Adjusting Q-jet carbs (specifcally the Edlebrock Q-jets)

[Leadfoot]

Dilemna:

I have been procrastinating working on the pulling truck for 8 months (I know my fault), but it did not run right last season and pissed me off, so I just let it sit.....
I had a Holley carb on it, and worked well for pulling, but I wanted an all purpose carb for inclines (hill climbs if I decide to do them) and daily driving. I know I probably won't win 1st place at the pulls, as I am under motored (compaired to the rest), so until I build the BBC, I wanted to have a DD that I could pull (and used to do well). When I got the 78 I really didn't need the 80 as a DD so I let it sit. Now I want to use it as a DD for the summer (better mileage with the OD), and pull with it for fun.

Problem:
It is running out of fuel at 3500-4000 RPM's. The carb is now a Edlebrock Q-jet. I replaced the LT1 mechanical fuel pump with a performance electric one (thinking the problem was the pump was starving the carb for fuel). That did not help, and I checked all the lines for obstructions (didn't find any). The performer manifold I have (dual pattern) allowed me to unbolt the Q-jet and bolt on a buddy's 750 double pumper set up for a built Ford (don't flame) 460 that tach's at 7200+ while pulling. Right now that carb works, but is WAY too rich. We deduced the problem was with the "new" (although it been on the motor for 14 months) Q-jet. It always had problems, but they have gotten worse. Does anyone (already tried a search), have any documentation (online or electronic) on how to tear apart, inspect, rebuild/reassemble a Q-jet and/or know what would be causing the starvation problems.

The carb is supposed to be 795 CFM and setup for a 350. I have a SBC 400 that is worked over but not a monster (flat top hyperutechnic pistons, 202 angle plug heads, Crane Fireball II cam, running ~9.5:1 compression). I could see if I turned it 7000+ and it started to run out of fuel, but not under 4000. It may be something simple but I do not know my Q-jets. I can do holley's, no problem, even the Carter/Edlbrock style, but Q-jets are foreign to me.....

Any help is appreciated.

 

[Leadfoot]

[ QUOTE ]
Something wrong w/ the secondaries? Maybe try a different size secondary metering rods.

[/ QUOTE ]

I will try that, but I have never dealt with metering rods on a Q-jet. What is the process (steps) to change them?

Thanks,
Chris

 

[dyeager535]

I had similar problems on my 403...I'd get up around 3500RPM at WOT (and with 2.56 gears, that took awhile) and the car would start to buck violently, just like running a tank empty. Anyways, let off the gas, problem gone.

I changed the secondary rods to a bit "leaner", and boom, problem gone. Personally I think the needle/seat is a major restriction in these carbs, and I've noticed there are at least two styles. You may want to ask Edelbrock what style they have, and if they can recommend a "better flowing" seat. (and needle if applicable)

Changing the secondary rods or hangar is extremely simple. Remove air cleaner. Look at the secondaries from above the carb. In between both "blades" of the secondaries will be a small flat or torx head screw. Remove it, and pull the piece it is retaining straight up. Thats the hangar, and the rods are attached. The hangars are also interchangeable, factory have an ID letter stamped on them.

 

[wrathORC]

Well, do you know what needle and seat it's got in it? Some of the small orifice seats for offroading don't flow enough under WOT to get enough fuel in the float bowl. This is more noticeable under WOT conditions at the bottom of the hill. The 25cc accelerator pump sucks out that much gas and then the metering rods are dumping gas down the secondaries and it isn't before long it feels like its running out of gas.

Next, remove the secondary metering rods and hanger. You can get it out by removing the little screw on the top towards the back. It's probably torx (I think T10 but don't quote me on it--channel locks work just as well) or flathead. On the top of the hanger on the end opposite the metering rods there will be a letter "B" through "V" stamped on it. You may have to use a scotch-brite pad to get the crud off to read it. On the side of the metering rods there will be a two letter code stamped on them.

Tell us those two codes.


Everything else involves removing the air horn (six screws I believe). You can set the fuel level or change the needle and seat by doing this. You can also find out what size primary jets you've got.

 

[sandy78]

There is a great Q-jet book by Doug Roe -- I got it at Barnes and Noble. It lists P/N's for the high-flow needle and seat. It also has great lists of the needle codes and their relative sizes so you know if you are going up or down. Q-jets are pretty simple.

 

[Leadfoot]

Thanks for all the replys, I am planning on buying a book or two (thanks for the Title), but I also need to see if I can get this thing "pull" worthy by this weekend. I will check those items and see what happens. Thanks again.

 

[pilgrim]

ther needs to be a "banging your head against the wall icon" because thats what i'am doing /forums/images/graemlins/grin.gif..

i'am having very similar problems with my "new" e-broke Q-jet.. first off get that book its the best /forums/images/graemlins/deal.gif /forums/images/graemlins/thumb.gif
i get hesitaion/bucking the more power i need. ie, the more pedal down the lower the vac signal the worse it is.. is this simlar to you problem???

after reajusting my vac advance, i felt a differance /forums/images/graemlins/cool.gif
maybe your having a timing problem /forums/images/graemlins/confused.gif.. if the motor isn't advanceing right than the mixture will not completely burn to make full power... just a thought....

 

[Leadfoot]

The book is on the way /forums/images/graemlins/thumb.gif

Timing is set per specs of the engine builder, although I may monkey with it a little (I can always set it back).

I'm beginning to think I have a simple problem, but it may be a while before I can check........

[possible idiot mode on]
My truck was originally a single tank with charcoal canister. When I tore it down and de-rivetted the frame, I decided to make it a dual tank truck without charcoal/emissions items (Dorian you can slap me here for changing things WITHOUT knowing what I was doing to the ENTIRE system). The original fuel pump had a feed, an out to carb, and return line, the corvette mechanical fuel pump that was spec'd by the engine builder only had a feed and out to carb line (no return line to tank). Now here is where I *might* have made a mistake. Both sending units on the tank had 3 lines. One was large and for feeding the pump (I used that to feed the selector valve which then goes to the fuel pump), one for the stock return line, and one for the charcoal canister (vent). I capped both the vent line and the return line. About a year ago, I put on locking gas-caps to keep people from F-ing with the rig. My thinking is the new caps are not vented (causing a vacuum lock, especially at high RPM's), there may be some "venting" going on, but at sustained high R's it can't breath enough, until I let off the throttle. It also explain why the other carb seemed to work better, as the floats are set higher, and have much bigger fuel bowls (more reserve) and I never really got onto a highway to keep that carb at sustained high R's, I just did it enough where the old carb would quit. When I get a chance, I am going to re-install the Q-jet and test without the caps on. It may not be it, but would explain alot. Another reason why NOT to F with your rig if you don't know EXACTLY what you are doing. I guess I am like Tim Allen........doing everything wrong, so you can see what not to do /forums/images/graemlins/blush.gif [possible idiot mode off]
I will see how it goes and keep you posted. I actually hoping it's that stupid, which would make it easy, although frustrating that I had to go through all of this.

P.S. Anybody know where I can get mid 80's style locking vented gas caps? LMC has different styles of locking gas caps, but none specify vented or non-vented. The only differences are Cali emissions, and non-Cali emissions which may be the difference between vented and non-vented, but which one is which.

Thanks.

 

[wrathORC]

Haha, this is a riot. Don't feel bad, I did something twice as bad as you.

One of the first things I did when I got my truck is get rid of the mechanical fuel pump for a Carter street/strip pump (100gph). I capped the return line.

My truck came with a single tank. I put a second one on it with a positive sealing cap (ie: 70s pickup truck tank) that you get to through the wheel well. I put in the solenoid and put "T"s in everything like you're supposed to. I also put a fuel pump between the tanks so I could fill the auxilary tank from the main tank. At the same time I changed to a 70s vapor removal system (two lines going to canister instead of four). I made the mistake of hooking it to manifold vacuum.

So what I ended up doing is trying to suck full vacuum through my gas cap all the time. End result: collapsed gas tanks. When I opened the gas cap it sounded like a crinkled pop can. It'd take about 30 minutes at expressway speeds with full tanks before I'd notice I was down on power but it was perfectly driveable. That Carter pump would suck like a... /forums/images/graemlins/grin.gif

I found out I had the wrong gas cap on the truck and got one for an 89 fullsize pickup. They let air in. I know Stant makes them. They made a lockable one too but I had to order them and I don't like to piss around with more keys. My truck had one that let air out (which I believe was wrong and I think my truck is supposed to have one that lets air in but not out). I also hooked my vapor evacuation system to ported vacuum. Under certain conditions I still get a vacuum in the tank but nothing like what I used to get. I assume I still have a vacuum source problem.