MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Testing a Rotor Arm

My V8 hasn't been running well and I was with a guy this afternoon who tunes cars for a living. He thought the spark was weak at the plugs, but the spark coming directly out of the coil was strong. He removed the dissy cap and held the coil HT lead about 1cm away from the metal top of the rotor arm and got me to crank the engine. A fat spark jumped from the lead to the rotor arm.

Surely this shouldn't happen? That must indicate that the rotor arm is tracking to earth doesn't it?

Thing is, this is a brand new 123-Tune dissy. Is that a fair test of a rotor arm, because if it is, I will need to get onto the supplier.
Mike Howlett

Yes, I have used that method to prove that a rotor arm is faulty.
Dave O'Neill 2

Yes, standard test, as described in Lucas manuals ( and in any number of books, magazine articles, etc. )

The Lucas instructions state a test gap ~ 3mm ~ 1/8".
( So I don't know if 1cm suggests a particularly strong spark, or a particularly bad rotor arm, or both ! )


J N Gibson

1cm indicates a strong spark, but I'm quite surprised the car runs at all. The failure inside the rotor must be just about equivalent to the plug gaps, so it's hit and miss as to whether it jumps the rotor or the plug. Ordinarily it will only jump the shortest of parallel gaps, as the voltage rises over time until there is a path to earth, then it stops rising.

For some time my V8 wouldn't cold start unless I put new plugs in. Each time it happened I had to be somewhere so only had time to swap the plugs and not do a proper diagnosis, and was having to swap plugs every few k. Until one day I did, and found (with a timing light) it was the cap breaking down, despite no visible evidence. In my case as the plugs age the voltage needed to jump the gap creeps up, and eventually jumped the cap gap instead of enough plug gaps to get it started.

Doesn't say much for your new distributor! Is the rotor red? Distributor Doctor started making red rotors some years ago as the standard black ones had become useless. Said to be due to a high carbon contact from the carbon black to give the colour, but carbon black is not conductive, it's effectively soot. I put it down to the rivet securing the blade which original rotors didn't have, being too close to the spindle as in the attached on the left, which is inside the base circle of the rotor. The one on the right still has the rivet but it's outside the base circle so should be OK. Dist Doc's red rotors have the blade moulded into the carrier as originally, i.e. no rivet. The problem is that due to the success of those every Tom, Dick and Harry started making their rotors in red, but to the same standard as before i.e. just as bad from a functional point of view.

If you have to buy a replacement then have a look here http://www.distributordoctor.com/rotorarms.html. But there is a complication. Apparently there are two different types for the V8, and because of other people making red ones which are a 'half-way' house between the two types which don't work properly with either, he is making the two types in black rather than red. A further complication is that whilst he says which to use for the original distributors, you don't necessarily know which you need for your 123! However he may, so talk to him.



paulh4

Hi Mike
What sort of running problem/symptoms are you getting?
We have noticed the 123s seem to have different rotor arms in lately. The earlier ones had good looking Beru ones in.
Peter
Peter Burgess Tuning

Just looking at the instruction manual for the V8 tune on the 123ignition website it shows a Beru NVL 177 (stretched out black rotor arm).

The genuine Beru and Bosch arm I've bought have the manufacturers model numbers on them and look and feel better finished and studier. For example, for my 123 (non-tune) the Beru arm is 26g against "red" rotor arm from two suppliers that are acceptable appearance but not as neat finish and weigh 18g.

(scroll to bottom) - http://www.123ignition.nl/files/manuals/123manual_TUNEPLUS.pdf

ETA: Just seen Peter's post, perhaps they've changed, they might not have but most manufacturers don't keep their websites up to date even the very large wealthy ones let alone smaller or sub-divided concerns.
Nigel Atkins

Mike,
it looks like it might be arm 54401050 / 54424762 / DRB195 of the Dissy Doc's range (but check) which shows as available now, I'd go with Paul's advice and buy from that source.

ETA: I've had a dissy cap that was a problem in the past, on a RV8, and have read for many years about rotor arms so have been carefully to regularly check or change both and buy with care.

The rotor could be your running problem from previous thread.

Nigel Atkins

"regularly check or change both"

Aaargh no! If it's running right leave it alone! Good ones will last for ever - apart from the sprung contact in the cap which is under very light load there is nothing to 'wear out'.
paulh4

I have an original part in my V8 that has lasted since I did the conversion 30 odd years ago, it is still working fine and it was secondhand with the engine even then!
Chris at Octarine Services

Here is a picture of the rotor arm as supplied with the 123-Tune dissy.

As it happens I have a Distributor Doctor rotor arm for the 35DLM8 dissy. It is even moulded with DD on the top, and it is black plastic. I have put that in but was unable to do the spark test as I had no assistant to turn the key while I held the coil lead near the arm. My arms didn't seem to be long enough!

However, the engine still has the same issues. It is perfectly driveable but feels like it misfires, particularly at idling speed. Pulling off plug leads, or even disconnecting various injectors, makes no difference to the way it runs, which seems very strange. It also has a slight hesitation when you press the loud pedal.

Peter B, I would welcome your input and will happily send you the full spec of the engine and injection system if you can tell me where to email. As you know, I am using your cylinder heads on the car, which has now covered 33,000 miles since I put it together. Gee, I wish I lived closer to your workshop!



Mike Howlett

Mike
Have you tested for manifold gasket leaks
With that rotor button, I'd borrow someone's fingernail polish or some epoxy and fill that rivet hole up flush with the body and let it set, If you've got a decent coil on board it will be strong enough to jump a spark from the rivet to earth on the shaft
It'd be interesting to do the coil lead test in the semi dark and see if a spark is visible underneath there
"Don't forget to use plastic pliers to hold the lead"
willy
William Revit

Mike, the 123s can have a problem whereby an oil film is spread between two plates by capillary action, one of which is the circuit board plate. If you take out the dizzy, having marked it carefully, you could try giving it a really good swill out with brake cleaner or isopropyl alcohol then see how it runs. Teaching you to suck eggs but make sure the drive connects to the oil pump again.
Symptoms tend to be (and /or others!)the misfires you describe.
Peter
Peter Burgess Tuning

Thanks Peter, but I bought the 123-Tune to try and cure the problem! The 35DLM8 was 28 years old and had covered goodness knows how many miles in a Range Rover before I got it, so I figured it had to be worth replacing it with a modern bit of kit. Unfortunately the modern bit of kit has made no difference. But I can play with the advance curve!

Willy, I see where you are coming from and I'll do that, but for now the Distributor Doctor rotor can stay in place.

One guy I spoke to who runs a rolling road near Glasgow wondered if the camshaft might be in trouble, even though it has only done 33,000 miles. He suggested I remove the rocker covers and have a look at the valve lift. Good idea???
Mike Howlett

Easy enough to see them, although I can't see that causing missing and hesitancy etc. under some conditions only - unless something is sticking. There is also the question of how to do it if you have hydraulic tappets, won't the lift need the engine running to be consistent? You could use a dial gauge I suppose, and try and spot the values. I've also used a dial-back timing light to 'freeze' valves. Depending on which valve you are looking at and which plug lead you connect the light to, you should be able to see each valve (one at a time!) in the fully up and fully down position by turning the dial, with a measure beside them.

Disconnecting injectors and shorting plugs not making any difference at all is definitely strange, although a missing cylinder is harder to spot on a V8 compared to a 4-cylinder! I'd still expect a change in the exhaust note though.
paulh4

Mike, don't assume you haven't got a miss with the 123.
Cam wear usually shows as a valve bouncy rasp foot flat at high rpms, eventually as wear worsens it shows up at modest rpms even part throttle.

Peter

Peter Burgess Tuning

Paul,
my 123 at least is quite wearing on the dissy cap and rotor arm. When I forgot to check them at a service I paid for it with a slight misfire at the end of a tour and had to do a scratch polish on the arm on what would have been the roadside had I not already had two lifetimes' worth of roadside repairs so stop at a pub and looked at it after I'd had a half. I prefer prevention to the dubious joy of roadside repairs.

To be fair I remembered wrong in my previous post it wasn't the dissy cap at fault it was a HT lead, another item that can last many years but might be passed it's best for a number of those years and another item I usually change before it goes off best too far or fails.

I only enjoy driving the car, solving problems and working on my car I loathe, if I need to spend a few extra pounds on earlier replacement I don't mind - course bloody parts quality always kicks my ar*e.

Nigel Atkins

Mike,
me remembering wrong about the RV8 diss cap being a problem might be of interest to you if not directly related. The problem turned out to be a HT lead, Sod's Law as normally I would have change them as I didn't know their age and wasn't sure about their condition but I didn't give them the priority they needed.

The problem was on a 1992 TVR V8S which had the Rover 3.9 engine - twice over a periods of many weeks the engine would cut out as I was driving along but on pulling over and checking what I could it would start straight back up. After having it on a diagnostics machine it turned out to be one faulty HT lead that caused this - I'd have never thought this. That cost me more to find out than if I'd changed the HT lead set when I got the car and I woouldn't have had the engine cut out on the road, luckily both times I was able to park up easily.
Nigel Atkins

Peter,
following on from story below, unless I've got a false memory or been visiting Fantasy Island, I found out then that TVR V8S has the leads connected on opposite banks to usual Rover layout numbering, i.e. left bank is right bank. I think it's the same for the Chims and Griffs but check Steve Heath's book in case I've remembered wrong on those.
Nigel Atkins

Mike
Not always, but mostly, if it's the cam worn doing it, at idle the missfire will cycle in and out mostly noticeable sitting in the car say at the traffic lights, you will get a poof poof poof brm poof poof poof type of idle as the worn lifter rotates around into different spots and also the engine will hit a wall up in the revs and make a hammering noise through the inlet
Actually, come to think of it we've been through this a couple of months ago
Still having trouble then-----------
Are you 100% sure you got that airflow meter adjusted correctly--can you borrow another for a try
Have you tried spraying some starting fluid around the inlet gaskets while it's idling to check for vac. leaks

While it's idling -cheats method, you could try a very small spray (very small) of starting fluid into the inlet, Just a minor trickle spray and if the revs pick up a bit and the idle smoothes out you could be looking at blocked injectors causing it

willy
William Revit

Nigel - what do you mean by 'scratch polish on the arm'?

If you mean the burning that develops on the edge of the blade where it passes the cap contacts, and on the cap contacts, that is because the spark is jumping a gap there as well as at the plug and is completely normal. For 50+ years I've been seeing burning there but have never yet had to change a rotor because of it, or any other problem that developed from use.

Can't speak for the 123, but if it causes problems for that, as well as oil as Peter has said, then as as I'm concerned that's yet more reasons to avoid them.

As far as HT leads go following a post here or elsewhere I decided to check the resistance of a couple of leads on the V8 and on the roadster, to find that the V8 coil lead (the shortest) was 80k whereas the others were in the 5-6k per foot area. No running problems, replaced it, no difference. Those leads have been on since I bought the car 24 years and 100k ago. Funnily enough I had the coil lead on the roadster go open-circuit many years ago, so replaced the set. Those are the only two failures I've had since I started using silicone leads in the late 70s.
paulh4

Willy said "Are you 100% sure you got that airflow meter adjusted correctly--can you borrow another for a try "

This is the one component that I am most suspicious of. Adjusting it without a decent exhaust gas CO meter seems to be impossible. I have twiddled with the adjustment - there are multiple turns on the adjuster - but no matter how much I turn it, there seems to be no effect on the engine. I always end up turning it back the same number of turns to where it was originally. The spark plugs are uniformly pale coloured, almost looking as though the engine is running lean. The exhaust note at idle is "splashy", not at all smooth.

Any ideas the best way to tackle this adjustment? I have sprayed the wire sensors within the AFM with carb cleaner - they don't look dirty.

A pal who lives in France is bringing a spare one when he visits the UK later this month. Will that make any difference? I'll have to wait and see.
Mike Howlett

Which air flow meter is it Mike?

Peter
Peter Burgess Tuning

The hot wire one Peter.
Mike Howlett

I don't suppose it is the same meter as on an MG Maestro?
Peter Burgess Tuning

I've no idea Peter. It is this one.


Mike Howlett

Bum, not like the Maestro ones I have laying around :(

PS are you running the high pressure fuel pump with a ballast resistor?
Peter Burgess Tuning

Paul fair enough, if you say it doesn't need changing then I'll go with that. Would be nice to have your mobile number so that you can be the first to know if I ever get another roadside repair from it. :)

What would you do with the rotor, if anything, at services then?

The 123 I have does seem to scar up the posts on the dissy cap.

As for leads, on the V8S it caused the car to suddenly cut out, not a good idea. I go by just because something is working doesn't mean it's working passed its best and optimum, I expect a sports car to perform very well not just drive.

Next time I go to Peter's I'll see if I can take my old set of HT leads to see if there is any difference shown at all on the rollers, I'm sure Peter would doubt it too, no offense meant but your statement of no difference is just your perception unless you had other testing equipment.

Nigel Atkins

Peter, I'm not sure what you mean by a ballast resistor. I have a low pressure pump feeding a swirl pot, and a Bosch high pressure pump feeding the engine through an adjustable regulator . The return goes back to swirl pot and then to the tank. There's no resistor in the circuit.
Mike Howlett

Just checking Mike. The old efi systems I worked on used a ballast resistor on the fuel pump, whne the pump became a little tired the pressure could be upped by bypassing the resistor. if you haven't got one this will not help!
It may be worth checking fuel pressure.
Does your efi have a vac operated system for lowering pressure when not foot flat. The older efi I am used to had a valve operated by vac on the fuel rail.
Peter Burgess Tuning

Thanks Peter. There is a vacuum line connected to the pressure regulator - is that what you mean? I have an adjustable fuel pressure regulator with gauge and it is set to the recommended 2.5 bar with the its vac line temporarily disconnected.
Mike Howlett

It may be worth trying more pressure and/or driving with the vac removed to see if that makes a difference.
Peter Burgess Tuning

Just a thought,
If it is an airflow meter problem, they usually go off lean----same with clogged injectors
Just for a try, if it's sitting there idling fluffy--gradually pinch off the return hose from the reg to the swirl pot, If it's running lean this will temporarily richen the mixture and it should smooth out ----IF lean mixture is the problem
Can't remember what plugs you have in there but have a little read of the -sparkplug- thread in this MGB Tech. section
First time I've ever had this happen, so don't discount the plugs being an issue
I'm really wary of using resistor plugs now

A bit semi off subject but worth keeping in mind--

willy
William Revit

Plugs are NGK BP6ES which I have used since the car was built. They are currently new.
Mike Howlett

All good, it was just a thought
Your plugs being non resistor shouldn't be affected by this problem
William Revit

This thread was discussed between 02/10/2018 and 06/10/2018

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.