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MG MG Y Type - Engine timing,etc

My YT engine has always ticked over little roughly, it's noisy (possibly typical XPAG) and is lacking in power. We carried out some tuning, balancing and setting up the tappet gaps, carburettors and ignition timing, installed new plugs, points, condenser and HT leads with some improvement. The plug colours suggest it's running rich but that's not reflected in the carburettor set-up checks or exhaust gas analysis.

The best running setting on the distributor is 30degrees advance, anything less than 15degrees and it stalls.

The invoices I got with the car 3 months ago detail an engine and head rebuild, but no indication whether the water pump and timing chain were replaced or whether the distributer and carburettors were rebuilt, I suspect not.

When checking the timing the light advances a little with speed increase and jumps forward on occasions.

The car has only done 1,050 miles in the last 20 years. I had an exhaust valve stick open on one occasion, which I managed to free. It doesn't overheat on a run. It's running on E5 petrol.

Any advise on how I progress would be gratefully received.
I Anderson

There seems to be something wrong. Perhaps it is the automatic advance/retard mechanism? Is your 15 and 30 degrees static or with engine running?
Check that the distributor cam/advance is well lubricated and can swivel well on the shaft. The light spring should offer some initial resistance before the heavy spring takes over. Static timing should not be far from TDC. When the engine is revving, it should advance to somewhere in the low 30s. Of course modern fuels and your compression ratio will affect optimum settings but that should be a good starting point.
Advancing a little with speed and jumping forward might mean the advance mechanism is sticking. If the distributor has a lot of wear in the top bush that can cause a lot of spark "scatter" too.
Mixture might be a separate issue. Are the jets centralised so the carby pistons drop down nicely? They need to be centralised by someone who knows SU carbys as the process and result are a bit complicated to describe here.
Good luck with it.
Bob Schapel
Bob Schapel

Hi Twenty odd years ago I got a ford falcon with the same problem setting the timing.. After checking the usual points, ignition timing, and yes valve timing I was still scratching my head. So I started swopping parts from a different car..The coil fixed ?????
Bob Wood

Bob's. Thanks for your comments.

I have made some progress. I acquired another DKY4A complete distributor, mine was a 40193B, the other was 40193A, so slightly different cam weights. I swapped them over, but no joy initially. I also got a new coil from Moss. It turned out both distributors had duff condensers, fitted a new condenser and new HT leads and refitted old distributor and got the car running. I also swapped over the coil connections which improved matters.

Set up the timing, still well advanced but less erratic, rebalanced and adjusted the carburettors. Took it out for a run, it's running better than it was but still lacking some power when pushed and is still running rich, even with the weakest needles.

I think I've done everything I can without getting it looked at by a professional.
I Anderson

The weakest needles will do little to fix a rich mixture if the jets are worn. Jets and needles wear if they are not centralized properly because the needle rubs inside the jet making the jet bigger and needle thinner. The only cure is new jets and needles and a good assembly process to centralise the jets so they don't touch each other.
This is one of the things which make some people think that SU carbys go out of tune easily. They don't if set up properly. If the jet is only slightly worn, idle mixture can be compensated with the adjustment "nut" but that does not weaken the mixture further up the rev range.
Bob
Bob Schapel

From your original post, it sounds as if the engine has had a fairly major rebuild recently. Rather than repeat the excellent points already made, which should help, I wonder if there's a possibility (and I hate to say this) that the valve timing has been set wrongly, so that the relationship between crank timing and valve position is incorrect? I ask because the ignition timing settings you have mentioned sound extreme - an XPAG should idle happily with firing point at TDC or 5deg BTDC. If the mixture is roughly right, as it sounds to be, and there aren't any intake air leaks, you shouldn't need much, or indeed any, static advance. Fingers crossed for you...
Tim Griggs

This thread was discussed between 29/07/2023 and 18/08/2023

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