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Bad power valve? Did you properly adjust the gap on the pertronix?
Bad idle/stall when in drive
- Thread starter wpdj61
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wpdj61
Well-known member
Keep forgetting to mention that the car wasn't right prior to the installation of the Pertronix distributor and coil. It has been getting progressively worse.
What do you mean adjust the gap on the Pertronix? I am a bit confused by that.
Was going to rebuild the carb and with that, the power valve would be replaced.
Scott
What do you mean adjust the gap on the Pertronix? I am a bit confused by that.
Was going to rebuild the carb and with that, the power valve would be replaced.
Scott
- Joined
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- 807
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- 1971 Mach 1 Completed
1972 Mach 1 Future Project
1972 Mach 1 Doner Parts Car
1973 302 Current Project
eohhhhhh, sorry bout that Scott! Just realized you've got the Flamethrower 3 dizzy and I'm not familiar with it and from what I've seen on the net, you may not need to air gap it the way older models do. Rebuild the carb and see if that fixes the problem. You could have a leaking or torn power valve. Hope you find the problem soon!
72HCODE
"My World is Fire and Blood"
Ok so the the problem was present before you tried to install the electronic ignition to see if the problem stopped and it just got worse and is continuing to get worse.
a blown power valve will actually not effect idle, this is because when the butterflies are closed they draw fuel and air from the transition slots it is a separate circuit.
coming off idle a blown power valve will present itself as a bog and go, with lots of black smoke out the tailpipes, and bad fuel economy.
A vacuum leak is first on the list of things to check. the first place to look is the brake booster.
to test the brake booster you can use a hand vacuum pump normally used to bleed brakes. hook up the hand vacuum pump to the hose going to the check valve on the brake booster, the hand pump will have a vacuum gauge on it. Pump the air out to about 15" HG and let it sit for a few minutes. come back and confirm the vacuum is holding. when that test is done. Remove the check valve from the brake booster with the hose and blow into it, then suck air through the hose and repeat a few times with your palm holding against the check valve you should hear the check valve open and close and only allow air movement in one direction.
if you can blow air and suck air through the check valve it is blown get another one.
Now if you had a A/C system the next thing to check is the vacuum canister mounted on the passenger side apron. the vacuum canister also has a check valve in it just like the break booster and it must be tested as well. we can skip that on your car if you really do not have A/C or a vacuum canister installed in the car.
Next check your transmission modulator if you have a automatic transmission, usually the lower hose rots out and causes a big leak. you can check the modulator with the same hand vacuum pump you used for the brake booster.
Moving on, you need to check the Vacuum advance on the distributor make sure it is not blown. this depends on if you are using Ported manifold vacuum or full manifold vacuum, our mustangs use ported vacuum timing, that means at idle there is no vaccum going to the vacuum advance. coming off idle you can have a leak. Pop the distributor cap, and hook a hose on to the vaccum advance canister then use the hand vacuum pump and again pump to 15"HG watch the vacuum advance move and make sure it does not hang up on anything. let it sit for a few minutes and confirm it is not leaking.
If you have any emissions equipment on the car it should be looked at the biggest source for a vacuum leak is the PCV valve located on the valve cover it can get stuck open and cause a vacuum leak at idle. next a EGR valve can clog up and cause problems if you have a 73 that is more involved.
to check the pcv you can pop the emissions hose off it, put your finger over the hose to close the leak off and start the car. if it has the same problem then the pcv is not the source but you should clean it out anyway.
that takes care of any source of leaks from devices
next has anything on the intake manifold been changed the intake manifold can be a source of problems.
there are different ways to check for leaks around the carb or intake, you can spray carb cleaner around the intake manifold while the car is running just be careful you don't want to douse it and start a fire a quick spray around the intake base and listen for the engine rpms to rise up.
you can also Cup your hand over the Carburetor throat. choking the air off should stall the car out. if the car continues to run with the carb throat choked off then you know you have a major vacuum leak and you may hear the engine sucking the air into the engine through it.
Vacuum leaks checked the next is to suspect the carburetor or ignition.
first i would make sure all 8 cylinders are in fact firing. you can take a timing light and place it on all 8 ignition wires and check the strobe, if you find one cylinder is not strobing the same as the other 7 then you know there is a problem with the electronic ignition module. Happened to me on a petronix I the magnetic sleeve was defective and 2 cylinders stopped firing.
confirming the ignition is firing all 8 now we move to fuel.
the carb might have a clog in the idle circuit. check the accelerator pump make sure it is squirting down the carb throat correctly when the throttle is moved.
put a vacuum gauge on full manifold vacuum and see what the car is doing.
you could turn the idle air bleed screws in all the way then back out 1.5 turns if your carb is equipped with an adjustable one and see if the idle improved or not.
you can also try checking for weak spark, run a full 12 volt power tap to the electronic ignition and bypass the resistance wire and see if things improve.
if all else fails then you start thinking about pulling the carb opening it up and looking for particles in the fuel or clogs.
if you find the carb does have some kind of build up inside it or you find particles inside it, check your external fuel filter if you have one. carefully cut it open or if you have a inspectable version open it up and see how clogged up the filter it. if its full of rust and junk. Time to change out the fuel tank and sender, and think about new fuel lines, maybe a fuel pump as well.
if you clean up the carb then garbage from the fuel tank is going to clog it up again unless addressed.
otherwise rebuild the carb, clean it out inspect all fuel circuits. you may want to think about swapping a new carb on the car and see what happens. if instantly the car is cured then you know the old carb is clogged up.
Ah you can see there is no simple answer and as always the problem will be found in the very last place you look
a blown power valve will actually not effect idle, this is because when the butterflies are closed they draw fuel and air from the transition slots it is a separate circuit.
coming off idle a blown power valve will present itself as a bog and go, with lots of black smoke out the tailpipes, and bad fuel economy.
A vacuum leak is first on the list of things to check. the first place to look is the brake booster.
to test the brake booster you can use a hand vacuum pump normally used to bleed brakes. hook up the hand vacuum pump to the hose going to the check valve on the brake booster, the hand pump will have a vacuum gauge on it. Pump the air out to about 15" HG and let it sit for a few minutes. come back and confirm the vacuum is holding. when that test is done. Remove the check valve from the brake booster with the hose and blow into it, then suck air through the hose and repeat a few times with your palm holding against the check valve you should hear the check valve open and close and only allow air movement in one direction.
if you can blow air and suck air through the check valve it is blown get another one.
Now if you had a A/C system the next thing to check is the vacuum canister mounted on the passenger side apron. the vacuum canister also has a check valve in it just like the break booster and it must be tested as well. we can skip that on your car if you really do not have A/C or a vacuum canister installed in the car.
Next check your transmission modulator if you have a automatic transmission, usually the lower hose rots out and causes a big leak. you can check the modulator with the same hand vacuum pump you used for the brake booster.
Moving on, you need to check the Vacuum advance on the distributor make sure it is not blown. this depends on if you are using Ported manifold vacuum or full manifold vacuum, our mustangs use ported vacuum timing, that means at idle there is no vaccum going to the vacuum advance. coming off idle you can have a leak. Pop the distributor cap, and hook a hose on to the vaccum advance canister then use the hand vacuum pump and again pump to 15"HG watch the vacuum advance move and make sure it does not hang up on anything. let it sit for a few minutes and confirm it is not leaking.
If you have any emissions equipment on the car it should be looked at the biggest source for a vacuum leak is the PCV valve located on the valve cover it can get stuck open and cause a vacuum leak at idle. next a EGR valve can clog up and cause problems if you have a 73 that is more involved.
to check the pcv you can pop the emissions hose off it, put your finger over the hose to close the leak off and start the car. if it has the same problem then the pcv is not the source but you should clean it out anyway.
that takes care of any source of leaks from devices
next has anything on the intake manifold been changed the intake manifold can be a source of problems.
there are different ways to check for leaks around the carb or intake, you can spray carb cleaner around the intake manifold while the car is running just be careful you don't want to douse it and start a fire a quick spray around the intake base and listen for the engine rpms to rise up.
you can also Cup your hand over the Carburetor throat. choking the air off should stall the car out. if the car continues to run with the carb throat choked off then you know you have a major vacuum leak and you may hear the engine sucking the air into the engine through it.
Vacuum leaks checked the next is to suspect the carburetor or ignition.
first i would make sure all 8 cylinders are in fact firing. you can take a timing light and place it on all 8 ignition wires and check the strobe, if you find one cylinder is not strobing the same as the other 7 then you know there is a problem with the electronic ignition module. Happened to me on a petronix I the magnetic sleeve was defective and 2 cylinders stopped firing.
confirming the ignition is firing all 8 now we move to fuel.
the carb might have a clog in the idle circuit. check the accelerator pump make sure it is squirting down the carb throat correctly when the throttle is moved.
put a vacuum gauge on full manifold vacuum and see what the car is doing.
you could turn the idle air bleed screws in all the way then back out 1.5 turns if your carb is equipped with an adjustable one and see if the idle improved or not.
you can also try checking for weak spark, run a full 12 volt power tap to the electronic ignition and bypass the resistance wire and see if things improve.
if all else fails then you start thinking about pulling the carb opening it up and looking for particles in the fuel or clogs.
if you find the carb does have some kind of build up inside it or you find particles inside it, check your external fuel filter if you have one. carefully cut it open or if you have a inspectable version open it up and see how clogged up the filter it. if its full of rust and junk. Time to change out the fuel tank and sender, and think about new fuel lines, maybe a fuel pump as well.
if you clean up the carb then garbage from the fuel tank is going to clog it up again unless addressed.
otherwise rebuild the carb, clean it out inspect all fuel circuits. you may want to think about swapping a new carb on the car and see what happens. if instantly the car is cured then you know the old carb is clogged up.
Ah you can see there is no simple answer and as always the problem will be found in the very last place you look
wpdj61
Well-known member
H-Code, thanks for that great post. I had the car running a bit last night and noticed black splatter marks behind the exhaust pipes on the garage floor. It is looking more and more like the carb is running way rich.
Scott
Scott
72HCODE
"My World is Fire and Blood"
i get that also. make sure any adjustments you make to the carb are when the engine is fully warmed up. the choke has to be fully opened to make sure the issue is not with the choke not opening all the way. you want that engine at operating temperature before messing with mixtures. I've always had rooster tails of soot and carbon behind my 72. it clears up when the engine warms up.
there are so many things you can play with like Spark plug temperature and back pressure to compensate for tuning issues. the first is just getting it in the ball park.
if that carb is clogged up with particles from the tank then the first order of business is clean out the carb. then inspect the quality of fuel from the tank.
you can drain some gas from the tank into a glass cup and if you see floaters then its time to replace the fuel tank and sender, maybe the fuel lines as well.
once your sure the tank is clean, providing good fuel to the cleaned out carb then the real work can begin. There is no point in rebuilding the carb if dirty gas continues to fowl it up.
there are so many things you can play with like Spark plug temperature and back pressure to compensate for tuning issues. the first is just getting it in the ball park.
if that carb is clogged up with particles from the tank then the first order of business is clean out the carb. then inspect the quality of fuel from the tank.
you can drain some gas from the tank into a glass cup and if you see floaters then its time to replace the fuel tank and sender, maybe the fuel lines as well.
once your sure the tank is clean, providing good fuel to the cleaned out carb then the real work can begin. There is no point in rebuilding the carb if dirty gas continues to fowl it up.
wpdj61
Well-known member
I finished rebuilding the carb and noticed that the power valve was not looking good. It looks to me like that was the source of the trouble. That and a small issue with one of the gaskets. Hope to get the car running again today to see if I made any headway.
Scott
Scott
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Good luck with your work on the carb. A failed power valve can make the engine run pig rich. Don't overlook the air bleeds in the main body they can affect the way the fuel and air mixes in the main well or idle circuit. Plugged or restricted air bleeds will richen the mixture significantly.
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wpdj61
Well-known member
drop, I got you on the bleeds. Went through them and all of the passages. Everything is cleaned out and compressed air moved through all passageways freely. I got the carb back on the car but didn't have a chance to start it up.
Scott
Scott
wpdj61
Well-known member
Well, the problem is solved. I replaced the car with a Holley 650 and all is well now. I went through that 750 over and over and I couldn't find anything that was causing the problem. New carb on the car and the problem is gone. I can live with that. I missed driving my mustang so life is good again. Thanks for all of the help with this problem.
Scott
Scott
Machman
Well-known member
I'm not sure if this will help but on my car (302 motor) the carb has a flying saucer shaped dohicky on the right side that is linked to the accelerator linkage near the carb. It has a vacuum line connected to it and is suppose to prevent the car from stalling at idle / decelleration The rubber boot on mine was ripped so the p/o had disconnected and blocked the line to it.
Perhaps this is your problem (just a thought)
AJ
Perhaps this is your problem (just a thought)
AJ
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