12/4 will not start / run correctly

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Austin-App73-77
Posts: 47
Joined: Fri Nov 11, 2016 3:43 pm
Location: Dumbleton, Worcestershire
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12/4 will not start / run correctly

Post by Austin-App73-77 » Sun Jul 12, 2020 7:32 pm

I have a 1928 12/4 which I am restoring as a special as it came to me without a body.
I have spent literally days trying to get it to run correctly.
The valve timing is correct - No 1 at TDC with both valves closed and No 4 inlet valve just about to open.
I have adjusted the clearances.
I have set the magneto timing as the VAR instructions of the memory stick.
I have done the above over and over again.
I have managed to get it running on occasions with different timing, but only on either 1 & 4 or 2 & 3 Firing and always back firing through the carburettor.
I have has the head off at least twice.
All valves are opening & closing correctly.
I have had the carburettor apart and it seems clear and not blocked.
There is a spark and there is fuel.
The exhaust is the only none standard part fitted

Frustration beyond belief..
In 50 years of maintaining , restoring and building cars I have never been so at a loss what to do next.
Any help would be appreciated.
Rob 07720 557182
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Martin27
Posts: 37
Joined: Thu May 26, 2016 9:38 pm

Re: 12/4 will not start / run correctly

Post by Martin27 » Mon Jul 13, 2020 11:58 am

Hello Rob
It does sound like it is going to be something so simple that you will wonder why you missed it! At risk of being included in Grandma's egg sucking competition, I can only suggest firing on either 1&4 or 2&3 and spitting back through the carb is a result of plug leads being on the wrong plugs. Remove distributer cap to check rotation of the rotor arm and connection of plug leads in correct firing order.
Best Regards
Martin

Neil McL
Posts: 4
Joined: Thu Nov 17, 2016 2:55 pm

Re: 12/4 will not start / run correctly

Post by Neil McL » Mon Jul 13, 2020 6:20 pm

I have a Bosch 009 distributor conversion on my 12/4 rather than the magneto. If I recall correctly, it uses a 2:1 reduction gearbox to drive the distributor. This means that it is possible for the input shaft to the distributor gearbox to be connected when it is 360° out-of-sync (I do mean a whole turn) with where it needs to be for correct ignition timing.

Is it possible to have the same problem with the magneto setup ?

When "I tried this arrangement" it produced equally puzzling behaviour including backfires through the carb.

Have you confirmed that the #1 cylinder spark is definitely at the correct phase of the four-stroke cycle ?

Best regards,
Neil

Austin-App73-77
Posts: 47
Joined: Fri Nov 11, 2016 3:43 pm
Location: Dumbleton, Worcestershire
Contact:

Re: 12/4 will not start / run correctly

Post by Austin-App73-77 » Mon Jul 13, 2020 8:47 pm

Thanks for the suggestions so far.
I have just come in from having another try - it is now running on all 4 cylinders, but still with back pressure through the carb. I’m thinking maybe the head gasket may be passing between the pots? It was an old gasket - or maybe I have not set the valve clearances correctly which will be my first job to check on the weekend.
I will be stripping the engine to fit an over bored & modified block with twin SU carbs fitted, but I just wanted to see how it went in standard form.
I think I had the leads on wrong as the rotor goes in the opposite direction to the engine so 2 was firing when 3 should have - schoolboy error I guess.
I have considered fitting a coil & distributor as I’m used to tuning the timing on a running engine by ear which can’t be done with the magneto I have fitted. I’m not sure about how it would run with the wasted spark which some modern engInes run with.
Ada can be seen running on my Austin 12/4 FaceBook page here.

https://www.facebook.com/18504201585712 ... 040335335/

Thanks again and I’m not giving up 😁
Rob.
Measure twice -Cut once!

Austin-App73-77
Posts: 47
Joined: Fri Nov 11, 2016 3:43 pm
Location: Dumbleton, Worcestershire
Contact:

Re: 12/4 will not start / run correctly

Post by Austin-App73-77 » Sat Jul 18, 2020 8:02 pm

Latest update.
I gave up with the Zenith Carb and fitted an 1 1/4“ SU Carb which I am more familiar with.
Still will not start.
Not trusting the old magneto I made up a mounting for a Lucas distributor & fitted.
It still will not run ( I know it is wasted spark as it runs at the same rotation as the crank, but some cars run this way)
Finally took off the head again can see I have the valve timing a tooth too advanced - see photo - number 1 cylinder at TDC and the exhaust on number 4 has already started to open.
The biggest discovery is that someone has fitted the flywheel Clutch cover 60 deg out from the main flywheel and I can only see the clutch cover markings through the bell housing window. This means I have been setting the ignition timing 60 degrees out I think?
I have given up for today - going to try again tomorrow- watch this space.
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