Intake plenum gaskets

[Ricko1966]

Yes, take the carburetor off of the truck before taking it apart. Take a few pictures of the linkages and get a magnet to catch the clips and screws when you take them out.

Okay, if it's your first time taking apart,take multiple pictures. Before it comes off put white out stripes on the hoses at the carbureator end. 1st hose 1 stripe next hose 2 stripes 3 stripes,etc then take a pic. You can look at your pic and it's real quick and accurate hose 1 here,2 there etc. You will have to remove some components to get the linkage off. Do you have a book? If you are buying a kit, I'd recommend a non windowed needle and seat, the correct fuel filter,which has a check valve and new float.

 

[Catbox]

Where in Oregon are you located?

 

[Bextreme04]

Where in Oregon are you located?

She is south of Eugene. This is the same person I pointed to the Eugene squarebodies group on facebook and she had made contact with someone down there that was going to come out and look at it. Apparently he wasn't much help either I guess.

 

[DoubleDingo]

Also, use line wrenches for the fuel inlet and brake booster fittings, you don't want to deform the fittings. And be careful with the fitting that holds the filter in the carb, it might need a 1" open end wrench, but it's easy to booger up those fine threads. Don't use teflon tape on the fuel inlet fitting.

Using ultra black on the china walls, make sure to build it up thick enough, and follow the directions on the tube as to guarantee good seal. You don't want to redo the intake because of leaks. It's a great learning experience but one that can be avoided.

 

[Grit dog]

Ok separate the issues.
1. Pumping the throttle 80x to start is 90% likely carburetor related. Accelerator pump worn out. It’s the little plunger thingy under the top lever. Not difficult. (I am also NOT an expert on Q jet carbs so anyone feel free to correct me. Just trying to be helpful.).
2. Ensure the choke works but now it’s warm and your issue sounds like acc pump to me.
3. You said runs rough up to 45mph. That “could” be a vacuum leak. A massive vac leak will not idle. Have to be partial throttle minimum just to get it to run. And will smooth out with higher rpm’s. Again long time since I’ve dealt with this.
Small Vac leaks are easy to diagnose. Big ones are obvious, symptoms above. Assuming it will idle. As mentioned, get a can of carb n choke cleaner or starter fluid. Spray small amounts around the intake manifold. Especially front and rear. The where the hokey gaskets are. Also base plate under carb and all vac line connections and vac lines. If a vac leak it will suck in the starter fluid and you’ll hear the rpm’s go up because it’s inhaling flammables.
4. Could be ignition or timing related but not likely.
More context like how long from fine to what symptom started always helps when getting “remote” advice.

Or carb could just have plugged orifices or circuits or.
Best of luck!

 

[75gmck25]

I have a ‘75 K25 350 with an 80’s vintage Quadrajet that is similar to yours. Some suggestions.

- if you can’t see fuel squirting in, you definitely need a new accelerator pump assembly. Get one that has seals designed for new ethanol mix fuel. Old pumps had rubber gasket that do not hold up well to new fuel.

- are you sure you got the fuel filter and spring in the proper order and orientation. It’s very easy to forget how they came apart, and the wrong installation will really screw up fuel flow.

- I see two T fittings in the line to the PCV. Do they go to a vacuum canister to vent the tanks? If they do, there is also a purge valve in the system and the purge is vacuum controlled. If not working correctly it could cause a big vacuum leak.

- have you changed the PCV valve? I had mine pop out once from a backfire and it caused the engine to run really bad at idle. Mine was laying on its side, so it was open and causing a big vacuum leak.

- have you verified base timing and that the mechanical advance is working? Factory base timing recommendation is probably about 6 BTDC, but the engine will run much better with about 12 BTDC at 650 rpm, with the vacuum advance line unplugged and capped. Then use a setback timing light to verify that you get about 18-20 degrees more (so it’s now 30-32 total) when rpm is at about 2500-2800 rpm.

Reconnect the vacuum advance line at idle and check to see that you get about 18 degrees from vacuum advance. You need this to help your highway gas mileage.

- do you have a 3 port fuel pump with a return line to the tanks? It will work with a 2 port, but you may have vapor lock issues in hot weather. The tank return keeps circulating fuel and cools it down.

- you said the fuel tank lines and solenoid were worked on recently. Did that mechanic understand the system? It’s not real complicated, but hooking up a few lines incorrectly can screw up fuel flow. For example, if you have a fuel return line they must use a 6 port valve that switches both fuel feed and return. 6 port valves are no longer available new. The vent lines for the tanks T together and run up to the vapor canister.

 

[yellowdog5]

I've been hauling a load of hay every day in spite of the problems while waiting for parts. This morning the truck starts but dies when I try to move it. Is this probably related to the same problem or is it something new? Reminds me of how it acted a long time ago when the vacuum line on the back of the carb came loose, but I called myself looking really carefully and can't see a loose vacuum line anywhere. Accelerator pump will be here today, I got a new mounting gasket. Is there a torque value for tightening the bolts or do I just get them snug and leave it there?

 

[DoubleDingo]

I've been hauling a load of hay every day in spite of the problems while waiting for parts. This morning the truck starts but dies when I try to move it. Is this probably related to the same problem or is it something new? Reminds me of how it acted a long time ago when the vacuum line on the back of the carb came loose, but I called myself looking really carefully and can't see a loose vacuum line anywhere. Accelerator pump will be here today, I got a new mounting gasket. Is there a torque value for tightening the bolts or do I just get them snug and leave it there?

I don't know if there is a torque number for the carb bolts. Basically tight enough to NOT have any vacuum leaks at the base gasket, and loose enough to not warp the air horn and base plate.

 

[DoubleDingo]

Do you know if the fuel filter is installed in the correct orientation in the carb?

 

[Bextreme04]

I've been hauling a load of hay every day in spite of the problems while waiting for parts. This morning the truck starts but dies when I try to move it. Is this probably related to the same problem or is it something new? Reminds me of how it acted a long time ago when the vacuum line on the back of the carb came loose, but I called myself looking really carefully and can't see a loose vacuum line anywhere. Accelerator pump will be here today, I got a new mounting gasket. Is there a torque value for tightening the bolts or do I just get them snug and leave it there?

The correct method for tightening the carb bolts is:

Tighten one diagonal set of bolts to 36 in-lbs (3 ft-lbs); tighten the other diagonal set of bolts to 36 in-lbs (3 ft-lbs); tighten all four bolts to 144 in-lbs (12 ft-lbs).

This is shown on an inset drawing in the Chevrolet Chassis Service Manual, Engine Fuel System section.

P.S. After operating the car for a few days, retighten the bolts to 12 ft-lbs, as the gasket will compress some under load and with heat/use.

 

[yellowdog5]

Do you know if the fuel filter is installed in the correct orientation in the carb?

I can't see myself putting it in backwards, but I'll check.

 

[Ricko1966]

If you verify timing as in post 51,and you definatly should that method of checking vacuum advance must be done on manifold vacuum,not ported vacuum. The vacuum advance is important for highway milage but that is not all,it's important that it functions. It is also important that the centrifugal advance functions,when you are setting base timing blip the throttle and make sure your timing mark moves.

 

[DoubleDingo]

I can't see myself putting it in backwards, but I'll check.

And making sure the spring is in there too.

 

[yellowdog5]

And making sure the spring is in there too.

Ok. pretty sure all is good there, but won't hurt to look.

 

[yellowdog5]

If you verify timing as in post 51,and you definatly should that method of checking vacuum advance must be done on manifold vacuum,not ported vacuum. The vacuum advance is important for highway milage but that is not all,it's important that it functions. It is also important that the centrifugal advance functions,when you are setting base timing blip the throttle and make sure your timing mark moves.

I have never checked timing on my own, I do have a timing light but need specific instruction. Can't do it right now but tomorrow I will quote that post (51) and answer and ask questions. It's sounding to me from the comments on this posts and from what the guy that looked at it did spraying around the gaskets, that I might need to replace intake manifold gaskets also, but would like to check again, if I can keep the truck idling long enough to do that. Accelerator pump should be here today but I want to know everything I can before working on it. Thanks.