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MG MGA - Ignition switch by-pass

Hi guys, as an MGA owner I've found this forum invaluable during my 1958 Coupe restoration, but this question concerns problems I've been having with my MGC. Although I'm more than happy with driving my MGA, I also enjoy the odd trip in my MGC, but a persistent problem has caused me hours of grief and quite a lot of expense. There is so much knowledge on this forum, that I hope you guys can assist with solving my MGC problem as I'm sure it could also occur in an MGA.
The problem concerns rough running, but intermittently. The car can run perfectly one day, then sound as if it's running with one or two cylinders not firing on the next day.
I've replaced the distributor with a CSI unit from Moss, the coil, plugs, a full set of leads, the ignition switch, the heads been off to check that a valve hadn't burnt out and been subject to a full overhaul before refitting. There's no blockage apparent in the fuel lines, as I have a glass filter fitted near the carbs and I can see the fuel is adequate.
I want to try one other thing, and this is where I need your input. I want to eliminate any possible problem with the feed from the ignition switch to the coil by connecting the coil directly to a 12v supply, probably from the starter motor connection. I realise that the ignition switch will not then be able to turn off the engine, but I have a battery isolator switch which can be used to do that during the test period. Am I in danger of causing any damage to the electrical system by trying this. I'd appreciate your input, and sorry for such a long-winded post. Regards David
D Jacobs

Shouldn't cause any problems...when I test started my TC (not Twin Cam) I didn't have any gauges or switches...hard wired everything.
Gene Gillam

I'd be tempted to fit a simple on-off switch inside the car with your new wiring from the starter to the coil---reason being that although you turn your isolator switch off, all that does is disconnect the battery from the system--
--With the engine running and turning the battery isolator off the generator/alternator will still produce current and the car will continue to run until you turn the ign. switch off as well--but if it's hotwired and the ign switch isn't in the circuit it will just simply keep going until the ign circuit is opened
Also you run the risk of damaging an alternator by diconecting the battery while running
A simple switch in your test wire is the answer

Also--If your car has a brake booster it might be worth blocking off the vac hose for a try--If there is a sticky/leaking valve in the booster, then that 'could' cause an idle issue ----- just a random thought

willy
William Revit

What Willy says about the isolator switch. They should only be used when the engine is not running, unless you have the 'race' version, which also cuts the ignition and ground the alternator output through a resistor.
Dave O'Neill 2

Thanks for your help guys. I'm also interested in trying Willy's suggestion about the brake booster (servo). Would blocking off the outlet from the inlet manifold achieve this, or should I by-pass the servo from the brake system by making up a short piece of brake pipe to join the servo inlet and outlet together? The problem with that is I'd have to bleed the system, I guess. Regards David
D Jacobs

David
The booster thought was that it might be sucking air intermittently
Simply removing the hose from the manifold and blocking the port off for a try is all that is needed--no need to get into the hydraulic side of things
Carefull if you drive it with the vac. disconnected though as the brake pedal will be a lot firmer than with it hooked up

Another nasty little thought I had was that maybe it has a camshaft problem
I havn't had this happen on a C but definately on a couple of B's
What happens is the lobe wears and also the cam follower can wear so that the surface of it is uneven and as it rotates in the engine the tappet clearance changes and can cause misfiring----usually if this is a problem you will hear the tappets changing sound from tight to rattly as the follower rotates
Probably if you hold it just above idle, like at about 1200/1500 revs if there is a problem there you will hear the rockers sort of cycling through this loose/tight thing
If the sound doesn't alter at all there probably and most likely isn't an issue there
Just thought i'd mention it though just in case you had heard some oscilating type noises from the rockers
willy
William Revit

Hi Willy. Thanks for your reply. I'll certainly give the booster-isolate idea a try and report back. Regarding your thoughts on the cam, I've recently had the head overhauled and I never thought of removing the cam followers to check their condition - too late now, but I'll readjust the tappets as they are a bit noisy and listen for the change after that. The rough running comes and goes - some days I can drive 40 miles and it goes like a dream, and on other days the rough running problem returns. Would I be right in thinking that if it was a cam issue, then the problem would be occurring more regularly on every trip?
Regards
David
D Jacobs

Yes David, you are correct in thinking it would be more noticeable than just every now and then---It's fairly rare and probably not the cause of your problem if you can go 40 mile without a problem--If it were a follower problem, you tend to feel it more, like when sitting at the traffic lights waiting and the sound/missfire will cycle in and out while sitting there
Just thought I'd throw it in there in case you had heard some strange valve train noises
willy
William Revit

Have disconnected the brake booster and been out on several longish runs covering a total of over 250 miles with no sign of the intermittent problem.
Willy, it looks like your hunch was right. So do you reckon it's the non-return valve on the booster causing the problem, or the little double valve assembly in the valve chest? I'd appreciate your thoughts.
Regards
David
D Jacobs

David
If that has eliminated the problem---
Next I'd refit the hose and see if it returns
Then double check by blocking off again, just to make sure
If you are convinced this is where the problem is then,

The check valve won't be the problem as it is there simply to stop the booster loosing vac. when going uphill or when you turn the engine off--unless the plastic of the valve is badly cracked and leaking vac. externally
The problem would have to be either a split hose between the booster and manifold or an internal problem with the booster
At this stage, I'd give the hose a good checkover and if that's ok put it back together and start it up and get it missing then while it's missing have a good listen around the booster and give the hose a good pull around, listening for a vac leak to lead you to the problem area
If the hose is good it almost has to be a fault with the valving in the booster

willy
William Revit

Thanks for that Willy. I'll get time to give this a try over the weekend. David
D Jacobs

Willy. Thought you might like to know that have tried your suggestion of replacing the booster hose, and so far the car is running ok. I'll need to take it out a few more times to make sure that the intermittent fault doesn't reappear, but I'm keeping my fingers crossed. Thanks for your help.
David
D Jacobs

Cheers -- Good Luck
willy
William Revit

This thread was discussed between 13/05/2018 and 28/05/2018

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