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MG MGA - Need help with diagnosis

Hello all. Car is '62 mga with complete restoration about 6 years ago including new Burlen carbies. Only about 7k miles on car. We had a rather aggressive "tune and test" session at the track last Sunday. Car ran fantastic then I started noticing a miss at all speeds which got worse. Made it home ok but felt like the car was running on two cylinders. I've got spark to all plugs but when I pulled the wires off of 1 and 2, there was no difference. Substituted some spare plugs but still no change. I'm guessing that fuel is not getting to 1 and 2. Float bowl for the front carbie does has fuel. What should I check next?

Gracias,
Tom
Tom Custer

they say that 80-90% of all carb problems are ingnition related. My past experience has supported this. Once set-up, i generally never touch the carbs again (for more than 10 years).

That being said, it is suspicious that this occurred right after you had your tune and test session. And that you have unplugged #1 and #2 spark plugs with no effect. Did you adjust the carbs during the session?

The first area that I would look at is the linkage between the front and rear carbies. 2nd, pull the dampers off the carbs and, using the SU wire tool, verify that the carb pistons are functioning. If that is all good, based on your previous experiment, then a full tuneup including points, condensor, cap plugs etc followed by a carb tune would be in order.

Chuck
C Schaefer

Have you checked #1 and #2 for compression?
You could have blown the head gasket between the two adjacent cylinders.

Mick
Mick Anderson

I am an advocate for the electrical side of things. However, on this occasion I am wondering if the shaft between the 2 carbs has disconnected and only the rear carb is responding to throttle demands.

Steve
Steve Gyles

Thanks for your replies gents. Carbie linkage is ok, compression is 140 all around, valve clearances ok. Also substituted spare coil, capacitor, points, plug wires, rotor, and distributor cap. Carbie pistons move freely. What's the possibility of an obstruction downstream of the float bowl?

Thanks,
Tom
Tom Custer

Tom,


Check the distributer cap for cracks. Check for a loose condenser.
mike parker

I'm going to third the motion to consider the ignition system.

In addition to the comments earlier, also check to see if the vacuum advance has experienced metal fatigue and is leaking.

Thx
T McCarthy

Tom, you have tried and tested just about everything. When you said "Carbie pistons move freely" was that observed while running, or by manually lifting the pistons and watching them fall? I suggested testing by using the wire tool. This will show any un-balance condition between the 1-2 and 3-4 cylinders, whether related to carb tune, throttle shaft slip or valve problems.

When you said "I pulled the wires off of 1 and 2, there was no difference" does that mean you tested 3&4 and there WAS a noticable running change?

Give us a hint as to what was touched-modified-tuned during your "tune and test" session. Did you forget to re-attach the vacuum line to the disty? Is the car currently static timed properly? Can you hear that the airflow in carb throats are in balance?

One way to check if you have a blockage in the downstream side of the carb needle valve, is to attach a tube to the overflow tube and blow on it with your mouth. You should see fuel come out the jet in the carb throat.

Chuck
C Schaefer

OK, problem solved, thanks to John Twist who I called today. After I explained the situation, he thought that fuel was not getting to 1 and 2. He said perhaps the carbie needle had come loose, dropped down, and was blocking fuel flow. Guess what? He was spot on! He also asked if the carbies were new SU's, which they are. He said he's seen this problem with the new ones. I mentioned that I really had to apply a lot of torque to that little brass screw to hold the needle tight. He suggested getting a ball bearing to insert in the hole and then tighten the screw.

Thanks for all of your suggestions!

Tom
Tom Custer

This thread was discussed between 04/06/2008 and 05/06/2008

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