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MG TD TF 1500 - Engine Spec Research

I'm trying to sort a few things on my XPAG engine. The most important is a lumpy idle, which I cannot seem to get below 1,000 RPM.

I've tried all combos of distributor components, from points to cap to rotor to wires to condensor to coil to pertronix. Same effects in all cases. Checked the advance springs, adjusted the timing all over the band. I've tried carb adjustments, added a supercharger, etc. Checked and rechecked the valve lash to .012 (car is a very late 53). Set various gaps in the plugs. No vacuum leaks. Still the same idle. Gets worse as the car warms up further.

May be heat against the air mixture, so I am experimenting with a heat shield.

I'm suspecting that someone before me changed the cam, but before I convince myself, I would like to know if there is a way to tell from observation. I seem to remember a process for this in the archives but I cannot locate it.

Also, is there a real correlation between compression and compression ratio? My compression checks out near 135 pounds warm, across all cylinders. Seems pretty normal to me, so I believe I have a stock head, although it does have signs of being ported.


Tom Balutis


Do your throttle shafts leak? Spray some engine start fluid by the carbs and see if the idle increases.

Dave DuBois posted this in 2004 regarding the stock cams:

Two different camshafts were supplied as original equipment for the XPAG and XPEG engines. The earliest type, part number AAA5776 (earlier numbers MG862/171 or X24084), was used in all TB and TC engines, and in TD engines up to engine number XPAG/TD2/24l15. This early cam requires a valve clearance of 0.019 in. The later type, part number AAA3096 (earlier number 168553) was used in all later TD and TF engines and requires a valve clearance of 0.012 in.

Unfortunately, now that even the newest TF is almost 40 years old, the engine numbers quoted above are not necessarily a valid guide for purpose of camshaft identification. Engines originally equipped with AAA5776 may have been fitted with AAA3096 in the course of an overhaul.

With the engine cold, remove the rocker cover and turn the engine over with the crank until #1 intake valve (2nd valve from front of engine) is wide open. At this point, #4 intake valve (7th from front) will be fully closed and on the exact center of the heel of the cam lobe. Set #4 intake to 0.021 in. clearance. This is the correct checking clearance for both cams, regardless of whether you use the degree wheel and dial indicator method or the simplified method which follows.

Now, wipe all the accumulated gook off the crankshaft pulley, and get out your measuring tape and a piece of chalk. Measuring around the rim of the pulley. make one chalk mark between 1/32 in. and 3/64 in. to the right (as seen from the front) of the top dead centre mark on the pulley. This corresponds to 5° BTDC, which is when the intake opens on the later cam (AAA3096). Make a second chalk mark 23/64 in. to the right of the TDC mark. This corresponds to 11° BTDC, which is when the intake opens on the earlier cam (AAA5776).

Next, turn the engine over with the crank (it’s easier with the spark plugs out) until the pushrod for #4 intake valve just barely locks up enough so that you can’t spin it between your fingers. At this point, the timing pointer on the timing chain cover should be more or less aligned with one or the other of the chalk marks.

If the pointer aligns with the 11° mark, then you have the early cam (AAA5776) and you should set the valve clearance to 0.019 in. with the engine hot. If the pointer aligns with the 5° mark then you have the later cam (AAA3096) and you should set the valve clearance to 0.012 in. hot. If the pointer does not align with either mark, go back through the entire procedure again to make sure you did it right.
What effect does the wrong valve clearance setting have on the engine? This is a bit difficult for the novice to pinpoint unless he can compare performance to a properly set up car. In general, however, an engine equipped with AAA5776, but with the valve clearance set at 0.012 in. will be very low on power. An engine equipped with AAA3096, but with the clearance met at 0.019 in. will be slightly down on power and will produce a really awful amount of valve clatter. You should also bear in mind the fact that while clearance which is too large is noisy, it doesn’t necessarily produce any mechanical damage. On the other hand, burnt exhaust valves will almost surely result from unduly small valve clearance, as in the case of an AAA5776 cam set at 0.012 in.

Hope this helps,
Dave Braun

During my engine start-up I also found a too high idle speed. In my case, it was due to a low pre-tension of the two springs at both ends of the throttle shaft although I found them already pretty loaded. Actually, those guys need quite some pretension in order to be able to keep the throttle discs fully closed at idle.
Good luck and greetings, Huib
Huib Bruijstens

Worse when warm could be too rich? What needles are in the carbs? Have carbs been rebuilt? Wrong needle, leaking top cork jet seal, sticking pistion, mis-postitioned needle in piston all could cause this. As far as the cam, the most common cams supplied since at least 1987 are made by Crane, and most use .018-.019 clearance, with different than original valve timing. You may want to adjust clearance looser to see if any difference. George
George Butz

Doubt that it's carbs...he's added a supercharger and nothing has changed (unless he's using one of the original carbs on it).

I'd go for the cam...maybe a worn lobe? Can you get a dial indicator and check how far each rocker is lifting? If there's much of a disparity between the rise on the exhausts (or on the intakes) it might indicate a worn camshaft.
Gene Gillam

Good point Gene. You could also install a degree wheel from an auto parts store, or make one out of a plastic coffee can lid, and as above roughly degree the opening/closing angle of the valves to get an idea of what cam is in there. George
George Butz

I volunteer to test the supercharger for you. Just send it along, I'll get it back to you after a thorough test.

Seriously, I would degree the cam as suggested above. Post the values and someone will be able to identify which one you have.

How did you test the advance springs? Any chance the distributor bushings are shot?

Good luck,

Evan Ford - TD 27621

All good suggestions, guys. Thank you. I do have my homework to do.

Best regards,

Tom Balutis

Tom - We had the same problem. Turns out 53's have a special distributor, yes. THAT was our our problem. TTFN sara
Sara D.

This thread was discussed between 13/03/2008 and 16/03/2008

MG TD TF 1500 index

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