69 BB won't rev past 4,200 in 3rd gear
03-18-2016, 05:30 PM
#21
Instructor
Thread Starter
totally agree that it's not manifold vacuum vs. ported vacuum or anything else to do with the vacuum advance. Will look into a possible carb issue, fuel pump issue or bad filter and any other ideas people have - thanks
03-18-2016, 05:56 PM
#22
Burning Brakes
I'm having similar issue with my car but its in a sb. had carb rebuilt by lars and sent back to double check everything and still the same.... if you find solution please post
03-18-2016, 06:00 PM
#23
Intermediate
It would matter if he was running ported vacuum to the vacuum advance and was pulling in too much advance under a heavy load. I agree that the most likely cause is the secondaries not opening but I was trying to help him cover all bases. Don't be a douche.
03-18-2016, 06:05 PM
#24
Instructor
Thread Starter
Forum members have been super helpful on this topic...sometimes typed stuff sounds disparaging but I don't think it was intended that way...
03-19-2016, 01:13 AM
#25
Team Owner
Once the primary throttle plates open just a bit, 'ported' vacuum level is the same as 'manifold' vacuum level. The only difference between them is at or just above idle speed. The purpose of ported vacuum is to eliminate vacuum advance at idle, thus retarding the spark and making the cylinder temps increase to burn off excess hydrocarbons. Once throttle is opened, there is no difference between ported and manifold vacuum.
If the mechanical advance is not increasing with rpm, the engine would run poorly at elevated rpms...but it would still rev above 4000 level. You are just running out of fuel at that speed; hence, the high probability that the secondary throttle plates are not opening at all.
If the mechanical advance is not increasing with rpm, the engine would run poorly at elevated rpms...but it would still rev above 4000 level. You are just running out of fuel at that speed; hence, the high probability that the secondary throttle plates are not opening at all.
Last edited by 7T1vette; 03-19-2016 at 01:18 AM.
03-19-2016, 09:50 AM
#26
Intermediate
My original question, "are you running manifold vacuum to the vacuum advance?" was just to insure that ignition timing was not being advanced under a load. I appreciate your correction on my use of incorrect terms.
03-19-2016, 10:57 AM
#27
Team Owner
No problem. To my knowledge the Q-Jet does not have a 'venturi' vacuum outlet; in fact I don't know what venturi vacuum is, since the vacuum level in the venturi would be different everywhere along the length of the venturi and would be compounded by the amount the throttle plates were open.
The 'manifold' vacuum orifices are located just below the throttle plates (when those plates are fully closed). This allows them to create a vacuum level that is proportional to the throttle movement. 'Ported' vacuum orifices are just above the throttle plates, so that ZERO vacuum level is created when throttle plates are closed; but once the throttle is opened, ported vacuum level approaches manifold vacuum level very quickly.
If there is such a thing as 'venturi' vacuum, I suspect that it is only used internal to a carb to regulate off-idle fuel mixture.
The 'manifold' vacuum orifices are located just below the throttle plates (when those plates are fully closed). This allows them to create a vacuum level that is proportional to the throttle movement. 'Ported' vacuum orifices are just above the throttle plates, so that ZERO vacuum level is created when throttle plates are closed; but once the throttle is opened, ported vacuum level approaches manifold vacuum level very quickly.
If there is such a thing as 'venturi' vacuum, I suspect that it is only used internal to a carb to regulate off-idle fuel mixture.
03-19-2016, 11:50 AM
#28
Intermediate
No problem. To my knowledge the Q-Jet does not have a 'venturi' vacuum outlet; in fact I don't know what venturi vacuum is, since the vacuum level in the venturi would be different everywhere along the length of the venturi and would be compounded by the amount the throttle plates were open.
The 'manifold' vacuum orifices are located just below the throttle plates (when those plates are fully closed). This allows them to create a vacuum level that is proportional to the throttle movement. 'Ported' vacuum orifices are just above the throttle plates, so that ZERO vacuum level is created when throttle plates are closed; but once the throttle is opened, ported vacuum level approaches manifold vacuum level very quickly.
If there is such a thing as 'venturi' vacuum, I suspect that it is only used internal to a carb to regulate off-idle fuel mixture.
The 'manifold' vacuum orifices are located just below the throttle plates (when those plates are fully closed). This allows them to create a vacuum level that is proportional to the throttle movement. 'Ported' vacuum orifices are just above the throttle plates, so that ZERO vacuum level is created when throttle plates are closed; but once the throttle is opened, ported vacuum level approaches manifold vacuum level very quickly.
If there is such a thing as 'venturi' vacuum, I suspect that it is only used internal to a carb to regulate off-idle fuel mixture.
03-19-2016, 05:12 PM
#30
Le Mans Master
Member Since: Jul 2000
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Have you considered the fuel pickup inside the gas tank? The original pickup has a "sock" that can become fouled over time and restrict the flow of fuel to the carburetor.
Jim
Jim
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Peterbuilt (03-20-2016)
03-19-2016, 05:29 PM
#31
Burning Brakes
4,200 RPM in 3rd requires a lot more fuel and horsepower than in the lower gears. You're essentially saying the car won't go more than 120 MPH. This could be insufficient fuel flow, a too small carburetor (or not fully opening primaries and/or secondaries), a restrictive exhaust system, or anything else that limits or degrades horsepower. What happens at 4,200 rpm in 3rd, i.e., what does the motor sound like when it flattens out at that speed?
03-19-2016, 05:52 PM
#32
Team Owner
It is very simple to check out the function of the secondary throttle plates.
Warm the engine to normal temps. Shut down engine.
Open hood and remove air cleaner.
While manually holding the upper air valve open (so you can see down into the carb with a flashlight), look down at the throttle valves while you exercise the throttle to full wide open position. If the secondary throttle plates open to vertical position, then the carb is working as intended.
If the throttle plates do not move or move very little, there is a problem with the carb/linkage/choke/lockout somewhere.
Warm the engine to normal temps. Shut down engine.
Open hood and remove air cleaner.
While manually holding the upper air valve open (so you can see down into the carb with a flashlight), look down at the throttle valves while you exercise the throttle to full wide open position. If the secondary throttle plates open to vertical position, then the carb is working as intended.
If the throttle plates do not move or move very little, there is a problem with the carb/linkage/choke/lockout somewhere.
Last edited by 7T1vette; 03-19-2016 at 05:52 PM.
03-20-2016, 09:35 AM
#33
Drifting
If it runs pretty strong (your words)in the lower gears I would assume the secondaries are opening.Pretty easy to tell if they are opening on a spread bore carb by seat of the pants.Do make sure they are opening all the way.
I would start with removing the fuel filter at the inlet and see if that makes any difference.It sounds to me like the float bowl are being sucked dry.I had that happen on a Qjet on a 383 small block years ago.It would run out of gas at the top of second gear.
You can also rig a temporary fuel pressure gauge up where you can watch it while driving to see what your actual pressure is.
I would concentrate on fuel flow first.
I would start with removing the fuel filter at the inlet and see if that makes any difference.It sounds to me like the float bowl are being sucked dry.I had that happen on a Qjet on a 383 small block years ago.It would run out of gas at the top of second gear.
You can also rig a temporary fuel pressure gauge up where you can watch it while driving to see what your actual pressure is.
I would concentrate on fuel flow first.
03-21-2016, 10:02 AM
#35
Safety Car
Lower gears you will get more RPM even on just the primaries . CLearly your not opening up the rear butterflies as many have indicated . Check to ensure they are operating properly and let us know what ya find
03-24-2016, 07:46 PM
#36
Instructor
if the butterflies are opening, then check the timing, we have a 452 that run good till about 4 grand then it started to fall on its face. bumped the timing one way then the other till it ran like a striped you know what.
Last edited by colesweat; 03-24-2016 at 07:47 PM.
03-26-2016, 09:21 PM
#37
Racer
My 71 big block did the same thing. Made me crazy, I kept rebuilding the carb, retiming, even finally sent the carb off and had it professionally rebuilt. Still would fall flat on it face as soon as the secondaries opened. Turned out to be a bad fuel pump, wasn't getting enough fuel.
03-27-2016, 07:54 PM
#38
Le Mans Master
I had a similar problem on my 396. After going through the carb, then changing carbs, it turned out that my new repro fuel pump wasn't keeping up. Ended up going to Holley 110gph mechanical pump, and bending new stainless fuel lines to utilitize the stock fuel filter and carb line.. Looks stock from the top and pulls clean to 6500 in 1st/2nd/3rd....and I've had it to about 5500 in 4th. This is on a 440hp engine.
03-27-2016, 09:05 PM
#39
Safety Car
