Carb problems.

[Matthew40566]

Hello,

Last December I picked up a 1988 GMC C1500. It has a recently swapped and rebuilt 5.7 with a 700r4 trans.

The engine was rebuilt with a Holley 4175 that's missing most of the choke components. Now that it's starting to get cold it takes about 45 seconds to start and has to be warmed up for about 10 minutes before it's drivable. Even when warm (it only gets to about 140 degrees) it hesitates when accelerating and often dies. Also, it often sputters between 1500 and 300 rpm.

I have checked the following:

• Timing (put it about 10 degrees BTDC)
• Idle (set to about 800 RPM, it's much lower while warming up. No fast idle)
• Accelerator pump discharge nozzle upgraded to 121-145

Is there anything I'm missing? Could this be due to the degraded state of the carb?

Also, would it be worth it to swap back to TBI, it seems to have all the electronics/ecu, not sure what I would need to put it back.

Would a cheaper option be to rebuild the carb? Is it possible to find the components for the choke?

Thank you,

 

[Erik the Awful]

The lack of a choke will make it more difficult to get started, but since it's still troublesome after you warm it up, you have a driveability problem.

What do you know about the carb? Was it a good running carb twenty years ago when Uncle Fred had it on his slant-6 drag car, or is it something you pulled off the shelf and rebuilt, but don't have a choke for?

After making sure the carb is good, I'd do a full tune-up. If you've only had the truck for a year, you need to replace the fuel filter and consider the condition of the cap, rotor, wires, plugs, and air filter. If any of them look rough, I'd replace them.

You'll need to know your fuel pressure. If you've already increased your accelerator pump shot and it still hesitates, I'd suspect a fuel pump problem over a vacuum leak, but you need to check for both.

 

[Schurkey]

Even when warm (it only gets to about 140 degrees),

Fix that first.




Having a defective carb on what should be a feedback-fuel-injected, computer-controlled vehicle is no way to operate. You're going to need serious tuning to get the carb and ignition to work, and then you have no hope of passing emissions requirements if they test in your area.

Put the TBI back on.

 

[Matthew40566]

The lack of a choke will make it more difficult to get started, but since it's still troublesome after you warm it up, you have a driveability problem.

What do you know about the carb? Was it a good running carb twenty years ago when Uncle Fred had it on his slant-6 drag car, or is it something you pulled off the shelf and rebuilt, but don't have a choke for?

After making sure the carb is good, I'd do a full tune-up. If you've only had the truck for a year, you need to replace the fuel filter and consider the condition of the cap, rotor, wires, plugs, and air filter. If any of them look rough, I'd replace them.

You'll need to know your fuel pressure. If you've already increased your accelerator pump shot and it still hesitates, I'd suspect a fuel pump problem over a vacuum leak, but you need to check for both.

The carb seems to be Frankensteined together, I'm betting is was something the mechanic had around and threw on. I think I'll rebuild it.

I'll replace the fuel and air filter and get a fuel pressure guage, maybe I can rent one?

Thank you!

 

[Matthew40566]

Fix that first.




Having a defective carb on what should be a feedback-fuel-injected, computer-controlled vehicle is no way to operate. You're going to need serious tuning to get the carb and ignition to work, and then you have no hope of passing emissions requirements if they test in your area.

Put the TBI back on.

Fortunately no emissions in my area.

I do not have the TBI, would I be able to purchase one on ebay and put it on? I need to check and see if the original wiring is still there.

 

[kenh]

Fortunately no emissions in my area.

I do not have the TBI, would I be able to purchase one on ebay and put it on? I need to check and see if the original wiring is still there.

Try your local junk yard. You may very well find all you need there. A rebuild kit for a TB is in the $35 to $40 range. If most of the wiring harness is there on your truck, you can fill in the missing pieces from the parts donor in the junk yard.

Ken

 

[PlayingWithTBI]

I do not have the TBI, would I be able to purchase one on ebay and put it on? I need to check and see if the original wiring is still there.

Check it out and see if you have the sensors and wires still for MAP, EGR, and EST (AIR if it was so equipped). Do you have the intake manifold too? I have a working TB with sensors (AIC & TPS) off my 88 without the air cleaner hold down rod, at least it worked 2 years ago, ha ha. Been sitting in my shop in a tote. I also have an OE intake manifold (aluminum) but, that may cost too much to ship from AZ to OH. Send me a DM or reply here if you're interested.