QuestionQUESTION: I recently overhauled my MGB. I have the valves set by the book, timing, and points are correct but it spits back out the carbs when I give it gas and dies down. I noticed that when I place the palm of my hand over the rear carb. slightly blocking the port it seems to run pretty level and not die down. Can you please give me some idea of what is causing this? Could I have disturbed something when removing the carbs for the overhaul?? It' a 1971 MGB. Thanks so much. Willie
ANSWER: Hi Willie There are many reasons why your car spits back. First off, check the wires to the coil. The plain white wire must go to the + terminal, and the White/Black wire to the - terminal.
Secondly, have a look at the rear carb float chamber. It is possible that your float is incorrectly set, or that it is starting to sink because it has a hole in it. The plastic originally used to make these floats can be attacked by today's gasoline, especially if it has Ethanol in it. Burlen Fuel Systems in the UK have just intriduced unsinkable floats for the HS type carburetter to solve this very problem.
Thirdly, run the engine for a few minutes, then gently raise the piston that you can see inside the air intake of the rear carb. Only raise it about 6mm (1/4 of an inch). What happens to the speed of the engine? If the engine speed drops, the carb is set too weak. If the engine speed goes up and stays up, it is set too rich. Ideally, it should rise slightly, and then drop back to a steady speed slightly slower than it started.
4) Check the points gap in the distributor, and then re-check the ignition timing.
If all this fails to solve the problem, get back to me.
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QUESTION: Thanks so much for your help Barrie. I did as you said and checked the wires at the coil and they were correct. I had even installed a new coil. I took the float chamber top off and found that I had sludge in both front and rear carbs. The car has sit for about a year! I'll take carbs back off and clean them out. Let me ask you this please. The piston in the rear carb. intake seems to sit a little higher that the one in the front carb. intake. Is that normal? Also is it normal to be able to blow air through the front float lid gas inlet line when holding the float upside down and sitting on the ball of the needle and seat. I was able to blow air through it and air would come out the spout where you hook the hose for the rear carb. Thanks so much. Willie
AnswerHi Willie. You should be able to get a test rod between the upturned float and the lid. A drill will do nicely. It should be at least 1/8 of an inch and not more than 3/16 (between 3mm and 5mm). Ideally, the two floats should be roughly the same. If the floats have a metal hinge, bend them a little. If they are all plastic, you can use a thin fibre washer under the valve housing.
Your lungs have enough pressure to blow the float valve off its seat. It is only designed to take 3.5 psi when buoyed up with fuel. 5psi or more will overwhelm it.
