Stumped

[78C30]

Hello everyone,

I am at the end of a 5 month build on a 1978 Chevrolet C30 crew cab dually camper special and now it is all done except for two issues. The first and most important issue is the engine. It idles great and accelerates decent but when i floor it or get out of second gear it starts to buck as if its starving for gas. keep in mind i have put a re manufactured carb quadrajet on it and a new fuel pump, filter, sending units and drained and flushed the tanks.

It also has a new coil, intake gasket, vacuum lines, plugs and wires. I cant seem to find the cause of it. I've switched carbs and it still does it when i hit around 40 mph and wont go any faster without dying. I am stumped.

I will also post in the timing section as I'm not sure if it is timed correctly. That vacuum advance just screws with my head.

lastly I know I'm in the wrong section for this but when the truck goes about 3-4 miles it develops this vibration/slamming as if the drive shaft is smacking the floorboards I changed the front wheel bearings and all the u joints and carrier bearing are solid so I'm confused.

any advice or solutions for the above issues would be greatly appreciated along with any tips to make this a daily driver.

Thanks, Jamison

 

[Quadrajet Power]

So, it does this bucking with different carbs on it?

If so, set up a fuel pressure gauge to measure your pressure at the carb while driving and having this issue. It could be a fuel delivery issue. Float can be too low, or fuel not getting to the carb to fill it.
Check all of your rubber fuel hoses for cracks. Make sure any return lines or vapor lines in fuel system are good, no leaks.

Hope this helps.

 

[78C30]

yes with different carbs on it, it does. i didnt think it was a fuel delivery issue as i put a new pump and pump pushrod, sending units flushed tanks and new fuel filters. ill check the rest of the vac lines this weekend, no fuel leaks as i replaced all rubber fuel lines but original fuel lines are still on it.

Thanks, Jamison

After thought: Vapor lines? not sure what they are i should also say that i have no emissions on this vehicle, no cats or charcoal canisters or anything.

 

[rich weyand]

Yup, 1978 is last year for no pollution controls on C20, C30, K10, K20, K30.

Make sure the timing is set right. If the distributor is not clamped tight, it will drag retarded and you will see this issue. BTDT.

Timing is simple. Disconnect the vac advance line. You don't have to plug it just to set timing. With no pollution controls, set the timing at 12* BTDC. That's a good place to start. You can actually go up from there. I am running 17* BTDC. More is better right up to the threshold of knocking. Reconnect the vac advance line. For no pollution controls, hook the vac advance line to manifold vacuum, NOT ported vacuum. That will help idle and off-idle performance.

 

[MadOgre]

Hello everyone,

I am at the end of a 5 month build on a 1978 Chevrolet C30 crew cab dually camper special and now it is all done except for two issues. The first and most important issue is the engine. It idles great and accelerates decent but when i floor it or get out of second gear it starts to buck as if its starving for gas. keep in mind i have put a re manufactured carb quadrajet on it and a new fuel pump, filter, sending units and drained and flushed the tanks.

It also has a new coil, intake gasket, vacuum lines, plugs and wires. I cant seem to find the cause of it. I've switched carbs and it still does it when i hit around 40 mph and wont go any faster without dying. I am stumped.

I will also post in the timing section as I'm not sure if it is timed correctly. That vacuum advance just screws with my head.

lastly I know I'm in the wrong section for this but when the truck goes about 3-4 miles it develops this vibration/slamming as if the drive shaft is smacking the floorboards I changed the front wheel bearings and all the u joints and carrier bearing are solid so I'm confused.

any advice or solutions for the above issues would be greatly appreciated along with any tips to make this a daily driver.

Thanks, Jamison

Got any Pics ???

 

[78C30]

Yup, 1978 is last year for no pollution controls on C20, C30, K10, K20, K30.

Make sure the timing is set right. If the distributor is not clamped tight, it will drag retarded and you will see this issue. BTDT.

For no pollution controls, hook the vac advance line to manifold vacuum, NOT ported vacuum. That will help idle and off-idle performance.

Whats the ported vacuum? the vacuum advance right now is hooked to the side of the carb. is there a vacuum hose diagram for my truck somewhere?

also yes i do have picture i will post them soon

Thanks, Jamison

 

[78C30]

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[78C30]

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[MadOgre]

Sweet that thing looks awesome!

If you have the divorced choke. The vacuum port on the passenger side that protrudes through the choke assembly is for the temperature valve in the air cleaner. Whether it is full or ported vacuum Im not sure off the top of my head.

 

[78C30]

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here is my carburetor this was the old one since then i have a new one on it but its the same exact carburetor

 

[Quadrajet Power]

That is correct carb number for the 78 454 so that is good.

In the second picture, the choke is engaged partially. Make sure it is not engaged when the engine is warm.

 

[Georgeb]

Are you running the paper filter in the carb? I have had lots of trouble with them in the past. Mine would run fine until I got on the interstate then it would run out of fuel. If I slowed down it would run ok again. If I shut it down it was like the crud would fall away from the filter, it would run fine for a while and then as it slowly picked up more crud it would act up. Just a thought.

 

[Georgeb]

Are you running the paper filter in the carb? I have had lots of trouble with them in the past. Mine would run fine until I got on the interstate then it would run out of fuel. If I slowed down it would run ok again. If I shut it down it was like the crud would fall away from the filter, it would run fine for a while and then as it slowly picked up more crud it would act up. Just a thought.

My tank was so full of crap I ended up running a big sediment bowl for a tractor inline to catch the crap until I could fix it right.

 

[78C30]

I am using the stock replacements so I think so is there a better replacement? I also added an inline filter and how do ya know if it's the correct carb? I know by the numbers but is there a reference guide? Since then I put a new carb on it that looks the same but has different numbers on it also what is the lever for in the front right of the carb? On the old one it is on the farther hole and the new one is on the inside hole
Also I have already flushed the tanks I'm starting to think the tank solenoid is gummed up or restricting flow and I can't find the right replacement every place I check it's either the wrong one or it says discontinued or won't let me purchase
Thanks, Jamison

 

[Quadrajet Power]

The carb you have in this picture is 17058212 which indicates a 1978 Chevrolet 454 truck quadrajet. This particular one was built on 258th day of 1977. If the other quadrajet you have is a different code stamp, let me know what that is. Quadrajets are great carbs, but they were built unique for their application. So they don't swap around to different engines well, without modifications.

The lever you are referring to is the accelerator pump arm. In this picture it is on the outside hole. I prefer to place them on the inside hole for better, quicker accelerator pump discharge.