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MG MGB Technical - Dwell Comparison

Last night I replaced my points & condenser (72 B)with an electronic unit (Penorix, sp?), but before doing so I thought I'd check the dwell and timing on the points system (just in case I have to put it all back in again).

I wanted to know the settings, because the car always started so easy. (yeah I know... why am I replacing something that works!!!!).

First I found that my 30 year old dwell meter only does 6 cyl and 8 cyl. The dwell read 24 on 8 cyl line.

My Main Question:
Is a dwell of 24 on an 8 cyl reading actually 12 or 48 for a 4 cyl?

If its 12 the points must have been only slightly opening. I never did check the timing.

After installation the car did fire right up, but it popped through carb(s) a couple times until warmed up. Then I went to drive it and all appears fine, but I think the power is off a tad. Went to steepest hill in town, put it in 4th gear as quick as I could and there was no laboring or misfiring on the climb back up - so I think things can't be too bad.

I still need to check timing.

Second Quetion:
Should I stick with stock timing settings from manual?

Just an FYI - I still plan on pulling head to check for need for shaving or valve job - gasket already leaks between #2 and #3.

Thanks for any comments. I really enjoy this website.

BobA
Stillwater, MN
R.W Anderson

Your dwell was indeed 48 degrees. Set the timing to stock and then check for any pinging under load. Ray
RAY

RW

Lots on timing and such in the archives. Bottom line, from what I understand, is that the engines are likely no longer to original spec (shaved head or block) and the fuel now available has different combustion characteristics. Thus the original timing, and distributor advance, curves may no longer be as close to optimum as they originally were.

One way to set the timing is to use what I call "the real world" approach. Set initially to factory specs. Find a long hill and pull in 4th. If it pings, retard the timing slightly and try again. If it doesn't advance the timing slightly. Repeat until you find the point at which it the most advanced without pinging.

FWIW

Larry
49 YT
58 A
72 B GT
Larry Hallanger

Bob. Yes, use the eight cylinder scales/settings when testing a four cylinder engine and double the indicated scale values to find your actual values. The Lucas 25D4 had a dwell of 60 deg when properly adjusted and the Lucas 45D4 had a 51 deg dwell when properly adjusted.

Assuming that your car still has the original Lucas 25D4 distributor which was fitted to the car, your points gap is excessive, resulting in the points opening sooner and staying open longer. Dwell is an indication of how long the points are closed and charging the coil.

As to timing, use the basic figures in the book as your starting point. Change of coil triggering device does not cause a change in the timing requirement. It can cause a change in the actual timing, hence the need to reset the initial timing after both replacing points and installing an electronic points replacement device. Larry is correct that engine requirements can vary and some testing is in order. If you have an adjustable timing light, it is worth checking the overall advance range as indicated in your workshop manual. Weakened springs can lead to setting the dynamic timing at the proper specification, then, having very little further advance as the engine speed increases.

As to the cylinder head, have it crack inspected (or do it yourself). I have seen too many people have work done on the cylinder head, then find the cracks. If your machine shop does not crack inspect the cylinder head prior to working on it, and many do not, make sure they do so before they begin any other work.

Les
Les Bengtson

Even changing the gap or dwell of points will change the timing, as you move the opening point up or down the ramp of the cam, which is angled, it moves the trigger point backwards or forwards. The moral being timing should always be checked after doing anything with points. The big problem with replacement triggers is that some seem to to result in the timing being way off, so much so that the engine won't even start in order to do a dynamic test and so must be set statically first. In these cases there is a real risk that the trigger point moves so far that the rotor arm is no longer adjacent to the contact in the cap under certain conditions, which can lead to weak sparking or even mis-firing.
Paul Hunt 2

Bob, feel free to give me a call if you need, since I'm relatively close by. Do you know about the Saturday morning british car club meetings at Square Peg diner in Minneapolis? It'd be a great place to "borrow" a little wisdom...
Jeff Schlemmer

This thread was discussed between 14/03/2007 and 17/03/2007

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