MG Car Repair: 1961 mga, jaguar dealership, float bowl

Question
QUESTION: i have a 1961 mga i got about 3 weeks ago.the car has been running great except for rough idle.a few days ago it started running rough while i was driving. it acted like it was not getting gas or the choke was on.it cleared up and ran fine for  the rest the trip. 10-15 miles.it now has gotten worse. it runs very rough at low rpm. sometimes i can get the rpm up to 3000 and in a few seconds the rpm will go way down.the car now is undrivable. i have drained the fuel tank and replaced the fuel pump.i just noticed the tachometer is not working. i feel because it runs rough at idol and there is now gas leaking from the bottom of the float bowl the carbs need rebuilt. the car sat for 6 years.i know there is gas getting to the carbs. i just wonder because of the tachometer if i am facing some other issues.also, i am a good parts changer and mechanically inclined. is rebuilding the carbs something i should try. if not do you recommend anyone? thanks very much for your help in the past and now.  gary

ANSWER: Hi Gary,
All of your symptoms don't clearly tell what is wrong. You need to do some testing. The tach on that car should be cable driven so that would have nothing to do with the way it runs. Fuel leaking from the carbs does though. First is the fuel leaking from the over flow pipes or the bottom of the carbs themselves? Either way I would go through the carbs first as that needs to be corrected whether it is the cause of the poor running or not.

You should be able to rebuild the SU carbs yourself. You would be wise to purchase a manual on the car. I was hired in a Jaguar dealership once and the service manager asked me if I had any manuals and if so he wanted to see them. He wanted to look at my tools and the manuals. He said he could judge a mechanic by looking at his tools and by the pages of the manual. If the manual had a lot of grease smudges especially on the pages that had the specs he felt safe to hire him. That service manager always said "ALL mechanics forget specs, but that manual never forgets specs".

I can walk you through the rebuild but it would be a good idea for you to have a manual as we go. If you have an SU apart on the bench you might be amazed as how few parts there are in it. However, there are a few odd things that can make the rebuild turn out bad.

The SU is a constant velocity carburetor meaning that the air that goes across the ventory is held at a constant speed from idle to wide open throttle. There is no idle circuit and no accelerator pump, no power valve. It runs off of one jet from idle all the way to top RPM. It controls the mixture with a tapered needle into that one jet. The needle is controlled by a piston that is vacuum controlled by the volume of air going across that ventory. The volume of air is controlled by a conventional throttle plate (butterfly). The fuel level is controlled by a float chamber mounted outside of the carburetor and it is just a float that opens and closes a needle and seat to keep the float constant.

I would advise you to purchase carb kits for the carbs and a service manual if you want to do the work yourself and when you have them, contact me again and I will walk you through the rebuild procedure.

Because you have fuel leaking form the carbs you need not test any further until this is fixed even though this may or may not be all of your problems.

Let me know,
Howard

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QUESTION: thanks so much for your response.the gas is leaking out of the bottom of the carbs.i have an mga workshop manual but it really does not go into carb rebuilding.i think i have the wrong one. i have also seen company's that rebuild these things anywhere from 3 to 500 dollars.i have done some research on the internet and realize there are suttle things in the rebuild process that make a real difference.worn throttle shafts etc.i am nervous that these company's may not have the attention to detail.i have gone full circle and think i should do the rebuild. moss motors has a su work shop manual and also an su carb rebuild video. can you recommend a book, shop manual or video that will be complete and cover all aspects of rebuilding an old carb. i do tend to over think stuff so i could be over complicating this.
thanks gary

Answer
Hi Gary,
I have not seen the Moss rebuild book but Moss has a lot of experience and I would not be afraid to purchase a rebuild book from them. The reason you need a book even though there are very few parts in an SU, is that there are many things that are not evident when looking into an SU. For example Jet alignment and jet adjustment procedure also the top pot operation and care.
The function alone is strange to a mechanic who has no experience with SU type carburetors.
Howard