View Full Version : My clunk is gone too and Backfire question

06-21-2006, 06:20 PM
Well I boxed in the front differential mounts and finally got everything back together today and YAHOOO the clunk is gone. All I have left to do is get rid of the slight backfiring/rumbling when declerating. I removed the vaccum retard line from the dustributor and plugged it and took it out for a test run. Well the backfiring is gone and all thats left is a slight rumbling. Can anyone tell me why removing the vaccum retard line might have cured the problem. Am I masking something else by doing this? Thanks

06-21-2006, 07:25 PM
Read this first post regarding vaccume advance on distributors. I'm not sure, but it might have something to do with mixture.



06-21-2006, 07:40 PM
Glad to hear you mastered the clunk.

Keep in mind that the dashpot on the distributor is there to give MORE PERFORMANCE by adjusting the advance utilizing the engine vacuum AND the centrifugal advance so as to allow the BEST timing either at partial throttle or WOT. If you do not use the vacuum system then the BEST timing will only be when the throttle is wide open; regardless of the RPM.

Now, with that said, I don't know why the rumble has disappeared when you plugged the vacuum line! /ubbthreads/images/graemlins/hammer.gif Cars with carburetors AND glass packs or relatively open exhaust all ways rumbled at least a bit with deceleration of the engine. FI cars seem to have much better manors, particularly when they are computer controlled.

06-21-2006, 08:34 PM
I should add that I have electronic ignition. Also mine is a 73 TR6 and they didn't have vaccum advance they have vaccum retard which if I understand it correctly it retards the engine at idle to lower the emissions. I'm not sure if it works when your crusing. Maybe someone can answer that question. Thanks

06-21-2006, 08:50 PM
I am not sure on the logic of this...but check your coil...I had similar backfiring on my 80 MGB until I changed the coil, now it purrrrs.

06-22-2006, 12:34 AM
Do you have two dashpots or 1? If you have the 1 dashpot dizzy, then the vaccum advance/retard works constantly until you get the RPM's high enough to cut out the need for the vaccum portion of you timing control.

06-22-2006, 06:29 AM
I have 1.

06-22-2006, 06:52 AM
The vacuume retard setup was purely emissions. It did very little for performance. What happens is that on De-cel you get your strongest vacuume, so the dashpot pulls the timing instantly to its most retarded position. this causes late/ incomplete firing and the unburnt fuel gets into the exhaust along with sometimes some still burning gases from the firing being late so then you get the backfire.
THe rumble could just be the design of the exhaust, but you may also want to check your base timing. it may need to be bumped up a bit. follow the book to a "T" and make sure you have everything disconnected that should be.
And like Bruno said. definatly check the mixture setting. if it's rich it sould be causing this, or if like R6MGS said, a bad coil can do the same.
Anything that results in incomplete firing/ unburnt fuel in the exhaust.

06-22-2006, 07:49 AM
If you have the one dash pot type dizzy, then your vacuum advance does BOTH, retard AND advance, keeping the timing just right until the centrifugal advance takes full control.

06-22-2006, 04:54 PM
I wear a size 14 shoe therefore I don't need no stinkin' vacuum advance. Straight centrifugal advance.


06-22-2006, 06:02 PM
As long as you "mash the gas peddle" between the fire wall and the sole of that sz 14, your motor will not need/benefit from vacuum advance.