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MG TD TF 1500 - ignition timing

I have read that the ignition timing on xpag engines should be slightly advanced due to modern fuels being alot better than those in the 40s and 50s, could anyone tell me how much it should be advanced and the best way of doing this (bearing in mind I am using British petrol) - I have a timing light. I hope this will stop the crackling noise from my exhaust when I am going downhill with the throtles closed.
Ian

Ian
I'm new to the TF, but I have been working on the engine set-up since I picked it up last week. The mixture was very rich when I got the car. Apart from difficulty in maintaining a constant idle rpm the engine starts and runs well. I will fine tune the ignition over the next few days using the technique I employed with my MGB. Loosen the distributor clamp bolt. Warm up the engine and then slowly move the distributor a couple of degree at a time in a clockwise direction (advance). The revs should increase initially. When you reach the maximum you can then annoy the neibours doing multiple test drives. If it pinks, retard a bit, then test again. Repeat the procedure until the set up is optimum for performance and non-pinking.
John
John James

John,

If she is running rich and you are unable to sort the problem by winding up the jet nuts check the rod which links the choke leavers. If the adjustment is out one carb will run rich even with the nut screwed to the top.

Cheers

Jan T
J Targosz

Here is the way I usually set my timing.
Set the intital timing at 10 Degrees BTDC. (premium fuel CA)
Then take it out and drive her with the correct wrench for the distributor.
Drive her until she gets hot and then drive her uphill full throttle.
That is the time she will ping most.
Keep driving up and down the hill while moving the distributor to find the point just before she pings.
Go home and reset the idle.
Recheck the timing and write it down for future settings of timing.

The 'poping' or backfiring is not caused by the ignition timing, it is caused by too much petrol entering the cylnder, and then some of the unburnt petrol exits out of the cylnder into the exhaust and fires.
It is not really a big deal, if it is not too bad, it is sometimes a unwanted side effect of richening the mixture to get more power.

If you want to get rid of the backfiring when you go down hill, just keep your foot all the way down on the throttle, and hope you drum brakes don't overheat. :)


B J Simpson

How do you know when it's 10 deg btdc? I have only a single pointer and a notch in the pulley which I assume is tdc. I know sophictictaed stroboscopes are available in which you can dial in the degs but all my cash has gone on spares. I suppose I could measure around the circumference of the pulley and file in a second notch but has anyone got the distance I should use?

Cheers


Jan T
J Targosz

Jan,
The simple answer is the pulley circumference/36.
George B.

Measure down below the TDC mark on the pully 30mm on the generator side and make a mark with white out or similar.

Set the timing to this mark with a timing light at idle and use this as a starting point.

Then do the driving test as previously described. The timing will be different for every car as we don't know what compression you are running or what octane fuel you are using. The higher the octane fuel used lets you advance the timing further. Basically as stated before, if it pings under load in top gear it is advanced too far.

The popping back through the exhaust generally means that there is air leaking into the exhaust and mixing with the unburnt fuel and igniting. This is usually through the flange gasket or the manifold gasket and is not related to ignition timing.

Hope this helps,

Paul.
Paul van Gool

A wise man told me (not so long ago from this BBS)...
set to TDC @ 30 dregrees advance @ 3000 RPM using a timming light with advanvance feature. I found a brand new timming light with advance "knob" on ebay for $15.00 (not a fancy "digital" unit).....it was worth the money! Mine was a getting to be a real "dawg" going uphill! I run "rich" and "hi-octain" with lead addative cause I don't have the cash for a burnt valve! pertronix Ing. and Bosh platinum plugs haven't hurt either!
Cheers,
David 55 TF1500 #7427
David Sheward

There is a lot of confusion on how to set the ignition timing.

Normally, the ignition timing is defined by the manufacturer so that the pressure of combustion gases develops the greater mechanical power. This generally happens when the maximum pressure in the cylinder occurs between 5 to 10 deg after TDC. The ignition timing is set this way, and the centrifugal advance allows to maintain the efficiency of the engine over the revs.

Now comes another problem which is knock. Knock must be avoided because it destroys the engine, and retarding the ignition timing is a trick to avoid knock. But giving the engine a high octane fuel does not make it producing more power, neither does advancing the ignition timing to the knock limit because then the work of the combustion gases cannot be at the optimum.

One exception of this is if the timing was originally limited to avoid knock, and this is generally for that reason that the centrifugal advance is blocked at thigh revs. If this is true for our engines, we can get more power advancing the timing at one end but loose at the other end.

Nowadays, most of our engines have higher CR that which have two effects: increasing the turbulence during the compression stroke and a better scavenging of the exhaust gases during the exhaust stroke. Both things leading to a faster combustion then to lower advance timing. I have tried 5 deg on my TF, but the engine produced so much vibration that the car started to loose bits.

The dynamic advance setting as described by David is the best way, but I am not sure of the value because XPAG1 and XPAG2 engines did not have the same centrifugal advance.

Jean-Marc
JM Thély

What Jean-Marc says makes a lot of sence!
One thing I failed to mention was that when I was having all my little "power problems" and made the decesion to go with the Pertronix Ign. I did discover that one of the springs on the centrifugal advance had broke! The dizzy was in bad need of a good cleaning and re-lubracation as well as removal of all the broken rusted parts! From this it might be good advise for anyone having a simular problem to remove your points "plate" and have a good look-see in there.
Cheers,
David 55 TF1500 #7427
David Sheward

Also check your coil and plug wires. Some years ago a wire had pulled loose from the cap and/or burned off. It gradually started popping more with throttle closed as Ian describes until it turned into a frank miss and I found the burned wire. I don't think timing would cause popping in the exhaust.
George Butz

I have checked the centrifugal advance in the Blower's manual, he indicates 32 deg for the XPAG/TD/501 and 25 deg for the XPAG/TF at 3100 rpm.
It is interresting also to note the compression ratios. It is 7.25 for the TD and 8.1 for the TF. It is a good example to illustrate that increased compression ratio requires less advance.
Jean-Marc
JM Thély

Ian,

Perhaps my recent experiences will help:

If you're crackling and popping while off the throttle going down or hill or just decelerating, I think you're running too rich. If you've leaned out your carbs by adjusting the jets and they're still too rich, there are some things you can do: replace the seals/gaskets on the jets; extend the needles from the pistons a bit; check the float level; anything to keep excess gasoline from getting sucked into the venturis.

In a recent TSO issue (the NEMGTR magazine), the advanced timing was described as about 1/4-inch BTDC (about 1/4-inch clockwise from the notch on the crankshaft pulley).

I think you're better off static timing. Static timing minimizes the possibility that you are inadvertantly retarding the spark, which will make your car run lousy and run too hot.

Greg
TF 6846 XPEG 797
TC 6669 XPAG 7179

Greg Van Hook

I have a simple mechanical connection to the dashboard that allows me to set the ignition timing manually. You'd be surprised what can be obtained by that, with the different types of fuels around these days and at different speeds. It used to be posted on the Web, but right now my server is down. If anybody is interested, send me mail. If not, stick to your incorrect timing.

Denis
Denis L. Baggi

This thread was discussed between 29/09/2003 and 12/10/2003

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