Ignition timing?

My TE21's distributor has the vacuum advance connected. Recently it has exhibited a barely perceptible sense of hunting (as though the power is going slightly up and down) when running on a light throttle. But push down on the throttle, and the car accelerates smoothly away. Also, the engine can briefly hesitate at around 1500 - 2000rpm when switching from decelerating to accelerating. While on holiday last week in the Scottish Borders, a new symptom emerged: when coming up to a junction, the engine revs would drop to 400 - 500 rpm and might or might not recover to normal idling speed. Restarting would be instant. The car starts from cold easily and generally runs well. It passes my favourite test of running at 1000rpm in 4th gear and pushing the accelerator right down; it pulls away smoothly and effectively. My hunch is that the vacuum advance is sticking or there's a fault with the vacuum advance diaphragm housing. But I thought I'd seek advice before tinkering! Posted by vaningen 02/11/2014 14:51:43

Charles, Does your distributor have the fine timing adjuster on the side? If it does then you can view the action of the vacuum advance as you rev the motor. As you have a tachometer you can also get someone to see at what point the plunger commences its inward travel. These items have a finite life and if the unit has not been replaced then it probably warrants a new one. In order to replace the unit the supplier will need the test code. This should be stamped on a tag located where the vacuum line enter the unit. The test code is usually 3 sets of numbers separated by a backslash. If the unit is the same as the TA21 then the numbers should read 7/18/12. The first number represents the vacuum depression in inches of mercury at which the unit commences to function. The second is the vacuum depression for maximum advance and the last is the maximum advance.  I purchased a new unit for my car on e-bay. The gentleman was very helpful.  Their e-mail address is <!-- e --><a href="mailto:britishvacuumunit@isp.com">britishvacuumunit@isp.com</a><!-- e -->  Richard Melbourne Posted by RichardWallach 03/11/2014 10:03:19

You can disconnect and blank-off the vacuum pipe fitting at the front carburettor, check the points gap / static advance is standard, and try a test-run.  See if symptoms persist, which could rule-out or confirm a problem with the vacuum unit.  Some TE's run with the vacuum unit blanked off (I believe it was a factory modififcation after warranty problems). My engine was blanked off in the carburettor port, but the vac. pipe was left connected. Out of curiosity I tried the vac. advance, but it was not an improvement. I went back to the blanked-off set-up, and adjusted the micrometer adjustment for minimum pinking with best acceleration under load.  If you need any parts for the 25D6 distributor it's worthwhile talking to Martin Jay who runs the company Distributor Doctor. He knows the Alvis set-up very well and is extremely helpful.  Martin Jay Distributor Doctor Ltd 01398 361 678 <!-- w --><a class="postlink" href="http://www.distributordoctor.com">www.distributordoctor.com</a><!-- w -->    Hope this helps,  Giles Lacey Posted by lacey giles 03/11/2014 11:22:36

Charles My TE had the pipe blocked and re-instated so looks original about 5 years ago by B Chrimes,no difference in running, also my hot ex RT TA/F has no vacuum pipe and she goes well so worth removing to eliminate it. Have you checked ? 1 Rotor arm, is it a tight fit on the spindle or can it move about, fixed mine with plumbers tape 2 Is the centrifugal system free with no tight spots, when did you last lubricate the mechanism, how are the springs 3 Assuming you still have the crankshaft damper and the timing marks are in the right place you can check the smooth or otherwise operation of the advance/retard with a a strobe lamp.Max advance is quoted at 22-26* at 6000rpm! and 0* at 300 rpm = cranking speed?  Let us know what you find  Peter Posted by Peter Martin 11/11/2014 10:49:14

Thanks to all for the advice. I have now blanked off the vacuum advance take-off at inlet manifold.  On an initial 15 mile mixed road test, the car runs well and smoothly. So I'll leave well alone. But, this being so, I am baffled as to why Alvis continued with the vacuum advance. Roland Simmonds told me that Alvis deleted this feature after the 5-speed gearbox was introduced. Owners were apparently were running around town in 5th and going through big-ends. Surely it must have made a positive difference under some operating conditions? Or is it something to do with the different petrol we have these days? Posted by vaningen 09/02/2015 22:45:58

Charles the blanking off is quoted as a factory fix in the Price-Williams book, the story I got was the engines were over advancing with the huge suction and running their big ends when then the proud owner lifted off the gas suddenly at 100+mph.. the mind boggles. Peter Posted by Peter Martin 22/02/2015 14:28:06