MG Midget Forum

Hi gang: I need some help with next steps here please. Recommissioning my donor car, the engine started first time but I have the following issue:

Major misfire / tractoring / running on 3 (2?) cylinders when under load. Coming up a hill in 2nd I can find some torque by juggling the throttle to a sweet spot but not much.

The engine idles sweet and revs freely in neutral

Diagnostic steps

The SUs DID need a tweak (rear was so lean the car was running on) and are now pretty-well set up (feel free to grill me on this)
SU dashpot oil experimented with: no change
Strong spark at all cylinders. No arcing or night-time glows
All spark plugs rotated out
Fuel flow strong - starvation does not seem to be an issue. I cleared a possible kink in the replaced front jet pipe that was too long
Fuel brand-new high-octane into a clean tank
EI timing, including "centrifugal" (speed, presumably) advance all good
Rocker gaps set to 12 thou
Air filters off (obviously)
Exhaust seems to be clear
Compression 13-12-12-13 Bar (call it 195-180-180-195 PSI)

I'm completely stumped: my next step is to swap the whole EI dissy into my running Midget but obviously a bit loath to do that (I value my knuckles). any other ideas?

Thanks for your help

WP

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Mk3 Midget 1970/1 UK spec. 1275 GAN5996*** and 2RWA wrecks "The Spice Girls" - "2 become 1"

Is it possible one of the floats is binding either open or closed?

Your SU pistons could be hanging up

Take them out, clean them up, perform the drop test (not the four foot kind) with the dome and match them up

Plenty of information here on how to perform the test

You are obviously well on top of things with what you have done and that it runs at all.

My initial thought is an air leak, easily checked with a spray of carb cleaner or damp start.

A change of dizzy is a good idea as it would at least eliminate ignition from the list of causes.

More deeply, I would run through the Burlen Fuel carb worksheets to check and tune each carb and then the pair. Damper oil would not make much of a difference if there was a problem missed earlier in the tuning process.

Misfiring under load is often a symptom of faulty spark plug wires.
Showing a strong spark while the engine is cranking over or idling does not prove the spark plug wires to be good when the engine is under load.

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In 1974 when I was 18, I passed on buying a running and driving 1966 E-Type coupe for $1,500, which is what they were selling for at the time.
Fifty years later, I'm still kicking myself...

Hi gang, especially @mgsprite , @TG2 , @ACW270 , and @pbcjr . Some great pointers up there. I have now been through them all. And tried a new coil for good measure. Tom: I am wearily familiar with making new (to me) SU pistons drop: this pair were no exception. I worked out there are 8 combos. I seem to inherit carbs where random chance has been used. Part of the fun. I changed the oil in the dashpots as well: the front one had the consistency of grease!

I am eying up the (Accuspark) EI. The spark is different strengths across different cylinders. Very suspicious. So is the fact that for the right advance I have the dissy rotated to the point where the vacuum chamber is against the oil banjo pipe.

With the car having sat a long time I can see how the poles of the rotating magnet might have changed and thrown off the timing and/or the reliability of the spark. So I am going to swap my spare points and condenser in there tomorrow (rather than swapping out the whole dizzy).

Wish me luck, thanks for the help so far and any other ideas please let me know.

W

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Mk3 Midget 1970/1 UK spec. 1275 GAN5996*** and 2RWA wrecks "The Spice Girls" - "2 become 1"

Update - then probably quiet for a week or so.

I got up and running with points (until the cam follower melted: are the coils different between EI and points? Please let me know) and it became clear* that the distributor drive is out one tooth clockwise so I cannot advance the ignition properly. I have a 4" 5/16 UNF bolt on the way and obviously need to sort this before I can go any further. All a bit scary really if that and the SUs reflect the standard of work done on the car. Oh well...

*WOW this forum is invaluable!!!

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Mk3 Midget 1970/1 UK spec. 1275 GAN5996*** and 2RWA wrecks "The Spice Girls" - "2 become 1"

Yes - The coils can be different.
It depends on the manufacturer and the specific model of electronic module, IIRC.

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In 1974 when I was 18, I passed on buying a running and driving 1966 E-Type coupe for $1,500, which is what they were selling for at the time.
Fifty years later, I'm still kicking myself...

Another update. Not good so again all ideas gratefully received.

The pause was while I got a 5/16" UNF bolt to stop the dissy drive falling into the sump (thank you, mgexp.com!) and after a couple of goes I have got the phasing of said drive right, turning it (and thus the dissy) one tooth anticlockwise. (There are 9 teeth so each is 40 deg at the dissy, 80 at the crank).

Now the dissy sits right and I have the right range of timing adjustment. Before I was not sure I could advance enough as I hit the oil pipe. (So we can say 90% of SU problems are electrical, and 5% are dissy drive rotation. How the hell that happened I'll worry about in another life.)

But no improvement. I'm completely stumped and am going to retire hurt. Bad misfire on any kind of exertion, ignition debugged to death and fuel flowing like water. Sigh.

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Mk3 Midget 1970/1 UK spec. 1275 GAN5996*** and 2RWA wrecks "The Spice Girls" - "2 become 1"

I think that your problem is the ignition timing so I'd go back to the basics and set the static timing to 7 deg BTDC. You can also use a 12 V light connected in parallel to the low tension lead on the distributor and ground to set the initial timing. This is outlined in the workshop manual
The car may idle at very low sped at this retarded a setting but take it for a drive and see if it's still misfiring. After that you can advance the timing a bit at a time to see what happens to the misfire.
This way you will have at least ruled out timing issues.

William, I understand your frustrations fully. I had a slightly similar situation when we first got our "MarGarita" Midget. When I advanced the throttle, it would seem to starve itself out of gas, and power seemed to be lacking. Never having had a car like this before, I thought it was just old, like me. I was trouble-shooting without knowing anything. I started the car, and manually tried to watch what happened when I pulled the throttle cable. I know it sounds weird, but when I did that, I noticed that the connecting shaft between the 2 carbs was slightly slipping out of position. When I looked further, I found that the front carb would not advance when I pulled on the throttle cable. The pin and lock assembly was loose. After tightening the screw that links the 2 carbs to the throttle cable, all worked as advertised. That was before I joined the mgexp, and I thought I was a genius for figuring that out. It was so simple! Ha! I'm discovering I'm definitely not a genius, just a lucky guy who found an easy solution.

How clean was your gas tank?

Did you just suck the old gas out and fill with new? Or did you remove the tank and clean it like that? Cleaning a suspect tank on a car is tough to do, lot of places crap can hide. What does the inside of the tank look like? Any rust spots?

What do you know about its history? You could have bad adjustments on top of bad adjustments on top of bad adjustments, before you even got the car.

Like stacking tolerances.

This is how engine rebuilds start