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MG MGB Technical - SU Carbs, first try..

Hi!

I did a rebuilt of my SU carbs on my '71 B GT, but now the car doesn't start anymore..
The timing is good, the fire order is good, the spark is OK..

so..my question:
What should I do to get a basic setting, before to go further and synchronize the two carbs?
I mean, first, I want to start the engine and what's the first thing to check on my carbs?
I checked everywhere on the net and in my books and the only thing I can find is to adjust them with the engine running...
Mine doesn't want to start...
it does some tiny little backfires, there's some little smoke comming from the air filters, it started and ran for about 30 seconds, but probably on 2 or 3 cylinders..it was very rough..and now, nothing..
I checked the fuel pump, fuel is reaching the carbs..The floats are not sticking.
so I don't know what to check first..

It's my first experience with SU carbs...
thanks
J-F
JF

You'll get a better answer from somebody who knows what they are doing soon, no doubt.

I made certain that my choke was fully opening the linkage. Then I closed the jets and backed off 8 flats to get a beginning point. Had a friend crank the engine while I sprayed starting fluid in the carbs. Once it was running, I returned to normal adjust procedure.
Dan

you said about 8 turns of the main jet flat nut?
from fully screwed to the top?
I will try that, and put a bit of gas into each carb intake...
Thanks for the help, I will start with that..
it was running correctly when you did that? I mean not perfectly tuned, but running straight?
actualy, for the few seconds is ran, it was very very roughly

J-F
JF

JF,
I always start from when the jets are perfectly level with the bridge, then turn down 8 flats. That should get you close enough for it to run, but you'll have to tune it from there. The starting fluid (ether) is a good suggestion as well....

You say the firing order etc is correct? Did you touch it since before you worked on the carbs?

HTH!
Rob
Rob Edwards

Rob's is a good procedure because it ensures that both carbs are starting from a mechanically equal point in relation to the bridge and rest of the throttle body. You probably should check the fuel level in each jet as well, just to double check that your float settings actually came out to where they should be.

Both of these are simply mechanical starting points, but THEN dropping each jet the 8 flats recommended, you should be ready to start and your engine ought to run well enough for the dynamic adjustments.

fYou could also simply measure the drop of each jet below the bridge with a dial micrometer to .060" but the main point is really just to ensure that you have a true mechanical zero starting point in both carbs before you start the running/dynamic adjusting.
Bob Muenchausen

I did it as you guys suggested me, so about 8 flats from the zero (jet flush with bridge)..
actualy, the car started very well, but I can't hold it running under 3000 RPM..
also, the throttle doesn't have any effect...I can floor the pedal or never touch it, it won't change anything.
It only runs with the choke almost pulled all the way..
Air leak?
I replaced the gaskets..I don't know..
But I'm sure that I will find the trouble!


JF
JF

JF, Here are some sites that cover setting up SU carbs. The first site is Paul Hunt's and it tells you how to do about anything you need to do to an MG. On the left side click on Spanners, under Hammer and Spanner, click on fuel then SU Carbs.
http://www.mgb-stuff.org.uk/wn_suspensionframe.htm
http://www.mgcars.org.uk/cgi-bin/gen5?runprog=mgoc&p=emg/basic4.html
http://www.chicagolandmgclub.com/techtips/552.html
http://www.teglerizer.com/sucarbs/garrettech.html
It sounds like you have too much slack in the throttle cable or something in the linkage isn't hooked up. Have you tried operating the throttle linkage in the engine bay to see if the linkage is actually moving the throttle shafts? If the throttles can be turned from the engine bay your cable isn't tight enough or it could have come out of the pedal slot if there is no split pin in the pedal to retain the cable. Any MG book including Haynes has instructions for setting up SU carbs. In general you preset the jets as described by Rob and Bob. You should also preset the throttle plates, usually about 1/2 turn from the fully closed position will work. If your choke is set so it pulls the jets down fully the car should start and run without having to inject starting fluid or gas into the carbs. In fact injecting fuel or starting into an unfiltered carb is a good way to get burned. Good Luck, Clifton
Clifton Gordon

Starting point should be two turns down from flush i.e. 12 flats, 8 flats would be too weak. When tuning from first principles all the throttle and choke (once the engine has warmed up enough) spindle clamps should be slackened so one carb can't interfere with another and nor can the cable, and the idle speed should be capable of being set with the idle screw. If the screw comes away from the stop on the body and the engine is still running, let alone racing, it implies that the butterfly isn't seating in the throat or if poppet valves are fitted they are leaking.
Paul Hunt

I feel that I'm very close to get something right with my carbs!
From your help and from the sites offered by Clifton, I managed to keep the motor running..from a 12 flats from flush with the bridge, with the two carbs free (throttle rod clamps loose), now the engine is running smoothly..But keeps a very high idle (around 2500 RPM), I can't lower the idle..
but from now, with all your help, I know that i'm very close to at least a basic setting..
any hint on the reason why the engine is running so high?

thanks
J-F
JF

Check the ignition timing - too far advanced will increase idle speed.

The mixture may also still be too weak.
Chris Betson

Is a "flat" a 180 degree turn? Thanks.
Alex

60 degrees
Each flat being one edge of the nut.

Efectivley you are turning the nut round twice. It's just easier to count the edges (flats) than the full turns.
andrew.horrocks

This suggestion may be obvious, but sometimes outter choke cable housing does not seat into the cylinder on the fast idle control arm. It means that the fast idle is engaged with the choke control in the off position. Check it out!
Steve

One other area to check is to make sure the throttle plates will fully close. I have seen a similar problem to yours when the plates did not fully close and the throttle was held open by the plate jambed against the side of the venturi. Les
Les Bengtson

thanks to everyone who helped me with my carb problem.
Now the setting is just perfect, I read the instructions on the sites suggested by Clifton, it wasn't so hard to do, I had lot of fun to adjust that..
Now I can feel a huge difference on the acceleration, the motor was doing backfires after an high rev, now it runs just perfectly..i'm very happy!
thanks again..
J-F
JF

This thread was discussed between 12/04/2003 and 18/04/2003

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