Triumph Repair: Backfire through carb, volt coil, gasoline engine

Question
QUESTION: Hello Howard,

I have a '79 Spit, I had rebuilt the carburetor (150D), re-installed and started car, engine ran really bad then quit. Re-checked the carb thinking I had done something wrong, but found I had no spark. Coil was good. So the original Opus had died on me. Replaced with a Pertronix breakerless. Hooked up and now will backfire through carb and quit. I did nothing with the timing and was told this is a direct swap. Have not checked the static timing yet, can't see as that would be the problem. This car does have the 6 volt coil, could this be a problem with the Pertronix?

ANSWER: Hi Steve,

I fund while working in dealerships that it is a waste of time to guess what is wrong when an engine don't start or runs badly.

You need to test systems first and there are only three systems that make a gasoline engine run.

Compression, Fire and Fuel. (with conditions on each)

Your tests must be done in that order and not skipping anything. It does not matter that any system was working right a week ago, a day ago or a hour ago. You need to test it for now.

Run a compression test. (compression gauges are not expensive) You need to see 125 PSI to 170 PSI on all cylinders with very little difference between cylinders and the test MUST be done with the throttle open. If you have a cylinder close to 100 or below it is no use proceeding until that is corrected. If you have a cylinder down close to or below 100 PSI let me know and I will give you further tests.

If and ONLY if the Compression is ok, you can proceed to "Fire" (ignition).
You can't change from the Opus/Lucas system to any aftermarket system like Pertronix without resetting the timing. And confirming that it is firing the correct plug wire.
Here is an old mechanics trick to do a rough test of ignition without any test equipment.
Remove all the plugs and place each on it's plug wire and lay all the plugs on a metal part of the engine so you can see the sparks. Pick a cylinder (any cylinder) and put your thumb over that plug hole and watch the spark of that plug while some one spins the engine over.
The compression will blow your thumb off of the plug hole on the compression stroke and it will make a "Pop" sound. At the same time the spark will make a "Click" sound and it should be a thick spark as thick as a pencil lead and be blue in color (not is bright sun light).
If it seems as though the "Pop" is causing the "Click" the timing is close enough to start. It does not mean the timing is correct, just close enough to start. If it goes "Pop"-"Click" or "Click"-"Pop" the timing is off and it may not start.
If you check each cylinder the same way and they all are correct, you have confirmed the firing order is correct too. (Very important)

Only if the first two systems are correct as outlined can you even think about Fuel.
This one is easy as all you need to do is get a spray can of starting fluid from any auto parts store and spray it into the intake when trying to start it. If it starts and quits but will keep running only if you keep spraying into the intake, you have a Fuel related problem. If it does not start for a second or two then you didn't do the first two systems testing correctly. If it starts and only runs if you keep spraying then let me know and I will give you the next tests for the Fuel system.

Howard

---------- FOLLOW-UP ----------

QUESTION: Thanks Howard, very good advice. However I was able to borrow a timing light and I think confirmed what I thought. Timing is all over the place. Started out good then slipped and I lost it. So I am assuming that I have a timing chain that skiped a tooth or more due to a bad tensioner?? Also concerned over what I think is a weak spark, coil checks OK but I don't have what I would call a healthy spark.

Answer
It is unusual for a timing chain to jump a tooth but it can happen. If it does it don't just effect the ignition timing, it changes cam timing and can bend valves and cause other damage. I would check for excess free play in the distributor shaft in rotation and for any side play too.
Check the spark from the coil not a plug wire to see if it has a thick blue spark (not in bright sun light).

Howard