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MG MGA - Distributor arcing and rough idle.

Hi --

I have a '61 MGA 1600 with a rough idle problem. The engine was rebuilt prior to my purchasing it a few years ago, and relatively little mileage < 5,000 miles on it (I think) since the rebuild. Since owning it, I have had the carbs professionally rebuilt, and have adjusted the valves, new cap, fresh wires, clean plugs, pertronix ignition, compression is strong and even in all 4 cylinders, dry (150 psi) or wet (175 psi).

When the engine is idling (smoothly at around 1500 RPM - it gets rough below that), I removed spark wire #1 and the engine ran a bit rougher, as expected. But I also heard a faint snapping sound coming from somewhere around the distributor. When I removed wire #2, the snapping sound was a little louder. On removing wire #3, visible arcing began from where #3 wire enters the cap to the distributor snap clamp. The arcing was even more pronounced when I removed wire #4, arcing from where #4 wire enters the cap.

I believe the distributor is an original DM2.P4 distributor, but the engine is a question mark, as both the engine ID plate and the vehicle ID plate have been removed (so much for a points car). It is a 1622 head. Without leading to the answer (ie "sounds like the dizzy is shot"), is there something I may be overlooking?

If all roads lead to the dizzy, the distributor shaft has a very, very slight wiggle to it, but apart from that I haven't diagnosed it further. Are there any specific tests I should do to conclude that the dizzy is the likely cause of the arcing and rough idle?

Thanks in advance,
Glenn
Glenn (Ohio)

We had a similar problem with our vintage racers. Discovered the bushing in distributor was going bad causing the rotor to just a tad off center when rotating causing the cap to crack. Replaced the cap and the fix was good for the weekend.

The crak was not visible to the eye but enough to allow an arch.
skip

While there MAY be an arcing problem with the wires connected, what you are observing is CAUSED by removing the wire. Under normal conditions, the coil only develops enough voltage to fire the plug,usually around 10KV, maybe up to 20 KV under max load. With the wire removed, the coil can generate its max voltage, which may be around 30KV with the stock MGA coil, which is a "semi sports" (HA marking)type. "Sports" coils (SA) can generate 40KV, and modern HO coils up to 60KV. Whatever it is, the coil WILL increase output until the spark goes somewhere, and where it goes is the point of least resistance - across the cap, across or through the rotor, across the coil terminals, or internally in the coil. Every time it arcs across something it will tend to "cook" the insulator it is crossing, leading to a permanent carbon track, which will render the component useless, as the voltage required to follow the carbon track drops. It is extremely bad prasctice to disconnect wires without grounding them to prevent this. Some, but not all, coils have an internal spark gap to prevent max voltage from exceeding the coil design voltage, but you are still subjecting the rest of the system to unnecessary loads. Your "test" does show that for now at least, the coil, rotor and internal cap are better than the exterior cap, but that is likely to change if you persist. All of this is why bad (high resistance)plug leads frequently cause cap/rotor/coil problems.
FRM
FR Millmore

FRM and skip,

Thanks - helpful, and makes a lot of sense. I suspected the cap might have been the problem and did put a new one on today before I did the wire removal testing. The wires are relatively new as well -- all good rubber, no cracks, nicks, etc.

I have also tested for vacuum leaks at the manifold and non could be detected. The carbs are freshly rebuilt, so I don't suspect them to be the problem, either.

So, if I assume the arcing is 'normal', for the test I did, and there doesn't seem to be a dead cylinder (apart from the arcing, the engine ran rougher for all 4 cylinders I disconnected, then smoothed out when I reconnected the plug wire) -- would it seem logical that the roughness at idle/low rpm is due to variations in timing from a potentially worn distributor, or should I investigate valvetrain-related problems?

Thanks,
Glenn
Glenn (Ohio)

Glenn-
Wire insulation is not an issue if all is dry and wires are not near metal - they would work just as well if bare. Conductivity is an issue, if they are carbon or plastic "string" type wires. Check ohms from dist cap terminal to plug terminal; they should all be about the same, maybe 5K ohms or less. Frequently the plug terminals are not correctly attached, or the "conductor" may be damaged. What kind of plug connectors are they?
Distributor shaft wobble is not likely to be an issue, unless severe. Check point gap on all 4 lobes, and check with shaft pushed toward and away from points. If gap is within .003 on all lobes, and not varying more than this when pushed sideways, you are likely OK. Actual timing can be checked by reading off cyls 1 & 4, should be the same. You can also make a mark on the crank pulley 180deg from the regular one and check timing on #2 & 3; all should be the same or very close. This will reveal a bent or excessively sloppy dist shaft.
There are a lot of reports of defective condensors, might be worth a switch test.
Valve train problems are not likely to cause low speed trouble, if clearances are correct & compression is even.
My guess is that you have a carb problem, despite (or because of) the rebuild. Go back to basic carb setup.
FRM
FR Millmore

The arcing can come from a bad cap OR rotor. There have been several reports on this board about microscopic holes being burned through the rotor and shorting coil power to the shaft. Another possibility that hasn't been mentioned is poor insulation on the LT lead to the points. Its not as likely in a DM2 as it is in a 25D, but its always possible.

It is also possible that the light spring in your distributor has fatigued beyond its useable lifespan. This would be detectable by checking your timing at idle. You would notice the timing mark bouncing and varying more than 1 or 2 degrees. While this can be caused by other tungin issues, more than 3 degrees of jumping almost guarantees a distributor issue.

To check your shaft bushing, remove the rotor from the distributor and pull out on the shaft as you shake it side to side. This will help eliminate the slop in the cam assembly that often feels like shaft wear.

Also check that your carbs are balanced. All the money in the world spent on a distributor won't help if the carbs aren't properly tuned.
Jeff
Jeff Schlemmer

FRM and Jeff,

Thanks for the advice - I am using a Pertronix ignition, so no points/condensor. I understand the carbs can be an issue, and have worked to smooth the idle out - it's getting there, but just not where I think it should be at.

I checked resistance on all 5 silicone wires last weekend and found them to be ~5M ohms. (5.6, 5.3, 4.3, 4.7 and 4.6 for the coil wire). Plug ends are "straight on", not 90-degree angle types and all plugs are well seated in their terminal ends.

I will investigate the rotor, distributor shaft and bushing, and the spring fatigue issues you have both raised, as well as futher balancing the carbs.

While I'm not expecting to get BMW-level engine smoothness, I would think a stable idle at around 800-1000 rpm is possible on these, particuarly with a rebuilt engine and carb. Maybe I'm asking too much!

Regardless, thanks so much for your speedy and experienced responses. This BBS is a treasure!

-Glenn
Glenn (Ohio)

If the rotor were bad, then the arcing would NOT be visible at the cap. Testing with removed wires is an excellent way to MAKE the rotor bad, once it goes south it is likely to not run at all. Insulation failure on the LT side, or a broken ground lead from points plate to body, may cause poor running, but NOT audible arcing, as it is LT.
FRM
FR Millmore

I heard some of the same comments today at one of the seminars. The speaker stated that most problems are not carb but timing. Have you set your timing to 28-32 degrees at 3000-3500rpm. Once this is done then start to check out your problems. Because you have a pertronix unless your dist shaft moves a great amount, that is not your problem as you have no points and it uses a magnetic pick up. It is very cheap to replace rotor and cap. Some parts have infant mortailty death. When my car was first done, in less than 50 miles the rotor went bad. It just happens sometimes. You also could find a mechanic who has a scope to read your wires, plugs, and coil to see that all is working well and then have him test your exhaust to see that everything is tuned correctly
Jeff Becker

As a side note to Jeff Becker's comments - I just tore down a "40897" distributor which is supposed to give great performance. Its a great unit with low miles, but slight internal wear and factory misalignments. This caused a 10 degree distributor to give 15 degrees of advance - that's 10 extra crank degrees. That's more than enough reason for everyone to check their total timing! Chance are very good that its not correct!
Jeff
Jeff Schlemmer

I agree!
Total timing is important.
Yars and yars ago, I 'crewed' for a guy in my town, SCCA GP 1098 Spidget.
We alway checked total timing not just the base.

SF
Dwight
DCM McCullough

Glenn-
I totally missed the reference to Pertronix in your 1st post, but agree with Jeff that that removes a lot of variables. Your wire readings are OK.
Note that if the dist is sloppy and you set timing at full advanced postion, the idle timing will be off by the cumulative errors, likely giving poor idle. No question that timing at speed is more important, but for sorting out idle it would be best to set it at idle.
Have you cleaned the carb pistons/chambers, and done a piston drop test to ensure that they are still correctly matched? Cars like the A which feed oil fumes through the filter are prone to oiling up the vac chambers, which really screws up idle and low end carb setup.
FRM
FR Millmore

FRM and others,
Thanks again for your advice. I recall when I installed the Pertronix unit 2 years ago that I had trouble getting the engine to start when the timing was set up statically. I needed to turn the distributor far away from where it needed to be get it to start (too long ago to remember if I needed to advance/retard it). I believe I roughed-in the timing from there.

I began to reivestigate timing last weekend, starting with dynamic when I remembered another problem - the timing mark indicator from the chain cover is missing! Regardless, I chaulked up the timing mark on the pully and ran the strobe on it. Most notable was the variability of the timing mark when the engine was running. I would guess between +/- 5-10 degrees, so quite a bit.

Given the newness of the carb rebuilds, I really don't think the chambers are gunked up. I had one off last weekend and clean as a whistle. I'm more convinced of the dizzy being worn as Jeff suggests, given the amount of bouncing around the timing mark is making.

FRM, you mentioned you posted the names of distributor rebuilders (Jeff Shlemmer, are you one of them?), but I wasn't able to find it on the bbs. Can you re-post or e-mail them to me (see email address at left)?

thanks,
Glenn
Glenn (Ohio)

Glenn-
Wasn't me who posted the dist rebuilders, but Jeff is one of them. From your description of the variation, the dist is worn out. The initial set-up trouble indicates there is a problem with the Pertronix installation. Only other thing that could cause that variation would be a worn out timing chain or maybe the timing sprockets were left loose on assembly. Suggest you send the dist to Jeff, as he has the machine to correctly test it.
Has it ever run/idled properly since rebuild?
FRM
FR Millmore

This thread was discussed between 24/06/2006 and 01/07/2006

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