I am still fighting with this engine trying to get it to pull. I checked the compression today and all cylinders are 120-130 is that good? I don't know what the compression should be on a 30 year old six cylinder but they are all fairly close to each other.

I have also been playing with the timing and reading as much on vac advance as I can find on the web trying to teach myself the basics. when my vac advance is on ported vac it doesn't move every time the throttle is cracked or revved and I have plenty of vac on that port its very in consistent but when its set to manifold vac it works properly other than it appears to be advancing the spark at high rpm and I thought it was supposed to go the other way retard the timing at high rpm or WOT. I tried setting it to the factory timing specs 8 degrees BTDC with the vac advance hooked to ported vac, I hooked our empty16' lowboy utility trailer (1200-1500lbs)to the truck and and took her for a road test and again it failed miserably pulling small grades at 30-35 mph and sometimes having to shift into 3rd gear. I am thinking maybe a bad vac advance can or maybe the 3.54 gears are just to high for the mountains around here. the thing gets me about this truck is it runs and pulls great by itself but if you put the slightest load on it the truck falls on its face and if she even smells a hill she quits pulling.

I mainly bought the truck to tow with but now I want to restore the truck so I'm gonna keep it and I like tinkering around with the truck but this pulling problem is getting the best of me. maybe I'm expecting too much out of the engine and gear combo but I have a buddy with an 88 F150 with a 300 six although it is EFI it has 3.08 gears (special ordered for fuel economy)and it will out pull this truck when towing similar loads.

sorry for the long winded post just needed to vent a little I suppose

 

The spark advance does just that,your engine runs at the most retarded timing when at idle,then when you step on it,the timing advances to help the engine build power.If the timing was advanced when you started it,your starter would run into a brick wall.If everything is right,with the vacuum line blocked,your timing light will walk right up the crank damper when you rev it,away from the timing marks,if it doesn't,your centrifical advance in the dizzy isn't working,may be froze up,and if it is,it would account for a power loss.There is a spec for total advance (I don't know it off hand) and that is how many degrees with the vacuum advance and centrifical /or mechanical advance all full on.So when your pulling that trailer and you have your foot in it,the spark needs to be at total advance or it will not have the power.All of this is different for the newer ignition systems,but in '79,it would apply.There is a type of timing light that has a dial on it for checking total advance,you rev the engine to about 2000 rpm and turn the **** untill your TDC comes back and then read the dial and it tells you how many total degrees of advance you have.A quick way to check that centrifical advance is to take hold of the rotor and try to twist it,if the advance is free,it will move back and forth a few degrees,if it's solid,something has it froze.If your truck is computorized none of this may apply.I'm taking for granted that it's a '79 or close.Hope this helps,that timing means a lot.

 

I just read your post again,always set your timing with the vacuum line disconnected and plugged,and even if your timing is set and the vacuum advance is working proper,it still may not be getting total advance if the centrifical/mechanical advance isn't working ( it's located under the plate in the dizzy that your vacuum advance pulls around.

 

thanks for the help buster and yes it is a 79 it's the one in my avatar, I was wondering how to check the centrifugal advance I'll try it tomorrow. I have Snap On timing light with a digital display and you can increase or decrease the timing on the light and it will also tell you the rpm its a nice light it belongs to the shop I work for. with this light I should be able to tell the total advance like the light you were talking about.

by the way everything you just said goes against what I was reading on the web. don't get me wrong please I'm not saying what you said was wrong just what I have been reading was the exact opposite and I've been told both ways by mechanics.

I am a diesel mechanic by trade but I am a complete idiot when it comes to gas engines and all of my diesel experience (9 years) is on electronic/turbo charged diesels which I understand and know very well but I have never worked on gas engines very much other than regular maintenance and external parts like starters,alts, and water pumps etc.. I am actually enjoying learning gas basics and on these older simpler engines it fun to me to learn something new. I got some info off of a mustang forum I will post it after this. but the info is opposite of what you just said please read it and tell me if it makes sense. the author was originally a GM man converted to Blue Oval so he references the timing on a small block chebby some but I am asumming that the basic principles are the same.

thanks again

 

The way I read this is that timing retards at high rpm and advances at idle you guys tell me what you think and if its right or wrong cause I sure don't know

ARTICLE FROM THE MUSTANG FORUM.


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.

 

Good article

 

I think the guy is right on the vacuum advance and I was wrong to include it in the total advance,It's been a while,but when your done setting the timing and you reconnect the vac line,the engine speeds up,so it makes sense what he is saying.I have experienced frozen centrifical advances on car engines and Harleys both and they can really rob power.Of course the bike engines didn't have a vac advance.I would try going with the manifold vacuum and make sure you use no vacuum when setting your initial timing,it will pull your static timing off and you have no idea where it is at.Sometimes the centrifical advance come apart or have springs come off or break,so if it is frozen,be sure to remove the plate and look at it.If you take it apart,clean it and lube the shaft with a little lithium grease,lightly,If the wieghts are worn at the pin holes,replace them and new springs are also nice.The total advance check will tell the story.If the spec calls for a certain amount and you don't have it,you won't get the power.Sorry for the wrong info on the vacuum advance.That vacuum line I would try the manifold and if evrything else checks out and it's acting bad,try the port,but make sure all the mechanical timing is right first,static and centrifical.One oddball sneaker is if you have power brakes,the boosters on older Fords get rust holes in the lower back side and leak vacuum bad,usually a gross leak,and can really fool with everything,less vacuum means less mixture to the engine and less vacuum to everything else.Good thing to check.I've seen a few of them.

 

Oh by the way,your compression doesn't sound bad,if they are pretty even.I would think a 20# diff in one of them or more would start concern.

 

Great information offered in the Timing and Vacuum Avance 101 article.

My 1986 with 4.9L engine has been converted to Duraspark with non-feed back carburetor and has no EGR. The vacuum advance is connected to ported source (as original). The truck runs well with 14 degrees initial timing without pinging, but the coolant temperature creeps up at idle with or without the A/C being on. I will try using full manifold vacuum this weekend.

 

IIRC, the gentleman that has the gofastforless.com site was experiencing the same problem on his car when idling around and waiting at the race track and moved to a full vac source and it took care of the problem. I run on full vac and the temp gauge actually goes down when idling.

 

OK tell me if I got it right? I took the dizzy off and tore it a apart found no frozen cam weights/breaker plate or missing or broke springs. so I cleaned it up and put it back in the truck. set the the timing at 8 degrees BTDC and 700 rpm with the vac advance unhooked I pluged the vac advance up and the timing advanced about 15 degrees which by what I read is correct then I revved the engine to around 2000 rpms and dialed my timing light up to 34-36 and the timing mark on the crank pulley hits right at the TDC mark on the front cover. so by my figuring the timing should be correct.
at 2 grand the centrifugal advance has the spark at 34-36 and at idle the vac takes over. the truck starts great and idles smooth. I haven't got to road test her yet but by what we have been discussing and the article that I read it should be pretty close. I hope to drive her tomorrow and let y'all know something

 

Sounds to me like you did it up right.If it doesn't perform now,I'd start looking elsewhere.Your compression sounds good,so maybe I would start looking at fuel getting to it,starting with a vacuum check,I don't know off hand what it's supposed to be,but if you don't have enough,you won't get the mixture in the cylinders.And that would probably mean a leak somewhere.Hopefully you cured the problem.That electronic ignition you have is great,you get a good hot spark without points,and none off the later computer problems,but get yourself a spare module and stash it somewhere in the truck,because when they go,they just won't start sometime and it only takes a few minutes to check for spark and replace the module.I've had them go on 2 vehicles,one shut off long enough to blow a muffler baffle back and plug the exhaust,if it just skips a beat,that's the only warning your going to have,usually though,there is no warning,you just go to start your truck and it never hits a lick.I still think it's a great ignition.Have you looked at the brake booster yet?That one happens a lot on older Ford trucks,if it's getting badly rusted underneath,replace it,because it'll sneak up and steal your vacuum and you will wonder what's going on,may be a pin hole,may be a big hole.By cleaning and checking that advance,you know what you have and won't have to worry about it now for a long time.That timing can be confusing until you learn it,some of the older bikes had a point set,condensor,and coil for each cylinder,and had to be timed on each cylinder,lots of fun.

 

yeah I had a small vac leak on the booster but it was the grommet where the fitting plugs in for the vacuum line. I know what you mean about the duraspark module I've heard people say get a spare one and put it in the glove box. all the GM boys that cuss the duraspark system don't understand it and say it's too many wires I personally think it's a pretty good system and it is a "High Energy Ignition" I gotta tell ya it kills me to go to a car show a see a 55 F100 with a small block chebby sitting in it because they say it's easier to put an sbc in it than a ford motor and ignition system. If your gonna build a ford put a ford in it dang! Pace sell the harness sytems for a duraspark and if you understand it its pretty simple

If this doesn't fix it I'm gonna check my fuel pressure I probably should have checked it before now. I rebuilt the carb a while back too but it didn't help. I hope this timing gets its straight

 

Hey Buster, I forgot to mention that I have 21 in" of vac at idle I thought that was pretty good too!

 

I just skimmed through this thread and wanted to chime in.
First off, your compression numbers are pretty strong.

I was chasing my tail a while back, thinking that I had an ignition timing issue.
I didn't have any means to measure my mechanical advance, so I
had to take my distributor to a speed shop to have it spun and measured.
Turned out that it is advancing my timing 14* at 4000 rpm... very conservative.

With a vacuum gauge and my timing light, I measured my vacuum advance
to be "all in", retarding my timing by 12* at 6" of negative pressure... yes,
retarding the timing with the throttle wide open. (My measurements
were verified by the guy at the speed shop.) It seemed backwards to me at
the time, too. My Ford shop manual explained that to be the correct function
of the vacuum advance though.

And smoke20, 21" at idle is real strong. What elevation is your area?

Murph.