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Triumph TR6 - Timing: A poll.

The car is running great: CNB! (Currently not Broken) but ...
I occasionally experience a shotgun-like blast of a backfire on deceleration. I have been tweaking the timing for optimum performance vs. fuel economy and have to correct it to alleviate this problem. It's definitely out the exhaust rather than intakes. So:
advance or retard?
Any thoughts or comments would be appreciated.


Sounds like you should advance it but before that you might want to check the tolerances on the tappets. If too tight they may be opening the exhaust valve too early. I believe the setting should be 0.010 inches when cold but please confirm from before proceeding.

Michael S. Petryschuk

Thanks Mike. That's likely the problem! I adjusted the valve lash last week and one gap was defineitely too tight.

Could be a carb problem. Getting a build up of too much raw fuel in the exhaust system. Have you checked your mixture?

By the look of the plugs, the mixture is good. I'm still getting a lot of "grumbling" out the back during deceleration, particularly down hill after a fast stretch at temp. Not those nasty BANGS anymore but still angry dog rather than purring kitten. Does anyone else notice this? Maybe I'm being overly fussy.


......."a lot of grumbling out the back during deceleration" is generally a reliable indication of a mixture a bit on the lean side. Try checking this.
Good Luck!


Had the same backfire problem with with my '75 TR6. After I installed a set of Monza Exhaust, the problem really got bad. I tried all the timing, carb-tuning, valve, etc. suggestions and it only improved a small amount. I found the solution on page 231 of the Haynes Triumph TR5&6 Owners Workshop Manual.

According to Item 15b, "The combined diverter and relief valve is incorporated to divert the air from the pump during deceleration to prevent backfiring. The relief valve allows excessive air pressure at high engine speeds to discharge into the atmosphere". What this means is that if your vacuum is not strong enough to activate the diverter, your smog pump is still pumping cold air into the exhaust manifold during deceleration instead of being diverted away from the exhaust. Cold air plus hot exhaust equals a lot of popping.

I connected the diverter vacuum line to the intake manifold for a stronger vaccum and now it runs like a top. I'll troubleshoot the weak vacuum later. Even though we don't like all the smog equipment, if it's there we've got to make sure it works or you'll have problems and end up trouble shooting elsewhere.

Good Luck
B Hinson

Are the dashpots topped off with their favorite beverage? Mine was backfiring on decel, dashpot oil was leaking out almost overnite.... Rick C might jump in here about replacing o-rings!
Rod Nichols

Diverter vacuum? Air pump? I didn't know I had either. It sounds like the right track. I'll look it up in the manual but I'm certain there's no air pump on my engine.

I've tried many things; carb check, dash pots, timing but I still get a massive shotgun-like blast after heavy-on-the-gas deceleration. I disconnected the PCV years ago. Otherwise running well at 21 mpg.

Still seeking solutions,


This thread was discussed between 12/08/2005 and 31/08/2005

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