That awful colour MGB GT

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Re: That awful colour MGB GT

Post by BenHar »

PhilA wrote: Wed May 05, 2021 4:15 am I had similar stupid issues with my timing and crappy running due to the quite major vacuum leak through the vacuum pump, which gave impossible to tune idle and poor low to midrange performance and economy.
"vacuum pump"?

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Re: That awful colour MGB GT

Post by PhilA »

BenHar wrote: Wed May 05, 2021 7:17 pm "vacuum pump"?

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For vacuum wipers; things like bad pcv valves, brake boosters etc can also have a similar effect. Vacuum leaks in general.
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Re: That awful colour MGB GT

Post by SiC »

Last night was a rare time in the last week where it wasn't peeing it down. Rather annoying as the car has been out in this weather all week. Not had a chance to run it down to storage yet either.

I've been wanting to fit my 123 Ignition unit for a while, but not had a chance. This one is a fully programmable unit that the curves can be altered on. The idea is that eventually I can run the car down to a Dyno and have it tuned up properly. Problem is, there is no curves supplied and you have you make your own. So after a fair bit of reading on exactly what this unit works on (basically crankshaft advance), I made up two curves - one for the Lucas 40897 that everyone goes on about and the Lucas 41391 that was originally on the car. These have a wire that you can switch between two curves. At the moment I've hard wired it to 40897 but may rig up a switch eventually.

One thing of note I've read is that under 500rpm (i.e. engine starting) it basically doesn't apply any advance at all. This is important as you can't use a static timing of 0 degrees if you want reasonably easy engine starting. I used 5 degrees static as that allowed me to fit both of the above curves in. Some seem to use 10 degrees static and I fully expected to redo them to have that. However in the event, it hasn't seemed necessary.

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First thing was removing the old dizzy.

After taking it out and looking at it, I noticed that the points wire had a nick out of it. Possibly this got caught after the cap had been off previously and this caused the hesitation? Plausible that if it was stuck, it would stop the vacuum advance plate moving properly.
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Old dizzy is going in the boot fully prepared as a backup. I don't ever expect to use it, but always useful to have there.
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123 unit went in easy enough. I did lube up the shaft, especially around the o-ring. It went in as a snug fit so well worth doing.
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I took the time to wrap electrical tape along the length of dizzy wires (there are four of them) and zip tie it to the existing loom. Keeps the installation tidy and less likely for accidental snagging of the wires.
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This morning I gave it a quick start. Started up perfectly and ran far smoother than it ever has done. On the original Lucas dizzy there was always a slight missfire and stumble that was especially pronounced at idle. Completely gone now as far as I can hear and revs very cleanly. Not sure if it's me but sounds a tad less tappy too...
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Re: That awful colour MGB GT

Post by The Reverend Bluejeans »

Points and condensers are so utterly shit. Like carbs, there's a good reason why they were abandoned and not a day too soon.
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Re: That awful colour MGB GT

Post by Hooli »

Points are great, the problem these days is trying to get decent ones.
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Re: That awful colour MGB GT

Post by The Reverend Bluejeans »

They're shit. Unreliable, maintenance intensive and inaccurate.
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Re: That awful colour MGB GT

Post by The Reverend Bluejeans »

Bosch Motronic arrived in '79, combined fuel and ignition. Probably the greatest single leap forward in car technology.
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Re: That awful colour MGB GT

Post by Hooli »

Still better than a lot of after market systems, as they work & are reliable.

That's why I upgraded my Bonnie back to points, the after market (well spoken of) replacement was a bag of wank that kept breaking. Points gives me better running, easier starting & has cured the main source of breakdowns that bike had.
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Re: That awful colour MGB GT

Post by SiC »

I like SUs for their simplicity and getting the job done. But then I'm using them as a toy. I'm sure my opinion would be different if I was trying to start a car on sub-zero morning when already late for work.

The 123 dizzy however is a big improvement over a Lucas electromechanical unit. It current senses the coil, so can properly auto dwell, multiple configurable timing points allow much greater flexibility, MAP can be configured for both vacuum and boost if an engine is turbo'd and the general construction is far superior too (appears to be fully machined stock rather than cast). As there is no side loads on the shaft from points pressing against it, there is going to be minimal wear. Parts wise it uses the Bosch or Beru parts commonly used on German stuff. This is good as those manufacturers still have the proper know how for exactly making ignition parts, unlike the cheap Chinese sourced Lucas and Intermotor branded stuff on a 25d.
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Re: That awful colour MGB GT

Post by Hooli »

The only thing I know of simpler than a SU is an Amal carb on bikes, think the same thing without the oil in the dashpot & no butterfly (the cable lifts the slide directly). They do a good job too, I'm not a fan of complicated stuff for the sake of it.
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