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Hey, guys. Crowd-sourcing a little diagnosis here before I start throwing parts and money around. I took my '69 4-4-2 convertible (400G, THM400, Q-jet) out for a blast yesterday, Fairly hot (80-85F) and I drove on the highway for about 20 minutes and then in a bit of traffic. Shut it down and went to the gym for a couple of hours. Took a bit of cranking to get it started again, but this is frequently the case when the weather starts to heat up. But then as I was exiting the parking lot and starting to accelerate onto the street, I got a bit of hesitation. At this point I'm still thinking possible fuel percolation -- no biggie.
It's a twenty minute highway trip home with lots of room to stretch out, so I decided to see if I could blow out a few cobwebs. At approximately a 60 MPH cruise, I stomped on it. The THM 400 dutifully dropped a gear and the car lunged forward. All good. But then around 4400-4600 RPM (70? MPH -- just before it would normally upshift at full throttle) it was as if I hit a wall of Jello. The engine made kind of a "WHUF!" sound and did one lurch before I got off it. I did this twice more and the same thing happened in the same way each time. Low speed operation seems unaffected as long as I stay out of the secondaries. Just a few days before this was not an issue, so I think I can say it happened suddenly. I haven't recently changed anything. Now I'm thinking fuel pump. I'm sure the current one isn't original but it appears to be factory-style.
What's the best way to troubleshoot this issue, and if I turn out to be right what would be the best replacement? It's been years since I bought a fuel pump..
Any opinions/experiences/suggestions/questions are welcome.
Obviously, this is only a suggestion. My first suspicion would be the Q-Jet accelerator pump. Both the linkage &/or the grommet on the pump are known to get sticky in various positions. Very inexpensive and easy fix.
EDIT: Sometimes that spring can bind/wear out, etc.
Last edited by Vintage Chief; May 20th, 2023 at 10:36 AM.
Obviously, this is only a suggestion. My first suspicion would be the Q-Jet accelerator pump. Both the linkage &/or the grommet on the pump are known to get sticky in various positions. Very inexpensive and easy fix.
The accelerator pump is only active while the throttle is being moved. Once the pedal is floored, the accelerator pump is doing nothing. If the engine accelerates up to a certain RPM then dies out without moving the accelerator pedal, the most likely cause is a drop in fuel level in the carb. This can be due to an improperly set float, a clogged fuel filter, a weak fuel pump, old rubber fuel lines, or crud in the tank being sucked up against the inlet sock.
I should also add that this can also be an ignition-related problem; a weak coil. a failing HEI module (if you have HEI), or if you have points, it could be a weak spring on the points. It can also be caused by older plug wires or carbon tracks in the cap that are arcing at higher RPMs.
Last edited by joe_padavano; May 20th, 2023 at 11:12 AM.
The accelerator pump is only active while the throttle is being moved. Once the pedal is floored, the accelerator pump is doing nothing. If the engine accelerates up to a certain RPM then dies out without moving the accelerator pedal, the most likely cause is a drop in fuel level in the carb. This can be due to an improperly set float, a clogged fuel filter, a weak fuel pump, old rubber fuel lines, or crud in the tank being sucked up against the inlet sock.
I should also add that this can also be an ignition-related problem; a weak coil. a failing HEI module (if you have HEI), or if you have points, it could be a weak spring on the points. It can also be caused by older plug wires or carbon tracks in the cap that are arcing at higher RPMs.
I think you left out - mice turds (although you covered it with "crud" maybe).
I think you left out - mice turds (although you covered it with "crud" maybe).
Fair point in that crud in the float bowl can clog the main jets and also restrict fuel flow. Most likely is the spooge that forms when that ethanol-laced cat urine they pass off as gasoline sits too long.
Well, those are definitely a few things to think about, Joe. Before I start randomly tearing into stuff, can you help with a little triage ... i.e. where's the most likely place to start? Thanks!
Well, those are definitely a few things to think about, Joe. Before I start randomly tearing into stuff, can you help with a little triage ... i.e. where's the most likely place to start? Thanks!
It kind of depends on when was the last time you changed fuel filter, fuel pump, HEI module, etc. If the filter is old, I'd start there, since it needs to be replaced regularly anyway. Same for the pump. The check valves in the pump wear out and don't seal as well as they should, which causes fuel pressure to drop, especially at higher RPMs. A fuel pressure gauge would be a useful diagnostic tool here. If pressure drops off about the time the hesitation happens, that's a smoking gun.
Fair point in that crud in the float bowl can clog the main jets and also restrict fuel flow. Most likely is the spooge that forms when that ethanol-laced cat urine they pass off as gasoline sits too long.
It's been on ethanol-free Chevron 94 for many years now. I have, however, frequently added an octane booster. Could that lead to crud or spooge?
It kind of depends on when was the last time you changed fuel filter, fuel pump, HEI module, etc. If the filter is old, I'd start there, since it needs to be replaced regularly anyway. Same for the pump. The check valves in the pump wear out and don't seal as well as they should, which causes fuel pressure to drop, especially at higher RPMs. A fuel pressure gauge would be a useful diagnostic tool here. If pressure drops off about the time the hesitation happens, that's a smoking gun.
I would start with the simple things first. As Joe mentioned, if you can’t recall the last time you replaced the fuel filter, it’s probably been too long. Don’t get the “anti-drain back” style filter with the check valve, those do NOTHING but restrict fuel flow.
If replacing the fuel filter doesn’t help, carefully inspect the fuel lines. Look for kinks, rusty lines (especially around the clamps) the hoses, vents, etc. You probably won’t see fuel leaking from rusty lines, but you might see the staining or discoloration on the frame. If there are pinholes in the lines, that means the fuel pump is sucking air instead of fuel.
Inspect the ignition system carefully. Plugs, points, plug wires, cap, etc. Replace anything that’s questionable.
Still noses over? Time to get more serious. Most vacuum gauges also read pressure. You might have to get a little creative with the plumbing, but it’s time for a fuel pump pressure test. Don’t put the gauge in the car, I’m betting it takes a lot of effort to get the smell of gas out of the upholstery!! Tape the gauge to the windshield where you can see it while driving. If the pressure falls off when the car noses over, then you know the problem is fuel delivery.
As for the hot restart issue, is your car air conditioned? Is the fuel return line capped off? Most a/c cars have fuel pumps with 3 lines, one suction, a supply line, and a return line. The return line is suppose to release pressure back to the tank, if the line is caped off or restricted fuel in the line to the carb can boil, raising the pressure enough to open the needle in the carb, which floods the engine.
It's not an A/C car, and I should clarify: there's not really a problem with hot starting, at least not to the extent that I'd complain about it.. When I said it took a bit of cranking, I meant like 5-6 seconds and it's often done that in the past in hot weather. I'm kind of hoping if I get this other problem sorted out, the starting issue might improve too.
Way back in the 80s my car would do similar if I got a bit of wheelspin off the line. Ran hard up to near redline then fell off. Didn’t do that with no wheelspin. I reset the carburetor float slightly to have more fuel on the bowl and that fixed it.
Since this is a sudden issue with yours, I doubt it’s the same thing but there may be something restricting fuel delivery. Check the things mentioned above and hopefully you’ll find and correct what’s wrong.
Okay, I'm gonna do a complete fuel system service using suggestions above because it's been many years and everything could probably use attention. And while I'm at it, I MAW replace the HEI ignition module which leads me to this ... I have an old Accel module that I've been carrying around as a spare for the last ~15 years. IIRC it was a premium one when I bought it and says "Made in USA" on the package. Possibly a stupid question, but should I use this, or do they deteriorate just from age? It seems like I have had electronics fail in the past apparently from disuse. Conversely, I have read lots of stories on CO about new Chinese modules being bad out of the box.
What do you guys think -- is this a dumb way to save fifty bucks?
At approximately a 60 MPH cruise, I stomped on it. The THM 400 dutifully dropped a gear and the car lunged forward. All good. But then around 4400-4600 RPM (70? MPH -- just before it would normally upshift at full throttle) it was as if I hit a wall of Jello. The engine made kind of a "WHUF!" sound and did one lurch before I got off it. I did this twice more and the same thing happened in the same way each time. Low speed operation seems unaffected as long as I stay out of the secondaries. Just a few days before this was not an issue, so I think I can say it happened suddenly.
Any opinions/experiences/suggestions/questions are welcome.
Your description would make me check the vacuum diaphragm that controls the opening of the secondary air valves. A very common issue with the Q JET's "back in the day". The diaphragm fails causing the secondary air valves to not open but at the same time the secondary throttle plates do open ("WHUF"). Very easy to check in your driveway by quickly snaping and fully opening the throttle (like you are flooring it) while observing the secondary air valves. My money is on the valves not opening. The secondary air valve diaphragm chamber is located at the right front side of the Q JET, is inexpensive and very easy to change.
Your description would make me check the vacuum diaphragm that controls the opening of the secondary air valves. A very common issue with the Q JET's "back in the day". The diaphragm fails causing the secondary air valves to not open but at the same time the secondary throttle plates do open ("WHUF"). Very easy to check in your driveway by quickly snaping and fully opening the throttle (like you are flooring it) while observing the secondary air valves. My money is on the valves not opening. The secondary air valve diaphragm chamber is located at the right front side of the Q JET, is inexpensive and very easy to change.
There is no vacuum diaphragm on the Quadrajet. The secondaries open mechanically, if the engine air flow is not enough the air valve (door on top of the carb) stays closed, if it is enough it opens and lift the rods in the jets to start fuel flow. It is possible that the spring that closes the air valve is not adjusted properly, but that would just introduce a hesitation. But I don't think that would cause the engine to go whump and have no power. Sounds more like a fuel delivery problem to me.
There is no vacuum diaphragm on the Quadrajet. The secondaries open mechanically, if the engine air flow is not enough the air valve (door on top of the carb) stays closed, if it is enough it opens and lift the rods in the jets to start fuel flow. It is possible that the spring that closes the air valve is not adjusted properly, but that would just introduce a hesitation. But I don't think that would cause the engine to go whump and have no power. Sounds more like a fuel delivery problem to me.
Just to clarify, there IS a vacuum diaphragm on the Qjet secondary air valves, but as correctly noted, it doesn't control opening, it actually holds them closed under high manifold vacuum conditions.
There is no vacuum diaphragm on the Quadrajet. The secondaries open mechanically, if the engine air flow is not enough the air valve (door on top of the carb) stays closed, if it is enough it opens and lift the rods in the jets to start fuel flow. It is possible that the spring that closes the air valve is not adjusted properly, but that would just introduce a hesitation. But I don't think that would cause the engine to go whump and have no power. Sounds more like a fuel delivery problem to me.
Please brush up on Q JET operation before commenting. I stand by my description of operation and repair suggestion.
Please brush up on Q JET operation before commenting. I stand by my description of operation and repair suggestion.
And your description would still be incorrect. The vacuum diaphram is only used to retard the opening of the secondary air valves. It does NOT cause them to open. You can disconnect and plug the hose to that diaphragm and the secondary air valves will open even more quickly. Your statement from above is completely incorrect.
Originally Posted by Dynoking
The diaphragm fails causing the secondary air valves to not open
Of course, rather than take my word for it, let's "brush up on Qjet operation" by going to the Rochester Qjet manual. Note in particular the second sentence in the first paragraph.
And your description would still be incorrect. The vacuum diaphram is only used to retard the opening of the secondary air valves. It does NOT cause them to open. You can disconnect and plug the hose to that diaphragm and the secondary air valves will open even more quickly. Your statement from above is completely incorrect.
Of course, rather than take my word for it, let's "brush up on Qjet operation" by going to the Rochester Qjet manual. Note in particular the second sentence in the first paragraph.
And your description would still be incorrect. The vacuum diaphram is only used to retard the opening of the secondary air valves. It does NOT cause them to open. You can disconnect and plug the hose to that diaphragm and the secondary air valves will open even more quickly. Your statement from above is completely incorrect.
Of course, rather than take my word for it, let's "brush up on Qjet operation" by going to the Rochester Qjet manual. Note in particular the second sentence in the first paragraph.
Joe P does it again !!! He destroyed another "story" with facts.
And your description would still be incorrect. The vacuum diaphram is only used to retard the opening of the secondary air valves. It does NOT cause them to open. You can disconnect and plug the hose to that diaphragm and the secondary air valves will open even more quickly. Your statement from above is completely incorrect.
Of course, rather than take my word for it, let's "brush up on Qjet operation" by going to the Rochester Qjet manual. Note in particular the second sentence in the first paragraph.
in his first message he specifically says “it controls the opening”
here’s your link pick, it specifically says “it controls the opening “
what it does is controls the opening rate…that’s why the dashpots have a tiny oriface in the opening to gradually bleed off the vacuum and allow the air door to open at a predetermined rate.
his second part is wrong…not the controls the opening part because you are saying that same thing
Kinda difficult to have an adult conversation with someone who ignores things they don't like.
Let's try this one more time. Please help my obviously incompetent brain understand this statement from above:
Originally Posted by Dynoking
The diaphragm fails causing the secondary air valves to not open
In fact, I'll sweeten the pot. I'll contribute $100 to the charity of your choice if you can provide a credible failure of the diaphragm that can cause the secondary air valves not to open.
Kinda difficult to have an adult conversation with someone who ignores things they don't like.
Let's try this one more time. Please help my obviously incompetent brain understand this statement from above:
In fact, I'll sweeten the pot. I'll contribute $100 to the charity of your choice if you can provide a credible failure of the diaphragm that can cause the secondary air valves not to open.
Did you conveniently leave out the part where I said his second statement was WRONG? Ya, you did.
again, back to what I initially said…..”it controls the opening” that’s his first statement..it’s also yours.
does it control the opening? i know what happens if the diaphragm doesn’t work. I said he’s wrong on that.
Wanna try again?
Last edited by CANADIANOLDS; May 23rd, 2023 at 05:12 PM.
the dashpot DOES control the secondary air valve opening…he said it doesn’t
if the dashpot is defective, like the diaphragm has a hole in it…it won’t allow a any controlled opening of the air valve ,, the air valve door would only be controlled by the air door spring, which could cause a hesitation/lean condition when secondaries open open to fast
Last edited by CANADIANOLDS; May 23rd, 2023 at 06:15 PM.
Just to clarify, there IS a vacuum diaphragm on the Qjet secondary air valves, but as correctly noted, it doesn't control opening, it actually holds them closed under high manifold vacuum conditions.
this is false.
the diaphragm DOES control the opening rate..that’s it’s sole purpose. Holding them closed is NOT its main function. The air door is held closed by the spring tension on the air door shaft. The car with run exactly the same with or without a defective diaphragm, until the secondaries are opened.
the diaphragm DOES control the opening rate..that’s it’s sole purpose. Holding them closed is NOT its main function. The air door is held closed by the spring tension on the air door shaft. The car with run exactly the same with or without a defective diaphragm, until the secondaries are opened.
wanna rethink this?
My statement would have been clearer if I had said that the vacuum diaphragm doesn't ACTUATE opening. Yes, I could have worded it better. More to the point, I was specifically replying to a prior post (NOT yours, by the way, despite your need to make everything about you) that stated that a failure of the diaphragm could PREVENT opening, which it cannot.
May 20th, 2023, 10:26 AM
