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MG MGB Technical - Gunsons gastester, also Q re: ballast resistor

I bought a gunsons gastester yesterday cause I was tired of this fiddling about trying to get the mixture right on my carbs. I set it up, calibrated it and set my carbs up using it to 3% CO and I have to say Im very impressed. The car runs absolutely perfect now. There's no flat spots. It pulls hard and sounds fantastic. It was money well spent in my opinion. The only thing I would say is that 8 minutes isnt long enough to leave it before calibrating it. I left it 15 minutes and it was spot on then once I set it.

Another thing though, and by putting this right I think I might get much cleaner idling and smooth running... my coil is only getting 8v. The spark is quite weak for a 40kV coil so there must be a ballast resistor somewhere. Where abouts is this (physically) and what does it look like so I can bypass it or remove it?
Ross Kelly

Thanks for the review, I have been tempted, but I knew no one who had used one. Now I am tempted again...

What year is your car? The real question is whether your coil is internally ballasted or actually expecting that external ballast resistor that appears to be there. If you disconnect at least one side of the primary of the coil and take an ohm reading across the primary windings, you should see either about 1.5ohm or about 3ohms of resistance. If you have 3ohms, you should bypass that resistor, but if you have only 1.5ohms, it needs the resistor there.

If it is like the late midgets, the resistance comes in the form of a special wire which would have to be bypassed... Might be easier to buy the appropriate coil?
David "Not sure which years of MGB came stock with a ballast" Lieb
David Lieb

Ross,

On my 78 BGT, the ballast resistor is a wire in the front part of the harness. It goes from the engine bay forwards, in with the wiring to the lights, horns, etc, and crosses to the other side of the radiator grill before coming back to more or less the same place. My one is pink.

David
David Balkwill

LOL what an odd resistor! Lucas electrics never cease to amaze me. Well mine's a 78 GT and its a 3 ohm coil. I know that because it says on the coil. Its a flamethrower coil. So I'll try to get this bypassed tomorrow then as well. It might end up making a massive difference in fact! It tends to be a bit rattly on idle and doesnt fire evenly.
Ross Kelly

Ross, General Motors used the same thing in the 70's, not really a Lucas thing. The Flamethrower comes in both flavors; assuming you have the proper starter solenoid that bypasses the ballast when cranking the starter, I would suggest that you get the coil that expects an external ballast and take advantage of the potential benefits.
David "not bashing Lucas this time" Lieb
David Lieb

My 123 distributor prefers a coil with a higher primary resistance. What are the advantages with the ballast anyway? For convenience and cost saving sake, I'll be bypassing the ballast anyway but Im curious as to what it was for. To bypass it, all Im going to do, after studying the schematic is disconnect the feed to the ballast wire from the ignition relay and take a wire directly from the relay to the coil.
Ross Kelly

Ah, you didn't mention the 123! In that case, I would run a wire from the white connection on the fuse box to the + side of the Flamethrower.
David "My Midget also has the 123 and Flamethrower..." Lieb
David Lieb

The ballast is a neat idea. When the car is running and charging the coil will see 14+ volts and deliver a good energy spark. When you are cranking and really need a good spark to ignite the cold mixture the battery volts will drop, maybe down to 6, as the 100+ amps of starter current crowbars it down, so just when you need it the spark is down. The ballast resistor in circuit drops 6 volts and the coil is designed to give a good spark on the 6 that it sees when running. When you are cranking the ballast is bypassed and so the coil still sees the 6v it expects and so you get a good spark and easier starting. That said I have no probs starting without but I do keep the car on a smart trickle charger now.
Stan Best

oh and a ps, there are a few watts dropped in the ballast and the resisitive wire is the cheapes way to dissipate them as they are spread out over a distance and need no thermal managment hardware.
Stan Best

Ah ok. It is a clever idea alright. I'll give it that. Its not going to work for me though so I'll go with the idea of taking the wire from the switched live on the fusebox. I'm going to do that after work and see if it makes a big difference. I reckon it will, especially cause Im using 3 prong spark plugs.
Ross Kelly

A 78, in fact any rubber bumper if wired as per the factory, should have a *1.5 ohm* coil as it also have a ballast resistance in the harness. A 3 ohm coil is for chrome bumper cars that don't have the ballast resistance. Rather than bypassing the resistance really you should get the correct coil, as it is part of the system that boosts ignition voltage on cranking for easier starting. Rather than being as low as 6v during cranking the norm is about 10v - with a good battery anyway - and so during cranking when it sees all the battery voltage there is the coil gets *more* voltage than when running and hence a better spark.

Whether you use a 3ohm and no ballast, or a 1.5 ohm with ballast is irrelevant as far as the 123 is concerned, as the ballast is also about 1.5 ohms and so the 123 'sees' the same conditions.

I've had a Gastester for many years as my V8 has to meet an emissions limit. It seemed fine for many years, but then I started noticing it gave different readings if tilted slightly (the sensor didn't have any water in it), higher readings if sitting on warm tarmac as opposed to cold, and even worse I couldn't calibrate it down to the required 2% starting level. I compensated for the latter by making sure it didn't go more than 4% above whatever I *could* calibrate it to, but found I was always reducing the mixture by the same 1/4-turn or so from ideal running (the V8 has K&Ns and tubular manifolds which need a richer needle and have upset things a bit), and as I get a print-out of the actual from the test station I was always getting confirmation that turning it down by that much was all I needed to do. Since then I just turn it down for the test, turn them dup again afterwards, and haven't used the gastester since. I did write to Gunsons about the problems I was having but heard nothing back.
Paul Hunt

I rechecked and in fact I was getting 12v at my coil so maybe the earth I used for my meter was no good the last time. So obviously the ballast was removed at an earlier stage. Anyway the car is running great now so Im happy. Well almost... Ive a different problem with my indicators now which Ill say more about in a different thread.

Paul, with the gastester, did you leave it on for around 10 minutes after every time you used it? The hot wire might be all coked up on it. Just a thought.
Ross Kelly

This thread was discussed between 06/06/2008 and 08/06/2008

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