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Initial settings for rebuilt SU's

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matchlessman Avatar
matchlessman Ken M
Eastern Ontario, Ontario, Canada   CAN
I've just completely rebuilt two very tired SU's on from my '53 TD. I want to get them set up to a point where the car will start and run, then I'll do the final tuning. I've read the jet should be set at 1&1/2 turns out (nine flats) and I've read the jets should be set at one turn out (six flats of the jet adjusting nut). When I bought the car it was running very rich (lots of black smoke so I didn't bother to make a note of those carb settings knowing they'd be rebuilt. What experience do people have with general settings for start up after a carb rebuild? I should add the ignition system has been completely rebuilt, including new plug wires and plug caps. I've retained the points for now.

Ken
Eastern Ontario



Edited 1 time(s). Last edit at 2017-04-19 07:34 PM by matchlessman.

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bobs77vet Avatar
bobs77vet bob K.
northern Va, Va, USA   USA
one thing to remember is the choke can be used to make the mixture richer,,,,but no magic can be applied to make it run leaner.


so erring on the side of being lean isnt such a bad thing

LaVerne Avatar
LaVerne LaVerne Downey
Fruita, Colorado, USA   USA
1954 MG TF "Green Hornet"
1969 MG MGB
Start with one full turn down and work from there.

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Mitchman2 Avatar
Mitchman2 Mitchell Andrus
Mills River, North Carolina, USA   USA
Well.... in reality the height of the gas in the bowl has a lot to do with it. If the level of the gas in the bowl (and therefore the jet) is too high or too low, it won't matter what you do with the height of the jet.

Wind the jet down 1 1/2 to 2 turns down and have a look for the level of the gas in the jet. IIRC, it should be 1/16" below the opening. If so, you have some room to play with the jet's height.



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aebreard Armand Breard
Monroe, Louisiana, USA   USA
If these are the original jets, they also could be worn from needle up & down...You have good advice from others, and you could also try Moss Motors videos or others on rebuilding SU's or buy the kit from Moss or Joe Curto. They are basically simple
but very effective carbs. One other I would suggest: I have been successfully using AT fluid in the pots instead of engine oil, which is usually recommended, and it has worked well for me in TC's, Sprites, & MGB's. good luck.

Buckdendave David Hill
St Neots, Cambridgeshire, UK   GBR
1954 MG TF
The level of the fuel below the top of the jet, when fully raised (no choke) should be in the range 1/8" to 3/16".
Dave H

plus4moggie Tom Lange
Bar Harbor, Maine, USA   USA
Ken - can you be more specific when you say that you have "completely rebuilt two very tired SU's"? How did you deal with the normal throttle shaft wear? Also, did you use the re-build kit's copper bottom jet washer, or re-use the original?

Bob - I hate to be a curmudgeon, but your statement that " erring on the side of being lean isnt such a bad thing" is incorrect and potentially dangerous. Erring on the lean side IS bad, leading to burned valves and damaged pistons.

With a proper rebuild there should be plenty of carb adjustment to guarantee that they can be properly tuned - if there is not, there is something wrong with what was done to the carb in the rebuild.

Tom Lange
MGT Repair

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bobs77vet Avatar
bobs77vet bob K.
northern Va, Va, USA   USA
In reply to # 3494545 by plus4moggie
Bob - I hate to be a curmudgeon, but your statement that " erring on the side of being lean isnt such a bad thing" is incorrect and potentially dangerous. Erring on the lean side IS bad, leading to burned valves and damaged pistons.



Tom Lange
MGT Repair

just to get the car running so you can tune it?

Buckdendave David Hill
St Neots, Cambridgeshire, UK   GBR
1954 MG TF
With modern fuel, if you start out with everything standard, i.e. per the WSM, including ignition, you will nearly always find that it appears to be running rich. As said, this is a safer starting position than starting lean.
Dave H

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DaveZ david zipkin
bay shore, NY, USA   USA
My own experience with SU's - mostly Volvos but I have owned 2 MGB Roadsters and 2 MGBGTs - is to start the adjustment about 1.5-2 turns down. As already stated, the float level is of key importance-and discussions rage on all forums that deal with cars with SU or Stromberg CD carbs over the "best" fluid to use in the dampers. I mostly rely on ATF - but boy, 3-1 oil comes in those very handy containers :-)
As for where to start--rich or lean -- it is far better to start rich and work your adjustment to lean. When you start lean every adjustment you make seems to make the car run better--until you get to the point you realize it's way to rich. Start rich--when you get to the point of too lean the motor/car will exhibit signs--delay in throttle response-back firing, etc. -- at that point you know to go one step back richer and you're probably right there. Volvo From Heck Dave

bobs77vet Avatar
bobs77vet bob K.
northern Va, Va, USA   USA
In reply to # 3494955 by DaveZ
As for where to start--rich or lean -- it is far better to start rich and work your adjustment to lean. When you start lean every adjustment you make seems to make the car run better--until you get to the point you realize it's way to rich. Start rich--when you get to the point of too lean the motor/car will exhibit signs--delay in throttle response-back firing, etc. -- at that point you know to go one step back richer and you're probably right there. Volvo From Heck Dave

well there you go start rich and go lean...its hard to argue with the above logic.

ozieagle Avatar
ozieagle Gold Member Herb Adler
Geelong Victoria, Australia   AUS
1958 Wolseley 1500 "Wooly"
1966 MG MGB "Bl**dy B"
HHi Ken,

I'm cruising from the B forum, when I read your post. I have written articles on setting SUs, from scratch and attach them for your info. They are mainly based on HS4s, but should be applicable to yours, too.

Herb



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Attachments:
SU idiot guide.PDF    1.5 MB

Buckdendave David Hill
St Neots, Cambridgeshire, UK   GBR
1954 MG TF
In my opinion 3/8" below the jet for the fuel level is lower than was originally intended, though the engine will still run. The method employed for balancing the carbs seems to be a matter of personal preference - I've used all of them from time to time. Moving the jet up and down only has a significant effect at idle. If you want to change the mixture throughout the rev range, you need to change the needle, though getting that choice right can be difficult, so few do it. Modern high octane fuel tends to give rich running on std settings, so lowering the fuel in the jet may actually be beneficial in this respect, though sub optimal. I would like to know if others have found this an improvement, though you won't really know if you set it by the method in the WSM.
Dave H

ozieagle Avatar
ozieagle Gold Member Herb Adler
Geelong Victoria, Australia   AUS
1958 Wolseley 1500 "Wooly"
1966 MG MGB "Bl**dy B"
In reply to # 3495305 by Buckdendave In my opinion 3/8" below the jet for the fuel level

It's not below the jet, but below the bridge.

Herb



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LaVerne Avatar
LaVerne LaVerne Downey
Fruita, Colorado, USA   USA
1954 MG TF "Green Hornet"
1969 MG MGB
I prefer to start out on the lean side and work the other direction myself....less plug fouling that way...but to each there own.

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