Rebuilt 1275-running a bit "out of sorts"

[ndevans]

Finally got my 1275 rebuilt and back on the road. I had it ruining ok for a couple of years after the first rebuild, with HIF38 carb and Howley 38mm manifold. A couple of broken piston rings forced a rebore and new pistons, and I took the opportunity to rebuild my spare HIF44 carb and fit it. I had to source a suitable 44mm manifold, which I did, it's an ex-Metro manifold (thanks John C!).
It runs ok, but isn't "quite right." I can't get it to start without a fair bit of choke, it doesn't idle very smoothly, and in 4th at 30mph, with foot virtually off the gas, it feels "jerky", like it's going to stall.
The needle in the carb is a BDL, I have cleaned and gapped the points, the timing is about 4°BTDC, which seems to be the "sweet spot", and I have adjusted the mixture as best I can, using a colortune set. The carb dashpot oil is mostly light machine oil, with a little engine oil. I'm running standard points and distributor, the vacuum advance is connected to the manifold, although there is a vacuum advance take off on the carb which I have blocked off.

Any ideas, please?

Cheers N

[Chipper]

Sounds as though it may be running too lean, if you're having to use choke at this time of year. What colour are the spark plug tips, and have you carried out a compression test?

[ndevans]

Plug tips were light grey, but that was shortly after rebuild. Compression test was ok, but I'll have to test it again, it's still not fully run in though.
I could try enriching, but that would leave the flame colour an orange, it should be blue I believe.
I'll have another play with the mixture tomorrow, I have adjusted the timing since I last did it.

[liammonty]

Adjusting the mixture won’t help a lot if the needle’s wrong, other than at idle. I’d suggest a rolling road session to get it set up right. Have you got the timing set right, with a distributor with a suitable advance curve?

[ndevans]

Don't know about whether the advance curve is correct, it's a 25D, I have the advance curve recorded somewhere, but don't have it to hand. I do intend to get it on a rolling road, but I think I ought to get it run in properly first.
How would the advance curve for a 1275 with big valve head and HIF44 need to differ from a 1098?

[philthehill]

The distributer needs to be suitable for the engine specification as the modifications to the engine have effected the fuel mixture/ ignition/burn pattern and the 1098cc distributer advance/retard pattern will not match the fuel mixture/ignition/burn pattern required.
In a nutshell the standard 1098cc distributer is totally unsuitable for the modified engine spec.
If you want a off the shelf distributer that is more aligned to the new fuel mixture/ignition/burn pattern the Cooper S 23D4 distributer would be a suitable candidate.

[ndevans]

Found it. It's off a Riley Elf, max 12° mechanical advance at 2800rpm, advance starts at 300rpm. The vacuum unit is a 4/7/8 unit, i.e. vacuum advance starts at 4"Hg, ends at 7" Hg, and has 8° total advance.
I have another distributor, this is 10° max mechanical advance at 2500rpm, starts at 150rpm, with a 5/11/7 vacuum unit.
@philthehill -regarding fitting a 23D4 - would you go with or without electronic ignition?

Cheers N

[philthehill]

As I have Lumintion electronic ignition fitted to my Minor I would go for electronic ignition.

The main point of my post above is that the distributer especially the advance curve has to be suitable for and match the engine spec.

[ndevans]

Just swapped the distributor for the spare, it's a lot smoother now. I'll get a 23D4 anyway though.
Cheers N

[liammonty]

Just swapped the distributor for the spare, it's a lot smoother now. I'll get a 23D4 anyway though.
Cheers N

Pretty certain they don’t have a vac advance, so I wouldn’t get one if that bothers you

[philthehill]

The 23D4 does not have a vacuum advance but the mechanical advance compensates for the lack of vacuum advance. The 23D4 distributer was fitted to the 1275cc Cooper S. I ran a 23D4 distributor for many years with a similar engine spec to the OPs and never had a problem.

[ndevans]

150 miles done today, at 50mph max, and it's run pretty smoothly. Still not perfect, but a step change for the better.
Vac advance doesn't bother me, as long as it's compensated. I'll just have to plug two vac take off ports instead of one!
The engine block came off a Marina, didn't they have 45D4's?

Cheers N

[liammonty]

The 23D4 does not have a vacuum advance but the mechanical advance compensates for the lack of vacuum advance. The 23D4 distributer was fitted to the 1275cc Cooper S. I ran a 23D4 distributor for many years with a similar engine spec to the OPs and never had a problem.

Agreed that it will run fine, but it will lose a little bit of economy at light throttle cruising, for example. The mechanical advance can’t really compensate for vacuum advance as as centrifugal advance is purely dependent on engine speed, not throttle position, like vacuum advance is. Race engines don’t need vacuum advance as one doesn’t pootle around at part throttle, so it would never do anything in that setting anyway. I guess that may have been behind the thinking of dispensing with it on the Cooper. Unlike on the 1275 Marinas, which I understand omitted the vacuum advance as the bottom ends didn’t like it!

[philthehill]

The 23D4 distributer has a different mounting to the Marina distributer and is not easily interchangeable.
There is a thread on here (use the search facility) on how to fit a non Marina distributer to a Marina block.

[ndevans]

The 23D4 does not have a vacuum advance but the mechanical advance compensates for the lack of vacuum advance. The 23D4 distributer was fitted to the 1275cc Cooper S. I ran a 23D4 distributor for many years with a similar engine spec to the OPs and never had a problem.

Agreed that it will run fine, but it will lose a little bit of economy at light throttle cruising, for example. The mechanical advance can’t really compensate for vacuum advance as as centrifugal advance is purely dependent on engine speed, not throttle position, like vacuum advance is. Race engines don’t need vacuum advance as one doesn’t pootle around at part throttle, so it would never do anything in that setting anyway. I guess that may have been behind the thinking of dispensing with it on the Cooper. Unlike on the 1275 Marinas, which I understand omitted the vacuum advance as the bottom ends didn’t like it!

So does that mean I'd be ok running without vacuum advance, if it was omitted on Marina engines?

[philthehill]

The problem you have is that you do not have a standard engine.
All the distributers quoted have advances set against known standard parameters. Once you go away from standard you do need a bespoke distributer.
The 23D4 Cooper S was and in some cases still is the preferred distributer when a distributer was required for modified engines from 998cc to 1400cc capacity. The internal weights and advance curve can be modified to suit the engine spec. See Vizard tech books.
Your engine will come to no harm when running with a distributer with no vacuum advance.
My 1400cc MG Midget engine ran with a 23D4 distributer matched to a 649 cam and it ran very well.

[ndevans]

Another 107 miles today with no problem, 45 of them motorway at a steady 50. It ran smoothly, nice and responsive pulling away, maybe a little weak in the lower engine speeds, but generally I'm pleased.
I have a standard Marina cam, but have considered a Piper, can't remember which one. I have Vizard's book, so I'll have a look in there. Now it's run in, I can have a play. Would I be wasting my money putting it on a rolling road before changing the camshaft & distributor? I don't particularly want a racing machine, and it will spend a fair bit of time on shorter journeys, but we tend to go camping, using it to tow a small trailer, so it does get long journeys. I also have a 3.9 diff to fit, but it requires overhaul first.

Cheers N

[ndevans]

Just looked in my Haynes Marina 1.3 manual, it says in there that the distributor was a 23D4 for HC engines, 25D4 for LC engines. However, that I guess will be for a standard 1275 with 12G940 head, and an HS4 (effectively 38mm) carb, whereas mine is a big valve head as fitted to MG Metros, and an HIF44 carb.
I'm looking at a Powerspark 45D4 now, with advance curve for a 1275.

Cheers N

[ndevans]

https://youtu.be/WX1e-OFkePg

A Powerspark 45D4 has made a big difference. Still not quite there, it's losing water through the radiator cap. I believe this is because I have a 4lb cap, I will try a 7lb cap.
There's also a slight misfire, just noticeable. I don't believe it's pinking though, and it's run very smoothly on a camping trip this weekend, 300 miles in total, a mixture of motorway, local and cross country driving. Pulls well from 40mph in 4th.
I need to get it in a rolling road now.

Cheers N

[oliver90owner]

it's losing water through the radiator cap. I believe this is because I have a 4lb cap, I will try a 7lb cap.

I suspect there is more to it than 3psi will solve - if it is actually losing water. Water boils at 100 degrees Celsius at one atmosphere. 4psi above atmospheric pressure and it will boil at about 107 degrees; 7psi above atmospheric pressure increases the boiling point by about another 6 degrees. Antifreeze will modify those boiling points. Your thermostat likely opens at 88C?

The coolant should not be boiling, so the pressure should make little difference.

Water expands by about 4%, so you may just be losing that volume as the engine heats up. No radiator cap will resist that increase in system pressure.

I suspect the cap is faulty, you have filled the radiator too full, the thermostat is faulty, radiator is blocked, water pump is shot, or you will be pulling the engine apart before long. All provided you have the timing set correctly and the engine is not running with a weak mixture.