1968 L-71 Tuning

[Gary Noser]

I took out the power valve bock-off and went with stock jets (#63) and power valve (#65). I checked the plugs and they where fine. Now the car runs pretty well. However it does still backfire out the tail pipe but much less frequently. Over all it is running well. So maybe leaning out the mixture a bit could help the occasional backfiring? The problem of run on still exists as well. I think first I will set the timing from 6, where it is now, back to 4. This should help the run on correct? And from there if I still have backfiring check the mixture. I set the mixture using a vacuum gage with the highest vacuum reading at idle being 13 inches.

Thanks for all the help,

Gary

[Duke W.]

Re: 1968 L-71 Tuning

The "backfiring" you refer to is actually "afterfiring". The mixture is not ignited by the spark plug and then autoignites in the exhaust system.

"Backfiring" is when the engine spits back through the carburetor.

Afterfiring can be caused by TOO LEAN a mixture or a leaky valve, which can be verified with a compression or leakdown test.

Running a SHP engine, espcecially a BB on the street without a vacuum advance is a recipe for overheating.

Even the OE vacuum advance setup is less than ideal because all L-72/71 were set up for CA emissions with a ported vacuum advance to avoid having two configurations of a low voluem engine.

An easy way to implement a full time vacuum advance on these engines it to tee the vacuum can signal line into the choke vacuum break line.

L-72's have a suitable OE spec vacuum can that provides full vacuum advance at 12", which is less that idle vacuum. L-71 and L-78 OE vacuum advances don't pull the plugger all the way until 15.5" which is less than the typical 14" idle vacuum, so I recommend either a L-72 vacuum can or the NAPA/Echlin VC1765 (16 deg. @ 12").

Also, L-71 has less initial timing but a quicker curve than L-72, so it might be a good idea to slow the centrifugal and increase the initial to about 8-10. This may yield better overall performance, but a key point is that the initial and centrifugal combination must be suitable to the octane fuel you are running.You can't run a very aggressive ignition curve on pump gas.

Duke