74 GMC Problem

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  #1  
Old 04-15-07, 03:03 PM
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Cool 74 GMC Problem

I have a 74 GMC truck that will not start. It cranks but I can't get the darn thing to start. I replaced the fuel pump, thinking that was my problem, but that did not help. What is the best way to trouble shoot the fuel system. I did check the fuel filter and it looked good. Any advice and help would be greatly appreciated.

Thanks in advance
 
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  #2  
Old 04-15-07, 03:42 PM
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I'm assuming it has a carburetor on it. Hold the choke open at the top and work the throttle by hand. You should be able to see two little jets of fuel squirting from the accelerator pump. If you do, you have fuel and need to look at a few other things.

Make sure you have the common tuneup items in good condition - cap, rotor, points/condenser (if so equipped), plug wires, fuel filter, pcv valve. Also have good fuel in the tank.

Pull a plug and ground it with a vice grips. Have some one crank the engine and see if you have spark at the plug. If yes, you have two of the things you need.

If all is well at that point, pull the lead from the coil and check for spark from the coil lead when cranking. If no spark, the coil may be bad.

If you have spark .....

Next check compression and you will need a compression tester (rent one at a parts store - or borrow one. You should get around 110 to 140 lbs, depending on the condition of the engine.

If everything is ok to this point, that leaves the timing on the ignition and possibly a major vacuum leak.

This should get you started,

Hope this helps,

Bob
 
  #3  
Old 04-15-07, 04:12 PM
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Thumbs up 74 GMC Problem

Marbobj,

Thanks I really appreciate your advice! I'll try some of the things you suggested. You'll be hearing from me again.

Thanks
 
  #4  
Old 04-19-07, 12:33 PM
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Question 74 GMC Problem

Hey Marbobj,

Okay I tried the fuel jetting and there is none. Do I need to replace the, "Accelerator pump"?

Thanks in advance
 
  #5  
Old 04-19-07, 04:36 PM
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try this

Pour a cap of gas in the carb and see if it starts for a few seconds. Just a little will do it. If you have starting fluid that will work also either one.
 
  #6  
Old 04-19-07, 04:44 PM
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I agree, see if it will run off a little gas before going further. You did verify it's getting spark, right?
 
  #7  
Old 04-19-07, 08:10 PM
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bdc:

No don't replace anything just yet, like micheal and mark indicated. Checking the accelerator pump is more of an indicator of fuel in the carburetor bowl. If you had cranked the engine a few times the fuel pump should have filled the bowl and you would have fuel ejected from the accelerator pump (providing it was working).

On that year of vehicle, often the starting procedure was to "pump" the throttle a couple of times then hold it in some intermediate position while trying to start the engine. Pumping the throttle was actually to prime the engine via the accelerator pump.

Even if the accelerator pump was faulty, the engine could still start, using the choke to give the engine the extra fuel it would need when cold. If you find, after manually priming the engine that it runs for a short time and dies, it would be an indication fuel either isn't getting to the carburetor or getting through the carburetor to the engine.

To check the first condition, I would take off the fuel line at the carb and direct the fuel flow into a container. Then crank the engine and see if you do have fuel entering the carburetor.

If you get it to start and do have fuel at the carburetor you would have an internal carb issue (plugged jets or plugged float valve)
Do you know if you have the two or four barrel Rochester carb?

Let us know - those are easier to fix than the new ones,

Later,

Bob
 
  #8  
Old 04-20-07, 01:46 AM
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thats right

Thats right you are going to have a system to troubleshooting. Start at the end and go back till you find the problem. You are lucky that you still have a truck that does not have all the new computer controled gizzmos makes it a LOT more easy to trouble shoot.
Let us know what is going on I bet we can get the old beast going again.
 
  #9  
Old 04-20-07, 10:02 AM
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Thumbs up 74 GMC Problem

Marbobj, Michael Van, and MarkSr

Hey guys thanks for all the advice.

Marbobj,

When you say , "Pull a plug and ground it with a vice grips", I am going to pull the plug leave it connected to the rubber boot, place a pair of vise grips at the exposed end, crank the motor and see if I get spark. Is that the correct procedure?

MarkSr,

I have not checked for spark yet I was not sure how to do that, so hopefully the vise grip grounding procedure above is correct and I can give a try.

Michael Van,

The top of the carburetor has 4 openings 2 large side by side and then in front of that 2 smaller openins which set of openings do I pour the gas into. I am going to say the 2 larger openings.


Guys I really appreciate your help. This truck is my one and only daily driver. I work at a grocery store , don't make a whole lot of money so I can't really afford to take to an automotive shop and have it fixed. So for what it is worth I really do appreciate all your help. Thanks again guys.

I'll try the spark thing first.
 
  #10  
Old 04-20-07, 03:21 PM
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Do you have points or electronic ignition?

You can pour the gas in any of the openings on top of the carb - it will all go to the same place. Just be sure not to pour in TOO much.
 
  #11  
Old 04-20-07, 03:49 PM
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make sure its grounded

Clamp the vice grips onto the flats were the wrench goes. Make sure the vice grips touch a good solid ground ( no paint) engine block or exaust pipe will work.
 
  #12  
Old 04-20-07, 05:09 PM
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You would have a 4 barrel quadrajet. Considering the year you should have the points/condenser ignition unless it has a retrofit. Chevy started the electronic stuff after '74 I believe. The first truck with it I remember was a 77 chevy one ton.

If you don't get the spark, take a look at the points/condenser, and rotor. Those are typical maintenance items to go bad.

Hope this helps,

Bob
 
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