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quadrajet: the fuel pump, fuel bowl, and accelerator pump

That could be another can of worms - I read up on the electric pump + fuel return, it is not the end all solution. Fuel pumps are noisy, they fail, if you crash they keep pumping fuel everywhere. The return line: I think a pressure sensor can be added to the fuel pump so it turns on and off, then a return line is not necessary - (EDIT: except to reduce fuel temperature). I know, all modern cars run electric pumps without a problem.

Lets be honest, some fuel injection would fix it. It is not entirely about practicality, the hunt is part of it.

I am starting to share the view that a mechanical pump and quadrajet cannot accommodate E10 fuel. From the technical report I posted at the beginning of this thread, it is clear the fuel bowl on the q-jet was barely adequate with '60s fuel. As they presented it: 1) The fuel bowl volume is barely adequate to allow enough fuel for startup after several days of evaporation. 2) It is basically a direct to meter carburetor. They added a larger orifice to the fuel shutoff and increased the fuel pump volume - both to feed fuel to the q-jet despite the lack of a sizable fuel bowl.

I have one last test before closing the book: So far I have the carburetor bolted directly to the intake manifold. My holley had a 1/2" spacer under it for insulation and had no problem with fuel evaporation (probably has more to do with where the fuel bowls are located and the volume of fuel). I need to put a spacer under the quadrajet and see if that takes care of it. Cliff advised that the spacer will cause more problems than it solves. I bought a wood 4-hole spacer awhile back. I plan to shape the bores with a taper. I am also playing with the idea of drilling cooling holes around the perimeter of the spacer to increase the cooling surface area. The spacer should lower the temperature of the carburetor body, and reduce heat soak after shutdown, allowing the fuel to remain in the bowl for more than a day or two. I need to calculate the evaporation and see if that is true before trying it. So, that is what's left on the table. The first step toward reduced fuel temp is shielding the fuel line from the headers. I need to measure the new fuel temperature and then go from there.

Failing that - nobody should be running mechanical pumps with q-jets. It is just something that is true.
 
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Well all I can say is I know for a fact that as soon as you turn the key it gets fuel and if you don't dead head the pump and have a bypass to allow fuel pressure to regulate the pump lasts much longer. E10 wreaks havoc on carbs it's that simple but I do not believe its a widespread issue or else's here (Palm Springs) my buddies hot rod shop would be crawling with this issue. I've been working on these engines for a pretty long time and haven't seen an issue like yours. So you either have another issue of your qjet is junk. Filing the body and having that help some confirms that. Do yourself a favor and check out the jet stage 1 they are new cores and it might be worth trying.
 
sreidmx, thank you for reading my posts

Do you have an idea what might be making the qjet junk. I am right here with you on this one. I want to verify it is junk before changing carbs.
 
I don't think drilling holes in the spacer is necessary. Can't decide if I think it would HURT any, but I don't see it really helping much.

The wood is an insulator, it's not going to radiate heat for squat. A finned aluminum unit might do something, but seeing as GM used an auxillary fan on the later setups with aluminum intakes and Q-jets, I'm not sure any insulator will make a huge difference since the engine bay is relatively sealed, and would still be heat soaking for quite some time as the engine cools.

On the EFI Buick GN's GM ran the radiator fan after shutdown, would be interesting (but a lot of work just to test) to find out what, if any, difference running a radiator fan for a few minutes after shutdown would have on the underhood temps on various components. Based on how quickly the heater core cools off with fan on/engine off, it would seem the radiator fan would quickly drop the radiator temp, which would also provide for much cooler air being blown over the engine.
 
What was Cliff's reasoning for the spacer hurting more than it helps?

I always figured that other than potentially causing cold weather issues (icing) a spacer could never hurt, at least in terms of carb heat/fuel boiling. Obviously you could run into geometry issues with things like a TV or throttle cable, choke operation if divorced choke, etc.
 
I think he meant cold weather. I ran my holley with a spacer in the sierras, usually between 10 deg F and 30 deg F, no problems. The colder the better. Open air cleaner. So, on this one, I planned to learn for myself what Cliff meant. I have a hot crossover so that might help.
 
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On this site, for the couple of years (lol) I've been here I've heard one person say they had icing issues. Must be very, very specific conditions for it to happen or there would be a lot more occurrences IMO.
 
No problem just trying to help, I had always been told that a qjet is junk once the body has been overtightened the avg joe doesn't know it requires very little to cinch these down and the fact the bolt goes threw the body and twists down on it makes things worse. If it was me I'd try a new not reman carb and see if the symptoms stay
 
sreidmx said:
No problem just trying to help, I had always been told that a qjet is junk once the body has been overtightened the avg joe doesn't know it requires very little to cinch these down and the fact the bolt goes threw the body and twists down on it makes things worse. If it was me I'd try a new not reman carb and see if the symptoms stay
Click to expand...

When I filed it I checked the base plate and body. The top plate was warped and i got that flattened down. The base plate looked ok. I dont see how the top is bent and the bottom is not but that is what it was.

If the surfaces are flat then the rest should be ok. I did not get it glass plate flat so it is possible it is better but still having sealing problems. Let me think about this and see if I can think of a test to check.
 
I think that is a good question here, is the body distorted. The top plate certainly was. The bottom plate is not. The body seems ok. I think I touched it up with a file on the top of the body, but I dont remember now. If i did it didnt need much. The top plate is a different story. I will check my images, I took some pictures.

Edit: I found the pic of the body warp - I had it posted back on page 5 http://coloradok5.com/forums/attachment.php?attachmentid=165898&d=1416770577

I did not file the body, just the top plate. If I cannot get things to work with the choke then I will return to the question about getting the body flat. Sounds like I'd have to bundle it up and send it in - no truck for a bit or run my Holley to cover the 'vacationed' q-jet. My feeling is I got the warp out of the top plate and the top plate is pulled 'into' the body warp by the top plate bolts. Any difference remaining between the body warp and the topplate flex is compensated by the gasket.


  • My top gasket looks dry - I will take a closer look at it and post some pics
  • next time I have the carb apart I will use a feeler gauge to measure exactly how much the body is warped

Some related posts on warp:

 
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Checked the well plugs, there is a video (http://youtu.be/tomrmd8S4wI) posted - page 5 - of me blasting them with compressed air and soap. No bubbles. At that time dyeager535 suggested (he's been walking along with me in this thread for awhile now) that I put fuel in the bowl and see if it leaks - he was not sold on the compressed air test. If I get the chance I will do the gas in the fuel bowl leak test, I read somewhere to put a sheet of clean paper under the carb to highlight any drips.
 
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Cold start resolved
dyeager535, I think you have talked me through the choke tuning. For the first time since I changed my holley for the q-jet, this morning it fired right up, fast idled, but not so fast it was annoying and undrivable, after a short warmup I drove it easy and it then came off fast idle within a few blocks. (Edit: started perfectly again today) I drove yesterday so there was fuel in the carb, probably helped with the startup.

I think cold start is now good.

Warm start problem
Next, I need to figure out the warm start. Yesterday warm start needed a little gas to get going and the idle needed holding the gas lightly until it got going. The fast idle did not engage so I assume the choke had not cooled down equally with the engine and was open. I searched and I cannot find anything about when the fast idle should reengage after cool down. After 2 hours my fast idle does not engage - on startup the idle is too low and I have to tap the gas, the idle comes back quick a few seconds at 450rpm --> picks up to 500rpm --> within a minute back to 600rpm, and then after a few minutes steady at 650rpm. It is those first few seconds at 450rpm it wants to stall and I'd like the fast idle.

Should I switch to a new thread for tuning q-jet warm startup or keep it going here?


Next Step - fuel evaporation, cold cold start

I need to go back to the original theme of this thread, "where is my gas?" After a few days, there is no gas for startup. The current advice is an electric fuel pump. I want to test a carb insulator first and see if that is enough.

On page 3 of this thread I posted some papers where they quantified E10 evaporation, basically as a component of emissions. I tried to build an excel spreadsheet to calc evaporation as a function of fuel temperature, but, after doing my best, I cannot understand fuel evaporation, and apparently nobody can explain it (usually means they don't understand it either and helps explain why all the experiments are based on setting some fuel out an measuring how much evaporated then building a table).

At 120 deg F a quarter of the fuel evaporates within 2 hours and then over the next 24 hours another quarter evaporates, for a total of half the fuel. This is consistent with what I have measured with my q-jet. After two days there is not enough fuel to start.

In one of the papers linked on page 3 they found that at 70 deg F 5% of the fuel evaporated after 2 hours. I'd say 70 deg is my target. Follow me on this:

  • The carb is 70 deg during operation and only heats up over 100 deg after shutdown. With the insulator under the carb I think it is possible to keep the heat soak out.
  • I insulated the fuel line so it does not pickup as much heat (still need to measure this) before the carb, the fuel was going from 100 deg to 150 deg passing the headers before cooling back down to 100 deg just before the carb. If the fuel is holding at 100 deg past the headers, or even cooling, then 80 deg fuel at the carb will help keep the internal temps down.
  • Combining a cooler carb operating temp due to reduced heat transfer from the intake manifold and reduced heat from incoming fuel should give an operating temperature closer to 50 deg F (was 70 deg F). This will give a lower starting point for the heat soak and the heat soak (currently 40 deg) will be reduced by the carb insulator. Reality check, I think reducing heat soak by 50% to 20 deg with a insulator is possible. So a 70 deg internal temp after heat soak is possible.
  • That means 5% evaporation in the first 2 hours and presumably another 5% in the next day. So I start with 90% of my fuel bowl capacity instead of the current 50%.
  • That should give me four days of evaporation instead of the current two. Anything longer than four days and the carb will basically be empty regardless of what I do; a quarter cup of gas set in a warm garage should be evaporated after a week (I should test this).
  • To have a clean startup after a week of non-operation, a fuel pump is the only solution. Or a time machine to let me do my fuel-ups in the '60s. I am Ok with this, I drive once a week so it wont be a problem.

Next week is taxes and I go to the in laws for help watching the kids. I will sneak out and put the insulator on during a break from the 1040 nightmare. We will know soon enough if that resolves the evaporation.
 
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forest_peterson said:
I think cold start is now good.
Click to expand...

Until the weather changes, lol.

Are there any resources online about tuning/setting up the Q-jet? For some reason I want to think there was a Pontiac page that was real in-depth on this. Setting up the choke properly should be pretty easy to find online. With the electric choke, I don't know how it "resets" itself. You still deal with "preload" on the choke coil, right? If you carefully mark the housing where it is now, and manually adjust the choke coil when it's warm, does the problem go away?
 
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