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Emissions and Timing

Started by september2013, September 29, 2013, 11:05:35 PM

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I have continued to have problems with my SMOG checks here in California on my 1979 MGB roadster.  I found that when I disconnect the hose from the charcoal canisters that connects to the Zenith Stromberg carb the car checks fine.  Of course this is against the rules, but the smog people didn't catch me, so I guess I have a couple more years before I have to do this again.

I have played with the timing in an attempt to get the car to pass with the hose connected and I have timed the car without a timing light.  I just advanced the distributor until the engine ran at it's smoothest.  It is running great and has very good power and not overheating problems.

The question is if I have timing advanced too far can I damage valves or piston even though I have no pinging and the car is running better than it has in a long time.  I have noticed when idling that it will act like it loses spark and then fires normally and continues to run smoothly.  I notice this only when idling.  I have a new solid state ignition installed.  I will call this a hiccup as it stumbles and acts like it is going to die, but catches itself and runs fine.

Sorry that this is so long, but tried to cover the basics of what is going on. Thanks in advance for your response...

Lyle Abel


The MGB is a difficult car to meet stringent emission standards.  The oil-damped carbs are finicky and components of the arcane patched-into-place emissions systems actually work better when removed, as you are finding out.

If disconnecting the charcoal canister helps the emissions situation, you can always stealthily plug the line (i.e.: a piece of tubing inserted at the canister end with a pencil or sealed with silicon inside the hose somewhere, so it it not visible from the outside, with a connector between the end of the current line and the canister).  If allowing air into the open disconnected line is necessary, then a 'T' inserted between the plugged line and the end of the original line instead of a connector would accomplish this without being obvious.

The "T" and plugged section of line can be removed and the original connection made after it passes.

On your question of timing, normally advancing the timing too far will result in pinging, higher operating temperatures and eventual engine damage.  If the distributor is not working up to standard and advancing the timing results only in better acceleration, but the engine temp remains normal and no pinging results, it is compensating for a deficiency which will manifest in other ways (the stalling/stumbling you are experiencing), but may not present any potential damage.

The timing question and what you are seeing has more interest to me than the damage you are concerned about. The 79 should have a fully electronic "Opus" distributor that had a vacuum advance system. While these where better than the previous version, they were prone to failure and have often been replaced with earlier points-type distributors, also with vacuum advance systems. I mention both these more than fully aftermarket distributors as the vacuum and mechanical systems on these were particularly prone to deterioration and failure, so the need to advance the timing becomes necessary to compensate for their lack of performance.
You might find rebuilding or replacing the distributor will help with performance, including the emissions issues.
Art Isaacs
AMGBA Tech Staff


Thanks so much for your detailed response Art. I am going to back off the timing a little.  It is running great with good power at higher RPM, but maybe I can eliminate the "stumbling" at idle if I retard the timing some.  This is a new issue and I probably have the timing advanced too far.

The smog issue is a continuing problem for me.  I have replaced the anti run on valve, the canisters, had the carb rebuilt several times, have new ignition etc etc....but disconnecting the hose on the input to the carb does the trick.  

Thanks again for taking the time to respond...

Lyle Abel


Always glad to try to help.  Just a note of interest.  At a show this past weekend, I saw a Triumph GT6 with fuel injection hidden in the Stromberg carb body.  An interesting approach, in this case to keep the car looking original while improving performance, but could also be a fix for your issues.

If interested, I can contact him for details.

Safety Fast!

Art Isaacs
AMGBA Tech Staff

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