Should I worry about this "total time" thing? Supposedly 32 * to 36* at 3000 RPMs???
CK5
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Spark Advance, Timing and Air/Fuel mixture Ported or Full Manifold Vacuum
- Thread starter Chief Brody
- Start date
That is adjusted with the springs in the distributor. Set idle timing at 8 to 10. Adjust idle, check timing. Then fine tune the idle air mixture. Set idle rpms and check timing.
Could be idle was too high and mechanical advance was in play when you set the timing specially if you had in neutral.
This would cause mechanical advance to drop out when placed in gear and rpm drops and retard timing too low and cause your issue.
Rpm drop of say 200 or more would indicate this.
Reving would cause the mechanical advance to come back into play and run better as you say.
You might want to look inside the distributor for missing/broken advance springs.
10 deg should be plenty for initial base timing with the advance plugged at the carb or vac source at say 650-750 rpm I believe in gear. Would have to look it up.
Reset your rpm to say 650 in gear, vac advance disconnected and plugged at the source. Set your timing to 10 initial then recheck your rpm and adjust then check timing again. More timing equals more rpm.
Reconnect vac advance and reset rpm if increases.
12 deg might cause hot start problems.
Don’t worry about that 36deg thing.
Are you running the point distributor, if so check your point gap and condition.
This would cause mechanical advance to drop out when placed in gear and rpm drops and retard timing too low and cause your issue.
Rpm drop of say 200 or more would indicate this.
Reving would cause the mechanical advance to come back into play and run better as you say.
You might want to look inside the distributor for missing/broken advance springs.
10 deg should be plenty for initial base timing with the advance plugged at the carb or vac source at say 650-750 rpm I believe in gear. Would have to look it up.
Reset your rpm to say 650 in gear, vac advance disconnected and plugged at the source. Set your timing to 10 initial then recheck your rpm and adjust then check timing again. More timing equals more rpm.
Reconnect vac advance and reset rpm if increases.
12 deg might cause hot start problems.
Don’t worry about that 36deg thing.
Are you running the point distributor, if so check your point gap and condition.
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Setting the timing with a vacuum gauge is usually a better method,many things can throw off the timing like a harmonic balancer that "slipped" on its rubber mounting and now shows a false reading when you use the timing marks to set timing with a light..or if a "mismatched" timing pointer and balancer was installed..
Also many old engines run better with a bit more advance than factory specs,especially on ethanol laced gas,which is leaner than the old leaded gas these old engines were designed to use..
A book I have on using alcohol fuel as a gas substitute has chapters on advancing the timing,and how to enlarge the carb jets,as alcohol has less BTU's than gas and the engine will need the mixture to be richer to run right and not overheat.and the timing advanced..
Todays 10% ethanol isn't a drastic change but can still affect the older engines ..they will tend to run hotter and leaner with ethanol in the fuel..
I never set the timing "in gear",for safety reasons,and never had a false reading leaving it idling in park or neutral..as long as the engine is idling slow,the distributor weights wont come into play..you can turn the idle speed screw on the carb to slow it down more if you suspect its idling too fast..
I once forgot I had my truck in gear while setting the timing,with the wheels chocked (E-brake wasn't worth a dam),and I absent minded-ly gave the carb linkage a rev and the truck nearly ran me over..ever since that day I time engines in neutral..
Plugging the vacuum advance hose isn't needed,all you need to do is disconnect it--but you should cork off the nipple on the carb or wherever it was attached to so a vacuum leak wont affect your carb mixture settings..a quick way to test the vacuum advance is to put the hose on a manifold vacuum source that has vacuum at idle--you'll hear the engine speed up a bit if its working..
To me it sounds like you may have turned the distributor clockwise ,which retards the timing--moving it counter clockwise advances it..
An engine with the timing retarded will idle smooth as silk,but be a real dog,lack power and possibly spit back thru the carb or exhaust under acceleration..
A quick test for the advance weight springs under the rotor can be done by twisting the rotor clockwise,it should move about 1" and snap back when you let go of it..
The weights themselves often rust up and can get stuck in either full advance or retarded position,also the pins they pivot on often get "sawed into" and the hole in the weights get egged out..this leads to erratic timing and or no advance..
Mr.Gasket makes an advance weight kit with new weights ,a few choices of springs,and little plastic bushings to fit where the pins ride on the weights..I've cheated and just used the bushing on the stock weights when I didn't feel like experimenting with the advance curve and when it'll kick in..
The carb mixture screws affect the idle mixture mostly,I doubt them being set too lean would affect the power after 1200 rpms or so,if you set the mixture with the vacuum gauge as I described in a previous post,it should be close to spot on..
Also many old engines run better with a bit more advance than factory specs,especially on ethanol laced gas,which is leaner than the old leaded gas these old engines were designed to use..
A book I have on using alcohol fuel as a gas substitute has chapters on advancing the timing,and how to enlarge the carb jets,as alcohol has less BTU's than gas and the engine will need the mixture to be richer to run right and not overheat.and the timing advanced..
Todays 10% ethanol isn't a drastic change but can still affect the older engines ..they will tend to run hotter and leaner with ethanol in the fuel..
I never set the timing "in gear",for safety reasons,and never had a false reading leaving it idling in park or neutral..as long as the engine is idling slow,the distributor weights wont come into play..you can turn the idle speed screw on the carb to slow it down more if you suspect its idling too fast..
I once forgot I had my truck in gear while setting the timing,with the wheels chocked (E-brake wasn't worth a dam),and I absent minded-ly gave the carb linkage a rev and the truck nearly ran me over..ever since that day I time engines in neutral..
Plugging the vacuum advance hose isn't needed,all you need to do is disconnect it--but you should cork off the nipple on the carb or wherever it was attached to so a vacuum leak wont affect your carb mixture settings..a quick way to test the vacuum advance is to put the hose on a manifold vacuum source that has vacuum at idle--you'll hear the engine speed up a bit if its working..
To me it sounds like you may have turned the distributor clockwise ,which retards the timing--moving it counter clockwise advances it..
An engine with the timing retarded will idle smooth as silk,but be a real dog,lack power and possibly spit back thru the carb or exhaust under acceleration..
A quick test for the advance weight springs under the rotor can be done by twisting the rotor clockwise,it should move about 1" and snap back when you let go of it..
The weights themselves often rust up and can get stuck in either full advance or retarded position,also the pins they pivot on often get "sawed into" and the hole in the weights get egged out..this leads to erratic timing and or no advance..
Mr.Gasket makes an advance weight kit with new weights ,a few choices of springs,and little plastic bushings to fit where the pins ride on the weights..I've cheated and just used the bushing on the stock weights when I didn't feel like experimenting with the advance curve and when it'll kick in..
The carb mixture screws affect the idle mixture mostly,I doubt them being set too lean would affect the power after 1200 rpms or so,if you set the mixture with the vacuum gauge as I described in a previous post,it should be close to spot on..
It's advanced too far. You set your idle timing *close*, then screwed it up by turning the distributor again. You can't/don't set total timing that way.
- Idle timing - set by turning distributor with vacuum advance disconnected. Exact RPM doesn't matter because mechanical advance won't be active till much higher. As name implies it affects the idle and off-idle.
- Mechanical advance - adjusted by changing the weights and springs in the distributor. You test it by increasing RPM until the advance stops going up. You want to know when it starts advancing, when it stops and how quickly it increases. This is "curving" the distributor. This affects mid and high RPM and determines how much power you can make.
- Vacuum advance - hook to full vacuum and measure the timing change. This is only active at cruise and is only there for MPG.
A couple other things to note:
The advance weights like to stick. There is a path from the crankcase into the distributor and this tends to coke them up. Take a look at the weights and lube if necessary so they move freely and "snap back" when released. Make sure the holes aren't ovaled out. Until that's done, there's no point messing with the timing curve.
Some carbs are a little lean in the off-idle circuit and depend on extra fuel from the idle circuit. If it stumbles a bit just off idle, you might try richening the idle a bit to see if it helps. I think sometimes the idle is fattened up too much because the off-idle circuit just needs cleaning.
The advance weights like to stick. There is a path from the crankcase into the distributor and this tends to coke them up. Take a look at the weights and lube if necessary so they move freely and "snap back" when released. Make sure the holes aren't ovaled out. Until that's done, there's no point messing with the timing curve.
Some carbs are a little lean in the off-idle circuit and depend on extra fuel from the idle circuit. If it stumbles a bit just off idle, you might try richening the idle a bit to see if it helps. I think sometimes the idle is fattened up too much because the off-idle circuit just needs cleaning.
Just to make sure the OP doesn't get confused. If you have the vacuum advance hooked up to manifold vacuum (not "ported" or "timed"), vacuum advance will change idle timing. It's important to understand the difference between ported/timed and manifold vacuum when talking about timing, and how they work.
Regardless, when setting curb idle, distributor vacuum disconnected and plugged.
I got a timing light that has tachometer built in, hope to use it tomorrow. Will report the results.
If timing is advanced too far, engines usually crank hard or at least not consistent rpm.
10* at 750 RPM , adjusted the air / fuel mix screws this time and everything worked. No messing with "total timing".
Runs good now
Runs good now
What was the timing at?
you mean before...it was at 20* BTDC
And the air / fuel mix screws actually gave me the feedback they were supposed to after the timing was set.
Now I just need to find a diagram of my carburetor and make sure I have the spark advance on the correct port.
Now I just need to find a diagram of my carburetor and make sure I have the spark advance on the correct port.
Full manifold vacuum is what you want--if you lack a vacuum gauge,an easy way to tell if its full manifold vacuum ,is to take the hose off the vacuum advance with the engine idling--it'll drop in rpm a bit if it had manifold vacuum,and you can feel the suction with your finger over the end of the hose..the nipple on the carb "should" be located at the very base,below the throttle plates,but some have it located further up..
How strong does the exhaust smell from your old 350's?
I haven't had a gas powered Chevy in quite a while,but they always stunk like a combo of blow by and a over rich mixture,even with them tuned to near perfection...just the nature of the beasts back then I guess..
When they added catalitic converters the smell sometimes was more a sulpher or cabbage fart type stink..
When they added catalitic converters the smell sometimes was more a sulpher or cabbage fart type stink..
interesting...I just read this article where it says spark advance SHOULD be connected to full manifold vacuum...I have always heard it should be ported.
As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.
TIMING AND VACUUM ADVANCE 101
The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.
The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.
At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).
When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.
The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.
Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.
If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.
What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.
Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.
For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.
Tom
Wonder if I could go straight to the manifold with the spark advance and plug the carburetor?
That article is spot on ,on all counts IMO...
Hooking your vacuum advance to the manifold itself,or a port on the carb that has manifold vacuum will make no difference,its your choice...
GM did diddle with ported vacuum in the early days ,then as they started getting more concerned with emissions they added all kinds of thermal vacuum switches to vary when the distributor got vacuum or was denied,they even had a "transmission controlled spark" solenoid on some vehicles to deny vacuum in third gear to lower emissions,thermal vacuum switches to deny vacuum to the distributor while the engine was cold--some also switched it to manifold vacuum at idle only ..the resulting jungle of hoses,prone to cracking and vacuum leaks,contributed to the poor running ..
The chapter about the "race car" distributors is also correct..
A friend of mine had a 62 Nova that he restored and he put a mildly built 283 in it...despite him being an excellent mechanic,he did not always make the right choices when it came to parts..he learned the hard way 1.6 ratio rockers and a hotter cam don't play well together,and he chose a MSD distributor that had NO vacuum advance,just a centrifugal advance..it was intended for full throttle,race track type use,not daily driving..
The car ran like dog crap with that distributor--he'd set the initial timing so it idled good,and it was a dog as far as acceleration..
He'd advance it until it pinged while power braking it,and it would be much more powerful,but it would spark knock badly when warmed up under load,and idle suffered badly..and when hot,it often wouldn't crank over without binding up till it cooled off some..
I tried to tell him to get one with vacuum advance,but he listened to the tech "expert" at Summit instead..after a few months of disgust he was ready to yank the engine out in favor of a good 350 he had gotten,but I suggested he swap in a stock HEI distributor forst for the hell of it..I gave him an extra I had been carrying around in my van ..
It was like night and day,the car ran sweet after that was put in and timed right...at first he also had tried an ACCEL HEI module in the HEI distributor,which was a printed circuit board,not the usual 4 prong plastic module,that supposedly allowed it to run at RPMS much higher than stock--I think it was defective,after he ditched that for the stock GM module it ran great...
Hooking your vacuum advance to the manifold itself,or a port on the carb that has manifold vacuum will make no difference,its your choice...
GM did diddle with ported vacuum in the early days ,then as they started getting more concerned with emissions they added all kinds of thermal vacuum switches to vary when the distributor got vacuum or was denied,they even had a "transmission controlled spark" solenoid on some vehicles to deny vacuum in third gear to lower emissions,thermal vacuum switches to deny vacuum to the distributor while the engine was cold--some also switched it to manifold vacuum at idle only ..the resulting jungle of hoses,prone to cracking and vacuum leaks,contributed to the poor running ..
The chapter about the "race car" distributors is also correct..
A friend of mine had a 62 Nova that he restored and he put a mildly built 283 in it...despite him being an excellent mechanic,he did not always make the right choices when it came to parts..he learned the hard way 1.6 ratio rockers and a hotter cam don't play well together,and he chose a MSD distributor that had NO vacuum advance,just a centrifugal advance..it was intended for full throttle,race track type use,not daily driving..
The car ran like dog crap with that distributor--he'd set the initial timing so it idled good,and it was a dog as far as acceleration..
He'd advance it until it pinged while power braking it,and it would be much more powerful,but it would spark knock badly when warmed up under load,and idle suffered badly..and when hot,it often wouldn't crank over without binding up till it cooled off some..
I tried to tell him to get one with vacuum advance,but he listened to the tech "expert" at Summit instead..after a few months of disgust he was ready to yank the engine out in favor of a good 350 he had gotten,but I suggested he swap in a stock HEI distributor forst for the hell of it..I gave him an extra I had been carrying around in my van ..
It was like night and day,the car ran sweet after that was put in and timed right...at first he also had tried an ACCEL HEI module in the HEI distributor,which was a printed circuit board,not the usual 4 prong plastic module,that supposedly allowed it to run at RPMS much higher than stock--I think it was defective,after he ditched that for the stock GM module it ran great...
So does the timing still need to be set without spark advanced hooked up?
I'll look for a Rochester that looks like mine so I can show the ports. There is one that appears to be manifold level right under the choke, but it doesn't seem to do anything...there was no vacuum at all from it at idle or acceleration
I'll look for a Rochester that looks like mine so I can show the ports. There is one that appears to be manifold level right under the choke, but it doesn't seem to do anything...there was no vacuum at all from it at idle or acceleration
