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MG MGB Technical - Loss of power under load

My MGB loses power under acceleration - have replaced plug leads, points, condenser, low tension lead. Have checked pump, carbs and float chambers and air filters
all to no avail - help please!
Derek Bull
Derek Bull

Check your timing and check your vacuum advance for proper operation. You didn't mention changing your distributor capacitor(condenser) either.
George B.

Derek-
Sometimes it's the simple things that are hardest to diagnose. Like George, I also suspect your vacuum advance, but here's a twist: it may not necessarily be the advance unit itself. Here's an 'ol-timey mechanic's trick: Remove the vacuum hose and bend it into a "U" shape. Place a finger over one end, stick it into a clear pitcher of water, and blow through the other end. If you see bubbles, the hose leaks and is useless. After you've done this procedure, remove it from the water and once again place your finger over one end and this time suck on the other. If the tube collapses, throw it away. A collapsed tube will not transfer vacuum properly and your vacuum advance unit will be clueless, giving inaccurate advance and retard characteristics that with drive an inexperienced diagnostician nuts.
Steve S.

Dear George and Steve,
Many thanks for your suggestions - have followed Steves advice and checked vacuum pipe -all ok- any ideas on further checks of the vacuum advance retard mechanism ( I have carefully sprayed some WD40 under the top plate of the distributor) but am loath to remove it (the distributor) if I can test it in situ.
Regards,
Derek
Derek Bull

Derek

Try accelerating without the fuel cap (or at least with it undone a bit). I had a similar problem on a Chevette years ago and it turned out to be fuel starvation as no air could get into the tank. It only showed under hard acceleration.

Obviously, be careful not to accelerate too quickly or you'll lose fuel onto the road!

Bob
Bob Everick

Derek,
If you have a timing light with a dial setting you can measure the advance at different engine speeds. You could then check the advance curves against what it should be (Paul Hunt keeps these on his web site).
Richard
Richard Atkinson

Check and make sure part of your exhaust was not collapsed by a speed bump.
.

Derek. One very basic check to see whether the vacuum advance is operating is to simply disconnect and plug it and check the timing, then connect it. You should see some advance if it is working. To see if it is working per specification requires a hand vacuum pump. WD-40 is a great penetrant, but a poor lubricant. One significant draw back is that it leaves a gummy residue when it sits over a period of time which will interfer with the proper operation of sliding or rotating parts. The very first thing you need to do is to pull the distributor, clean, inspect and lubricate it properly with a good light machine oil or teflon oil, then re-install it and check it for proper operation. The check will require a timing light (preferably adjustable) and a hand operated vacuum pump to do properly. As Bob notes, fuel problems can also cause the problem you describe. This is the result of an excessively lean mixture and can be caused by a plugged fuel tank vent, insufficient fuel pump volume (the pump can put out sufficient pressure, but insufficient volume), improper carb float levels or wrong needles. I think a rolling road session would provide the best troubleshooting information in the least amount of time. Especially if you do not have a lot of test equipment yourself. Les
Les Bengtson

This thread was discussed between 16/12/2001 and 20/12/2001

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