Falling Flat at high RPM
#1
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Thread Starter
Falling Flat at high RPM
1973 bone stock motor
Car has had high idle issues since i got it. never felt like it really ran well
i have good oil pressure, good voltage, dont burn oil and motor is actually pretty quite
i have been through the gambit
So lets start with what's new. some is new due to a mild restoration
New Fuel Lines front to rear and fuel filter
Distributor completely refurbished by Northwest Florida Distributor Service this month. top bushing came out and stuck on shaft when i tried to disassemble.
New Coil this week
new spark plugs couple weeks ago
Adjusted Valve lash this week
New vacuum lines two months
Intake manifold new install this week.
Carb rebuilt by Lars about 4 years ago, quadrajet, not original to car. we peices one together out of two carbs
fuel pump looks relatively new, ive owned car for 4.5 years.
car has only seen 2500 miles in that time
Timing set accordingly
Dwell set at 30, fluctuated 29.9 to 30.1 occasionally
vacuum is at 20 and holds steady. head lights work flawlessly after complete rehab
With no vacuum advance and carb plugged at advance connection, set initial at 12
remove one spring, bring to 36 before 2750 rpm with adjustments to distributor.
install new spring, timing at idle is at 22 degrees, connect vacuum, idle at 1000 rpm timing at 36 degrees
running with two silver springs from mr gasket as i could not get to 36 degrees otherwise.
i cannot adjust idle now as idle screw is all the way out.
carb fuel and air are at 2.5 turns out currently
car will run like this but fall completely flat at 3000 to 3500 rpm if mashed, can slowly persuade that way, but will start popping & sputtering.
one thing i cannot get is my secondaries to even start to pop open while revving in driveway.
im really starting to believe this carb is my issue.
idle will still fluctuate at times as well.
anyone have any others thoughts.
I also confirmed TDC on harmonic balancer is correct
Car has had high idle issues since i got it. never felt like it really ran well
i have good oil pressure, good voltage, dont burn oil and motor is actually pretty quite
i have been through the gambit
So lets start with what's new. some is new due to a mild restoration
New Fuel Lines front to rear and fuel filter
Distributor completely refurbished by Northwest Florida Distributor Service this month. top bushing came out and stuck on shaft when i tried to disassemble.
New Coil this week
new spark plugs couple weeks ago
Adjusted Valve lash this week
New vacuum lines two months
Intake manifold new install this week.
Carb rebuilt by Lars about 4 years ago, quadrajet, not original to car. we peices one together out of two carbs
fuel pump looks relatively new, ive owned car for 4.5 years.
car has only seen 2500 miles in that time
Timing set accordingly
Dwell set at 30, fluctuated 29.9 to 30.1 occasionally
vacuum is at 20 and holds steady. head lights work flawlessly after complete rehab
With no vacuum advance and carb plugged at advance connection, set initial at 12
remove one spring, bring to 36 before 2750 rpm with adjustments to distributor.
install new spring, timing at idle is at 22 degrees, connect vacuum, idle at 1000 rpm timing at 36 degrees
running with two silver springs from mr gasket as i could not get to 36 degrees otherwise.
i cannot adjust idle now as idle screw is all the way out.
carb fuel and air are at 2.5 turns out currently
car will run like this but fall completely flat at 3000 to 3500 rpm if mashed, can slowly persuade that way, but will start popping & sputtering.
one thing i cannot get is my secondaries to even start to pop open while revving in driveway.
im really starting to believe this carb is my issue.
idle will still fluctuate at times as well.
anyone have any others thoughts.
I also confirmed TDC on harmonic balancer is correct
Last edited by gmmeyerIII; 12-27-2018 at 05:54 PM. Reason: Also, i confirmed that my TDc on harmonic balancer is correct
#2
Burning Brakes
1973 bone stock motor
Car has had high idle issues since i got it. never felt like it really ran well
i have good oil pressure, good voltage, dont burn oil and motor is actually pretty quite
i have been through the gambit
So lets start with what's new. some is new due to a mild restoration
New Fuel Lines front to rear and fuel filter
Distributor completely refurbished by Northwest Florida Distributor Service this month. top bushing came out and stuck on shaft when i tried to disassemble.
New Coil this week
new spark plugs couple weeks ago
Adjusted Valve lash this week
New vacuum lines two months
Intake manifold new install this week.
Carb rebuilt by Lars about 4 years ago, quadrajet, not original to car. we peices one together out of two carbs
fuel pump looks relatively new, ive owned car for 4.5 years.
car has only seen 2500 miles in that time
Timing set accordingly
Dwell set at 30, fluctuated 29.9 to 30.1 occasionally
vacuum is at 20 and holds steady. head lights work flawlessly after complete rehab
With no vacuum advance and carb plugged at advance connection, set initial at 12
remove one spring, bring to 36 before 2750 rpm with adjustments to distributor.
install new spring, timing at idle is at 22 degrees, connect vacuum, idle at 1000 rpm timing at 36 degrees
running with two silver springs from mr gasket as i could not get to 36 degrees otherwise.
i cannot adjust idle now as idle screw is all the way out.
carb fuel and air are at 2.5 turns out currently
car will run like this but fall completely flat at 3000 to 3500 rpm if mashed, can slowly persuade that way, but will start popping & sputtering.
one thing i cannot get is my secondaries to even start to pop open while revving in driveway.
im really starting to believe this carb is my issue.
idle will still fluctuate at times as well.
anyone have any others thoughts.
I also confirmed TDC on harmonic balancer is correct
Car has had high idle issues since i got it. never felt like it really ran well
i have good oil pressure, good voltage, dont burn oil and motor is actually pretty quite
i have been through the gambit
So lets start with what's new. some is new due to a mild restoration
New Fuel Lines front to rear and fuel filter
Distributor completely refurbished by Northwest Florida Distributor Service this month. top bushing came out and stuck on shaft when i tried to disassemble.
New Coil this week
new spark plugs couple weeks ago
Adjusted Valve lash this week
New vacuum lines two months
Intake manifold new install this week.
Carb rebuilt by Lars about 4 years ago, quadrajet, not original to car. we peices one together out of two carbs
fuel pump looks relatively new, ive owned car for 4.5 years.
car has only seen 2500 miles in that time
Timing set accordingly
Dwell set at 30, fluctuated 29.9 to 30.1 occasionally
vacuum is at 20 and holds steady. head lights work flawlessly after complete rehab
With no vacuum advance and carb plugged at advance connection, set initial at 12
remove one spring, bring to 36 before 2750 rpm with adjustments to distributor.
install new spring, timing at idle is at 22 degrees, connect vacuum, idle at 1000 rpm timing at 36 degrees
running with two silver springs from mr gasket as i could not get to 36 degrees otherwise.
i cannot adjust idle now as idle screw is all the way out.
carb fuel and air are at 2.5 turns out currently
car will run like this but fall completely flat at 3000 to 3500 rpm if mashed, can slowly persuade that way, but will start popping & sputtering.
one thing i cannot get is my secondaries to even start to pop open while revving in driveway.
im really starting to believe this carb is my issue.
idle will still fluctuate at times as well.
anyone have any others thoughts.
I also confirmed TDC on harmonic balancer is correct
those are vacuum secondaries? They wont open untill your airspeed reaches a certain velocity under load, reving in neutral wont open them
Last edited by naramlee; 12-27-2018 at 07:08 PM.
#3
Tech Contributor
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Sounds like a fuel starvation issue, but the high idle is indicative of a vacuum leak, which will lean out the carb and cause problems. You cannot get the secondaries to open while "winging" the throttle in neutral - there's not enough airflow to open them in neutral, so what you're doing is an invalid and irrelevant test of secondary operation. If you suspect the carb, send it back and I'll re-test it. All carbs are tested extensively to assure correct operation. Part of my test procedure and acceptance is to verify that the engine can be "killed" by backing out the idle screw. If you are idling at 1000 with the screw backed out, you either have a faulty divorced choke keeping the idle on the fast idle cam, or you have a vacuum leak. Popping at elevated rpm and "falling on its face" is a sign of fuel starvation or vacuum leak.
No, the Q-Jet does not have vacuum secondaries. It is a mechanical secondary carb with a secondary airvalve. This is not a vacuum secondary carb.
Lars
No, the Q-Jet does not have vacuum secondaries. It is a mechanical secondary carb with a secondary airvalve. This is not a vacuum secondary carb.
Lars
Last edited by lars; 12-27-2018 at 07:28 PM.
The following users liked this post:
LenWoodruff (12-28-2018)
#4
Advanced
Thread Starter
Lars
i almost agree in fuel starvation. the vacuum leak i guess is a possibility, but i i have changed all hoses under hood including headlights actuators and so on.
just pulled intake thinking there could possibly be a leak despite not being able to source one.
would vacuum leak explain high timing when setting? that's the other thing that really threw me for a loop.
i almost agree in fuel starvation. the vacuum leak i guess is a possibility, but i i have changed all hoses under hood including headlights actuators and so on.
just pulled intake thinking there could possibly be a leak despite not being able to source one.
would vacuum leak explain high timing when setting? that's the other thing that really threw me for a loop.
#5
Le Mans Master
As LARS stated:
Quadrajet carburetors have mechanical secondary throttle plates operated by a progressive linkage; the primaries open before the secondaries, and use on-demand air valve plates above the secondary throttle plates. The air valves are connected by a cam and linkage to the secondary fuel metering rods. As the airflow increases through the secondary bores, the air valves are pushed down, rotating a cam that lifts the secondary metering rods. The secondary rods are tapered in a similar fashion to the primary metering rods, effectively increasing the size of the fuel metering holes as the rods are lifted and delivering more fuel. Therefore, the position of the air valve will control both fuel and air flow through the secondary venturis, even if the secondary throttle plates are fully opened. Thus the Quadrajet acts like a vacuum-secondary carburetor and only delivers more fuel as it is needed.[[i]citation needed]
Quadrajet carburetors have mechanical secondary throttle plates operated by a progressive linkage; the primaries open before the secondaries, and use on-demand air valve plates above the secondary throttle plates. The air valves are connected by a cam and linkage to the secondary fuel metering rods. As the airflow increases through the secondary bores, the air valves are pushed down, rotating a cam that lifts the secondary metering rods. The secondary rods are tapered in a similar fashion to the primary metering rods, effectively increasing the size of the fuel metering holes as the rods are lifted and delivering more fuel. Therefore, the position of the air valve will control both fuel and air flow through the secondary venturis, even if the secondary throttle plates are fully opened. Thus the Quadrajet acts like a vacuum-secondary carburetor and only delivers more fuel as it is needed.[[i]citation needed]
Last edited by jb78L-82; 12-27-2018 at 09:05 PM.
#6
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Lars
i almost agree in fuel starvation. the vacuum leak i guess is a possibility, but i i have changed all hoses under hood including headlights actuators and so on.just pulled intake thinking there could possibly be a leak despite not being able to source one.would vacuum leak explain high timing when setting? that's the other thing that really threw me for a loop.
i almost agree in fuel starvation. the vacuum leak i guess is a possibility, but i i have changed all hoses under hood including headlights actuators and so on.just pulled intake thinking there could possibly be a leak despite not being able to source one.would vacuum leak explain high timing when setting? that's the other thing that really threw me for a loop.
If the throttle plates are completely closed, the engine will die regardless of timing or any other issues, since the engine cannot idle without air. If the air is not going by the throttle plates (because they're closed), the air has to be coming from some other place other than through the carb. That source is a vacuum leak. The engine cannot idle if the throttles are fully closed unless air is leaking into the manifold from another source. Make sure your throttle cable is not holding the throttle open. Again, if in doubt, send the carb back and I'll re-do the entire test procedure and setup for the cost of the postage.
Lars
#7
Race Director
Before you start pulling parts, purchase or borrow a vacuum gauge and test your vacuum system.
It does sound like a classic vacuum leak.
Even though your hoses are new it could be one of the components causing the leak and not a hose.
If you can’t find a vacuum leak then send your carb to Lars to work his magic.
Good luck and keep us posted.
It does sound like a classic vacuum leak.
Even though your hoses are new it could be one of the components causing the leak and not a hose.
If you can’t find a vacuum leak then send your carb to Lars to work his magic.
Good luck and keep us posted.
#8
Burning Brakes
Before you start pulling parts, purchase or borrow a vacuum gauge and test your vacuum system.
It does sound like a classic vacuum leak.
Even though your hoses are new it could be one of the components causing the leak and not a hose.
If you can’t find a vacuum leak then send your carb to Lars to work his magic.
Good luck and keep us posted.
It does sound like a classic vacuum leak.
Even though your hoses are new it could be one of the components causing the leak and not a hose.
If you can’t find a vacuum leak then send your carb to Lars to work his magic.
Good luck and keep us posted.
Check around the base of the carb including the gasket for leaks. Anything that runs with vacuum (power brakes, heater control system, headlights, etc.) should also be checked for leaks. Make sure the fuel filter is clean and installed properly in the carb.
Fran
#9
Advanced
Thread Starter
Before you start pulling parts, purchase or borrow a vacuum gauge and test your vacuum system.
It does sound like a classic vacuum leak.
Even though your hoses are new it could be one of the components causing the leak and not a hose.
If you can’t find a vacuum leak then send your carb to Lars to work his magic.
Good luck and keep us posted.
It does sound like a classic vacuum leak.
Even though your hoses are new it could be one of the components causing the leak and not a hose.
If you can’t find a vacuum leak then send your carb to Lars to work his magic.
Good luck and keep us posted.
#10
Dr. Detroit
Member Since: Mar 2012
Location: New Braunfels Texas
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Go around with the engine at idle and use a propane torch (unlit) to 'sniff' out any potential vacuum leaks. The idle will change if it pulls in unburnt propane.
You need to fix the idle problem first. Are any of the pulloffs or linkages casing the blades not to shut? With the curb idle screw all the way out....are the blades shut?
Your explanation of how you did the timing confuses me as you are saying you are pulling more timing after the springs are installed. With a spring removed....you should see full advance WAY before 2750 rpm.
What is the story on the Q-jet? Was it run on the car before? Did it sit for a long period of time with pump gas in it?
Jebby
You need to fix the idle problem first. Are any of the pulloffs or linkages casing the blades not to shut? With the curb idle screw all the way out....are the blades shut?
Your explanation of how you did the timing confuses me as you are saying you are pulling more timing after the springs are installed. With a spring removed....you should see full advance WAY before 2750 rpm.
What is the story on the Q-jet? Was it run on the car before? Did it sit for a long period of time with pump gas in it?
Jebby
#12
Advanced
Thread Starter
Go around with the engine at idle and use a propane torch (unlit) to 'sniff' out any potential vacuum leaks. The idle will change if it pulls in unburnt propane.
You need to fix the idle problem first. Are any of the pulloffs or linkages casing the blades not to shut? With the curb idle screw all the way out....are the blades shut?
Your explanation of how you did the timing confuses me as you are saying you are pulling more timing after the springs are installed. With a spring removed....you should see full advance WAY before 2750 rpm.
What is the story on the Q-jet? Was it run on the car before? Did it sit for a long period of time with pump gas in it?
Jebby
You need to fix the idle problem first. Are any of the pulloffs or linkages casing the blades not to shut? With the curb idle screw all the way out....are the blades shut?
Your explanation of how you did the timing confuses me as you are saying you are pulling more timing after the springs are installed. With a spring removed....you should see full advance WAY before 2750 rpm.
What is the story on the Q-jet? Was it run on the car before? Did it sit for a long period of time with pump gas in it?
Jebby
I guess i should have mentioned as well, any start is rough, cold or hot. when hot takes a minute to get to correct idle speed, when cold its a rough start
i disconnected choke and accelerator cable to check for binding. curb idle does not allow blades to shut when screwed all the way out.
car idles well when without advanced timing, when i hook up advance canister my idle jumps dramatically and pulls in all timing it seems.
Q-jet has been on car for past four years, sat for last 4 months due to much needed rocker channel rear suspension rehab. car gets driven regularly before that. at least once a month or more for enjoyment purposes
it pulls more timing with vacuum advance hooked up. it raises my idle about 2000rpm
Last edited by gmmeyerIII; 12-28-2018 at 10:23 AM.
#13
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im not sure i even want to stick with the q-jet. its not original to car, which doesn't have all that much meaning anyway.
#14
Dr. Detroit
Member Since: Mar 2012
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Unless you plan on mods like cam and or heads......leave the Q-jet on it....you will not find a better fuel mixer.
Jebby
#15
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Even if it's gummed up or has some issues after 4 years of service, I'll "refresh" it, fix it, and check it out for you for the cost of the postage to assure it's running right.
If the throttles are not closing completely with the idle screw backed out and throttle cable disconnected, there is a problem.
Lars
Lars
Last edited by lars; 12-28-2018 at 11:05 AM.
#16
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Even if it's gummed up or has some issues after 4 years of service, I'll "refresh" it, fix it, and check it out for you for the cost of the postage to assure it's running right.
If the throttles are not closing completely with the idle screw backed out and throttle cable disconnected, there is a problem.
Lars
If the throttles are not closing completely with the idle screw backed out and throttle cable disconnected, there is a problem.
Lars
sending you a pm
#17
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#18
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It's in the PM here now, and I just checked my e-mail and I do not have anything from you.
Lars
Lars
#20
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My distributor machine is out of service, so I can no longer assist with distributor testing and setup. If I can get it repaired at a cost that makes sense I'll let everyone know. In the meantime, Gene has the carb on its way to me, and I'll give it a good test, checkout, and evaluation to see if it's the cause of the problems. With Gene's permission, I'll post up test results and progress in this thread along with some photos.
Lars
Lars
Last edited by lars; 01-03-2019 at 09:20 PM.