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 Midget and Sprite Technical - Poor running

Hi All,
No nothing to do with the London Marathon at the weekend. My new aquisition is not running at all well which has frustrated me big time. Let me give you the background.
The car is a 1275 mk3 with a fairly new Ivor Searle Remanufatured leadfree, standard engine. It has a LCB exhaust manifold, sports exhaust box and a weber 45 dcoe carb. It also has a 45d4 Lucas dizzy with the numbers etched off. The dizzy still uses points and condensor which look new as do all the other ignition bits.
When I picked it up it had a flat spot between 1500 and 2000 rpm and went ok up to around 60mph on the test drive.
I drove it the 200 miles home ok although it did not like going much higher than 60 as it started misfiring.
Since getting it home I have tried to sort the running and made it worse.
I have checked the points gap. It is fine.
I have tried to time the dizzy but there are no marks on the timing cover so I am going to have to find tdc from down the spark plug whole (unless someone can tell me an easier way).
I have replaced the fuel pump. It was only producing 1.8 psi, now it is up to 3.5 (after the regulator).
Carb jetting is -
Choke 34
Main 135
Air corrector 180
Emulsion F2
pump 50
idle 50F8
Aux 3.5

I have tried F2, F8 and F9 idle tubes all on a 50 jet and swapped from 150 to 135 main jets.

The problem I have is that I need to get it running over the week end well enough for a trip up to Peter Bugress for a new head, 123 tune and rolling road session (about 200 miles) on the 30th.

I have several options.
1) Keep fiddling with the carb and timing to try to get it near enough
2) Put on a pair of good HS2s and original dizzy and set up as standard (If I can time it in ok).
3) Any combination of the above.

Does anyone have any advice? I suppose without knowing the timing of the dizzy I am going to be guessing everything else so I should get this spot on first. Should I be using the newer 45d4 dizzy or an old one that may be worn but does have an electronic ignition mudule fitted? Should I continue with the weber of unknown history?
Too many questions. Are there any answers?
Thanks in advance for your replies.
Dave
Dave Brown

nothing wrong with 45D4 and points, introducing electronic module just gives another unknown. Yes find TDC and time sparks from there. As long as spark is about right and mixture not too weak it will be OK to drive to Peter's. If Pete can't get it right then you may need to drop back to twin SUs (or a big single).
David Smith

Dave,
Having elsewhere argued against "shot in the dark" diagnosis, can l ask if you have checked the tappet clearances. If they are tight it could explain the apparent misfiring. I only mention this as you do appear to have checked most other things.

I would however suggest you find a way of checking the ignition timing. 200 miles is too far to go without potentialy causing damage if it happens to be over advanced. Are you sure the timing marks are missing - you do know that they are under the crank pulley and not in the easiest of locations to see !
Guy

Hi Guy,
I forgot to mention that I did check the valve clearences and they were all a little loose. I adjusted them all to 12 thou.
I also checked the compression and they were between 185 and 195 psi which I thought was a little high for a standard engine.
I have timed a midget in before and know where the timing marks are/should be and I know how awkward it can be to see them, but I am sure they are not there. I will have another look tonight though.
Dave
Dave Brown

OK, valves was just a random suggestion, that as I said, went against the logical approach that I always try and favour!

And I wasn't meaning to be cheeky about finding the timing marks! Sorry. I would in that case definitely use pencil down the spark plug method that you mention to find and mark tdc, at least to get a check on it.

Have you checked for erratic waving of the tachometer needle when the problem occurs?
Guy

Guy,
I didn't take any of your comments as cheeky, so no need to appologise. I too realise how difficult they are to see and I always struggle to see them. In fact you have made me question myself. Did I look hard enough for them? So I am going to double check tonight.
The Tacho needle is rock solid so I have ruled out electrics for now.
But I may check the engine earth strap just in case Nigel ;) (I am sure he will be along in a mo).

Dave
Dave Brown

Have you checked the plugs? - I don't mean for being faulty themselves, but a diagnostic clue? I would run the car until it was misfiring, persist for a few minutes at that speed/ condition then clutch down, ignition off and coast to a stop. Remove plugs to see what colour they show.

Also, when it is misbehaving does adding a bit of choke make any difference?
Guy

I have had another look at the car tonight and I hate timing Midgets.
I do have timing marks on the cover. I just could not see them as the brake pipe was in the way. I didn't realise that the brake pipe was there as it all merged in to the cross member!.
Anyway, I set the static timing to 7 then checked the dynamic timing at 1200 as the handbook says and it showed 40. I then ran out of time and gave up.
I messed something up and will recheck tomorrow.
Setting the static timing did retard the ignition a little (dizzy moved anti clockwise). So I took a little road test. A little better now, it drives ok with very little throttle up to about 40mph but a little too much throttle and it misfires and continues to even at full throttle. Pulling the choke on makes on difference.
Dave
Dave Brown

Hi Dave

Firstly the setting of the timing should not be done by static. That will just get the car started and the value of static timing is totally dependent upon the advance of the dissy which can be an unknown.

The timing should be set to about 32/34 degrees before TDC at anything over around 4500RPM. This figure will only change if the engine has very high CR and requires less advance to quench any possibilities of detonation.

However I believe your major problem is carburation. Typically your engine is demonstrating what happens when a weber's main jets are not working correctly and the engine is running solely on pilot jets. Normally in these cases a top speed of about 60MPH is just about possible if the throttle is progressed very delicately :)

Your main jets appear to be about the right size for the choke size so will not be the problem you are experiencing so it must be that fuel fom the main jets is not reaching the venturis. This is normally caused when the venturi locating screws on the bottoms of the weber barrels have either fallen out or have not been put in correctly. These screws hold in what I call the flow straighteners (in the throats of the weber) that contain the injection points for fuel into the carb chokes. If these are not oriented correctly then fuel can not make its way from the main jets to the throats of the carb. They should line up with holes in the carb and also the dimples in these will be in line with the locating screws on the barells of the weber

Hope this is straightforward and you can get sorted.
Robert (Bob) Midget Turbo

Hi Bob,
My plan was to check the timing in the morning(I don't think I am too far out and I am off to the rolling road soon) and then swap the carbs for 2xHS2s. Once the weber is off, I will have a good look at it as I too am thinking that it is this that is causing the problem but I have not got the detailed knowledge you have. I have a feeling the HS2s may stay on until I get to Pete's.
Thanks
Dave
Dave Brown

Hi Bob,
Have you been round to my house when I hav ebeen out, playing with my car?
I have just removed the weber and stripped it and this is what I found - see photo.
Your diagnosis was spot on once again. Thank you very much.
Dave


Dave Brown

Thats good mate and I am very pleased to have helped

Sadly the reason I occasionally get the odd diagnosis correct is because I have messed with these things for many year.

This means I am getting far too old

And secondly I have made far too many mistakes during these years.

Both these are not things to be pleased about, :(
Robert (Bob) Midget Turbo

This thread was discussed between 20/04/2012 and 21/04/2012

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.