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MG MGB Technical - Idle with Pertronix

Normally I wouldn't bother asking this, but no sense in re-inventing the wheel if somebody else has the answer.

I put a new head on my 74.5 B/GT about two months and 1000 miles ago and of course completely retuned everything in the process. With new points, etc, it ran great. Today I switched over to pertronix. I've had some bad experiences in the past with other electronic ignitions and I never thought I'd try another. Well, I'm hooked! The only other thing I did was to bump the timing up to something more like 20 degrees BTC. I just got back from a 100-mile trip with it and LOVE the new throttle response and the way this thing grunts as low as 2500 rpm in 3rd climbing mountains. It pulls like a truck. And blipping the throttle on the level at 60 mph is an irristable new pleasure. On 89 octane, it doesn't even think about pinging.

But... it now idles like dog do-do. I really don't understand why it should perform so much better. After all, points - or Pertronix - are just a switch. And I don't understand why idle should be affected. Granted, advancing the timing threw another variable in the mix. I could back it down - but it performs so well and doesn't ping - so why? If it's mixture, it's gotta be too rich. Why would that change - it was fine before? My inclination is to experiment with leaning it out. What's your opinion?

Thanks!
Allen

Allen, Check the plugs,black soot, too rich.
Ken Knize

Thanks Ken. That's one of those answers that's so obvious I probably would have overlooked it. ' Will check them before I start tweaking carbs.
Allen

Also check out the amount of total advance you are getting at 4500 rpm - for performance you need around 36 - if your dizzy springs are worn or the weights are sticking you may be getting much less centrifugal advance than you ought to.

Hence the good performance at speed, but 20 degrees is twice what you should have at 1500 rpm.
Chris at Octarine Services

Allen,
If you did your advance by turning the distributor it may be masking a need to adjust the phasing of the electronic ignition. The paper work should explain the way its done.

Doug
D. Cook

Thanks again, folks. Doug, the Pertronix instructions say nothing about phasing; they specifically say to adjust the timing in the normal manner. Chris, I've long been in the habit of advancing the timing, particularly on these smogged cars, until pre-ignition occurs, then back off. But per your advice, I will back it off to around 10-13. It's hard to assess the condition of this dizzy. The car had but 39000 miles on it when I bought it and now shows 45000. The movement of the centrifugal advance feels free and there's no noticeable wear on the spindle. In other words, I'm presuming that this 25D4 is in pretty much like-new condition.

Again, I'm not complaining in the least about performance. I've driven it another 250-300 miles since installing the Pertronix and it continues to surprise me. But the engine now gallops at idle. A glance at the plugs will confirm if it's running too rich. Now that I have electronic ignition, should I be running .035" gap on the plugs rather than .025"? Oh yes, I'm running the original coil and have yet to measure the resistance (the Pertronix instructions specify 1.5 ohms). Should I be using a more powerful coil?

More thanks...
Allen

Allen; I believe you should have a 3 ohm/12 volt coil on your car. If the Petronix calls for a 1.5 ohm coil you should either add a ballast resistor of around 1.5 ohms or get a new 1.5 ohm coil. Clifton
Clifton Gordon

Allen: My advice in the above post is wrong. It should read. Your car probaply has a 3 ohm coil. Since the Pertronix calls for a 1.5 ohm coil you need to replace the 3 ohm coil. The 1.5 ohm coil will need a ballast resistor added between the coil white wire and the + terminal. Some coils, maybe the Pertronix, come with a ballast resistor attached. Sorry about the error, that's what happens when you are old and tired. Good luck, Clifton
Clifton Gordon

Clifton,

Thanks! Actually this car does have a ballasted coil. I guess that's a change they made with the switch to rubber bumpers. Positive lead on the primary side is light-green/white. I connected the Pertronix to the extra spade where the white lead connects to the fuse box. I'll do some measurements with the ohm meter today.
Allen

Speaking of phasing, if this relates to the position of the rotor wrt the cap contact when the ignition fires, then I have heard of people fitting one of the electronic triggers and having to change the timing quite a bit as a result. This implies the phasing has changed, and at one extremity or other of vacuum advance (the phase doesn't change with centrifugal advance) it is possible that the rotor has not yet reached or is passed the cap contact so the spark has to jump further than normal and may not jump at all. If you look at the rotor of a conventional points system I have noticed that the rotor shows the results of arcing over a significant length of the 'contact' surface, but interestingly is *not* centered with clean metal both sides, but only has clean metal one side. This implies the phasing is not ideal, but if you are not getting any missfiring is probably just withing limits. Altering the phasing could centralise this better, or could make it wortse causing misssfiring. And because manifold vacuum cars idle at max vacuum but carb vacuum cars idle at min vacuum this could also have an effect on whether individual cars experience the problem or not.
Paul Hunt

Thanks Paul. Here's what I've done today:

Set the timing back to 10-11 BTDC (with some improvement in idle).

Cleaned (sand-blasted) and reset plugs to .035. Plugs 2 & 3 were a little black, but not too bad. Plugs 1 & 4 were close to normal.

I did try leaning the mixture. No further improvement in idle. It now rather "gallops" - at an adjusted 900 rpm. If I lean it out further, when I lift the pistons with the pins, it dies.

There is no provision for phasing in the Pertronix instructions - so I did nothing about this.

It has now been sufficiently long since I converted to Pertronix, that I can no longer remember how well this car idled before. Once off idle, it performs great. Changing the timing from 20 degrees to 10+ doesn't seem to have made any measurable or noticeable change in performance.

This is a low compression 18V836 engine. Since it's delivering more power now than I ever expected, I've decided I'll just live with the rough idle, unless somebody comes forth with a solution.
Allen

Allen,
To set the electronic ignition trigger bring No. 1 to the firing position. Take the distributor cap off and note where the triggering mechanism is located. On the XR700 the the notch in the trigger should be just entering the light source, but Pertronix should have something similar. Loosen the attaching screws and slide the sensor one way or the other to get the trigger to the point that it is just begining to send the spark signal for a static timing at idle.

All this is just to make sure that your original problem is not related to a timing issue.

Doug
D. Cook

Pertronix conversions don't need phasing as do the Crane setups. The Pertronix is set up where all you have to do is drop it in unlike the overly complicated Crane setup which is a 'universal' kit. That being said, the Crane 3000 is far better than the 700.
Mike MaGee

Thanks again guys, but as Mike suggests, Pertronix not only do not need phasing, There is no provision on them for any adjustment. The bracket bolts in solid to the breaker plate in one position only, and similarly the sensor bolts solid to studs in the bracket. I'm suspecting that the "gallop" or "lope" in my idle has more to do with an HIF problem that I have yet to discover. If it were a coil problem, wouldn't it show up more under load? As I said earlier, this car is driving great - it's only the idle that's driving me nuts.
Allen

This thread was discussed between 22/10/2003 and 27/10/2003

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