Car Questions

Notifications
Clear all

I need help with this 1986 GMC 3500. Engine not running!

   RSS

0
Topic starter

I am about to pull my hair over this one!  My Grandfather has a 1986 GMC 3500 with the 454 in it and has had it since practically brand new.  My Dad and my Grandfather use to pull their tractor across the state with the truck, and it handled it like a champ.  But the last 10 to 15 years, it's had some issues.  Several years ago, it wasn't running and they towed it to our trusty mechanic a few miles away, and they did work to it (not sure what they did cause I was too young to understand then).  It has never run right since then.  It would start just fine, but then for a few minutes it would blow tons of black smoke.  After it warmed it would run ok, but would misfire a little.  Oh and, you couldn't floor it past 55 MPH because it would just cut off.  We never did anything about, and I really wanted it to run perfectly again.  I didn't know how to work on cars until about 3 years ago.  I've learned a lot by watching scotty's videos, working at a tire and auto shop, and actually doing work on some of our cars.  I rebuilt the carburetor on his 77 LTD with the 302 and it's now running like a clock.  Also replaced the tank and several other stuff.  I thought if I got the LTD running that good, I should work on the GMC.

 

I didn't want to rebuilt the carb because I knew how difficult these Quadrajets were to work on, but I thought it would be a good idea just to pull the top half off and just clean it up like I would on a lawn mower carb.  This was a terrible mistake, and I had so much trouble getting it back together that I wasn't even sure everything was together correctly.  Also pulled all the spark plugs to check them.  Put everything back together, and when I go to crank it up, big puffs of black smoke are just blowing out both sides (dual exhaust) and the engine is barely running and will hardly respond to any throttle.  So I disassembled it again, and put it back together, and it was even worse after that.  I thought I had messed up on the carburetor, and there was no way I could fix it.  So I had to tell my Grandfather we need a new or remanufactured carb and it would be over $500.  I assured to him this would fix the problem, so he agreed to buy it.

 

I get the remanufactured carb from autozone.  It's the Qaudrajet  ND4663, the same as the old one.  Did not do anything to the carb, no tuning, no disassembling.  I put everything on correctly with the new gasket.  When I tried to start it, it would only fire when I'm holding the starter even while pumping the gas.  As soon as I quit starting, the engine would just die. 

 

I don't understand why this did not fix the problem!  This is my luck with this truck.  Everything I have tried has failed to fix the problem, and I have no idea what I should do!  I need someone to help me.

6 Answers
2

You said you did no tuning.  Did you open the mixture control screws 2 1/2 turns to start?  Did you properly set the automatic choke?  You should a lot more research on carb set up and adjustment before trying to start it.   

I understand it needs proper adjustments, but what could possibly make it so bad that a cold engine doesn’t even stay running when I keep pumping the gas? I set the screws to 2 1/2 like you said, but it’s still doing the same thing.

There is a fairly high probability that the carb is defective right out of the box. (For example the float level could be way off or the core they used might be internally cracked, or it might be worn where the throttle plate shaft goes through which would lean out the mixture.)


However, check the simple stuff. Make sure that you have no vacuum leaks. Just so you have a complete picture of what you're dealing with a compression test of the engine would be a good idea.

2

I know this is not what you want to hear, but if it were mine, I'd 86 the Quadrajunk and go for an aftermarket fuel injection system that eliminates most all the problems you will continually have with a carburetor, especially since the fuel today contains at least 10% ethanol that eats up carburetor seals and rubber parts.

That sounds challenging

I’m thinking about installing an edlebrock or Holley because I am sick of these “quadrajunk”

Here's a pretty good rundown on Edelbrock carbs. Bear in mind that any carburetor you install needs to be tuned to your application. It's an analog device with no feedback for self-adjustment.

1

Open the choke, pull on the throttle and look down inside the carburetor. If the accelerator pump is working correctly, it should spray gas as the throttle moves. If you don't see a stream of gas, then there's probably an issue with the accelerator pump in the rebuild job.

Carburetors need primed like lawn mowers to start, so if the engine starts after you pump the gas 5x or so then stalls, there's probably an internal problem with the jets. 

Accelerator pump is working properly. I opened the choke and moved throttle and saw steady stream of gas.

@Doc and @Chucktobias can probably give you more troubleshooting tips.

Make sure you're not flooding out the mixture and crank with the throttle wide open for a few seconds, then try again. Try with fewer and more pumps of the pedal. Once the mixture hits the right ratio, it should fire for a second, providing the spark is good.

1

Part of the problem is even back in the day the quality of mass-produced parts store rebuilt carbs was very variable, tending to be poor. I can't imagine how bad they must be today! It would have been better to have an old-school carburetor rebuilder like https://www.carburetorfactory.com/ overhaul the original carb.

Another issue is you really can't count on it having been set up properly out of the box. (I've seen "rebuilt" carbs where the float level wasn't even right.)

Quadrajets can be particularly finicky. Do an internet search for "setting up quadrajet" with your favorite search engine for info on properly tuning the carb.

This is why they are lovingly referred to as "Quadrajunk" carburetors.

1

It's not anywhere near as challenging as trying to properly jet and set up a carburetor without access to a dyno tuning facility.  All the aftermarket systems I've used are self-learning systems.  You install the computer, throttle body that bolts to the carb flange, electric fuel pump on a frame rail and an 02 sensor on the exhaust manifold.  Some systems even come with their own complimentary electronic ignition systems.  You start the car and the computer figures all the rest out for you.  You get incredible reliability and optimal MPG.   

1
Posted by: @rcflashlight

Any idea what cfm I should go with for my 454? I'm thinking about getting an eldebrock 1406 performer 600 cfm, but not sure its good enough.

There's a lot of different factors, and there are carb size calculators online. With some assumptions as to volumetric efficiency (.85) and maximum rpm (5000), I'm seeing a suggestion of 558 cfm from this calculator, so  a 600 CFM carb should be fine:

http://www.gtsparkplugs.com/CarbCFMCalc.html

You could also contact Edelbrock and see what they recommend.

The Edelbrock Performer 1406 is basically an updated version of the classic Carter AFB. The latter came on my '67 Rambler 343 V8 from the factory. Has never given me any problems in the 30 years I've owned the car.

Share: