Ignition Timing Noob Issue...
Thread Starter
Racer
Joined: Jul 2017
Posts: 364
Likes: 103
From: Atlanta
Ignition Timing Noob Issue...
Hey guys, I have been trying to do a lot of reading on timing and tuning, but it has been a long time since I have messed around with a distributor. I have read articles, watched videos, and searched the forum for information. My stock drivetrain 1980 is having a part throttle cruise miss issue that I am trying to troubleshoot.
I suspect that it has to do with my ignition timing/vacuum advance. The vacuum advance can works. I tested it and it holds vacuum and the arm moves, no problem. When I check my timing with the vaccum advance disconnected and plugged I am getting 2* at idle, 12* at 2000rpms, and 16* at 3000rpms... but I am not seeing any change when I reconnect the vacuum advance...
I have been assuming that it means I have a vacuum leak somewhere in the line to the vacuum can, but even with that being said, those timing numbers don't seem to be correct. Does that sound right for a stock, smog equipped 1980?
I suspect that it has to do with my ignition timing/vacuum advance. The vacuum advance can works. I tested it and it holds vacuum and the arm moves, no problem. When I check my timing with the vaccum advance disconnected and plugged I am getting 2* at idle, 12* at 2000rpms, and 16* at 3000rpms... but I am not seeing any change when I reconnect the vacuum advance...
I have been assuming that it means I have a vacuum leak somewhere in the line to the vacuum can, but even with that being said, those timing numbers don't seem to be correct. Does that sound right for a stock, smog equipped 1980?
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,809
From: Cool Northern Michigan
Well, the Mechanical Adv is moving, that's a good sign. Your Vac Adv hose is likely at the wrong location. Should be hooked to manifold (full) Vac port and not the timed port.
Your initial is too low. Loosen the dizzy and shoot for at least 10-12*.
And most Ign problems are Carb issues.
Most Carb problems are IGN issues
Your initial is too low. Loosen the dizzy and shoot for at least 10-12*.
And most Ign problems are Carb issues.
Most Carb problems are IGN issues
Disabled by request 8 JUN 2025
Joined: May 2019
Posts: 329
Likes: 94
From: St. Augustine, FL
Vacuum Source
Do you have full vacuum at the port on the carburetor where you are hooking the vacuum advance up to??...If in doubt check with a vacuum gauge,,,,,,,.....Does your 1980 model have thermostatic vacuum ports on the thermostat housing, allowing vacuum to the distributor only when the engine is up to temperature..........Your timing numbers aren't too far off for a smog motor and show that at least the mechanical advance in the distributor is working.
Tech Contributor
Joined: Aug 1999
Posts: 14,373
Likes: 6,371
From: At my Bar drinking and wrenching in Lafayette Colorado
Your timing is certainly off (really retarded), but misadjusted timing won't cause a part-throttle miss at cruise. You have other issues causing the miss.
Lars
Lars
Thread Starter
Racer
Joined: Jul 2017
Posts: 364
Likes: 103
From: Atlanta
Well, after some tinkering, so far so good...
Thanks for mentioning the thermal valve, all of my emissions stuff is there, and I think the valve was never activating, so my vacuum advance was not doing anything. I plugged the valve and ran the vacuum advance directly to ported vacuum on the carb, and that fixed that.
I also adjusted the timing to 6*, and now it is a lot peppier, and the miss seems to be gone... So, a combination of adjusting the initial timing and connecting the vacuum advance to ported vacuum seems to have solved it...
Now, ported or manifold vacuum for the vacuum advance... I guess I'll have to drive a while and fiddle some more...
Btw, I am getting 16lbs vacuum at 600 rpm idle.
Thanks for mentioning the thermal valve, all of my emissions stuff is there, and I think the valve was never activating, so my vacuum advance was not doing anything. I plugged the valve and ran the vacuum advance directly to ported vacuum on the carb, and that fixed that.
I also adjusted the timing to 6*, and now it is a lot peppier, and the miss seems to be gone... So, a combination of adjusting the initial timing and connecting the vacuum advance to ported vacuum seems to have solved it...
Now, ported or manifold vacuum for the vacuum advance... I guess I'll have to drive a while and fiddle some more...
Btw, I am getting 16lbs vacuum at 600 rpm idle.
Last edited by Black72GTS; Apr 24, 2020 at 05:36 PM.
Thread Starter
Racer
Joined: Jul 2017
Posts: 364
Likes: 103
From: Atlanta
Ok, well I spoke too soon...
It was idling at 600rpm when I was done making adjustments, but as soon as I got in the car and drove it the idle jumped back up to 1000rpm... My idle screw is all the way out, not touching the throttle at all... Idle mixture screws are at 3 turns out, adjusted using a vacuum gauge.
Back to tinkering...
It was idling at 600rpm when I was done making adjustments, but as soon as I got in the car and drove it the idle jumped back up to 1000rpm... My idle screw is all the way out, not touching the throttle at all... Idle mixture screws are at 3 turns out, adjusted using a vacuum gauge.
Back to tinkering...
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
you went from 2 degrees to 16 with mech advance. at 3000. put the light back on. now that you are at 6 initial, you are around 20 at 3000. rev it higher. stays at same advance reading? now, put timing at 3500 or at 30 to 32 degrees. your initial is now gonna be 16 to 18. now feel the end of the vacuum hose. you can tell if it has vacuum or not. now when you hook up vacuum you should see idle jump up another 8 to 10 and 3000 light load should be over 40. and you are still too retarded.
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Thread Starter
Racer
Joined: Jul 2017
Posts: 364
Likes: 103
From: Atlanta
you went from 2 degrees to 16 with mech advance. at 3000. put the light back on. now that you are at 6 initial, you are around 20 at 3000. rev it higher. stays at same advance reading? now, put timing at 3500 or at 30 to 32 degrees. your initial is now gonna be 16 to 18. now feel the end of the vacuum hose. you can tell if it has vacuum or not. now when you hook up vacuum you should see idle jump up another 8 to 10 and 3000 light load should be over 40. and you are still too retarded.
I'm not exactly sure what you are telling me to do... Are you saying to set my initial to 16-18 degrees?
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,809
From: Cool Northern Michigan
Never use factory timing settings. That WAS all smog crap specs to keep the Feds off of GMs ***. Use the performance specs.
And, I bet your dizzy clamp is not as tight as you thought. Hint-hint.
And, I bet your dizzy clamp is not as tight as you thought. Hint-hint.
Last edited by HeadsU.P.; Apr 25, 2020 at 04:18 PM.
Racer
Joined: Sep 2019
Posts: 349
Likes: 44
From: Spokane, WA
Also, once you have set your timing, do pull every spark plug, check the gap, and install using factory torque specs unless you have aftermarket Aluminum heads.
I had a similar problem and found two plugs at .025 vice .035. Re-gapped them all to spec and now zero hesitation.
I had a similar problem and found two plugs at .025 vice .035. Re-gapped them all to spec and now zero hesitation.
Dr. Detroit
Joined: Mar 2012
Posts: 10,095
Likes: 4,026
From: New Braunfels Texas
You should be at 36 degrees total at or over 3000 rpm. The initial (idle) number means very little, but recorded it for reference. Your vacuum advance should be hooked to full manifold vacuum so it can take advantage of the advance at idle.
Jebby
Jebby
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
Ok, well I spoke too soon...
It was idling at 600rpm when I was done making adjustments, but as soon as I got in the car and drove it the idle jumped back up to 1000rpm... My idle screw is all the way out, not touching the throttle at all... Idle mixture screws are at 3 turns out, adjusted using a vacuum gauge.
Back to tinkering...
It was idling at 600rpm when I was done making adjustments, but as soon as I got in the car and drove it the idle jumped back up to 1000rpm... My idle screw is all the way out, not touching the throttle at all... Idle mixture screws are at 3 turns out, adjusted using a vacuum gauge.
Back to tinkering...
Race Director
Joined: Mar 2017
Posts: 11,923
Likes: 4,495
From: Virginia
You changed your vacuum advance from a null-source to a ported vacuum. You are halfway there! Find a manifold vacuum source on your carb (easy with the finger method!). It my 80, I used the port on the front driver side of the Q-Jet. I also got rid of the vacuum switches and many of the vacuum hoses.
With fewer vacuum hoses, it is a lot easier to find vacuum leaks, which could cause your idle issue.
Last edited by Bikespace; Apr 25, 2020 at 11:44 AM.
Old Pro Solo Guy
Joined: Dec 2017
Posts: 8,033
Likes: 4,388
From: Marlton NJ
.Sounds like a lean miss to me due to a vacuum leak. The idle screws 3 turns out sounds like another clue.
Get the timing correct first, use Lars 36 degree total method.
Then start trouble-shooting for vacuum leaks.
Lots of possibilites: carb base, intake manifold, lots of vacuum hoses, power booster, etc. etc
Disconnect all that stuff from the engine and see if driveabilty changes. Then start working thru it all with a vacuum gauge one at a time.
I found a hose with a huge crack just last week between the head and the firewall.
Get the timing correct first, use Lars 36 degree total method.
Then start trouble-shooting for vacuum leaks.
Lots of possibilites: carb base, intake manifold, lots of vacuum hoses, power booster, etc. etc
Disconnect all that stuff from the engine and see if driveabilty changes. Then start working thru it all with a vacuum gauge one at a time.
I found a hose with a huge crack just last week between the head and the firewall.
