The 302 in my 70 Bronco is not running as I would like. I have a basic engine with some minor mods. The block and heads have about 800 miles on them, as I swapped the motor recently. Anyway I have what seems to be a miss when running. You can feel it, when the engine is a idle, and or when it is under load. It does not cause the engine to fail, or sputter. You can just feel it there when the engine is running. The engine has plenty of power, but it still is not right. I have replaced the coil, wires, and plugs. No effect. I am thinking the distributor cap, which was brought over from my old engine.
Could the carb be the culprit? I have an Edelbrock Performer package on the whole engine, cam, manifold, and carb. The carb was working fine on the old engine, and I did have it tuned on the new engine. This problem just seemed to develop this spring after I brought it out for the summer.
Any ideas would be a help b/c I have tried everything I can think of.
Valve Timing is fine, as I checked the timing chain last month.
Ignition timing is fine at idle with Vacuum Advance removed
Vacuum Pressure is rock steady at 20 at 1000 rpm.

I am stumped. Thanks

 

change the distributor cap and rotor button, like you mentioned and see if there is an improvement. good luck

 

Thanks JW, I just ordered a set. (It is a Mallory Distributor) I inspected and cleaned the cap and rotor and they look worn but OK. Is there some unforseen performance loss that stems from the wear? I always thought that it had to be cracked or corroded to fail. Also this issue seems to be sudden, not gradual. I was thinking a failing distributor cap would slowly cause the engine to fail.

Well I suppose this is the most obvious and least expensive action at this point.

Thanks again!

 

Well when you wrote distributor from old engine, I was thinking oem and old. Now that you mention mallory I am not so sure, but its probably worth replacing just to see.

I don't know what the odds of my next suggestion being the problem before and after you changed them, but could the wires be faulty? Ever run the engine in the dark and look for spark from the wires to the block? Thats a long shot but figured I would suggest it.

If this occurs at idle have you tried richening the idle screws a bit, to see if the miss goes away?

You probably could look in your edelbrock book and the something to look for as far as a miss would be:

Too lean - adjust idle screws see what happens

improper timing - you stated you double checked this, possibly the cap(?)

choke adjustment - double check / readjust if needed

fuel pressure too low - check psi

vacuum leak - vacuum gauge test or the other means in which people spray aerosol products along the gaskets


Good luck

 

Thanks JW.
I am becoming suspicious of the carb at this point. I have perfect ignition timing (very smooth). I suppose some adjustments on the carb would be in order if the distributor cap and rotor are not the culprit. I have the Edelbrock tune up kit for the carb, and the book on how to tune it's rods and jets. So I guess that will be the next direction I go after I get the new cap and rotor. As you can see I am not optimistic about the cap solving the problem (too easy). After working on this rig since 1982 l learned one thing...nothing is ever easy!
Therefore I am going to see what happens with the cap go from there. I am losing my warm weather and I like to have it running and put away by November.
Thanks again for the help...I will post again when I get the cap and rotor later this week!

 

...yeah from my limited expereince...seems like the smallest problems have to create a headache before they can be figured out...or it just wouldn't seem right....haha...good luck

 

I at least discovered that the vaccum advance is no good. I tried sucking air through the hose and it does nothing but pull air through. I think that could be my problem as far as the poor performance goes. I am going to tune the carb as well, b/c it was set up for that torker II cam. Thanks for the help.

 

Whats the deal with aftermaket distributors lately, your the second person in two weeks that mentioned their aftermarket distributor has a faulty vacuum advance canister.


Sounds like you found it.

Do you have the vac advance connected to the timed or full vacuum port on the carb?

 

Not sure, but this distributor has been loyal since 1988, so I am not complaining! Actually I have two ports in my Edelbrock carb. One is constant vaccumm at idle the other starts pulling at throttle, I use that one. Is that what you called "timed"? I never knew what that was called. Which one is better? BTW I replaced the cap and button, better but not the problem!
Thanks JW!

 

I found a really helpful site for ordering parts and researching Mallory distributors, and this is where I am going to get my new vaccum advance, I thought it may help some other guys. It really helped me.

Thanks

http://www.centuryperformance.com/mallory/se47assm.asp

 

Timed is on the left and full vacuum is on the right.

Well I suspect your not using the emissions anymore. I have been playing around with the timing lately, I read an article on full manifold vacuum and well after doing a simple switch, I am hooked.

I had it set on timed port, 8 initial and adjusted the vac advance canister until it stopped pinging at any circumstance while driving. Thought I had it set and liking it, recently I re-read an article on full vacuum, feeling bored thought I will give it a shot. All I did was go out, adjusted the vac advance all the way in for minimal vac advance, switched the vacuum hose to full manifold (right), reset the idle rpm, obviously it jumped with the added advance. Connected a vacuum gauge and adjusted the idle mixture screws until I achieve max vac reading (I have it at a steady 19hg up from 17hg). I tried to move the distributor left or right to see which was best but it was happiest right were it was, weird. I don't have a timing light so I haven't tried advancing the distributor anymore, for all I know I have 30 BTDC at idle, and who knows how much total mechanical advace or where it comes in. There was a comment on hard starting with so much advance, in my case this thing idles and starts 100% better than it did before, lets put it this way I stopped using the choke. I was impressed all the way around, it has a positive feel to it from idle to wherever, the off idle grunt is much much better. This way you have all the advance at idle, as you accelerate into the rpm or go under load the advance drops, which is what you want to happen, then somewhere in that curve the mechanical takes over and picks it back up. Who knows? All it really requires is time, there were a couple others that did this on another forum the same time I did, generally the idle vacuum was up, off idle was improved and in a couple cases the idle temp dropped, I noticed it takes longer for my engine to warm as well. They used to set cars up like this from the factory until emissions was a concern, by using the timed port the engine runs hotter burning more pollutants, weird thing is some even report better mpg with the full manifold vacuum. I can't say how your experience will be but I can say that I am glad I did it. I can send you the link to the other website if you wish, I would rather e-mail it to you...sometimes they delete links to outside forums on this website. Later

 

Once I get the vaccum advance back on there I will try your suggestions, I can see what you're saying about it running hotter on the timed port. I remember when I used to have the stock carb and distributor on the factory engine, the timing would jump at idle when you hooked the advanced hose up to the port on the carb. So that is what I will do. I am going to order that abvance today, so I should have it on there by next weekend.
I would like to look at the website you mentioned. Do I just put email address on a post?
Thanks JW!

 

No...don't put your e-mail address in any of your posts. Seems you have been a member for less than 15 days so I cannot e-mail you, click on my username above my last post a window will pop up and then click on "send e-mail to jwtaylor". If an when I received the e-mail I will reply back to you. later

 

As found posted elsewhere, author unknown:


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.

 

Wow!
What a great read! I have been using timed advance for the past 10 years or so, come to think of it my engine has not run well for years... When my new vacuum advance diaphram arrives, I will be sure to place the hose on proper (non-timed) vaccum port.
Thanks Motorhead351!