MGB & GT Forum

Hi - I'm experiencing cranky cold starts on my 77 MGB Roadster. When she does start it sounds like she's running on two or three cylinders for about twenty seconds, then after a few throttle blips she settles down to normal running. On checking I noticed the gaps on my NGK BP6E plugs looked a bit large and I wondered what they're supposed to be? I'm running on electronic ignition.

Thanks

.035"

Andrew

It depends on a LOT of factors. Factory spec is a very conservative .025. .030 to .035 is more typical IF everything else is up to snuff. "right" is what runs in your car.

By "electronic ignition" that can mean either a simple igniter driving a 3 Ohm coil, or it can be a high energy ignition driving a low Ohm coil. It matters a lot.

Let me assume simple igniter. Otherwise you would not be having the question. If so, you have slightly less energy available than new points, so the same overall formula for both:

NON-RESISTER plugs.
Quality low resistance helical core suppression wires
Alternator putting out the correct 14.3V solid
Good tight connections from the battery to the ignition switch to the coil/Igniter
Coil on the higher end of the primary impedance side. My preference is the Bosch Blue made in Brazil ( not Mexico) The Pertronix is my second choice followed but the stock Lucas DLB101. Do NOT use the Lucas "sport coil" It is for over 6000 RPM running, as in sports like racing.
Good rotor without a rivet or high carbon black in the plastic. Red or blue.

Sure you don't have a bit of oil leaking down the valve guides? What does the exhaust look like as it first fires?

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Cogito ergo sum periculoso

Hey man!


.035 is what I put mine at, and they do great!

Normally it is 0.035; this is what I used for 35 years or so.

One year I tried 0.025, 0.030, 0.035, 0.040 and 0.045 it did not make much difference in term of fuel economy or even on power.
0.025 was maybe a bit harder to start when the engine was warm and 0.045 I was concerned about the distributor cap durability.

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1977 MGB Red 'Victoria' and 2018 Jaguar XE 'Meghan'
All Classics:
Married, 2 children, MG MGB May to December, Jaguar the rest of the year.

The differences have to do with varying conditions You want a car to start in Death Valley @ 2 in the afternoon and in Fargo in the winter. You want it to start hot, you want it to start cold, fresh tune up or 6K later. It is very difficult to notice misss-fires above 2000 or so. Many people are so used to miss-fires at idle they don't think about it. It is no accident one of the first things industry did to reduce emissions was to improve ignition systems. The lower the number of miss-fires, the lower the HC emissions. DUH! Fire every charge every time and you get more power, better mileage and lower emissions. MG spec of .025 was for worst case conditions, not optimum running.

One test I discovered is to put a second mark on the damper @ 180 degrees. Then with a timing light you can look at the spark of all four cylinders. I found you can notice spark dropout or excessive scatter as indicators of a weak ignition. I also found defective igniter magnet rings this way as the timing on each cylinder was not the same.

.045 ? Bet it would not start in the winter with a standard 3 Ohm ignition system. You could with the CEI as it is based on the GM HEI module and a 1 Ohm high voltage coil.

Why a big gap? Well, the goal of the ignition system is to put in enough energy to get the flame kernel up to about 0.1 inch, where it has enough energy itself to then rapidly ignite the rest of the fuel charge. Until it is that big, it is subject to just fizzling out, quench or long delay before burn. OK, so why not use big gaps on all cars? Well, the larger the gap the higher voltage is needed to ionize the gasses in the gap so the strikeover and glow ( arc) stage will happen. But, the higher the voltage, from a larger gap, means more energy is used up in the flashover and not left for the glow stage to help keep the ignition going as it transitions form endothermic to exothermic. So cars designed for large gaps have higher energy ignitions. i.e the CEI Lucas system GM HEI, Ford TFI and all the newer ones.

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Cogito ergo sum periculoso

GM's HEI started at 0.080" then quickly went to 0.060" then settled on 0.045" because warranty repairs couldn't keep up with the ignition rotors burning up prematurely.

Even the Lucas CEI was spec'd at 0.035 with a 1ohm coil.

In comparison, the GM HEI cap and rotor are massive compared to a Lucas cap and rotor.

Going any larger than 0.035" with a Lucas cap and rotor gets you no noticeable performance gain but puts you at risk of cross-firing and premature wear of ignition components.

I just installed a set on an MGB Thursday. Right out of the box, they measure .030. I have NEVER seen an NGK plug right out of the box, much different than that. I stuck them in at .030 Car ran like a top. How old are your plugs, ???????

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Robin, how old are your wires? distributor cap? rotor?

Any one of these (like a single wire) can give you an iffy start then settle down for the haul... til the day it fails.

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Michael J. Caputo
'79, '77, '76, '74.5 (rubber dual SU), and '73 owner. 12v Audio System design. Vendor of Regalia and Promotional Products since 1993. Supplier of Accessories to MOSS. Forum Member since 2009. Warped sense of humor since birth.
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I do wonder why newer cars with COP have not gone to larger gaps. My GTI is only .035. Searching I find .044 to about the max on new cars. Coil insulation breakdown maybe? How hard it is to provide protection to transistors for the back EMF glitch?

For the 8.5:1 CR, .025 plug gap takes about 14KV to ionize, .035 about 17K, and .045 about 20K on a comfortable day. When it is cold outside, it can be as much as 25KV. That is of course for a perfect system with nice clean sharp edge plugs. 9.5 takes a KV or so more.

I tried larger gaps in my Spit with an MSD and got cross-fire above .035. Our resident expert claims it was the fault of the MSD not the small Lucas cap. Whatever, It happened. It was not magic RF induced false triggering as it happened with points. If you calculate the voltages, you find it only takes about 5KV to ionize the adjacent cylinder so the combination of rotor gap and next/last cylinders not under pressure makes it attractive to cross-fire with high voltages. I believe this is why race cars with CDI reduce the width of the rotor tips. They get away with it because the run over a much narrower range of advance. Do also notice modern rotors are a full disk to prevent flash-over to the distributor body as not being under compression, an inch in free air only takes about 20KV. So too big a gap means you may find a better path to ground than the spark plug.

When gaping your plugs, the "squareness" of the electrode to the tip matters a lot. This is why most plugs come pre-gapped. They have slightly different numbers for different gaps. You have to look at the specs to find them. Modern fine tip plugs almost always are pre-gapped and they warn you not to mess with them. Just cranking a .025 gap to .035 may lead to unpredictable gaps as the plug wears.

A side note: Getting zapped with 15KV will knock you on your butt. Anyone who had been hit with secondary knows how bad it bites. But much over 20KV, it can kill you as your body has enough distributed capacitance that it will exceed the 32mA through the heart to stop it. I am not a fan of holding a plug wire near the block to see if I get a spark.

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Cogito ergo sum periculoso

Like others, I have been running my MGB and MGC with NGKs (BP6ES) at .035 for many years. No issues.

The wider gaps cause higher voltages on all the ignition components: rotor, condenser, coil, dist cap wires. These were all designed for 0.025" plug gap. raising it to 0.035", voltage stresses these components 40% more than they were originally designed for. Most don't have a problem with increasing the gap. Some do have problems. especialy in dirty, moist environments.

Scott mused "I do wonder why newer cars with COP have not gone to larger gaps." There are physical limitations on the wire insulation used in the manufacture of the coils. Typically COP's use smaller coils than traditional coils. As such, they, use thinner wire. Thinner wire has thinner insulation over the copper. The thinner the insulation, the lower the internal turn to turn breakdown voltage. This could be at least one consideration in the use of the lower gaps.



Edited 1 time(s). Last edit at 2019-04-15 10:53 AM by cschaefer.

The factory used .035 on cars with the 45D distributors.

Andrew

Wire thickness and insulation thickness are not proportional, but that was one of my thoughts, inter-winding shorting. .044 seems to be a common gap on modern cars. Of course, a COP does not have the losses of the distributor and HT leads, so it is inherently more efficient. Without the variables of the HT leads the engineer is free to manage the impedance better. On the other hand, it is back to an I-core which is less efficient than the E+I cores used in CNP and the HEI/TFI style coils. Might be why some engines use CNP.

FWIW, .025 to .035 is about 21%, higher voltage for ionization based on the Mega-squirt online calculator. Same effect, same issues. I would disagree with the "stress" issue for .035 and less. Going to 7 or 8mm wires will take the full secondary potential into an open. Not an issue. It probably was in the days of cotton/carbon and cloth/rubber HT leads with carbon black filler in Bakelite rotors and caps. Jeff's red rotors won't short to the post, and caps are fine. The Pertronix coil I run is rated at 40KV which it would only see with a CDI and an open plug.

Fun to ponder, at least for me. Ignitions look so simple but in fact are horribly complicated. Even the process from spark to complete burn is 7 stages of sequential chemical reactions. You have to strip those Hydrogens off the Carbons, bust apart that O2 and so on. People get PHD's in just that sequence of reactions! A transformer, i.e. coil, is one of the simplest to construct, yet most complex physics of any electrical component.

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Cogito ergo sum periculoso

.035
Your issue isnt with the SPG
more diagnostics are called for

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