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MG MGA - mallory dist. question

Ok this is a bit wierd but there might be a good explanation for it. Or it may not matter.
I bought a mallory dist for my 1800 from Princess Auto. It's a mechanical advance dist. designed for a tractor but it fits(with a few minor mods.) Other club members are also using this dist. but don't recall having this problems.
I set the point gap to 15 thou. installed it and set the timing. It ran great. Then I checked the dwell and it read 40 deg. Strange. So I kept resetting the gap down to 3 deg. and couldn't get it over 50 deg. So I reset the gap to 15 thou. and took it for a spin. WOW what a difference. It ran way better with a lot more power right up to 4000rpm.
Am I looking for a problem that doesn't exist. Is the point gap more important than the dwell???? The only real difference that I can see is that the lobes on the spindle are much larger than on the lucas dist. Would that have anything to do with this.
Sorry but I'm slowly climbing the learning curve on how all this stuff works.
Thanks
Kris
Kris Sorensen

Kris. No, the point gap is not more important than the dwell. Like torque and bolt stretch, they represent two sides of the same coin.

Dwell represents the period, measured in degrees of arc, when the points are closed and the coil is being charged. Points gap is a measurement which, when the points are new, should provide the specified dwell reading. The dwell angle is built into the points cam and, at the factory specified points gap, both the dwell and the points gap should be within factory specification.

Points gap has an inverse relationship with dwell. The larger the points opening, the lower the dwell reading--i.e. the less time the points spend in the closed condition. However, the contour of the points cam will have a very significant effect on how the dwell is affected by minor changes in the gap between the points.

Owning neither a tractor, nor a Mallory distributor, I cannot comment further on what you are experiencing. I would note, however, that if you are using an older dwell meter, make sure you read it properly. The old systems only had eight and six cylinder scales and the four cylinder engine was used with the eight cylinder scale, doubling the readings to get the correct values.

Les
Les Bengtson

About 40 degrees dwell is correct for the Mallory dual points distributor, while the Lucas dizzy has 60 degrees dwell. There are two major differences leading to this result.

The cam in the Lucas dizzy apears to be near square with rounded corners The points spend a lot of time with the rubbing foot in the vally and then the points pop open just long enough to hop over the bump. Result is 60 degrees closed.

The cam in the Mallory dizzy is more like round with four small flats. Here the points spend a lot of time with the rubbing foot on the cylinder and points open. Then the foot drops into the valley just long enough to make a good points contact, and pops open again. Result is about 32 degrees closed.

Additionally the dual points setup has the two sets of points on opposite sides of the cam, not exactly 180 degrees apart but with one offset 8 degrees from the other. In this manner one points set opens and closes 8 degrees before the other. End result of this is a total of 40 degrees when at least one of the points is closed to maintain ground contact, so you see 40 degrees showing on a dwell meter. If you stick a bit of cardboard in one points set to prevent closure contact, the dwell meter reads 32 degrees for a single operating points set.

The points set that opens last bears the small flash spark upon breaking the ground circuit which leads to the high tension spark event. The one that closes first bears the small flash spark upon making the ground connection to recharge the coil. In this manner the points sets split the electrical wear, about half for each, and will last longer in service without burning up and changing the points gap. End result of that is the Mallory Dual Points unit is more stable with time, is slower to change the points gap or dwell or timing with wear, and can run longer between maintenance calls.

I have a Mallory Dual Points distributor which has been in service for 180,000 miles on my MGA. It is still in very good condition giving negligable spark dance under a timing light. It doesn't mind running up to 20,000 miles between points adjustments and maybe twice that between points replacement. It is a very robust unit, and I am looking forward to many more years of use before I need worry about repair or replacement (easily a quarter million miles or nmore). Yes the Mallory tune up parts are more expensive than Lucas parts, but those bits are not needed very often (and I never worry about failure of an electronic unit). I think I'll keep it.
Barney Gaylord

Barney - you mentioned spark dance under a timing light. Is this something that can be easliy checked for the quality of a spark? I've never tried this and wonder if its worth doing?
J H Cole

Gentlemen
That certainly explains what is happening. So if Barney's dual point dist shows 40deg and my single point dist shows 40deg then that should be a sufficient amount of time to build up the correct amount of current. So I guess I am fine with the setup as unless there is something else that should be checked or verified.
Thanks
Kris
Kris Sorensen

What I casually refer to as spark dance is the variation of displayed position of the timing mark under a strobe light, or the tendency of the spark timing to vary from turn to turn. This can be caused by a worn parts in the distributor such as a loose mainshaft bushing that allows the shaft to wobble. With a new distributor this spark dance will be minimal, usually within a half dergree range at the timing mark, so the spark timing is nice and stable. If this dance of the timing mark under the strobe light exceeds 2 degrees total range you might want to investigate the mechanical condition of the distributor and distributor drive, including all of the camshaft drive parts. When I say my Mallory exhibits minimal spark dance, it means all of the drive parts are in good condition, including the distributor itself. This is supposed to be pretty impressive for a distributor with 180,000 miles in service.

Dwell time is not terribly important for strong spark, as the coil can charge and discharge very quickly. A distributor for an 8 cylinder engine might have dwell around 22 degrees. If those engines can readily run several thousand rpm with a strong spark, then a 4 cylinder with half as many sparks per revolution could run much faster with plenty of time to charge the coil for each spark.

If you know the base dwell specification for your distributor, and you check it with a dwell meter (with engine running), you can have a fairly accurate indication of the points gap setting. Small points gap makes long dwell. Large points gap makes short dwell. When dwell is correct and you have a strong spark, you may never feel the need to check or adjust the points.

Change of points gap (and dwell) also affects spark timing. If the rubbing foot on the contact set wears, the gap get smaller, the points open later and close earlier, and the dwell increases. Late opening of the points delays the spark, and the ignition timing will be retarded. Checking the dwell occasionally allows you to monitor the condition of the points, so you may not feel compelled to haul out the timing light so often.

The other thing to check with points is contact resistance. Connect a low range ohm meter between the distributor side terminal (or the same wire at the coil) and ground on the engine block. With points closed, resistance here should be less than 1/4 ohm (250 milli-ohm). High resistance inhibits current in the ignition coil and reduces spark power. High resistance at the points contact will also cause heating of the points and possible premature points failure. Cleaning the points can help. Low resistance also depends on having good ground connection between the distributor body and engine block, so keep that joint clean and snug.

It is common for an ignition tester to have dwell meter, tachometer, and points resistance test all on one instrument. Connect the two clip leads to distributor and ground, check speed and dwell with engine running, check points resistance with ignition off and the points closed.
Barney Gaylord

Top of the morning to you all, is spark dance anything like River Dance ?


Paddy
Paddy O'Dors

Now thats funny! Or perhaps a lap dance?
skip

Barney When I last timed my car with a strobe I definately saw what you describe as valve dance and probably more than 2 degrees. It got worse the higher the revs. At the time I put it down to the strobe now it seems I have to think again. Are there any dims to check the wear on the cam? If everything seems tight I'm not sure what else can be done unless you pull the distributer apart! I thought the distributer was Ok but perhaps not so. When I did the Dwell I reduced the points gap to achieve the dwell angle. Maybe its better to have a lower dwell and keep the specicified gap rather than the other way round.
J H Cole

This eratic variation of spark timing is caused by slop in the mechanism driving the contact points. Start by checking for wobble in the distributor shaft, which is caused by a worn bushing in the distributor body. If the shaft wobbles, replace the bushing.

If that looks snug, remove the distributor and check for float and backlash in the drive gear. Screw a long 5/16 fine thread bolt (rocker pedestal stud) into the distributor drive gear and pull/push to see if it has any significant end float. There should be very little float there, and this is usually not a problem. Also give it a little alternating twist to check for backlash in the drive gears. This too is usually not a problem, but the gears do sometimes wear if someone had been running abnormally high oil pressure, which puts a high load on the oil pump drive, which uses the same drive gear on the camshaft.

If that checks out the next step is to check for backlash in the cam drive timing chain. Put a wrench on the crankshaft pulley nut. Screw the long bolt into distributor drive gear. Apply a little side load on that bolt with one finger to create a bit of friction in the dizzy drive gear mount. Rotate the crankshaft back and forth slightly to see how much it may move without turning the dizzy drive gear. If you find more than one degree backlash at the crankshaft you may need to replace the worn timing chain tensioner and/or a worn timing chain, and maybe even a worn sprocket or two.

The worn tensioner is a common culprit. While you're in there, a new timing chain is cheap. The crank sprocket goes around twice as many turns as the cam sprocket and wears the teeth twice as fast, but check both sprockets anyway. Pick up a sprocket with your fingers on the ends of the sprocket teeth. If you cut your fingers on sharp ends of the teeth the sprocket needs to be replaced.

When you eliminate the drive backlash and any dizzy shaft wobble, the eratic spark timing problem should go away.
Barney Gaylord

I checked the advance by setting the idle to 3000rpm and eyeballing the timing mark. It seems to be about 20deg or so. How can I tell if the springs are holding it back. My initial thought was to remove the springs and recheck. Would that cause a problem.
Kris
Kris Sorensen

Okay to remove springs just to check for maximum advance. If you remove the springs it will go to maximum mechanical advance any time it's running. It could be hard to start, or run rough at low speed, or ping on accelleration, but no problem running full mechanical advence at 3000 rpm.

A nice feature on the Mallory distributor is the adjustable stop for maximum mechanical advance. If you want to run 20 degrees advance at idle, and limit total advance to 36 degrees, set the stop at 16 degrees.
Barney Gaylord

I did notice on the advance plate was a number 8 stamped on it. Am I to assume that is the amount of advance or is it a coincidence that my 12 deg. of timing plus the 8 happened to equal my advance of aprox. 20 at 3000rpm.
Still learning
Kris
Kris Sorensen

I contacted Mallory and they said the dist. I have can be recurved but they would have to do it. The cost would only be 30.00 plus shipping.
Thanks
Kris
Kris Sorensen

This thread was discussed between 27/10/2006 and 30/10/2006

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