MG Car Repair: MGB rough SU HS4 carbs, float bowls, float chamber

Question
Hi Barrie - my ongoing saga - 74 1 MG with SU Hs4 carbs is running rough. I suspected compression low on cyl 1&2 but all 4 check out at 140 psi. Next I replaced points with Petronix ignition. Still seems 1 and 2 are misfiring, but here is a clue - upon starting the engine hot it runs very smooth for about 3-5 seconds, it really purrs - then stumbles.. I checked fuel in float bowls and both about 1 full with float out. Is this right? I checked needle valve and gap is little bigger than 5" on both ( 3.9 mm ) Plugs look dry and black on 1 and 2 ... and brown on 3 and 4.
Could it be the jet is not centred on front carb? Stymied in Canada  - Paul

Answer
Hi Paul.  Your compressions are good and the Pertronix conversion is a good idea.  That leaves two possibilities.  Either you have an ignition problem with cylinders #1 and #2 (unlikely) or you have a fuelling problem.  I do not think that jet centring is a problem.  

I have always suspected over-fuelling of the front carb, as evidenced by the black plugs.  
Your observation of clean running for a few seconds followed by a stumble leads me to think that you have a faulty front SU float chamber needle valve.  Instead of removing the float chamber lids, take the tops off the carbs and carefully remove the pistons.  The chambers and the edges of the pistons should be clean and dry.  Try not to get them mixed up.  The pistons and chambers are matched pairs.  (Actually, it might be worth swapping them to see if someone has gotten them mixed up in the past).  

Turn the ignition on but do NOT try to start the engine.  Does the fuel pump stop or does it keep ticking away?  Look at the jets.  Is the front one flooding?  Can you see the fuel level in the front one but not in the back one? If so, you have a fuel level problem.  If so, replace both needle valves, preferably with Grose Jets, then do the above check all over again.  Don't forget to top up the dashpots when you put them back on.

Another thought.  Visually inspect the plastic floats.  They do sometimes go porous.  Can you SEE a quantity of fuel inside the front one?

Keep working on it.  You are nearly there!