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MG MGB Technical - Dwell Puzzle

Hi,

Have a 25D distributor in a 948cc midget GAN1, yeah I know post to the Midget BBS but 25D is a 25D and more traffic thru here. :)

It appears impossible to set the dwell on this distributor to 60 degrees. The Car seems to be able to be set to either 40 degrees or 90 degrees and nowhere in between. I have tried it twice (I own 4 MG's) thought I was going crazy as I could only see 90deg or 40deg with it running like a dog at 40 acceptably at 90deg. I've tried 12 thou to 15 thou gaps with a feeler gauge and between those variations I see 40-90deg no in between. Tiny adjustments don't change it either. At 40deg it runs like a dog.

New points, distributor shaft bearing is bloody perfect, no play. My mechanic and I spent another few hours on it and the same thing happens to him. I've used a digital meter, an analog meter and we both see the same thing. AFAIK the only think affecting dwell besides the setting of the points gap is the lobes on the distributor shaft.. Perhaps we were just both having bad weeks or something but I swear there is a problem. Could it be a shaft out of some sort of weird distributor, or a severely flattened shaft?! Grabbing at straws here. lol

She runs ok at 90deg but I as I am in the process of rebuilding my HS2's I don't want to start tuning her with the dwell set at this weird level, I mean 90 degs? Thats impossible?!? When do the points actually open?!!!! The car is Negative Earth. Tried swapping the coil out - no change. New condensor, grounding strap is mint. Vac advance car is buggered tho. Disconnected right now, can't see how that has any bearing anyway. Anyone's 5c well appreciated. <G>

Cheers,
Neil
61 Midget/61 MGA/68 CGT/74 Factory V8
http://www.apphosting.com/mgstuff/
Neil Cotty

The only way on God's own earth you can get 90 degrees (or rather 89.9999 degrees) of dwell on anything with four cylinders and points is with a capacitive discharge electronic system. Look for a small and inoffensive, aftermarket box somewhere in the ignition circuit......
Tim Cuthill

Neil ; not to be obtuse but... you do have the dwel meter on the 4 cyl. scale right ? .

I like to set the dwell with the plugs out and adjust it whilst running it on the starter - did it again just last night on an old 1300 , works a treat every time .

-Nate
Nate

And where are you putting the meter probes??
Chris Betson

I'd take it out of the car, set the points to the correct gap, disconnect the capacitor and all existing wiring, connect an analogue ohm-meter between the fixed and moving contacts, then jury-rig some kind of crank onto the rotor or where the rotor goes and rotate it by hand - 3 or 4 revs per sec should be enough. The ohmeter *should* indicate about 2/3rds full scale deflection which equates to about 60 degrees (60 degrees being 2/3rds of 90 degrees). If that is what you get then something you have disconnected is putting some electrical 'drag' on the dwell meter which is causing it to read incorrectly, and may also be affecting the spark and/or timing to some degree. Reconnect things bit by bit and see what happens.

If not, well, *something* must be delaying the points opening or making them close sooner, and slow rotation of the spindle might show it happening.

Now if you came to the NEC on Saturday instead of Sunday I could get a few minutes respite from my human dynamo of a granddaughter ...
Paul Hunt

Sorry the car is positive Earth not negative, not sure why I wrote the above! Obviously half asleep! lol

Tim, traced the wires, no Cap discharge system.

Nate - ahem yeah. <G>

Chris from memory pos lead on input side of the coil and neg lead on the dist output to the coil. As far as I remember could only get a reading one way.

Paul, yeah I'll bench test it. I pulled the carbs, dizzy and ex. manifold (cracked - anyone got a 1098cc manifold?) today.

My tacho is playing up too. From what I remember an incorrectly set dwell will make the gauge read high - is that right? Maybe I should disconnect the gauge.. Could this be the electical drag - a faulty tach perhaps?

N
Neil Cotty

Neil - If you are using a 3 wire (power, ground and distributor or signal wires), you need to take the complement of the dwell meter reading of your positive ground system. If you are trying to set your dwell at 60°, the meter should read 30° on a positive ground car. Good luck - Dave
David DuBois

Dave... Thank you. <G> I gather this must be it. Having never owned a positive earth car I didn't know this. Should've use my noggin. <G> Thanks again. N
Neil Cotty

I dunno about that - my'67 is posative ground and I just reverse the dwellmeter's leads and set the dwell normally .
I noticed you coil is wired backwards - + should go to the points , not the hot lead from the ign. switch . but that shouldn't make any dif. in the dwell .

-Nate
Nate

This thread was discussed on 07/11/2002

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.