turns in to go, so there’s some adjustment yet.

I’ll get it back on tomorrow and see how I get on- I’ve booked half an hour of workshop time before the mot so I can have a play with the car hooked up to the emissions monitor. If needs be I can use that time to remove the airfilter - thanks for the suggestion

Cheers

Oh, the needle in the spare carb is definitely bent- it looks bent and as it’s spring loaded I can twist it in my fingers I can feel the bend. I’ll post a photo when I get a chance.
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Old Aug 17th, 2018, 20:26   #17
Bugjam1999
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Passed the MOT this afternoon at 4.31% co

Took the car for a drive round the long way to get to the garage and then with it idling connected to the emissions tester I leaned the mixture out gradually until the above figure was reached. Didn’t run out of adjustment, so I didn’t need to resort to removing the airfilter or anything like that.

Success! Thanks for your help all, much appreciated.

Cheers
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