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MG MGA - Distributor Again

For those of you unfamiliar with my problem I have just had my original distributor totally rebuilt and fitted with the Ignitor (Petronix) ignition. I have been unable to set the timing accurately - the timing mark has jittered quite noticably at 4000RPM using a dynamic timing light (fitted with copper wire HT leads). Performance has been noticably down with an apparent flat spot at about 2500RPM.

Refitting my modern distributor returned performance to normal.

I sent the dizzy back for checking the curve etc - all ok.

So where is my problem?

Jeff said wire HT leads sometimes are not compatible with timing lights. However, I have not yet found suppressed graphite leads of 6.7mm diameter - Are they available? Also, my distributor has pointed screw contacts, not push-on fit. Is this ok for graphite leads?

The new Lucas rotor also has slight in and out movement of its blade of 0.009in - is this acceptable?

The vacuum pipe is clear (not blocked).

Thinking of sticking with my modern (Metro) dizzy with standard point ignition!!!

Steve
Steve Gyles

Steve,

Its the unblocked vacuum pipe. I had the same thing occur. Unblocked the timing mark jumped around while trying to time it. Corked it and it worked a charm.

I used an old wine cork (cork not plastic) and drilled a hole smaller than the diameter of the vacuum pipet and set in there.

Good luck
Tysen

You said you checked the vacum pip The vacuum pipe is clear (not blocked).
Now did you check to see if you have Vacumm??

Jones

Steve,
Could there be play in the dizzy drive system that isn't being adequately mitigated by the mechanical loading afforded by conventional points?
D Sjostrom

An unstable timing mark is usually due to wear in the distributor shaft bushing or a shaft with excessive runout, if found at high rpms. At idle instabile timing is a sign of a weak primary spring, which is as original in some cases, but not in the MGA. I've never seen a pertronix cause an instability issue with any distributor.

Check the mounting of the trigger wheel on the points cam of the distributor. Typically, they fit loose and can be rotated on the cam. If this is the case, use a bit of RTV sealant on the inside of the trigger wheel to hold it to the cam. RTV will release if you need to remove it later. This will cause a few degrees of timng drift, but I have not seen it cause a jittery condition.
Jeff
Jeff Schlemmer

Thanks all for your views.

The Vacuum being clear is a bit of a red herring in the timing issue as it is disconnected for dynamic timing. I just mentioned it as being clear so as not to generate comment. It also works fine with the Metro distributor.

The reconditioned unit has been rebushed and has absolutely no play. All the springs are new and the unit has been rechecked by Aldon on their equipment. It follows the MGB curve.

The Ignitor rotor is a solid fit with no play.

I am suspicious that one of the supressed plug caps is playing up and coincidently it is the one fitted to the No1 plug. The threaded contact inside the cap was loose when I inspected it last night. Perhaps an intermittent firing on No1 is causing the jumping about of the timing mark.

I am going to have another play with it today and report back.

Steve
Steve Gyles

Steve,

I must have misunderstood your comment about the blocked vacuum pipet.

I ment plugging it when undertaking the timing process. Plugging noticibly steadied the timing mark whilst timing.
Tysen

Steve - it is possible for a Pertronix unit to show more jitter than points, due to backlash in the drive gear. I've observed this with a rebuilt disti in my Magnette, although only at idle speed. If you twist the rotor arm you'll probably feel a small amount of free movement before the advance comes in, but points provide sufficient 'drag' that the shaft is always on the driving edge of the gear teeth. Without points, a small drop in engine speed can allow the momentum of the shaft to take up the free play, causing jitter.

It's quite simple to remove the pertronix and refit points if you want to check this. Probably not relevant to your flat-spot problem though
stephen

Steve, Check all cylinders with your timing light to see if one or all are experiencing problems. A steady flash at 1, 2, or 3 cylinders will tell you the problem is definitely HT related. A loose plug wire is a bad thing. If Aldon experienced no issues on their tester, then the problem is in your wiring, probably the HT side.
Jeff Schlemmer

Jeff

Sorted. Car running great. You were right with your comments about some timing lights not liking copper wire HT leads. Typically, with 32 degrees dialled in, at 4000RPM I was getting a dull mark at TDC and another 3 brighter marks at 15, 20 and 25 BTDC. As it turned out this was equating to horribly retarded ignition and performance as I have previously described.

I resorted to good old fashioned seat of the pants testing. I must have rotated the distributor a good 1-2cm, maybe more, until it finally pinked, then backed off. The difference in tone of the exhaust is also very noticable. In fact I now do not believe that I had blown the exhaust as previously stated - £50 ($90) down the drain! Anyone want a sound but used (10 years old) stainless steel box?!!

So much for relying on modern technology!

As an aside, I also followed Dominic's suggestion and closed the Ignitor ignition gap from 65thou to 45 thou by elongating the attachment holes.

Steve

Steve Gyles

Isn't it a blast going for that first drive after a revelation? Oh, the sweet seat-of-the-pants feeling of success!
Congratulations!
Jeff Schlemmer

Certainly is Jeff. The car has been more in the garage than on the roads these last 2 months. New distributor, electronic ignition, 3.9 diff conversion, diff oil leak and carb rebuild. What am I going to do this winter?!

I think Glenn has been suffering similar garage-bound problems. At least I will be able to keep driving while he is snowed up.

Oh my gawd, we are getting close to poem time again!

Steve
Steve Gyles

This thread was discussed between 25/09/2006 and 30/09/2006

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