MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGA - What is the cause?

A friend of mine recently replaced his MGA gearbox and clutch and since then his car has no power down low. To get out of his step driveway it is fine if you rev the engine hard and exit at speed - car goes well on the open road when you keep the revs up. It was okay before the gearbox replacement. Car has a fast road cam and 1 3/4" SUs, Bosch distributor
I have checked the following so far - tappets okay 0.015" gap, compression varies between 170-175 over 4 cylinders, cleaned and gapped plugs 0.028", distributor removed, checked centrifugal advance mechanism, cleaned and set points 0.015", tested fuel flow (strong), fuel in float chambers, float valves clear, plastic prism shaped floats not adjustable, removed suction bowels on carbs - clearances good (drop test), needles correctly affixed, no blockages between main jet and float chamber.
Any ideas on what could be the cause? Problem didn't exist prior to engine removal and replacement. Owner has since replaced spark plugs. Interestingly the SU dampers had no oil in them when I checked carbs and believe they may have been this way prior to engine removal as owner only ever add a couple of drops (thought for lubrication rather than dampening) - these were topped up with Penrite SU damper oil at time of reassembling carbs.
My next steps were to check timing with strobe light, look for air leaks in the inlet system (vacuum gauge), retune the carbs from scratch.
(Yes, Steve the air filters are attached the right way up!)
Mike
Mike Ellsmore

Oops - meant "steep driveway" and meant "my next steps will be"!
Mike Ellsmore

Any chance the gear ratios are different?

Matthew.
M Magilton

No they are the same - he gave the gearbox rebuilder the bits to overhaul and got the old bits back - they were same ratio.
Mike
Mike Ellsmore

Mike

I was going to say timing, but I see that is next on your list.

Steve
Steve Gyles

You don't mention condenser. I can't really see the logic but they can be an unpredictable pain so I would check/change anyway.

Paul
Paul Dean

Doesn't sound like conspdensor problems to me, try Srevie Gyle's one of having the air filter back plates on upside down.
dominic clancy

My 1600, "Alfred", was a "victim" of something similar, until I changed the exhaust system....It seems that there was a mechanical restriction caused by an over zealous mechanic, somewhere along the way....I would never have known the difference, as the car came that way...Especially, since I went from a TD to the "A"....
As soon as I put the new stainless exhaust system on the car, it was like I was driving someone else's MG....
Get under your car, and check along the pipes for any crushed areas or extreme bends in the exhaust....
It is also possible that the muffler internals have collapsed from age or use, and are blocking good flow.
Edward
E B Wesson

I might make some comments about fast road cam and oversize carbs causing reduction of torque at low engine speed, but you say it was okay before. I'd look for vacuum leak between carbs and head, or badly retarded ignition timing.
Barney Gaylord

Thanks all for your comments. My friend has told me that he has now pulled the suction chambers and pistons from the carbs and emptied out the damper oil and re-installed. He says it is now running as it was before the gearbox replacement - has the power to drive out of his steep driveway at slow speed, low revs. Is it feasible that this is what has corrected the problem? Would this indicate it was too rich on the first section of the needle?
Steve I will check the air filter installation when I next see the car just to be sure - the air filters are the tapered K&N type that has a built in sub stack on the back plate - not sure if these can be installed up the wrong way.
Mike
Mike Ellsmore

Problem solved - Barney was on the money (and Steve) - badly retarded ignition timing was the problem. Somewhere along the way the distributor turned in the clamp, we had marked the distributor flange location with "liquid paper" in a couple of spots so we thought we were putting it back in the same position. But after swapping the coil, distributor cap and leads and rotor button from a donor car and fitting new points and condenser there wasn't much else to alter so followed Barney's instructions for Spark Timing Adjustment

"If you do not have a timing light you can sill find a good setting for the spark timing. With engine running at idle speed turn the distributor body anti-clockwise to retard spark timing. Engine speed will slow down when you do this. Rotate the distributor clockwise to advance spark timing. Engine will speed up when you do this. Reset idle speed as necessary to keep engine speed under 1000 rpm. When you go too far advanced the engine will start to shake a bit. Turn slowly back anti-clockwise to retard timing just enough to make the shake go away. Then turn slightly more anti-clockwise to retard timing until you notice a slight drop in engine speed, not much, just 50 rpm drop is enough. Lock it down there and be prepared for a nice test drive. You might also do the lift-the-pin test on the carburetors again just to verify the carb setting is still valid."

The above is an extract from Engine Tune-up the Casual Way http://www.mgaguru.com/mgtech/care/cf120.htm

I am going to buy myself a set back timing light!
Mike
Mike Ellsmore

Well done Mike

A while back I invested about £50 ($90Aus) in an adjustable timing light. It has proven brilliant over the years. I dial up 33 degrees of advance on the light, then run the engine at about 3500 RPM with the vacuum disconnected, then turn the distributor until the timing mark shows TDC. I then accept whatever static or idle advance that gives.

Steve
Steve Gyles

I meant to say also that the owner reckoned he fixed it originally by taking the damper oil from the carbs that was a furphy as next time he drove it the problem was back - real problem was too far retarded.
Mike
Mike Ellsmore

This thread was discussed between 16/07/2014 and 29/07/2014

MG MGA index

This thread is from the archive. The Live MG MGA BBS is active now.