Vac advance/distributor issues

[Mark Wilson]

I'm having what I think are problems with my vacuum advance. I've got the engine (Midget 1275) running smoothly at tickover, but it loses power as soon as I try to take it out up my steepish driveway. I've set the timing by timing light to around 13 BTDC at approx 1000 revs (can't find my digital rev counter... ) with the vac pipe disconnected and the carb connection plugged, and the advance moves to about 20 when I raise the revs to approx 2000. If I connect the vac pipe the advance increases to around 20 at 1000 revs and over 25 at 2000. It sounds to me like too much vac advance, but I'm no expert.

The dizzy is a 45D 41270 B, which was the type fitted to the Midget, and I rebuilt it with new springs from the Distributor Doctor, a new vac advance from ebay and an accuspark electronic conversion. Although I took as much care as I could, don't discount the possibility that I made a total mess of the rebuild, I've no previous experience of distributors!

The engine has a standard cam, unleaded Mini Mpi head, HS4 on mini manifold.

Although I took care to find a dizzy with theoretically the "correct" advance profile I do realise there's no such thing as correct for a non standard engine. I'm tempted to cut my losses and buy a new Accuspark dizzy, knowing that people have had decent results from these in a variety of setups. I'm not looking for race performance!

[philthehill]

The distributer should be a 23D4 or 25D4 for the 1275cc Midget.

The 45D4 distributer was fitted to the 1500cc Midget.

The static ignition timing for the 1275cc Midget should be 7 degrees before TDC.

Dynamic timing should be 22 degrees BTDC at 1200rpm...............23D4 distributer. Vacuum line disconnected.
Dynamic timing should be 13 degrees BTDC at 1000rpm...............25D4 distributer. Vacuum line disconnected.

[liammonty]

If I'm not mistaken, the 23D4 that Phil has posted data from is a non-vacuum advance unit?

25D4 or 45D4 makes no odds - it's the curve that's important, as you've said. What you didn't mention, though, is whether your issue of the engine hesitating is resolved when you have disconnected the vacuum advance.

You need to remember that with vacuum advance, you'll see a lot when you are testing it as you are, with the engine under no load, as you're using extremely small throttle openings. As soon as you give the car some welly (such as up your drive) the throttle opening will be much wider, hence less vacuum advance. Vacuum advance is LOAD sensitive, whereas centrifugal advance is engine SPEED sensitive. Make sense?

Is there any chance you've introduced a problem somewhere else, and the distributor is a red herring?

[Mark Wilson]

Apologies, the distributor is indeed a 25D 41270 B, dated 05 72. So the mechanical advance sounds at least okayish?

I haven't been able to test the engine under load yet. Sob story alert - I've been on crutches since being on the losing side in an argument between my push bike and a van before Christmas, so haven't been able to drive it. I've only just made it back into the garage this week! Things are improving though, and I expect to be able to test it in the next few days, so will let you know how it goes with the vac line disconnected. The explanation of the difference in operation between the mechanical and vacuum is enlightening, not something I'd thought about properly before.

I'm not totally wedded to my theory of the distributor being the problem, but it did seem a strong possibility which should be investigated first. I do want to get it to a rolling road where the carb needle and settings can be looked at properly, but don't want to do this without ruling out faulty components.

[philthehill]

The 23D4 distributer (without vacuum advance) was originally fitted to the early 1275cc MG Midget as the engine was comparable to the Classic Mini 1275cc Cooper S which was also fitted with the 23D4 distributer. The materials used in the manufacture of the first 1275cc Midgets was the same spec i.e. crankshaft EN40 nitride and the conrods were the same as the Classic Mini Cooper S EN24V. Later parts were down graded in specification due to the cost of production.
I had a reworked 23D4 distributer fitted to my 1400cc Midget engine and it gave good service.

[Mark Wilson]

Solved! I tried to drive it up the drive as suggested, terrible with or without vac, on whatever advance adjustment I gave it. Checked the plugs, nice biscuity colour so mixture reasonably ok, at tickover at least. As suggested I considered what might have changed (since my sole road test back in August). Nothing likely to cause this issue.

Then I mentally rewound to about 1972, and remembered having problems with a distributor cap. I happened to have a new one, so swapped it over and voila, running fine.

Thanks guys. Your input helped me to rule out my kneejerk analysis of the problem and think a bit more laterally. And saved me the cost of an unnecessary new dizzy!

[liammonty]

Fantastic result Mark! It quite often is something simple! I hope your recovery continues and you can drive a bit further soon!

[myoldjalopy]

Well done. Nice, easy fix too - we like them!

[Chipper]

The 23D4 distributer (without vacuum advance) was originally fitted to the early 1275cc MG Midget as the engine was comparable to the Classic Mini 1275cc Cooper S which was also fitted with the 23D4 distributer. The materials used in the manufacture of the first 1275cc Midgets was the same spec i.e. crankshaft EN40 nitride and the conrods were the same as the Classic Mini Cooper S EN24V. Later parts were down graded in specification due to the cost of production.
I had a reworked 23D4 distributer fitted to my 1400cc Midget engine and it gave good service.

Would a 23D4 work well on a 1970(ish) 1275cc MG Midget engine, or would low-speed running fuel economy be badly affected by not having a vacuum advance? I currently have a 25D4 with Accuspark, which works well enough but always looking for possible improvements...

[olonas]

The 41270B is the correct distributor for that engine.
The vacuum advance should have the code 5-8-3.
That's with reference to the list of Lucas distributor numbers I have.

[Sleeper]

While we're on the subject, does anyone know how much advance the Vacuum provides ( distributor or crankshaft degrees ) ?

Thanks
John ;-)

[Andmurph]

Here’s the graph from my 25D4 from Distributor Doctor. Hope it helps.

For my 1973 1275 Midget.

[Sleeper]

Thanks

Looks like 11.5 dizzy advance at 3500 dizzy rpm

So , 23 degrees at 7000 crank rpm.

Plus maybe 6 degrees static advance.



John ;-)

[Chipper]

The dizzy advance should be 3 degrees at 400rpm, 7.5 degrees at 1,150rpm, 13 degrees at 2,800rpm, according to this chart:
https://www.starchak.ca/tech/pdfs/lucas.pdf

I have now fitted the correct Lucas 41270 MG Midget 25D4 distributor (with Lumenition Magnetronic) and it goes quite well, though not noticeably any better than the previous Lucas 41242 Wolseley/VDP Princess 1300 dizzy with Accuspark I previously had fitted.

Interestingly though, the MG Midget vacuum diaphragm is stamped 5-8-3, while the Wolseley/VDP Princess 1300 one is stamped 3-15-9, so a far greater degree of vacuum advance with the latter.