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I have basically stripped everything emissions related off of my AMC 258 i6 / 4.2l . I have the PCV hooked up to the large port under my BBD carb. I have a max vacuum from under the carb going to the vacuum advance on my distributor. Can anyone tell me if I should be hooked to a particular different port for vacuum advance? Also I am not too sure if my vacuum advance is even working to be honest. It is an electronic ignition distributor from Motorcraft. Can I purchase the vacuum advance from somewhere instead of the whole distributor if the advance is not functioning? Is there another better setup for distributor and carb other than the BBD and the Motorcraft? Not that they are bad just am open to other ideas. My throttle shaft is somewhat loose on my carb and does make my idle speed change but other than that it idles pretty good.
73 CJ5 4.0 head on a 258, HEI, Holley Projection w/ 4.56, Detroit lockers Narrowed D44 front and full float D44 rear T14 w/ D20 4 wheel discs, 38.5 Boggers
Put the hose in your mouth an suck. You should see the vacuum advance move. Easy check. The hose should be hooked up to the ported vaccum line on your carb.
Set the initial timing at 8 BTDC with the vacuum line removed and capped, then check for maximum mechanical advance by revving engine until the advance peaks. Then reconnect the vacuum hose and check for max advance again which should be the total of the mechanical and vacuum advance. You say the vacuum advance should be about 10 degrees, is there some chart or reference that covers this?? This whole thing is confusing to me for some reason. The way I have done mine is to set the initial at the Mfg. spec. and listen for pre ignition, If it is present I back the vacuum advance out by ¼ turns until it goes away. My observation on the ported vs. manifold , Most true hot rodders like the manifold vacuum but the ported will give you a lower Idle speed and better emissions at idle at the cost of a higher cylinder temperature and , perhaps , a greater chance of overheating at idle. I am using the ported vacuum but I have bypassed the coolant temperature switch that it originally used that only allowed the advance to work after the engine was warm.
I am a tad confused as well. Any idea which carb fitting is the ported vacuum and which is full manifold vacuum on my BBD? Haynes is not much help. Or at least it didn't strike me obviously in the Haynes. I like the ability to have a low idle speed and I do not care about emissions. Overheating really isn't a problem cuz the Jeep never sees traffic and isn't offroad much either.
I take it intial timing is at idle with vacuum line plugged and 5 deg +/- 2 deg (sticker above radiator)? Then install the hose back on and I should get what? How are you adjusting the advance with 1/4 turns? What are you turning? What do you mean by High reading please?
I think you will find a ported vacuum at the front of the carb. The vacuum advance can that the vacuum hose fits on has an Allen screw in the barb the hose fits on, pull the hose and insert the Allen wrench and in advances while out retards. If it knocks, back it out, reconnect the hose and try it again. If you are on a ported vacuum supply the idle advance will not change with the hose on it. the high reading will be at above 2000 RPM or better said, at whatever RPM the highest mechanical advance is measured.. You will need an advancing timing light to read it, you don't have enough timing marks to use a regular timing light when you’re talking about 30+- degrees.
this whole thing is making me crazy, right now things are working pretty well but I do not really understand why or how.
I guess you can adjust the mechanical advance with some clever machine work and you can change the rate that the mechanical advances with some spring/counter weight changes and I don’t see myself doing either of those things. From what I think I may understand, the greatest advance possible without pre ignition is the object to the exercise. And if you get too much advance it will be hard to start.
I think you will find a ported vacuum at the front of the carb. The vacuum advance can that the vacuum hose fits on has an Allen screw in the barb the hose fits on, pull the hose and insert the Allen wrench and in advances while out retards. If it knocks, back it out, reconnect the hose and try it again. If you are on a ported vacuum supply the idle advance will not change with the hose on it. the high reading will be at above 2000 RPM or better said, at whatever RPM the highest mechanical advance is measured.. You will need an advancing timing light to read it, you don't have enough timing marks to use a regular timing light when you’re talking about 30+- degrees.
this whole thing is making me crazy, right now things are working pretty well but I do not really understand why or how.
I guess you can adjust the mechanical advance with some clever machine work and you can change the rate that the mechanical advances with some spring/counter weight changes and I don’t see myself doing either of those things. From what I think I may understand, the greatest advance possible without pre ignition is the object to the exercise. And if you get too much advance it will be hard to start.
Re: 82 CJ5 distributor advance- Don't use ported vacuum!!
To answer your question about if the vacuum can is working on the distributor. Pull the vacuum line off of the carburetor. Pull a vacuum with your mouth and then stick your tongue in the hole in the vacuum line the vacuum canister should hold a vacuum or it is bad and needs to be replaced. (you can also use a hand held vacuum pump like I do)
With the vacuum line still connect to the distributor but disconnected from the carburetor start the vehicle. while the engine is idling use your tongue or vacuum pump to pull a vacuum on the line and the engine rpm should raise. If it doesn't it's either blocked off by something or sticking.
In most engines the timing at highway speeds of about 60 mph is pretty close to 40 degrees. This is because you have very little throttle opening at this speed. At this low throttle opening the engine is mostly running off of the idle circuits of the carburetor. The leaner the engine is running the more advance that you are going to need to completely cause the fuel to burn up the available oxygen. (I said that right. You aren't supplying the oxygen, you are supplying the fuel to cause the oxygen to support the flame. Liquid gasoline doesn't burn. You are attempting to burn all of the available oxygen. Once that's gone there will be no more flame. any more fuel will not have any positive effects on the performance of the cycle. Yes, top fuel dragsters dump fuel out of the exhaust the entire quarter mile run in order to burn up all of the available oxygen but they also rebuild much of the engine every 1/4 to 1/2 mile)
The timing is normally set about 6 degrees at idle with the vacuum line disconnected on most Jeeps. Hooking up the vacuum line at idle will fully advance the VACUUM SECTION of the ignition timing. This should give you somewhere around seventeen degrees of ignition timing to run the lean fuel mixture at a no load idle. The engine doesn't have any load on it so there is very little chance of it detonating. Almost as soon as you accelerate the engine the throttle plate opens and your intake manifold vacuum is lost because there is no throttle plate to suck against. With the loss in vacuum comes the loss in the VACUUM SECTION of the ignition timing and you are fully reliant on the mechanical advance springs for timing.
That's okay though because you are giving the engine more fuel to mix with the air so you will have a rich mixture that's a lot easier to burn. You can start the combustion process later in the engine cycle and still get the maximum expansion pressure close to the 18 degree after top dead center mark that every engine manufacturer has been aiming for since the four stroke engine was invented.
As soon as you back your foot off vacuum returns to the manifold under the closed throttle plate. this causes the VACUUM SECTION of the distributor advance to return and burn the now leaner mixture.
There are other factors that cause low vacuum in the engine such as LOAD! As you are driving you encounter a steep hill and the engine vacuum gets lowered by the decrease in RPM, (and generally the increase in your throttle plate angle). This lower vacuum again allows the spring in the vacuum diaphragm to close off the vacuum section of the distributor lowering your timing.
If you are using ported vacuum and you open the throttle plate up you are increasing the amount of vacuum to the distributor diaphragm as the load of the engine is increasing. That's not good!!
Here is a representation from a modern manufacturer, (GM) of the proper ignition timing for a given throttle opening versus RPM. this table does not take into consideration the load of the engine as that is done in another table. this is a table from a 2001 GM 1500 with a 5.3 liter LS engine but almost all gasoline engines are going to be in a similar range. (Yes each engine is different because the efficiency of the combustion chamber requires more of a head off start if it's bad and a later start if it's good. the higher the compression in the cylinder the later you have to start the spark. the lower the octane of the fuel the later you should start the spark. this is just a broad example of the ignition timing base table and it's similar to what SHOULD be going on in your Jeep
Load
(Throttle Percentage)
If you look at these numbers carefully you will see that it's not possible for a mechanical or vacuum operated distributor to pull timing correctly for every RPM. It gets worse when you calculate in the load. That's why manufacturers started using computers to adjust the timing. As soon as they started having the computer control the ignition timing the life expectancy of the engines went from about a hundred thousand mile to about two hundred and fifty thousand miles.
In Calif 1982 CJs (Calif only, 1982 only) had a weird timing sequence other than 49 state models. Is your emissions label still present and legible?
Don't know if yours could be a Calif model or not.
When it comes to old school carbed engines, ignition spark timing can be favorably advanced or retarded for improved fuel economy and drivability. This may sound redundant, but too often we don’t adjust or even set up our engines correctly to take full advantage of this.
First, and very important, you have to know the maximum timing advance (in degrees, of course) your engine will handle before going into destructive ignition timing.
Timing can be adjusted using two technologies (hey, let’s pretend we are in 1960), one of which is the clever centrifugal mechanism and the other the vacuum operated.
Let’s say that our engine can only take a maximum of 30 degrees of ignition advance. In general terms, the vacuum can advance timing by about 12 degrees. Add those to the 15 provided by the centrifugal. We have a total of 27 degrees. Add those to the initial baseline timing set point of about 5 degrees. We now have a total of 32 degrees of maximum adjustable timing. We are maxed out already.
Here is the key point to know about vacuum advance: It must engage fully as soon as you step on the accelerator pedal, AND must disengage fully as soon as you put your foot away from the accelerator pedal. This means it must be connected to a fully ported vacuum. The centrifugal advance, on the other hand, has a gradual operation based on engine rpm and can be adjusted by varying weight mass and spring tension. Therefore, it requires the right combination of “variables” in order to achieve the ideal timing advance curve. We are not going to get into this.
If we stay within these guidelines, we can expect decent performance from our outdated engines. Decent can be be replaced by great in many instances. I hope this basic information helps those of us with carbed engines. Good luck.
Here is the key point to know about vacuum advance: It must engage fully as soon as you step on the accelerator pedal, AND must disengage fully as soon as you put your foot away from the accelerator pedal. This means it must be connected to a fully ported vacuum. The centrifugal advance, on the other hand, has a gradual operation based on engine rpm and can be adjusted by varying weight mass and spring tension. Therefore, it requires the right combination of “variables” in order to achieve the ideal timing advance curve. We are not going to get into this.
If we stay within these guidelines, we can expect decent performance from our outdated engines. Decent can be be replaced by great in many instances. I hope this basic information helps those of us with carbed engines. Good luck.
This is a good post Patrick, but there are certainly good reasons for using manifold vacuum for spark advance. Throttle response on trails is one of them. Everything stated by IOPort51 on ported vs. manifold is true. When I had a carburetor on my AMC 304 it ran SOOOOO much better on manifold vacuum, especially off-road. I have a dash mounted vacuum gauge and the vacuum recovery upon pedal lift was virtually instantaneous. I'm sure there was some sort of barely measurable lag as opposed to having ported vacuum, but it would have had to have been in fractions of a second.
Also, a lot of these advance guidelines were made by very intelligent engineers indeed...but one thing that's changed enormously since then is the fuel we use. I'm not well-versed enough in the number of changes, the types of changes, the reasons for the changes, the benefits of the changes or the drawbacks of the changes - but it's for d@mn sure different fuel and it's more susceptible to tuning variations than it ever was 20-30 years ago.
Double o seven, thank you for your reply. And yes, as old as the post is, it still applies and I need some clarification. I like to keep my posts to the point without too much technicality. Keeping it simple always works.
Let us agree on some facts before moving on. Vacuum is maximum (disregard coasting, please) when throttle plate is in the closed position . Vacuum is reduced when the throttle plate moves past the closed position. Vacuum is regained but not maximized when at cruising speed. Vacuum is highly diminished during acceleration (power demand). Vacuum can be totally lost immediately following pedal to the metal position (very fun, also known as full throttle). Do you agree? If you do, then the logic of vacuum is put into perspective in the simplest of forms.
Advancing the spark timing within reason and at the right moment will generally increase power and throttle response, but can be detrimental if not retarded when the power is no longer needed (foot off the pedal). It is my understanding that the vacuum advance diaphragms normally mounted to the distributor are not designed for a “part” response, meaning they are almost immediately fully triggered upon the presence of any vacuum amount and fully retracted with no vacuum.
So, can you clarify to me how is it that vacuum advance can work effectively with manifold vacuum (constant) as opposed to ported (switched) vacuum? Wouldn’t it defeat the purpose for the existence of the vacuum advance all together? Wouldn’t it have not just a purpose defeating effect, but a counteracting one?
All the aforementioned engine dynamics are not affected by gasoline grades or from any decade or era. This is why keeping it simple and to the point always works. Thank you, sir.
Double o seven, thank you for your reply. And yes, as old as the post is, it still applies and I need some clarification. I like to keep my posts to the point without too much technicality. Keeping it simple always works.
Let us agree on some facts before moving on. Vacuum is maximum (disregard coasting, please) when throttle plate is in the closed position . Vacuum is reduced when the throttle plate moves past the closed position. Vacuum is regained but not maximized when at cruising speed. Vacuum is highly diminished during acceleration (power demand). Vacuum can be totally lost immediately following pedal to the metal position (very fun, also known as full throttle). Do you agree? If you do, then the logic of vacuum is put into perspective in the simplest of forms.
Advancing the spark timing within reason and at the right moment will generally increase power and throttle response, but can be detrimental if not retarded when the power is no longer needed (foot off the pedal). It is my understanding that the vacuum advance diaphragms normally mounted to the distributor are not designed for a “part” response, meaning they are almost immediately fully triggered upon the presence of any vacuum amount and fully retracted with no vacuum.
So, can you clarify to me how is it that vacuum advance can work effectively with manifold vacuum (constant) as opposed to ported (switched) vacuum? Wouldn’t it defeat the purpose for the existence of the vacuum advance all together? Wouldn’t it have not just a purpose defeating effect, but a counteracting one?
All the aforementioned engine dynamics are not affected by gasoline grades or from any decade or era. This is why keeping it simple and to the point always works. Thank you, sir.
Patrick - sorry for the slow reply. Virtually everything you've stated is accurate. I think the difference we have here is where you say that manifold vacuum is constant - it absolutely is not. I'd say that while it may have a fraction of a second (unnoticeable) less response, it ends up having greater "net" response - hence why rodders and some off-roader prefer it over ported.
The only reason I brought up fuel was that when it comes to spark and advance and curves and AFR, etc....degrees are degrees and percentages are percentages, but the fuel has changed drastically in the past 30 years. 10 degrees is still 10 degrees, but the net change isn't what it used to be. May have nothing to do with the OP's post, but it is what it is.
'85 CJ-7, 258/4.2L 6cyl. Bought new in 1985. Full cage, Warn 8274 winch, Ford 9" rear, front/rear Detroit Lockers w/4:88 R&P. T-5 tranny and 4:1 t'case. 33X12.50-15 BFG/AT, MSD ign, on board air---
Right on, 007. Done some research on this subject and come to find out this feature is meant to retard ignition timing, not advance. Hence the reason for using manifold instead of ported vacuum. Keep on jeepin’ and let us all stay safe.
