Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Tuning / Diagnosing and Edelbrock 1904 Q-Jet

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thefirebirdman:
Hey all, I've been on this forum before and Cliff has rebuilt a Quadrajet for my '73 Trans Am and the results were outstanding. I purchased a '68 Firebird convertible for my wife and it doesn't run that great. Idles really good and normal moderate driving is ok but it really has no power. After about half the pedal, increasing throttle really doesn't add any more power. I don't think the secondaries are working at all. I have warmed it up in the garage to ensure the choke disengages and have blipped the throttle and the secondaries do open. But out on the street, even floored, I never feel or hear them open.

Engine is the numbers matching 350. The previous owner put a new cam in it (RV cam). I can provide the specs a little later. He also installed Ram Air II exhaust manifolds. Otherwise I believe its stock. The carburetor on it is an Edelbrock 1904. I am running a new 200R4 from Monster Transmissions from Florida. I bought a TV cable and associated hardware for the 1904 from Bowtie Overdrives. I also installed a Lokar throttle cable to eliminate any slop in the throttle cable (also necessary to accommodate the TV cable setup).

I would like to ask the forum for help through your collective knowledge. I'd like to diagnose/tune the carburetor to see if the carburetor is the primary problem for the lack of power. I know almost everyone here will say to rebuild the carburetor as it likely needs it and its an unknown. If the recommendation is to not waste time trying to evaluate / troubleshoot the car and just go straight to a rebuild I can do that but I would like to do what I can to see if the carb is really the problem. I have never rebuilt a carb before but I am ready to try my first one. I think that will present its own challenges as I think it will take me some time to complete and I don't want my wife's car down for very long.

Given that, I don't really know where to start. I have read Cliff's book but I wanted to consult everyone in the forum on where to start so I am approaching this in an organized way rather than just conducting checks. I'd like to learn from the experience. To date the only thing I have done is to set the fast and slow idle. I did this using a tach connected directly to the engine. I've noticed that the fast idle speed always seems to be different than what I previously set it to but it has been close. Also, the engine idles at the slow idle speed I set but the car tries to stumble and die once put in gear, even if fully warmed up, until its been driven a bit.

So if there is specific information I need to provide on the carb or the cam etc... before getting started please let me know and I will post it up here. Otherwise I'd like to step through a good troubleshooting procedure with help from the forum to determine what's wrong with the carburetor and if needs to be rebuilt or replaced. Thanks!!!

omaha:
I would start with fuel pressure (verify that you have enough and/or not too much). you may need to rig some type of TEE fitting even if it would be temporary but you could make it permanent. Them I would use a vacuum gauge to fine tune the idle. check to see if your vacuum pulloff is working (with a vacuum source or handheld vacuum pump). You can also check to see if the vacuum advance is working with the vacuum pump. You'll need a timing light but you can get away with a cheap one (like from a thrift store or pawn shop). This will also help to determine if your timing advance is actually working. After this is done then you can go into the inside of the carb and check things like float level. You can do this with the carb on the car but you have to be very careful so it is beneficial to remove the carb. Make sure the choke is actually opening all the way as some of the carbs have a secondary lockout that prevents the secondaries from opening when the choke is applied. I had a 350 Pontiac and it was from a 1970 Firebird but I had put it in my 1968 chevelle 4 dr. Seems it liked the initial timing at about 14 degrees, but it was probably cause the engine was really tired. I ended up selling the car (still kicking myself). You can adjust the opening of the secondaries of course.

thefirebirdman:
Omaha, thanks for replying. Here's what you suggested with some notes from me. If anyone else would like to help fine tune this list please feel free to speak up. I'd definitely like to do as many checks as I can before taking the carb apart but I don't have any concern about pulling it off the car, its already been off several times and/or taking the air horn off to check a few things:

1) Check fuel pressure
 - I don't currently have a fuel pressure gauge but I will get one. Something I have probably needed to add to the toolbox for a long time. Currently stock fuel pump. I haven't experienced any fuel starvation issues but would be good to document.

2) Fine tune the idle
 - This Edelbrock carburetor has different idle set screws. They are recessed into the throttle body and don't just have a simple slot like most. I need to get the right tool from the parts store to work with these. I'll pick one up today and adjust them with a vacuum gauge and post results.

3) Verify vacuum pulloff is working
 - I have a vacuum gauge. Will check this per the instructions in Cliff's book.

4) Verify vacuum advance is working
- I can check this as well with a vacuum gauge but I've never done this before. Is this covered in Cliff's book. I don't recall. Will have to look. I am planning to replace the distributor and coil anyway. A friend with a good timing light checked the timing and advance with me and although the timing was set properly at idle the distributor was providing way too much advance at high rpms. I don't remember how much now but it was much higher than it should have been. I'm not an expert on distributors. Thinking the weights and springs are either wrong or work out? As mentioned I've ordered a new distributor and coil. My plan is to replace it and get the timing and advance where they need to be before continuing much further with the carburetor.

5) Check Float level
- Will check per Cliff's book when I get to this point.

6) Verify a secondary lockout is not preventing the choke from fully opening
- I'm pretty sure this car doesn't have a secondary lockout but I'll verify.

Shark Racer:
For the choke pull off, manually retracting it and covering the vacuum port with your finger should serve as a go/no-go gauge.

For vacuum advance, it should hold vacuum when vacuum is applied. In addition, you can test that your time jumps 15-20* when  it's applied (assuming manifold vacuum; if ported vac you'll need to hold the engine up in RPM a bit).

When you're testing centrifugal (mechanical) advance, make sure the vacuum advance is not connected and the line plugged. I don't know how much advance a Trans Am should have. I frequently find that people set these old Firebird engines up as if they were Chevy 350s... not the case.

thefirebirdman:
For #2 above I definitely have the late style adjustment screws that required the slotted tool. I will probably pickup the tool I need but I may pull them out and modify them for a screwdriver per cliff's procedures.

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