Vacuum timing
Thread Starter
Burning Brakes
Joined: Jul 2009
Posts: 1,237
Likes: 288
From: ESCONDIDO CA
2025 C2 of the Year ('63 and '67) Finalist - Unmodified
C2 of Year Finalist (appearance mods) 2019
Vacuum timing
I just had a minor tune-up done on my '69 big block. The tuner said he timed the motor using a vacuum gauge because the timing pointer was too far away from the harmonic balancer timing mark. Ultimately reporting that he had to retarded the timing.
All this is pretty unfamiliar ground for me but I do have a working vacuum gauge attached to my motor. Before the tune it read 19, it now reads 15 at idle (around 850rpm).
Is this a reasonable way to tune the car? and is the change in idle vacuum good, bad, or doesn't matter?
Would like some experienced comments please.
All this is pretty unfamiliar ground for me but I do have a working vacuum gauge attached to my motor. Before the tune it read 19, it now reads 15 at idle (around 850rpm).
Is this a reasonable way to tune the car? and is the change in idle vacuum good, bad, or doesn't matter?
Would like some experienced comments please.
Melting Slicks
Joined: Oct 2005
Posts: 2,056
Likes: 8
From: Redruth Cornwall
Not sure but if the guy you took it to knows how to time these cars correctly then it sounds as though you have a problemm with a slipped balancer. Quite a common problem and once fixed, timing will be a breeze! 
Tech Contributor
Joined: Aug 1999
Posts: 14,370
Likes: 6,364
From: At my Bar drinking and wrenching in Lafayette Colorado
I just had a minor tune-up done on my '69 big block. The tuner said he timed the motor using a vacuum gauge because the timing pointer was too far away from the harmonic balancer timing mark. Ultimately reporting that he had to retarded the timing.
All this is pretty unfamiliar ground for me but I do have a working vacuum gauge attached to my motor. Before the tune it read 19, it now reads 15 at idle (around 850rpm).
Is this a reasonable way to tune the car? and is the change in idle vacuum good, bad, or doesn't matter?
Would like some experienced comments please.
All this is pretty unfamiliar ground for me but I do have a working vacuum gauge attached to my motor. Before the tune it read 19, it now reads 15 at idle (around 850rpm).
Is this a reasonable way to tune the car? and is the change in idle vacuum good, bad, or doesn't matter?
Would like some experienced comments please.
If the engine does not have a timing tab, or if the tab is mislocated, you need to use a TDC finder (or follow the procedure outlined in my TDC Tech Paper) and mark TDC accurately. Once marked, you need to set the timing correctly and check all parameters of the timing and the timing curve. If the balancer has slipped, it needs to be replaced.
Lars
Tech Contributor
Joined: Jun 2004
Posts: 20,847
Likes: 959
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
No, you can't get the timing even close to the optimum spec by tuning for vacuum at idle. Timing needs to be set for total timing (36 degrees total), and setting for best vacuum at idle can have the total spec off by up to 20 degrees.
If the engine does not have a timing tab, or if the tab is mislocated, you need to use a TDC finder (or follow the procedure outlined in my TDC Tech Paper) and mark TDC accurately. Once marked, you need to set the timing correctly and check all parameters of the timing and the timing curve. If the balancer has slipped, it needs to be replaced.
Lars
If the engine does not have a timing tab, or if the tab is mislocated, you need to use a TDC finder (or follow the procedure outlined in my TDC Tech Paper) and mark TDC accurately. Once marked, you need to set the timing correctly and check all parameters of the timing and the timing curve. If the balancer has slipped, it needs to be replaced.
Lars
I agree that a 36* target at high RPM WOT is a viable target, but leaving the low RPM timing "wherever it falls" results in a soggy running, inefficient engine.
Tech Contributor
Joined: Aug 1999
Posts: 14,370
Likes: 6,364
From: At my Bar drinking and wrenching in Lafayette Colorado
Advanced
Joined: Nov 2013
Posts: 79
Likes: 0
From: Casper WY.
I have always been a timing light person myself but know quite a few people whom set their timing using a vacuum gauge. One even insisted it's the best way. I think you are probably alright with where he set it.
That being said I wouldn't worry about a 4lb drop in vacuum. My question to you is how does it run??
That being said I wouldn't worry about a 4lb drop in vacuum. My question to you is how does it run??
Thread Starter
Burning Brakes
Joined: Jul 2009
Posts: 1,237
Likes: 288
From: ESCONDIDO CA
2025 C2 of the Year ('63 and '67) Finalist - Unmodified
C2 of Year Finalist (appearance mods) 2019
Interesting comments. I gave permission to him to change the balancer and mark if it would benefit the outcome. He elected not to do the change. Also I don't think he set the timing at vacuum idle though not sure of the actual procedure.
It seem to run fine, though much like it did before the work. Ran it around this weekend a bunch without issue but worry some about overheating when the weather really warms up.
I'm running a 750 Demon, vacuum secondary's with a mild cam. The exhaust is a bit restricted with factory sidepipes so no race car.
Another reason I had the work done was to improve the exhaust smell, not really a lean stink but still a bit unpleasant. He told me that a mild cam (other than stock) will always contribute to stinker exhaust.
It seem to run fine, though much like it did before the work. Ran it around this weekend a bunch without issue but worry some about overheating when the weather really warms up.
I'm running a 750 Demon, vacuum secondary's with a mild cam. The exhaust is a bit restricted with factory sidepipes so no race car.
Another reason I had the work done was to improve the exhaust smell, not really a lean stink but still a bit unpleasant. He told me that a mild cam (other than stock) will always contribute to stinker exhaust.
Safety Car
Joined: Aug 2000
Posts: 3,711
Likes: 43
From: San Antonio TX
If the cam has an idle lope, you can get a bit of a stinky exhaust if you just leave things be. This is due to cyclic variation in the cylinders. It's the same thing that makes a 2-stroke at idle sound like a popcorn maker. There can be insufficient mixture and pressure in the cylinder to have full combustion. It ends up being a soft misfire but gets better saturated on the next cycle and does burn.
The factories made many performance cars with fairly large cams and they didn't have stinky idle exhaust so it would seem there is something in your tune up that is causing it to be that way.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
2027 Corvette vs The World: Every C8 vs Its Closest Competitor
Joe Kucinski
10 Most Common Corvette Problems of the Last 20 Years!
Joe Kucinski
Melting Slicks
Joined: Feb 2006
Posts: 3,105
Likes: 96
From: Sunrise FL
[QUOTE=1969RAY;1589558981]
All this is pretty unfamiliar ground for me but I do have a working vacuum gauge attached to my motor. Before the tune it read 19, it now reads 15 at idle (around 850rpm).
QUOTE]
This is NOT ok, a mild cam on healthy engine would give you about 19, or what you had before. Going to 15 is going in the wrong direction.
Have him advance the dizzy a little till it gets back to 19 (another clue is if the idle increases, then you are on the right track). Or even better, FIND ACTUAL TDC.
All this is pretty unfamiliar ground for me but I do have a working vacuum gauge attached to my motor. Before the tune it read 19, it now reads 15 at idle (around 850rpm).
QUOTE]
This is NOT ok, a mild cam on healthy engine would give you about 19, or what you had before. Going to 15 is going in the wrong direction.
Have him advance the dizzy a little till it gets back to 19 (another clue is if the idle increases, then you are on the right track). Or even better, FIND ACTUAL TDC.
Tech Contributor
Joined: Jun 2004
Posts: 20,847
Likes: 959
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
My apologies for the lack of clarity. The second paragraph was intended for the OP. He seems like a novice to the tuning world, and I didn't want him to infer that the lower RPM timing should be ignored as long as the upper RPM WOT timing is okay. There have been posters in other threads where the advice was to leave the idle timing wherever it happened to fall. I believe we share the same perspective that that isn't an efficient timing curve.
Thread Starter
Burning Brakes
Joined: Jul 2009
Posts: 1,237
Likes: 288
From: ESCONDIDO CA
2025 C2 of the Year ('63 and '67) Finalist - Unmodified
C2 of Year Finalist (appearance mods) 2019
We surely get used to less exhaust smell these days but my first car was a 55 ford back in 1965--my memory is still good enough to sort out the difference.
Melting Slicks
Joined: Jan 2014
Posts: 2,478
Likes: 254
From: Downers Grove Illinois
Thread Starter
Burning Brakes
Joined: Jul 2009
Posts: 1,237
Likes: 288
From: ESCONDIDO CA
2025 C2 of the Year ('63 and '67) Finalist - Unmodified
C2 of Year Finalist (appearance mods) 2019
Advanced
Joined: Nov 2013
Posts: 79
Likes: 0
From: Casper WY.
I find that sometimes the exhaust on mine smells and sometimes not. I never gave it a whole lot of thought until now. Could be an issue of where you get your fuel at. You may want to try different stations and fuels without alcohol and see if that makes a difference. Could go to the airport and pick up some really high octane fuel and see what that does..lol..Just kidding...As far as the timing if you are running fine. It isn't hard starting your fine. I would just run it..
Le Mans Master
Joined: Jul 2006
Posts: 8,117
Likes: 104
From: charlotte north carolina
Mea Culpa! I'm the poster that says "let the initial timing fall where it may". My question is, if the distributor is set up properly with the correct amount of mechanical advance, what's wrong with "letting it fall"? Not being an expert, I'm more concerned with total than initial advance.
Tech Contributor
Joined: Jun 2004
Posts: 20,847
Likes: 959
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
Mea Culpa! I'm the poster that says "let the initial timing fall where it may". My question is, if the distributor is set up properly with the correct amount of mechanical advance, what's wrong with "letting it fall"? Not being an expert, I'm more concerned with total than initial advance.
When you set the timing at 36* @WOT & 3000 RPM (or whatever point you choose), you are making a best guess adjustment of the timing where the engine will spend 1% of its time. Your engine (assuming it's not a race engine in a race car at a race track) spends 99% of its time at less than 100% throttle and less than 3000 RPM. Letting the lower load, lower RPM timing fall "where it may" just results in lower fuel mileage, crappier idle quality and drivability, and hotter exhaust valve and coolant temperatures.
Playing with the idle and low RPM timing to get higher/highest manifold vacuum and best drivability will give you the "other side" of your engine's desired timing curve. There's then members on this forum who can refurbish/recurve your distributor to match the total spectrum of your engine's timing requirements.
Thread Starter
Burning Brakes
Joined: Jul 2009
Posts: 1,237
Likes: 288
From: ESCONDIDO CA
2025 C2 of the Year ('63 and '67) Finalist - Unmodified
C2 of Year Finalist (appearance mods) 2019
So can I infer from this post that lower vacuum will result in higher coolant temperatures?
Race Director
Joined: May 2006
Posts: 18,665
Likes: 2,564
lower initial timing will cause higher coolant temperatures. As far as the low end timing number to aim for I go for 12 BTDC with no vacuum connected. Once the vacuum is connected I end up with about 22. Some people have numbers higher than 12 but it is doubtful it would be practical to go beyond a maximum of 20. The factory distributor came with a nylon/plastic advance stop. Over the years it wears out and most spring kits contain a brass replacement bushing. If you set your timing for a maximum of 36 and then find the initial is too low it maybe because the bushing is shot. The other cause is the slot that the bushing rides in is too long.