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MG TD TF 1500 - Misfire at 4500 RPM

Since I put the 4.3 rear end in, I haven't been at 4500 RPM too much, even on the freeway. However, last few days, reving to 4500 on the road has caused misfiring. In reading the archives, there seem to be many causes, from condensers, to fuel pump, dizzy shaft wobble, to point gap.

One of the references was to "too much" plug gap. Just before this started I regapped my plugs to .035 as I am running a sports coil and that seemed to be the recommendation. I am also running the pertronix, and the dizzy was rebuilt about 6 months ago.

As I said this came on suddenly (I think)and the only change I made prior to this was increasing the plug gap. Checked all plug connectors and dizzy/coil connectors and they are tight. Lucas bumble bee wires ok too.

Before I go pulling the plugs again, has anyone else experienced this due to plug gap too large, or should I be looking elsewhere?

TIA

Larry
Larry Karpman

Larry; Have not been where you are now,but most true "sport coils"should have enough output to carry a .035" plug gap. Would question the quality of the coil if reducing the gap by .010 resolves the issue. I would probably backtrack--if you believe the issue began with the increase in plug gap-go back to previous spec and try it. Look carefully at plugs,plug wires and ends at the same time-could have created an issue with these during the previous plug change(cracked plug insulators happen easily).
High RPM misfire can be caused by a multitude of things-ignition problems are the most common causes. Start with the simple stuff and keep us posted.
Hang in there Dan
D.R. Craig

Dan:

I regapped back to .030 and the misfire disappeared, and the car feels much livelier. When I first regapped to .030 form 0.25 several months ago, I was very pleased at the liveliness of the car. When I went to .035 (as I read in the archives as the commonly used gap with Sports coils) I noted the livliness seemed to disappear. I kind of wrote it off as "imaginary" for a while, until last night, when I regapped back to .030. the liveliness returned, no misfiring, and driving today was a joy in perfect weather (for a change :-)

I imagine that my modern Lucas Sports coil is somewhat on the weak side, and not up to the task, but I'd have no way to test it anyway. I believe I will leave things as they are, as I am quite happy with the car's performance. Thanks for your input!

Cheers

Larry
Larry Karpman

Larry - glad you sorted things out but I don't believe Lucas make any coils etc themselves. They're made goodness knows where and have a label put on them ! See archives but I'm using secondhand 1970s 80s coils that were made by Lucas and can last 30-40 yrs. There is still a touching faith in the Lucas name which is why they do it. A dealer/restorer here in the UK who I know quite well resorted to using coils from the USA because of probs. with European electrics. Look at Bosch - often made now in South America !

One of the most common items made incorrectly are rotor arms which use a metal pin to fasten the brass contact to the plastic. Original Lucas had a moulded plastic projection that held the contact - no chance of spark leakage to the base but not uncommon with the modern replicas, through the pin. Again the original Lucas ones for our cars were common and can still be found at car meets here for a Pound ! I collect them and sell them to other owners at cost if they've problems with ignition.

So go and check your rotor arms and replace the 'metal pin' ones you find ASAP !

Cheers John.
J.C Mitchell

Many of my Lucas ignition parts have the Lucas name but also a small Mercedes Benz Tri-Star imbossed on them. I wonder if they were made by M-B for Lucas.
Jim Merz

JC/Jim: The Lucas Sports Coil on the car now is the typical gold case modern one available worldwide and quite common. I do have an original "period" Lucas Sports Coils that are quite hard to get a hold of, but I have not installed it. It's the one with the large spiral red plastic connector on the front.

If I'm not mistaken, like all of the Lucas switches, etc. on the Series III Jaguar salons I used to collect, there was much made under the Lucas brand by Bosch.

Thanks for the tip on the rotors.

Cheers

Larry
Larry Karpman

Not a "Mercedes Benz Tri-Star" mark, Lucas stuff has had that since the beginning of time, don't know where it came from.

The increased plug gap causes the voltage to be much higher throughout the secondary side. This can result in arcing anywhere, from the coil - internally or externally, to cap, rotor, wires, or down the plug insulators. If you left it that way, eventually it would create the carbon path known as "tracking", and it would miss all the time. Most common place is from a plug wire hole to the cap clip.

The distributor cap/rotor is right at the edge of handling the high voltage at .030, which is why the cap size was increased when the plug gaps were opened up to .035 for emissions reasons - (change from 25D to 45D distributors, 1965). Lots of failures with modern rotors too, from poor material as well as design.

FRM
FR Millmore

FRM, you said, "The distributor cap/rotor is right at the edge of handling the high voltage at .030, which is why the cap size was increased when the plug gaps were opened up to .035 "

That leads me to ask how the consensus is to run at a .035 gap, with the standard cap, if that is beyond the handling capability of the standard TD cap? What am I missing?

Larry
Larry Karpman

What consensus? Not me!

It is frequently advised as worth a try - the coil should take it and it MAY give better running. Generally, it fires lean mixtures better at low speeds, which is why it works well for emissions cars. But, the high secondary voltage creates the tracking problem, with small diameter distributor caps. The pre 45D Lucas caps are about the smallest common caps in any cars, so it is more of a problem with ye olde Lucas stuff. Add the fact that many repro parts are poorly made, and you have trouble, for little benefit. If you examine many different caps, most will be larger and/or ridged inside to increase the distance a spark has to travel to ground. That's why real Sports Coils have ridged high tension towers, as do spark plugs and high voltage power transmission insulators. Since the outside of the side entry caps have very short distances from lower wire holes to clips, it is important to use a silicon grease in the wire holes to keep moisture and dirt out. These also build up along the corners of the clip groove, effectively making a ground wire out of it.

The problems frequently only surface when the other factors requiring high voltage add up, like high compression and high speed, and maybe a bit of additional wear on the plug electrodes. Once a carbon track is established, no matter if it is still invisible, the breakdown becomes increasingly more likely. This is also why it is very poor practice to "test" a coil or other parts by "seeing how far the spark jumps."

FRM
FR Millmore

FRM - you talk a lot of sense - the only time I would widen the plug gap would be as a temporary fix if a plug was fouling, especially important on a two cylinder classic motorcycle !

Larry - I hope you don't have one of the 'metal pin' repro rotor arms, but there are a lot out there leading to a lot of breakdowns !

Cheers John.
J.C Mitchell

JC-
Be careful widening gaps on classic bikes - the distances are even smaller. Also, the magneto equipped ones won't start with wide gaps, unless you are Superdude. My Royal Enfield 750 TT specifies .018 plug gaps. It reliably started first kick, even in winter; if the gaps got to .019, it took a really good kick, and at .020 you could pretty well forget it.

Regarding the three pointed mark, it does appear on Bosch stuff, but I think it's also on other manufacturer's parts. Perhaps some kind of standards sign? Or maybe a relic of the very early days, when Robert Bosch was one of the originators of automotive electrics? I always took it as being a symbol derived from a three phase alternator, such as generates AC for mains supplies. Would be interesting to hear from someone who actually knows.

FRM
FR Millmore

Hi FRM, I think I went up a blind alley with the motorcycle comment ! You're right re mag. equipped bikes - 12-14 thou. is often the norm, but I was thinking of coil bikes. What I meant was a gunge fouled plug will tend not to foul if gap is widened but only as a very temporary solution to get home ! Otherwise I stick to standard gaps.

Cheers John.
J.C Mitchell

Hi FRM - correction - I was thinking of Mag CB points ! As you say 18 thou. would be the norm for the plugs !

Cheers John.
J.C Mitchell

I had a similar problem once, misfiring starting at about 3,000 rpm - not your case, but - and it turned out it was that the points had lost elasticity. This was caused by a short I made accidentally which heated the points and gave them that typical brownish color. Changing the points solved the problem instantly. Again that's not exactly your case if you did not cause any short and burnt your points, and in fact you revv up to 4,500, but loss of elasticity does impair the ability of points to working fast.

Denis
1950 TD
Denis L Baggi

This thread was discussed between 11/09/2007 and 16/09/2007

MG TD TF 1500 index

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