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MG MGB Technical - Advance problem with a B


I have a slight timing problem with a MGB. First it has a delco distributor from somewhere.

At normal timing it does not pink under acceleration but it is hard to go higher than 5000 RPM. It yields to: "I do not want to rev. higher" impression
Distributor vacuum advance mechanism is working properly.

My basic interrogation: does too much advance could cause that problem or is it a symptom of insufficient advance.

Thanks all,

Jean Guy Catford

This hints of in sufficient advance but where has the Delco distributor come from. My recommendation would be to obtain the correct Lucas unit for the engine and start from there as without this everything is really a shot in the dark.
Iain MacKintosh

Thanks Iain,
In fact I do know the provenance ot the distributor. But as I am located in n-america, Spitfires were equipped with such distrib.

Finding a decent Lucas is not so easy.


Jean Guy Catford

If the Delco has high milage it is probable that the bushings are worn. This wear will cause a condition known as spark scatter. AT 5000 and above the points are bouncing so unevenly that the dwell is is all over the scale. This results in high speed misfire. Ray

Jean Guy. We do not have sufficient information to make a determination. Yes, a distributor who's curve does not match that required by our engines (and the exact required curve will vary slightly from engine to engine), will not perform as well as an engine having a more optimal advance curve. This is true of either an under advanced condition or an over advanced condition.

With an under advanced condition, the high engine speed means that the exhaust valves are opening too soon, thus not allowing the engine to develop its full power at the upper ranges of engine rpms. (Remember, burn time is a fixed period while the speed of operation of the valves is based on the engine rpm. Thus higher engine rpms mean the valves are opening at shorter periods of time and, often, not at the best portion of the burn cycle for maximum power.)

With an over advanced condition, the pistons are having to overcome the pressure of the burning fuel/air mixture which has been ignited too soon. Thus, the "pushing the boulder up the hill" problem.

You need to plot what the mechanical advance curve is on your distributor for the various engine speeds, then, make some determination as to how closely it follows the original factory specification for your car.

Ray is perfectly correct. A worn distributor is not as capable of producing a strong, reliable spark, especially under the "harsh" conditions of high engine rpm, as a tight, well maintained distributor is. The Lucas 25D4 distributors are available from Bob and Gil Schaulin here in Arizona for a reasonable price for a rebuildable core. (Have not purchased one for several years, so do not know the exact price today.) John Twist, and a couple of others, can rebuild the distributor, resulting in a good quality distributor of known, and reliable, advance characteristics.

However, the distributor is only one small portion of the engine (even if it is a very important part--there are other parts which play an equally important role). A less than optimal distributor can cause the engine not not want to keep reving when one is near the red line portion of the tach. So would marginal spark plugs, marginal spark plug wires, marginal rotor, marginal distributor cap, and/or a marginal coil. Add in marginal electrical connections and any source of water/dirt which could tend to cause a spark plug wire to ground out side of the cylinder head rather than through the spark plug and you see that, just in the ignition circuit, there are several areas to investigate.

Add in the carbs and intake manifold. Minor air leaks or a slightly lean mixture will show up at higher engine rpms more strongly than it will at lower engine rpms. So will weak valve springs (hard to get good firing at high engine rpms if the valves are bouncing and limiting the amount of fuel/air available to you), the condition of the valves and their seats, the condition of the cam lobes, the condition of the tappets, timing chain "slop" causing the valve timing to be incorrect in its relationship to the crankshaft--all of these can cause the problem you are describing either alone or, more commonly, in combination with each other.

Start with a good distributor, freshly rebuilt and having an advance curve suited to your engine, then, begin to check out other possible sources of your acceleration problem. Right now, there are too many possible influences to do any form of adequate, focused troubleshooting.

Les Bengtson

At 5000rpm your distributor mechanical advance should be well and truly at its peak no matter what distributor and the vacuum advance does not work at full throttle. This leaves the problem Ray mentioned or weak coil,condenser,
insufficient total advance (give it a couple of extra degrees). Have you checked that your getting full throttle, clogged air filters etc. Denis

The Delco was always a junk item; they were no good new and got rapidly worse. It was not unknown for them to saw themselves in half as the advance unit disintegrated. It was always difficult or impossible to get parts for them - we used to replace them with the correct Lucas item for about the price of cap, rotor, and points/condenser for the Delco.
Get rid of it!

FR Millmore

Fletcher is right, the Delco distributor is junk. There is no provision for lubricating the mechanical advance shaft as it cannot be removed from the lower shaft and so frequently becomes sticky with little you can do. I addition the points gap is highly variable leading to trimimg scatter. I have seen the points set at .020" on one side of the cam and on the directly opposite cam be less than .010". You really would be much better to obtain a Lucas unit possibly from Jeff Schlemmer and you then know that it's right for your motor.
Iain MacKintosh

Thanks guys,

For your input. I found a used Lucas here in decent condition. My parts seller offered me two options now. An used Lucas or a new (Lucas)repro distributor #143-115 (Moss).
Not an easy choice..

Jean Guy Catford

I think I would be inclined to go for the used unit and send it to Jeff Schlemmer for overhaul/recalibration, that way you know its right for the engine. However check first to see whether it's a 25D or a 45D you require. Engines up to and before the 74 1/2 year require the 25D after that its the 45D. Trouble with a new unit is that you never know whether its calibration is correct and that's why I recommended you go for the used unit
Iain MacKintosh

I purchased one of the new Moss units several years ago and it redefined the word junk. Go for the known rebuilt unit. Ray

Yup. Had one rebuilt by another reputable company and it was like new and has been great ever since. I hear Jeff does a great job as well and you have known quantity and someone who backs up his work.
J.T. Bamford

The Lucas 25D distributor had some minor problems, mainly with the distributor cap. The terminals are set fairly closely together which, under certain conditions, can cause the spark to break down. The Lucas 45D4 was introduced, probably because of US emissions standards requiring a somewhat hotter spark, to correct this problem. (The other reason would seem to be that the 45D is less expensive to produce.)

I cannot comment on Ray's observation about the quality of the distributor he purchased from Moss. I have purchased four Lucas 45D4 distributors from Bob Ford at Brit-Tek and have all of them installed on running cars. Some for a number of years now. No problems noted over the years. But, Bob is an MG owner himself and, if there were a picture in the "Dictionary of Management" under the "Customer Service" heading, they would use Bob for the illustration.

Jeff is relatively recent in the distributor rebuild field. From our discussions, he is very serious about doing the job correctly and takes a great deal of pride in his work. John Twist has been at this for a longer period and, also, takes a great deal of pride in the quality of his work. Both have made a significant investment in time, money, and tooling to do the job correctly and stand behind their products. Either would be a good choice for both information and a first class rebuild.

Les Bengtson

This thread was discussed between 04/07/2007 and 11/07/2007

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