I'm running a 351C, Aussie 3V heads, Comp Cams 292H cam. My idle vacuum around 1000 rpm is 5". Is this low? It's been recommended I pull the intake and replace the gasket. Could the timing be a factor? Thanks for the help.
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Mark,
(so many Marks on the bulletin board
) 5" Hg vacuum at idle is pretty low. I'm suspecting you don't have enough static ignition advance. Or at least that's the first thing to check.
I'm told the 3V heads require only about 30 degrees total ignition advance, which is about 6 degrees less than the iron heads. I don't know if that holds true at idle as well. An iron head with a cam like yours can run around 20 degrees static advance (with no vacuum advance, or with the vacuum advance connected to ported vacuum).
You can optimize the static setting in the driveway. With the engine up to temp and idling, vacuum advance connected normally, slowly turn the distributor in the advance direction (clockwise?). As you turn the distributor, the idle speed should increase, continue to turn the distributor until you just reach the point where the idle speed stops increasing, this is the ideal setting for the distributor at idle. The intake manifold vacuum will be at it's highest point, the exhaust temps will be at their lowest temps, you'll have more low end torque, better drivability, less heat load on the cooling system, and a better idle.
However, that will probably throw your total advance setting way off, ideal total advance can only be determined on a dyno.
If you arrive at 20 degrees static advance using the method I just described, and your engine only requires 30 degrees total advance at around 3000 rpm, then your distributor should only have 10 degrees centrifugal swing. Most distributors are set up with about 20 degrees of centrifugal advance.
After you determine the optimum static setting, and the optimum total advance (a trip to the dyno is required) then you can have your distributor recurved for the proper amount of centrifugal advance.
You can check for vacuum leaks with a can of starter fluid. With the engine idling, spray the starter fluid wherever you suspect there may be a leak, if there is a leak the starter fluid will make the engine speed increase.
your friend on the DTBB, George
(so many Marks on the bulletin board
) 5" Hg vacuum at idle is pretty low. I'm suspecting you don't have enough static ignition advance. Or at least that's the first thing to check.I'm told the 3V heads require only about 30 degrees total ignition advance, which is about 6 degrees less than the iron heads. I don't know if that holds true at idle as well. An iron head with a cam like yours can run around 20 degrees static advance (with no vacuum advance, or with the vacuum advance connected to ported vacuum).
You can optimize the static setting in the driveway. With the engine up to temp and idling, vacuum advance connected normally, slowly turn the distributor in the advance direction (clockwise?). As you turn the distributor, the idle speed should increase, continue to turn the distributor until you just reach the point where the idle speed stops increasing, this is the ideal setting for the distributor at idle. The intake manifold vacuum will be at it's highest point, the exhaust temps will be at their lowest temps, you'll have more low end torque, better drivability, less heat load on the cooling system, and a better idle.
However, that will probably throw your total advance setting way off, ideal total advance can only be determined on a dyno.
If you arrive at 20 degrees static advance using the method I just described, and your engine only requires 30 degrees total advance at around 3000 rpm, then your distributor should only have 10 degrees centrifugal swing. Most distributors are set up with about 20 degrees of centrifugal advance.
After you determine the optimum static setting, and the optimum total advance (a trip to the dyno is required) then you can have your distributor recurved for the proper amount of centrifugal advance.
You can check for vacuum leaks with a can of starter fluid. With the engine idling, spray the starter fluid wherever you suspect there may be a leak, if there is a leak the starter fluid will make the engine speed increase.
your friend on the DTBB, George
George, Thanks for the tips. Thorough as usual. What would it take to bribe you to coming to KY and line this thing out? I'm using a Unilite with mechanical advance. Could there be a problem there?
Mark -
You've got the big brother of my cam, a Compcams 282S! It is a stout cam and I'd expect a somewhat low vacuum, but am not qualified to say how low.
But let me ask the question - what are the symptoms you are trying to solve? Just low vacuum at idle? Or is the engine not running particularly well?
You've got the big brother of my cam, a Compcams 282S! It is a stout cam and I'd expect a somewhat low vacuum, but am not qualified to say how low.
But let me ask the question - what are the symptoms you are trying to solve? Just low vacuum at idle? Or is the engine not running particularly well?
Charlie, I'm having problems with maintaining a reasonable idle speed, somewhere around 1000 rpm. Don't know if I can get any lower with this cam. I'm not sure if low vacuum is the symptom or the cause, or maybe its normal. I just installed a Demon and am having fuel bowl leakage issues. The primary fuel bowl is draining overnight. Investigating this now.
Mark,
other than a low vacuum reading, you haven't mentioned any other problems. Tune up the timing, test drive the car if possible (weather) and get back to us on the BB if there any further problems.
I'll pass on the trip to KY right now, but once I get 6018 AND my finances into traveling condition, I plan to start doing long distance trips & visiting fellow Pantera owners around the country (maybe even Canada if they promise not to hurt me), so keep that in mind.
Any distributor with mechanical advance can have an advance mechanism hanging up. My experience with the Unilite system is that it either works or it don't. Don't forget to check the cap & rotor, they do wear out. also, the wire between the coil & the center terminal on the distributor cap will wear out much faster than the other 8 wires. Any wire that lays on an exhaust header will also fail quickly. If you run the gaps on the plugs really big, your car will run better, but the wires will fail quicker. If the wires are typical resistance wires, an ohm meter should read somewhere between 5000 and 20,000 ohms, depending upon the length of the wire.
your friend on the DTBB, George
other than a low vacuum reading, you haven't mentioned any other problems. Tune up the timing, test drive the car if possible (weather) and get back to us on the BB if there any further problems.
I'll pass on the trip to KY right now, but once I get 6018 AND my finances into traveling condition, I plan to start doing long distance trips & visiting fellow Pantera owners around the country (maybe even Canada if they promise not to hurt me), so keep that in mind.
Any distributor with mechanical advance can have an advance mechanism hanging up. My experience with the Unilite system is that it either works or it don't. Don't forget to check the cap & rotor, they do wear out. also, the wire between the coil & the center terminal on the distributor cap will wear out much faster than the other 8 wires. Any wire that lays on an exhaust header will also fail quickly. If you run the gaps on the plugs really big, your car will run better, but the wires will fail quicker. If the wires are typical resistance wires, an ohm meter should read somewhere between 5000 and 20,000 ohms, depending upon the length of the wire.
your friend on the DTBB, George
I would replace the gasket on the primary bowl and see what happens.
Maybe this is effecting the fuel pressure.
Maybe this is effecting the fuel pressure.
quote:Originally posted by Husker:
Charlie, I'm having problems with maintaining a reasonable idle speed, somewhere around 1000 rpm. Don't know if I can get any lower with this cam. I'm not sure if low vacuum is the symptom or the cause, or maybe its normal. I just installed a Demon and am having fuel bowl leakage issues. The primary fuel bowl is draining overnight. Investigating this now.
Mark,
you must have been writing at the same time as me!
Is the fuel leaking from the fuel bowl leaking externally, onto the manifold, or is it apparently leaking internally, into the manifold. An internal leak can result in any number of performance issues, rich OR lean conditions, poor metering, that sort of thing.
I normally set Clevelands to idle at 1200 rpm warm, so that it drops to 1000 rpm when the a/c compressor kicks in.
I believe 1000 rpm was the Ford spec for the 72 to 74 motors, the 70 to 71 motors were spec'd at 800 rpm, but they had smaller carbs & milder cams. This is all from memory. The point is, Cleveland engines like to idle fast.
George
you must have been writing at the same time as me!
Is the fuel leaking from the fuel bowl leaking externally, onto the manifold, or is it apparently leaking internally, into the manifold. An internal leak can result in any number of performance issues, rich OR lean conditions, poor metering, that sort of thing.
I normally set Clevelands to idle at 1200 rpm warm, so that it drops to 1000 rpm when the a/c compressor kicks in.
I believe 1000 rpm was the Ford spec for the 72 to 74 motors, the 70 to 71 motors were spec'd at 800 rpm, but they had smaller carbs & milder cams. This is all from memory. The point is, Cleveland engines like to idle fast.
George
Husker, I am also running the CompCasms 292H on my 351 cleveland. The very lowest that my Vacuum achieved was 5" at Idle. I have gone through the timing countless times. Checked and rechecked for Vacuum leaks. Taken apart the carb and carefully reassembled it. Went through all of the adjustments. NO changes! I don't know if your demon carb is similar to my Holley. Two things I have found: In my case the 'standard' power valve of 6.5", was open ALL the time at idle making for a very rich condition which makes the engine run poorly which makes low vacuum...Well you get the Idea. As of this writing am looking for a power valve with a rating of 3.5- 4.0. My ignition is a Mallory Unilite, Ive gone through it, except have not yet hooked it up to the Ossiliscope. The Vacuum advance was running of of straight Manifold Vacuum. Then I went on the internet and have found that Half run their Vac. Advance off of the 'Ported' Vacuum. I will be running more tests on the ported Vac. Have not gotten to it yet, because of the holidays. My intial timing has been at 12 Degrees BTDC...
...Also; My idle RPM is 1000. I can make it idle as low as 700 or so. OR 1000-1200 sounds good. This is by the 'Dash' Tach, which is not accurate enough for this kind of work. One thing you might try is to 'Kick-up' the set screw under your Secondary Butterflys to increase RPM and then close your ptimaries to bring the Vacuum up, more. This gained me aprox. 1 or 2 more INCHES. Not a whole lot. The only reason, I can think of; why your float bowl would be draining overnight, could be a crack in the Plate or Body, or leak in a gasket, perhaps the gasket is missing behind one of the lower 'Bowl Screws'. The only other time I saw a Bowl drain, and this was at a very Fast rate; and WHILE the engine was running, was from a Blown POWER VALVE. Good-Luck with it!...
Marlin & Mark,
from what I'm reading it sounds as though the 292H is a lumpy idle, high overlap type cam, but I'm not convinced 5" vacuum is the best you can do. Loosen your distributor clamping bolt & slowly twist the distributor in the advance direction until you find the spot where it idles the fastest, that will also be the spot where the highest intake manifold vacuum is achieved. Ignore anything you've ever been told before, that setting is the ideal static advance setting for your motor.
IF 5" vacuum is all the vacuum your motor can muster up, then forget about running a PCV valve, and forget about running vacuum advance on the distributor; ported, manifold, whatever.
I hate power valves! lol. They are the crudest device ever installed on an automobile! What was Smokey thinking when he included that in the design? I think Smokey was smokin' them cigarettes you don't buy at the store, know what I mean?
I want to make 2 points regarding setting a Holley carb (I've never worked on a Demon yet, but I'll bet this applies to them as well). (1) The power valve should only open at full throttle, so monitor your manifold vacuum while driving the car with the pedal mashed, and install a powervalve that opens at that vacuum. The power valve should never open during part throttle operation. (2) The idle stops for the secondary butterflies should be set so that at idle the small idle transfer slots in the bore are partially exposed below the butterflies. If the secondary butterflies are too far closed, this will force the primaries too be opened too far & the car will not idle properly.
The upper picture shows the secondary butterfly set properly, you can see the transfer slot exposed near the tip of the screwdriver, the lower picture the butterflies are set too far closed, the transfer slot is concealed.
I would appreciate feedback on the results, especially concerning the manifold vacuum.
Your friend on the DTBB, George
from what I'm reading it sounds as though the 292H is a lumpy idle, high overlap type cam, but I'm not convinced 5" vacuum is the best you can do. Loosen your distributor clamping bolt & slowly twist the distributor in the advance direction until you find the spot where it idles the fastest, that will also be the spot where the highest intake manifold vacuum is achieved. Ignore anything you've ever been told before, that setting is the ideal static advance setting for your motor.
IF 5" vacuum is all the vacuum your motor can muster up, then forget about running a PCV valve, and forget about running vacuum advance on the distributor; ported, manifold, whatever.
I hate power valves! lol. They are the crudest device ever installed on an automobile! What was Smokey thinking when he included that in the design? I think Smokey was smokin' them cigarettes you don't buy at the store, know what I mean?
I want to make 2 points regarding setting a Holley carb (I've never worked on a Demon yet, but I'll bet this applies to them as well). (1) The power valve should only open at full throttle, so monitor your manifold vacuum while driving the car with the pedal mashed, and install a powervalve that opens at that vacuum. The power valve should never open during part throttle operation. (2) The idle stops for the secondary butterflies should be set so that at idle the small idle transfer slots in the bore are partially exposed below the butterflies. If the secondary butterflies are too far closed, this will force the primaries too be opened too far & the car will not idle properly.
The upper picture shows the secondary butterfly set properly, you can see the transfer slot exposed near the tip of the screwdriver, the lower picture the butterflies are set too far closed, the transfer slot is concealed.
I would appreciate feedback on the results, especially concerning the manifold vacuum.
Your friend on the DTBB, George
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George, I'm still working on the problem. So far I've found several rocker arms loose and a spark plug insulator cracked top to bottom. I checked the idle stops and adjusted them per instructions. The power valve was replaced with a 3.5. The Demon's instructions differentiates on adjusting the idle stops based on idle speed. I think above it, the secondary is shut and below 1000 the primary and secondary are adjusted open the same amount. I might have these backwards. Just to make sure I'm not all backasswards, if the distributor turns counter clockwise, turning it clockwise will reduce the advance, right? (closer to TDC) Hopefully I'll be able to play with it a little more tomorrow, weather permitting.
Mark,
Turn the distributor clockwise to advance the timing.
Your friend on the DTBB, George
Turn the distributor clockwise to advance the timing.
Your friend on the DTBB, George
Any headway? Post what you're seeing, we'll help. Good Luck..
John
John