73/timing

[pizza3260]

hello!
i can not get my timing down to 12 btdc,it will backfire,spit gas out the carb, when i go to 14/15 btdc it fine. why can't i get it to specifications. it runs good & strong @ the higher setting.

tom b.

 

[Rob]

Hi Tom,

Please post your C3 questions in the C3 forum so that we can keep the Introductions Forum for our new members. Thanks.

Moving this thread to the 1968-1982 Forum.

 

[Jack]

/tom ... i can not get my timing down to 12 btdc,it will backfire,spit gas out the carb, when i go to 14/15 btdc it fine. why can't i get it to specifications. it runs good & strong @ the higher setting.

Hi Tom ... thanks for moving this over Rob.

First look at distributor ... assuming proper gap/dwell & good condensor ... (1) check that vac can advance/retard is truly operating & that it holds a vac ... also check there is no vac leak from can to manifold etc. (2) check mech advance ... both springs-both weights are moving freely thoughout their range of motion ... clean off rust/smutz. If really old/weak, consider replacing springs.

Next, harmonic balancer/damper's outer ring (where timing mark is) may have slipped ... it happens ... 'specially if old ... rubber degrades from heat/oil/time/mileage. If slipped, that zero-TDC mark/groove is no longer zero (becomes anyone's guess) ... mark will continue to slip ... will not give a true baseline. I have no proof, but physics of torque would seem to indictate that when outer ring does slip, it would slip more upon acceleration ... and that the timing mark would become more & more retarded in relation to true crankshaft position. If my hypothesis is correct, this would require the timing mark to be aligned in a (falsely) advanced position to set normal timing. G'luck!
JACK:gap

 

[JohnZ]

I'm assuming you have your vacuum advance disconnected and plugged when you're setting your base timing; you can't do it with the vacuum advance operating.

Do you know that the balancer and the timing cover are both original? The '69 and later balancers had the timing index mark shifted 9 degrees counterclockwise from the keyway, and the timing tab on the cover was moved to match. If it has a pre-'69 timing cover on it, what appears to be 14/15 degrees is actually 23/24 degrees, way over-advanced for base timing, which will cause the symptoms you describe.

I'd get a piston stop and verify exact #1 TDC vs. the index mark and its lineup with the "0" mark on the cover tab just to be sure you don't have either a slipped balancer outer ring or the wrong timing cover tab. If that checks out OK, you'll have a solid baseline to work from, and then it's time to look at the distributor.