Lumenition ignition systems

[Tinman]

Having only owned my TR6 (twin Carb) for about 8 months and garaged it for the winter I thought it about time I started to get it ready for next years happy times. The car is fitted with a lumenition system, what concerns me is the fact that the Vacuum advance pipe has been removed and blanked off, is this normal?

Can anyone comment on their experience with this type of ignition system?

Thanks

Tinman

Christmas wishes to one and all

[Victor1155]

Hi Tinman

 

Welcome to the Forum, I am sure that you will find that your decision to join the Register was the right one. As far as the Vac hose to your distributor, 6's for the UK market should not have a vacuum advance pipe fitted to the distributor however US ones usually have 2 fitted for advance and retard. If you do fit one it will advance the ignition timing too far causing severe pinking and possible damage. If you want further advice use the search facility within the forum and I'm sure that you will find more than enough information on the subject.

 

Brgs & Merry Christmas

 

Steven H

[ChrisR-4A]

Hi Tinman, I thinkyou will find, and others will soon agree or correct me, that the vacuum system moves the distributor base plate clockwise which actually retards the ignition as vacuum increases (at low revs) so without this feature the engine is more likely to pink and if retarded to combat this then at higher revs will be under advanced and lack power. I always assumed that all road engines using carbs required a vacuum advance/retard system.

 

Best regards, Chris

[jean]

Yes, in fact the advance is there to retard the ignition under engine load, some U.S spec Tr6 had advance and retard system to improve emission at idle.

The ignition advance itself is managed by bob weights and springs as far as I know. isn't it . As a dense mixture burns faster than a lean mixture the spark advance is progressively retarded with increased manifold pressure and to prevent flame front collision in the cylinder , to say knocking. The criterias for detonation are: Lean Mixture, High Compression and Low-octane Fuel. The reason that carburettor engines are more critical to detonation is because the vaporization and combustion is less efficient than on fuel injected engines.

[Tom Fremont]

Hi Tinman,

 

As I think the factory manuals will confirm, vacuum retard is supposed to be active only when idling; this achieved by a little plunger switch located beside the linkage bell crank under the front carb. This feature improved the emissions when idling, and was deactivated as soon as the throttle was depressed. Hence, the only effect from disabling it will be a higher idle speed which can easily be adjusted downward to compensate.

 

My experience with a stock TR250 was it ran more smoothly without any vacuum advance ( both advance and retard functions were fitted to these distributors ). I always got a kick out of the retard feature though, as it made the idle purr like a kitten. Difficult to maintain the switch, though, as it became wet with condensed vapours of who-knows-what and would cease to function until dried out over a period of days. Out of production, this switch is very hard to come by. I eventually learned the trick to keeping it dry, which I will reveal to any who apply with the right inducement .

 

As for pinging, I proffer the theory that it has to do with effective compression pressures more than ratio. When I went to a high overlap cam ( like the 150 BHP cam ) I found the pressures to be significantly lower than with the stock '250 cam, even though the c/r was higher in the former. Never had pinging problems with a high overlap cam .

[Tinman]

Thanks for the advice but I still don't know if I should re-connect the vac pipe or not?

[Tom Fremont]

Sorry about the surfeit of prose - here's a practical suggestion:

 

1) Ascertain whether the vacuum unit is advance or retard.

 

2) If advance, leave it disconnected, but plug the feed orifice so as to prevent vacuum leak and the attendent poor idling which can result.

 

3) If retard, connect it only if the vacuum switch referenced above is functioning; it should move visibly when the throttle linkage just comes off idle. If this doesn't happen, leave it off unless or until the vacuum circuit is rectified - you don't want the retard function operating except at idle. Again, be sure to plug the feed orifice.

 

Hope this helps