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MG MGB Technical - Ooomph

I've got a 72B that is stock and, as far as I know, has the original distributor and HIF4 car setup.

I've had the car for over 5 years and I do not know the history other than it has/had been maintained reasonably well. The car has 56k+ miles/

I'm guessing the carbs have been rebuilt at least once since they do not have the throttle plates with poppet valves.

The engine runs reliably - compression 150 on all four cylinders, burns no oil - but I've noticed a lack of oomph over the past few months. Timing is at 15 degrees btdc and the dwell is 62. Never fails to start, cruises smoothly but as I say just feels a little sluggish.

I've replaced the points, rotor, condensor and wires as well as kept the valves adjusted as I perform the annual tuneup. No vacuum leaks that I can find.

I'm sure the carbs could stand a rebuild but I suspect even moreso the distributor. Perhaps the springs are just tired or ??? There is virtually no wobble on the shaft.

So... I notice that Brittek has a sale on the "Ultimate Ignition Package" which includes a new Eurospec distributor and Sports coil.

Does it sound like that might solve the sluggishness?

Thanks, Greg
Greg Smela

Greg; If you have a timing light you can check the distributor advance to see if it's functioning. Hook the timing light to number 1 cylinder, disconnect and plug the vacuum hose to the vacuum advance. Start the engine and run it to around 3500 rpm, record the advance, it's easy if you have a dial on the timing light. If no dial you can estimate the advance by looking at the movement of the timing light beam. On you distributor the advance at 3300 to 3900 rpm should be about 35* 42370 distributor. The reading you see should be a little more than 35* due to the static timing. If it's about right and and the timing beam isn't bouncing around the distributor should be OK. I'm assuming your engine is stock. If it's good I would investigate the possibility of a worn camshaft.

Regarding the ultimate ignition pack, the distributor may help, but I doubt the sport coil will help. The stock coil is adequate for stock and mild performance modifications.

Clifton
Clifton Gordon

Greg, you probably still have the original 41491 distributor in your car? Its not much for offering performance with its timing curve. However, a very good cleaning of your distributor may free up the advance weights and improve performance. It is likely that less distributor advance and different springs will help dramatically. Your vacuum advance module, provided it is functional, is better for performance than the Eurospec model.

I hate to throw a sales pitch out there, but you could also check out www.advanceddistributors.com to have your distributor rebuilt and recurved for under $100. Check out the "testimonials" page!
Jeff
Jeff Schlemmer

Cam wear was mentioned by Clifton. I have stripped 3 MGB engines in the last 5 years and all of them have had significant cam wear. This is something that will increase over time so it would give a gradual tail off in performance.

Yes, measure your ignition timing as has been said and clean the dizzy. But if it is in good shape it will not be the cause of your problem so I would not spend money on a new one untill the cause of the problem is clearer. Investigate rather than spend.

Have you checked that the carbs are properly adjusted? The workshop manual has a proceedure for this. A tool like a gunson colour tune plug is a good addition to the workshop manual proceedure.

You can check for carb spindle wear by spraying a little water at the spindle ends at tick over. If there is a change in engine note then some wear is present. The HIF carb has spindle seals that can be changed if the wear is minor. However, I would not expect alot of carb wear on a 56K mile engine.
David Witham

Thanks all. The carbs aren't showing any wear and they aren't out of adjustment however my timing light is showing some bounce so I guess I will first pull the distributor and give it a cleaning.
Greg Smela

Timing wander can be caused by worn cam/distributor drive dog gears and slack timing chain as well as sloppy distributor bearings, but unless gross there is not likely to be much effect on 'seat of the pants'. Springs can be given a basic check by removing the distributor cap and twisting the rotor anti-clockwise by hand against spring pressure. There should be two springs - a weak and a strong, the strong coming in later than the weak, which you may be able to feel by hand. Releasing the rotor gradually should return all the way and leave no slop, which can also cause timing wander, but some slop is almost inevitable. Then you can do an advance check (vacuum disconnected and plugged at the carb) and plot it against rpm. It's possible you have the right timing at the strobe rpm, but if the springs are weak you will be getting the advance faster than you should (although that will probably be causing pinking) and if they have slop you probably aren't getting enough total advance, which will affect performance.

Also check the vacuum capsule by sucking on the pipe leading to it and check that a) the timing advances and b) you cannot suck air continuously through it.

A North American 72 should have either an 18V 584 or 585 engine with 25D4 41370 distributor, or an 18V 672 or 673 engine with a 25D4 41491 distributor. The former strobe setting is 16 degrees at 1500rpm, the latter 11 degrees at 1500 rpm, both with the vacuum disconnected and plugged at the carb. Other advance specs at http://www.mgb-stuff.org.uk/wn_ignitionframe.htm and click on 'Distributor Curves'.

Buying a replacement distributor can be tricky, what constitutes a 'Eurospec' varies from supplier to supplier, dare I say depending on what they have in stock. Whilst an original UK curve may give better performance on a North American engine, which were progressively tuned more to pass the emissions tests from 1968, that is only likely to be with a 'to spec' engine and original leaded fuel. The fuels today are so different that probably none of the original curves are ideal, the actual only meeting the ideal at one point on the curve at best. Unless you are going to spend a lot of time on a rolling-road determining the correct curve for your engine and fuel, which having vacuum advance multiplies the complexity of several fold, you are unlikely to do much better than by advancing the engine as much as you can short of pinking at any combination of revs, throttle opening and load.
Paul Hunt 2

Paul
As usual your comments are right on and most useful. I would add tho' that the late smog distributors have terrible vaccum advance settings and centrifugal advance settings. Almost any distributor will produce more power than those. The extra performance will be at the price of changing the emissions of the engine.
Barry
Barry Parkinson

Barry. I am running the "Euro Spec" dizzy, obtained from Brit-Tek and testing out properly on my distributor test machine, on all three of my cars. All are fully capable of passing the required Arizona Emissions Test, even my daughter's car which does not have a cat installed on it, but has to meet the same standards as if it did. Thus, the change in advance curve does not seem to cause a significant change in the amount of emissions produced by the engine. Both engines are rebuilt and kept in a good state of tune. That seems to be more important than the distributor used in producing a powerful, clean running engine.

Les
Les Bengtson

Greg,

Before diving intop[ossible causes and solutions, it may be an idea to run the car against the clock 0-60, or check it's top speed before assuming the power is at fault.

With modern cars it's easy to climb into a classic and wonder where the 'sports' performance is exactly.

If indeed the power is not up to scratch, I suggest going the V8 route, you will never regret it.

Regards, J
J E G Eastwood 1

J -

I wish I could go the V8 route! Not in this current life but maybe down the road...

No, I'm basing the feeling on my experience with the car itself and comparing it to my GT which is very peppy.

I finally have time tonight to pull the distributor and start looking. -Greg
Greg Smela

This thread was discussed between 18/06/2006 and 22/06/2006

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