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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: suzukiboy14 on January 23, 2013, 04:48:23 pm
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hey guys and gals, i recently purchased a 1986 k2500 4x4 with the stock 350. it is an L engine code. i took off the stock manifolds and put on long tube headers, 2.5" pipes into flowmaster original 40's and NO cats. it runs ok but back fires. the biggest issue is when i drive it, as soon as i give it gas it sputters like crazy and chokes.. i just put in all new plugs, plug wires, rotor, cap, coil and fuel filter and nothing changed... i checked the timing, and it appears to be around 6 degrees BTDC..
does anyone have any ideas what this could be?
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I have to ask, did it run okay before you changed to the headers and exhaust system ? If so the additions that you have made are more likely causing the problem.
This might sound strange, but your carburetor was tuned to your old exhaust, by changing exhaust you might have to retune your carburetor. Exhaust creates back pressure in a engine, by changing the amount or elimination of said back pressure it might require re-tuning.
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hey guys and gals, i recently purchased a 1986 k2500 4x4 with the stock 350. it is an L engine code.... it runs ok but back fires. the biggest issue is.... as soon as i give it gas it sputters like crazy and chokes.... i just put in all new plugs, plug wires, rotor, cap, coil and fuel filter and nothing changed.... i checked the timing, and it appears to be around 6 degrees BTDC....
- Does it backfire out the exhaust or up through the carburetor?
- Does it "pop - pop - pop - pop" through the carb with rapid throttle opening?
- If you apply vacuum to the distributor vacuum advance, does it advance the timing? Will the vacuum advance 'hold' vacuum?
- verify that the harmonic balancer hasn't spun on its hub.
- you could have a faulty distributor pick-up coil or module.
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yes it ran fine before i changed the exhaust.. when i bought it, it had the stock manifolds, 2 cats and mufflers. it was also a smaller diameter pipe like 2 or 2.25"... when i first start it, it will run ok for a few seconds then it wants to die so i have to hold the throttle.. it backfires a bit but sounds like it is comig out the exhaust. once the idles somewhat ok on its own i can rev it up and it seems to rev fine.. then when i put it in drive it will idle fine in gear but as soon as i give it gas it will spit and sputter, if i ease off gently on the gas it will drive a bit better. i checked the timing with the vacuum advance plugged and it showed roughly 6 degres BTDC and when the vacuum advance it plugged back in it shows that it is way more advanced (about an inch or more to the left of the tab).. he only thing i could think of was the carb not being tuned but i have never tuned one before... would it be running too rich or to lean with the exhaust upgrade?? what would be the best way to go about tuning it?
i appreciate the help.
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Your exhaust mods decreased backpressure, which probably leaned the engine slightly - more so at higher RPM. Unless the engine was already running a bit lean before the mods, I'm skeptical your exhaust changes by themselves would cause a backfire and 'sputtering.' Lean backfire is more common through the intake. Nevertheless, it's possible.
But, before making any calibration changes to the carb, verify the following:
- Make sure the choke is operating correctly.
- Check the idle mixture adjustment.
- Check fuel pump pressure and delivery volume.
- Check the carburetor float level (Photo 1).
QuadraJet tuning isn't especially difficult, but they are a moderately sophisticated carburetor. Your biggest hurdle with QJets is finding a reliable source for jets and rods and a good reference on QJet tuning. You might consider the How To... by Cliff Ruggles (Photo 2), among other publications. See if you can find a copy in your local library or bookstore and thumb through it. If you tear into the carburetor, at a minimum you'll incur the cost of an O/H kit.
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Is the idle mixture screw the one near the throttle cable linkage on the carb? I played with that one a bit to get the idle up because it was idling at a low rpm and sounded like it was going to die all the time. It is idling a bit higher now but almost seems to run worse when I drive it..
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(http://img.tapatalk.com/d/13/01/26/pypezatu.jpg)
The two screws on the bottom base plate are the idle mixture screws. I'd start with getting cliffs book on carbs before I open or adjust the carb you have. Read up so you don't get frustrated.:-) good luck!
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Is the idle mixture screw the one near the throttle cable linkage on the carb? I played with that one a bit to get the idle up because it was idling at a low rpm and sounded like it was going to die all the time. It is idling a bit higher now but almost seems to run worse when I drive it..
The single screw on the driver side of the carburetor that you adjusted is for the warm (curb) idle speed. Look at 454's image, above. The idle mixture screws are the two 'cone shaped' screws at the left and right sides of the image, on the front of the carburetor. When adjusting those screws they should be balanced, meaning they should be set to the same number of turns out from seated - typically anywhere between 1 and 3-1/2 turns out. Adjust for smoothest idle.
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I took a good look at my carb and it doesn't have either one of those screws.. In fact I can't find any screws on the entire carb that look like they adjust anyhing except the curb idle screw I already played with.. Do I have a different carb maybe? I thought maybe if I run the truck and back the adjustment screws out a quarter turn and try driving to see if it helps and maybe back them out again, trial and error, until it runs better and go from there.. But I can't seem to find them.. Any ideas?..
Also my fuel gage doesn't work.. I traced the pink wire back to the tank and its not hooked up to the sending unit. When I ground it , my gauge shows empty so I know it's connect at the gauge... Where on the sending unit does it connect and where does the ground wire connect to the sending unit? Looks like I might have to lift the box to get at it, rather then empty the tank and drop it.. Any advice on that job?
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I took a good look at my carb and it doesn't have either one of those screws.. In fact I can't find any screws on the entire carb that look like they adjust anyhing except the curb idle screw I already played with.. Do I have a different carb maybe? I thought maybe if I run the truck and back the adjustment screws out a quarter turn and try driving to see if it helps and maybe back them out again, trial and error, until it runs better and go from there.. But I can't seem to find them.. Any ideas?..
Also my fuel gage doesn't work.. I traced the pink wire back to the tank and its not hooked up to the sending unit. When I ground it , my gauge shows empty so I know it's connect at the gauge... Where on the sending unit does it connect and where does the ground wire connect to the sending unit? Looks like I might have to lift the box to get at it, rather then empty the tank and drop it.. Any advice on that job?
take a pic of the carb from the front and post it here. It will help
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Suzukiboy, the screws are hidden under hardened steel caps to make them "tamper proof." The caps will need to be removed to adjust idle mixture. Post pics showing the front and top of the carb.
The fuel tank sending unit should ground to the frame via a black wire that's welded to the sending unit. The sending unit lead should plug into the chassis harness through a Weatherpak connector.