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MG TD TF 1500 - Gearbox Rattle is GONE!

A couple of days ago I suffered the catastrophe of destroying a Pertronix LU146LS by messing up the routing of the wires. My first repair was to reinstall my 'old' LU146 (the one with the magnet collar). The engine fired up, so I didn't think much about it. Took Lazarus out for a short test run and noticed that my usual gearbox rattle was worse than usual, especially under load. Got back to the house and replaced the LU146 with a 'spare' LU146LS that I've had. I decided to check the timing while I was at it. Discovered that my guesswork for a slight advance at idle was way off. At 850 rpm I had about 25 degrees of spark advance! It was giving me about 27 degrees at 2000 rpm. I backed off the distributor to get 20 degrees at 2000 rpm and reset the idle speed to 800 rpm and took Lazarus out for a spin.

First thing that I noticed was that my usual gearbox rattle had COMPLETELY DISAPPEARED!!! I'd heard folks talk about the rattle being affected by advanced timing, but I never tried it. Now I'm a believer.

One of our members from Germany, perhaps Declan, suggested replacing the gasket between the gearbox cover and the gearbox with a piece of rubber. I was about to try this and obtained a piece of Tom Lange's gasket material. Guess I'm not going to use that extra piece, Tom. Thanks anyway. Bud
Bud Krueger

I to Bud, had a gearbox rattle and also found that my timing was to far advanced. I only had to back it off a couple degrees and the rattle disappeared. Good feeling to know the problem was caused by something so simple. PJ
PJ Jennings

Bud, thanks for posting. Same problem here. I kept advancing my timing waiting for the traditional ping that most cars exhibit. But it never occurred. But the gearbox rattle started. Since my car is just back on the road after who know how many years I thought it was a mechanical issue with the gearbox. After reading your post I checked the timing and found that I was 30 degrees at 2000 rpm. I set it at 20 degrees at 2000 rpm and the rattle is gone. BTW did you check your dwell? I checked mine and it was 90 degrees. I have the Ignitor Plus 9LU-146LS. That model adjusts the dwell based on engine speed. On another topic, since our cars do not have markings showing before and after TDC on the pulley I highly recommend a timing light that allows you dial in the degrees of advance or retard. It made making the adjustments above a piece of cake.

Tim
TD12524
Tim Burchfield

Tim, I use such a timing light. My LU146LS is not one the IgniterII or III varieties. Dwell is never a matter of concern. Bud
Bud Krueger

Bud, the timing light recommendation was not directed at you specifically, just a general recommendation. I was just curious about the variable dwell feature of the Ignitor 2. Thanks aging for the helpful post above.

Tim
TD12524
Tim Burchfield

But what is the physical reason for the relation between timing advance and gearbox rattle ?

Timing advance generates vibrations ?
Is that vibration occurring before or after the optimum advance for the maximum engine torque ?
What part of the gearbox is vibrating that way ?

In otherword, I'm sure that playing with timing to cancel a vibration is not the right way.
It only hides or compensates the actual cause of rattling.

These are only questions. I've got no answer.

Laurent.


LC Laurent31

Laurent, my professional career was spent in electro-optics, not in mechanical engineering. So I'm only making a SWAG.
My experience with with advanced ignition issues goes back to my first car, a 1929 Model A Ford, with a spark control lever. I learned about pinging ("pinking?") when going uphill under load. The same audible phenomenon occurs in modern engines under load. I surmise that the mechanical configuration of the XPAG system allows the pinging to resonate through the linkages and that the gearshift configuration amplifies this noise.

It's one of these things that I don't know 'why', I just know 'that'. Bud
Bud Krueger

It's probably a bit like a diesel
Most diesels now have a duel mass (damper) style flywheel to stop gear chatter noises coming from the gearbox from low to mid speeds
If you fit a solid flywheel to one of these that normally have the d/m flywheel you would swear that the gearbox has bearing problems
Maybe on the MG with the timing up too high it is causing a similar resonance problem

Or not

Also - There was mention of setting the timing by ear by giving it a bit of advance---
Normally if forced to set the timing by ear you would move the dizzy around to find a neutral spot and then slowly retard it back till you can 'just' hear the revs start to drop off, and lock it up there.
If you really have to--
willy
William Revit

So the gear box rattle is just an indicator of pinging and as such,
we must not manage to cancel it with other means than looking for pinging reason (too much advance, carbonized head, too low octane fuel, heating, compression ratio, plugs...).
That's wright ?

Laurent.


LC Laurent31

I didn't say that. I only said that MY gearbox rattle ended when I retarded my overly-advanced spark. Bud
Bud Krueger

Bud

in the first posting you mentioned values of degrees and rpm. Are they related to the distributor or to the crankshaft??
W_Mueller

The crankshaft. Degree readings were obtained through the use of a timing light with a digital advance control pointing at the timing mark on the crank pulley. Bud
Bud Krueger

My 2 cents
All engines experience some torsional vibration just from the difference of the timed explosions of each cylinder. The more cylinders, the smoother the engine. Older engines may have an additional impact on the torsional vibrations because they are more likely to have hot spots that cause secondary ignition points, adding to the magnitude and frequency of the vibrations. This is known as knock or pinging. Not all pinging is audible (cold knock) but it's there nonetheless. Engine timing affects the magnitude of the ping (audible or not) as shown in the attached figure. By changing the timing, you change the magnitude of the ping forces and thereby change the vibration frequency that the engine undergoes. Mechanical parts have a natural frequency which can be set off by the engine vibration which would explain why the shift lever rattle goes away when the vibration frequency is changed by changing the timing. However, if the lever connection to the shifter is not worn and the tolerances of the parts are tight, chances of a rattle due to changes is engine vibration are drastically reduced.
Mike Izzi


Mike I

Thanks Mike and Bud.
So the gear box rattle for our car does not imply a hazard for the engine.
Other way :
Solving the gearbox rattle by reducing the advance can lead to a too small advance hence less mechanical torque at the propshaft.

Laurent.
LC Laurent31

Laurent, on my test drive, when I discovered the rattle gone, there was no noticeable reduction in performance. An advance of 20 degrees at 2000 rpm is still greater than OEM advance for a 40368 distributor. OEM would call for about 17 degrees. But, I'd be happy to give up a bit of torque for the sake of not having to listen to that aggravating noise (and apologize to passengers). BTW, I'm running at about 9.1:1 c/r at 1308 cc. Bud
Bud Krueger

I agree with Bud, a couple of degrees of change in timing would not be noticeable. I don't know about apologizing to passengers though.
Mike Izzi
Mike I

I reactive the thread since winter is the time to take out everything and understand.
I really would like to understand what is vibrating? And to what other part it interferes. What is the root cause of ths rattle.
My rattle is only in 3rd.
Idea : what is the "support cap, 3rd and top selector shaft" ? Moss part 441-640.
It seems to be fitted on late cars. Waht was there before ?
In third, I guess that the selector shaft slides into this cap. Could it be the source of the rattle ? Can the car operate without it for a test ?

Laurent.
LC Laurent31

If the noise was caused by pre-ignition (pinging) making something in the drive train rattle, then wouldn't it be reduced or eliminated by using high octane premium? Not as a solution, but as a test for cause.
Ron Coates

My TF had the same problem, backing off the timing a bit solved the problem. No difference in power. When set too far advanced, the engine is not performing properly and can cause hot spots on the piston tops, especially if any carbon is present. I'v seen a couple pistons with holes burnt right through the tops. Higher octane fuel will sometimes cure the issue, Sometimes!
PJ Jennings

This thread was discussed between 05/10/2017 and 09/01/2018

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