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Old 05-08-2008, 03:28 PM
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intake leak?

could a intake leak make a vacume advance not work properly?

i have a mopar small block 366 i had built that im having vacume advance problems with i will try to make this as simple as possible since its kida hard to explain.


first i made shure the rotor was pointing at the #1 spark plug therminal while at tdc on the compression stroke.

than i plug off the vacume advance line and port on the carb.

i have my dad start up the truck and rev it to 3000-3300 than i set it to 34* than i let off and it set the idel and it runs and sounds awsome i can rev it and it has instant throttle resonce.

than i plug in the vacume advance and when i try to rev it the the engine pops and missis when i put the light on it and watch it the timing jumps all over.

i tried both port's on the carb one make the timing advance up to about 30 at idel and run like junk and the other idels normal but when u rev it the timing jumps all over.

pluged in to either port u cant rev it at all but when the vacume advance is pluged off it runs like normal.

i have a mopar eletric distributor and msd 6a box could it be a carb or intake leak? i have tried 2 dist and 3 vacume advances

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Old 05-08-2008, 04:48 PM
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Quote:
Originally Posted by cope
could a intake leak make a vacume advance not work properly?

i have a mopar small block 366 i had built that im having vacume advance problems with i will try to make this as simple as possible since its kida hard to explain.


first i made shure the rotor was pointing at the #1 spark plug therminal while at tdc on the compression stroke.

than i plug off the vacume advance line and port on the carb.

i have my dad start up the truck and rev it to 3000-3300 than i set it to 34* than i let off and it set the idel and it runs and sounds awsome i can rev it and it has instant throttle resonce.

than i plug in the vacume advance and when i try to rev it the the engine pops and missis when i put the light on it and watch it the timing jumps all over.

i tried both port's on the carb one make the timing advance up to about 30 at idel and run like junk and the other idels normal but when u rev it the timing jumps all over.

pluged in to either port u cant rev it at all but when the vacume advance is pluged off it runs like normal.

i have a mopar eletric distributor and msd 6a box could it be a carb or intake leak? i have tried 2 dist and 3 vacume advances
Way too much base advance. With the vacuum disconnected and plugged the engine is started and kept a slow as it will keep running. The distributor is moved to advance the timing to what ever the factory recommended which is usually around 10 degrees. There are some reasons for this.

- During cranking you don't want a lot of advance because the early firing puts a back load on the starter which can and will break parts.

- Once the engine is running with high vacuum, the vacuum advance will add another 14 to 20 degrees which improves low speed performance and economy.

- The centrifugal has about 25 to 35 degrees in it. established by a cam arrangement of some type. The springs and counterweights control the shape and speed of an advance curve against RPM where the cam establishes the top advance limit.

- The distributor has both a vacuum and a centrifugal curve, typically the vacuum curve reduces as the centrifugal comes up as a function of the opening throttle reducing vacuum and the engine's response to the more open throttle is increasing RPM which brings in the centrifugal. The total advance is the base timing plus whatever variable is being installed by the distributor. Typically the combination of all advances should not exceed 35 to 40 degrees. This should happen from 2000 to 3000 engine RPMs. If the base is set too high that is added to the centrifugal (actually to the vacuum as well) and the engine becomes way too advanced. It will react with bucking, surging, holes in it's power band, and RPM hunting.

The carb typically has two vacuum advance sources one is simple manifold vacuum all the time, the other is timed by throttle position. Generally the hotter the (street) cam the more you'd use the vacuum all the time port because hot cams reduce idle vacuum therefore some additional advance at idle can help the engine run more stable at idle. The timed port is more suitable for mild cams where idle vacuum is very high and that much advance at idle, especially with long idle periods in stop and go traffic can lead to overheating.

You cannot set the vacuum advance the way you are without loading the engine. The reason is that without the engine actually working against a load, it doesn't take much throttle opening to get to 3000 RPM, so the throttle blades are not coming clear of the timed port and the jumping timing is reflective of the system positioned in this transition zone.

Bogie
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Old 05-08-2008, 06:20 PM
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soo how do i fix this than how would u set the timing.

everbody told me that u have to plug off the vacume advance and rev the engine to 3000 to open the weight up in the dist than set the timing
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Old 05-08-2008, 07:01 PM
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Have your distributor hooked up to the manifold vacuum port, and raise the engine to over 3000 rpm with everything hooked up. Set your timing to 36-38 degrees advanced, at this point lock down the distributor and return the engine to idle. Unplug the distributor advance and look at the timing marks, this is your new base timing, note it, and hook it back up and test drive it to check performance.

You can use this procedure to fine tune the engine for normal driving conditions. You may need to increase, or decrease the maximum timing if it pings, or is sluggish.
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Old 05-08-2008, 07:59 PM
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Quote:
Originally Posted by cope
soo how do i fix this than how would u set the timing.

everbody told me that u have to plug off the vacume advance and rev the engine to 3000 to open the weight up in the dist
It will and with a timing tape on the damper or a dial back timing light you can see the centrifugal do it's thing, but rate is as important as amount and thats difficult to do as you need to call out the advance at specific RPMs to see the rate. For what its worth, on a mild engine this knowledge really isn't too important. But I don't know how modified, if any, this engine is so I'm sticking pretty close to generic principles.

The ignition timing for a street and even a street/strip engine consists of three parts, base, vacuum, and centrifugal.

- Base is the initial timing established with the enigne idling at it's minimum RPM and the vacuum disconnected and the port plugged. The minimum RPM assumes that the centrifugal will not be operative. This is achieved by moving the distributor till the timing marks on the damper and the pointer align to the proper lead. For a street engine this is usually from around 5 to 10 degrees. Some engines take more, some less. You just have to remember that the vacuum and centrifugal advances are values on top of this, so if you put in say 20 degrees base onto a distributor that moves 30 degrees internally and attached this to an engine that only needs 40 degree, the total of the advances would be 50 degrees which is 10 more than the engine needs. The problem with too much advance is it starts the combustion process too early which is trying to drive the piston backwards resulting in a loss of power and economy. It also results in an abnormal pressure and temperature spikes that lead to detonation and makes incredible amounts of NOx.

People often see race engines with extreme amounts of base spark advance and emulate this as a way to more power. It isn't! Race engines need a lot of lead because long duration short LSA cams contaminate the incoming charge with a lot of exhaust and pump a lot of the mixture back out the intake at lower RPMs. The extreme advance is there because burn speed is related to mixture density, when there isn't much trapped in the cylinder, a lot of advance is required to get the stuff burn't before the cycle ends. What you don't see is that for every degree of super advanced base, an equal amount is removed from the variable (usually just centrifugal for competition engines). Extreme engines run full advance all the time because they run above 3000 RPM all the time. So getting rid of the un-needed variable advance eliminates a potential failure point and you live with the hard starting by changing the start up process to where the ignition isn't turned on till the motor is well into cranking. But its important to see that all this initial advance has nothing to do with making an engine go fast. It's just a tool to solve a problem

- Centrifugal advance is there to make the engine easy to start. This takes load off the starter during cranking and protects it from back fires which can make the engine rotate in reverse when its being cranked. Such an event can and will break starters and their parts. In this age it's also used to manage emissions by delaying full advance at WOT till the RPMs get over 3000 to keep combustion temps down which reduces NOx formation. For a drag race or road course engine where the RPMs and gear selections are highly variable so you want the engine as close to peak torque as possible, but can't tolerate a locked at full advance situation because of detonation, a timing curve is developed for maximum power at any given RPM just under the detonation limit. This is hard to do because the detonation limit is reached under different RPMs and loading in different gears. Unfortunately the most severe detonation occurs at lower RPMs in higher gears. So without a computer in the circuit, this limits how much and how fast the advance can be in the lower gears where more of it sooner could be tolerated.

- Vacuum advance is there for good fuel economy. This is more of a solution to the problem of under powered street cars with automatic transmissions as with a stick it's fairly easy to select a gear that runs the RPMs up to where a centrifugal advance is optimized. This is hard on a less than race prepped automatic and will significantly shorten the gear boxes life. So vacuum advance is used to make the advance sensitive to load rather than RPM. Under in-town and typical moderate cruise conditions the engine isn't tuning fast enough to bring the centrifugal advance in far enough to where the low density mixture from not much throttle opening is burnt early enough to complete combustion of all the stuff that's in there. So vacuum advance is there in proportion to the engine vacuum, lots of vacuum gets lots of advance. Less vacuum as the throttles are opened up, gets less advance. The assumption here is that as vacuum goes down in proportion to the throttle being opened, the engine RPM goes up and the centrifugal picks up the advance. But we all know what "ASSUME" really means!

One can see the difficulty hiding under this assumption. Going up a steep grade, for example, may require a lot of throttle opening with no increase in RPM. In this case the engine is far from optimized and not only is getting lousy gas mileage but will over heat from its struggles. This is why shifting to a lower gear on a steep grade is so useful. Not only does the engine get a boost in mechanical advantage, but the throttle can be reduced which improves vacuum that in turns brings more advance for better efficiency or the RPMs go up for the same effect.

The carb has two vacuum ports, one at manifold vacuum all the time which means the port is always lower than the throttle blades. The other is timed which means it's slightly above the throttle blades when they are properly set for idle. Then as the blades are opened the port is exposed to the manifold side of the throttle. Usually a hot cammed engine prefers the always on port while a more mild cammed engine prefers the timed, but this is more a tuning feature than anything so you need to see which the engine prefers. The fact that you're seeing instability from the ported side is that without the engine having to carry a load, it doesn't take much throttle opening to get the RPMs up pretty high, so I suspect the timed port is seeing an instability that is switching back and forth between manifold vacuum and venturi vacuum the later is in reverse to the manifold, that is the faster you turn the engine, the higher is the vacuum thru the venturi but it's lower in the manifold. Lots of tricky stiff going on.

OK covered a lot of stuff to get simple:

- Base timing with the vacuum disconnected and the ports plugged. Probably some where in the range of 5 to 10 degrees, if it takes more than 12, you need to think about reducing the amount in the centrifugal, but first you need to know how much is in there. Remember this is only to make the engine easy to start and non-distructive to the starter. To keep the centrifugal out of this equation you want the idle to be as slow as possible.

- Centrifugal relates to engine speed, a typical distributor has 20 or so degrees in it. A street will have more centrifugal and less base, a performance motor will tend toward more in the base and less in the centrifugal. But both top out around 35 to 45 degrees. Head selection plays in this, modern fast burn chambers require less advance for optimum performance, older open chambers more.

- Vacuum advance is related to load. when vacuum is high the spark is at max advance ans vacuum comes down as the throttle is opened so does the amount of this advance. Timing this coming down with the centrifugal going up should be done but isn't easy, it takes many passes or a dyno to do this. It may or may not use ported vacuum, you'll have to see which it likes.

To answer your very first question, yes, a large enough vacuum leak will not only lean the mixture but will affect the vacuum advance.

Bogie
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