Stalls when pressing gas too fast

[Red Rex]

Fuel pump failure? Accelerator pump actuating rod in the wrong hole?

 

[thecantaloupeman]

To make sure you are getting the correct parts for your carburetor, there should be numbers stamped on the driver's side rear corner. He's mine. Your's should have 8 digits for a '78 and start with 1705****

http://www.carburetion.com/quadnumber.htm

#17084502 - Breakdown
1708 - 80's
4 - Production year - 1984
5 - Quadrajet (4 bbl) California standards
0 - Chevrolet
2 - Even - Auto trans

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Mine is 17058213
So I guess that means it is original. Mine is a federal standards one, so maybe the Calif standards one had longer filters.

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[1987 GMC Jimmy]

I just got into Tony Mopar’s channel, and it’s good. He did a Quadrajet video recently, and I really enjoyed the Slant 6 video.

I’ve got a Quadrajet laying around here with a short filter, but it’s a pre-‘73 model. Either way, the filter’s a picayune item in the grand scheme. The acc. pump diaphragm is a good one to check, which the trick I use is putting my phone on video with the light on and watching the screen. It makes the test a lot easier. You need to do a visual inspection of all the vacuum lines in the engine bay and get a manifold vacuum reading after you’ve exhausted all the advice. It’s fuel, spark, and air on a vehicle this old. The electronic components are either meager to the point of negligibility or totally absent. If you’ve got a good, white spark getting to the plugs and your timing advance is working, your pump and carb are in proper working order (you can test the suction and distribution sides of the pump with the vacuum gauge, lines off and with someone hitting the starter to move the pump, while it is still installed), and you’ve got a stable idle vacuum of no less than 17” of Hg, you shouldn’t have any problems.

I can empathize with you just getting started and I’m 22 myself, but I had the luxury of a dad who was mechanically savvy, and I grew up in an airplane hangar. He didn’t do squat with automobiles, and cars are way more complicated IMO. You have to go and acquire the basic knowledge by studying it yourself or experimenting with the truck. If things break (accidentally of course) and curse words are said, you’re doing it right because that helps you fine tune your approach where things will break less as you do it more.

I would suggest bringing yourself up to speed a little more because the regular guys on here know their stuff and expect you to bring a little something to the table. If I were you, I’d go to Autozone, buy a 73-87 pickup Haynes manual, and read it like a novel. When you’re done, keep it in the truck to reference as you need. There’s lots of good info in there. If anything confuses you, Google it and see if there’s a write up or Youtube video on it. That way when something goes wrong that’s beyond your knowledge, which is a phenomenon that happens to even the best of them, you’ve got some background on it, and you can apply more complex advice/understand what’s going on. I hope you stick with what you’re doing here, and I would only suggest that you attack it a little more proactively.

 

[thecantaloupeman]

I just got into Tony Mopar’s channel, and it’s good. He did a Quadrajet video recently, and I really enjoyed the Slant 6 video.

I’ve got a Quadrajet laying around here with a short filter, but it’s a pre-‘73 model. Either way, the filter’s a picayune item in the grand scheme. The acc. pump diaphragm is a good one to check, which the trick I use is putting my phone on video with the light on and watching the screen. It makes the test a lot easier. You need to do a visual inspection of all the vacuum lines in the engine bay and get a manifold vacuum reading after you’ve exhausted all the advice. It’s fuel, spark, and air on a vehicle this old. The electronic components are either meager to the point of negligibility or totally absent. If you’ve got a good, white spark getting to the plugs and your timing advance is working, your pump and carb are in proper working order (you can test the suction and distribution sides of the pump with the vacuum gauge, lines off and with someone hitting the starter to move the pump, while it is still installed), and you’ve got a stable idle vacuum of no less than 17” of Hg, you shouldn’t have any problems.

I can empathize with you just getting started and I’m 22 myself, but I had the luxury of a dad who was mechanically savvy, and I grew up in an airplane hangar. He didn’t do squat with automobiles, and cars are way more complicated IMO. You have to go and acquire the basic knowledge by studying it yourself or experimenting with the truck. If things break (accidentally of course) and curse words are said, you’re doing it right because that helps you fine tune your approach where things will break less as you do it more.

I would suggest bringing yourself up to speed a little more because the regular guys on here know their stuff and expect you to bring a little something to the table. If I were you, I’d go to Autozone, buy a 73-87 pickup Haynes manual, and read it like a novel. When you’re done, keep it in the truck to reference as you need. There’s lots of good info in there. If anything confuses you, Google it and see if there’s a write up or Youtube video on it. That way when something goes wrong that’s beyond your knowledge, which is a phenomenon that happens to even the best of them, you’ve got some background on it, and you can apply more complex advice/understand what’s going on. I hope you stick with what you’re doing here, and I would only suggest that you attack it a little more proactively.

Thanks for the advice. I actually already have the Haynes manual and recently I bought the factory service manual off of eBay. I've been giving them a read for a few of things I need, but not like a novel. It probably is a good idea to do that. But you're right I should try to bring something else to the table. I'll try to do as much as I can.

On a different note I was able to check the accelerator pump and it was bone dry when I moved the throttle. I think I've found my issue. What are some things I can try to check to diagnose it?

Thanks to everyone who being really supportive of me btw. It's been really nice to have this resource. I've been exploring the forum a bit more getting a feel for some other projects going on. I'm watching Dingo's Crusty build and a few others. It's pretty cool to see some other GM trucks because they really are some super awesome trucks. I would take any GM truck over a Ford/Dodge any day. I really have become a squarebody enthusiast and it's cool to be part of such a huge community like this!

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[Nonstop]

All left to do is make sure there is fuel to the carburetor and move the throttle a few more times to make sure it is bad. Once it is confirmed, best suggestion I can give you is to remove the carburetor and bring it to a shop to be rebuilt. Rebuilding a quadrajet can be intimidating.

 

[thecantaloupeman]

All left to do is make sure there is fuel to the carburetor and move the throttle a few more times to make sure it is bad. Once it is confirmed, best suggestion I can give you is to remove the carburetor and bring it to a shop to be rebuilt. Rebuilding a quadrajet can be intimidating.

Why do you think the accelerator pump could be bad if the previous owner just rebuilt it? Did he do it wrong or something? Also how do I verify there is fuel to the carb? It is definitely starting up pretty easily. And the pumps of the pedal seem to do something so does that mean my accelerator pump is fine?

 

[73c20jim]

When something goes wrong, always check the last thing that was done.

 

[1987 GMC Jimmy]

I would do the test immediately after the truck had been run. If fuel isn't coming out then, there's a problem.

 

[thecantaloupeman]

I would do the test immediately after the truck had been run. If fuel isn't coming out then, there's a problem.

That very well may have been my mistake. I'll give it a go in the morning. Thanks!

 

[thecantaloupeman]

I would do the test immediately after the truck had been run. If fuel isn't coming out then, there's a problem.

So no fuel came out, but it doesn't even look like there was fuel in the bowl. I ran the engine for maybe 5 mins or more. How could it have started so easily of there was no fuel in the bowl? Like I said pumping the gas seems to help it start so shouldn't that mean it's pumping has into the carb?

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[1987 GMC Jimmy]

It sounds like you’ve got fuel coming out of your idle discharge, but the main discharge might be having issues related to the accelerator pump or a clog/check valve issue. You checked both sides, primaries, choke open or held open? You went all the way to wide open throttle? In some failures, the discharge will start at a part-throttle position, and this can be why pumping it to start it up is doing something, and if you baby it and get it going, it won’t leave you stranded because you’re getting fuel at cruise.

 

[4WDKC]

Fuel pump failure? Accelerator pump actuating rod in the wrong hole?

I dont know **** about carbs so this is what I was thinking, then he said the carb was already rebuilt, that tells me there was a problem before the carb was rebuilt and thought it was the issue. Get a fuel pressure tester and check it.

 

[thecantaloupeman]

I dont know **** about carbs so this is what I was thinking, then he said the carb was already rebuilt, that tells me there was a problem before the carb was rebuilt and thought it was the issue. Get a fuel pressure tester and check it.

Well the fuel pump looks new too, so maybe they tried that too and still couldn't fix it. I'll check it anyway tho

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[Snoots]

Get a fuel pressure tester and check it.

X2

 

[thecantaloupeman]

I dont know **** about carbs so this is what I was thinking, then he said the carb was already rebuilt, that tells me there was a problem before the carb was rebuilt and thought it was the issue. Get a fuel pressure tester and check it.

I haven't done the fuel pressure test, but I had a friend come and take a look. He reckons it's my accelerator pump. He had a look inside the carb while it was running and pumped the throttle and didn't see the accelerator pump spray. Tried the same while it was turned off. So I have a feeling my best shot is to rebuild it or find a cheap used one. If I give the scrapyard a good long look I might be able to find a nice rebuilt carb and they only charge 30 bucks.

I have a retired mechanic visiting in a few days as well, so he might be able to help me if I decide to rebuild it.

What rebuild kits are out there that are decent for the quadrajet? I have no idea how much they cost, but it may be a good opportunity to learn to rebuild a carb.

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