Backfire thru carb
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Backfire thru carb
Hi guys. Y'all helped previously with getting my truck running. '85 F150 4x4 351HO was parked over a yr ago because of no fuel to carb. With your help, after replacing mech fuel pump, found that prrmay bowl valve was stuck in off position. Replaced that and the accel pump by removing front bowl without removing carb. Put back together and, after replacing o-rings at transfer tube because of leak, truck cranked right up. Adjusted bowl level by sightglass hole. Everything worked except truck stalled when shifted into gear. Idle was very good, up til normal temp, then back down to about 800rpm. But when I punched the throttle, it bogged down and backfired thru the carb. I know timing can cause this, but before it was parked the truck ran well at speed but with a rough idle, which I believe was caused by the fuel problem. If timing is assumed good, what could cause the backfire? Have the carb off and rebuilt, but want to know anything in particular to look at before reinstalling.
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Joined: Jan 2001
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From: Virginia
Take the aircleaner off, and shine a flashlight down the carb throat as you push the throttle wide open with your hand. You should see two strong streams of gas pour down into the engine. If not, that's your problem. You replaced the accel pump, but that does not mean the passages and the check ***** are not stuck with crud, which will make the accel pump malfunction.
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From: Virginia
Your secondaries should not open at all revving it in the driveway. It sounds like they are malfunctioning if they are, or someone has been messing with this carb, modifying it and fouling it up. I would check the secondaries, the secondary diaphragm should keep them closed. If someone has added a screw in the linkage to make them mechanically open(common thing to do) take the screw out. If you are missing some parts, then get a wire and tie the secondaries shut and see if your problem goes away.
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Putting the carb back on today. If problem persists, I'll unhook the secondary diaphram linkage and try again.
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Update from before the holidays: Okay, the issues with the carb backfiring caused me to pull the carb and put in the two-stage power valve that came with my rebuild kit (AutoZone, $28). Also installed a new secondary diaphram, and adjusted the accel pump as per instructions. Accel pump was really stiff. Also, I had to pull the primary metering block to the throttle body with the bolts because the new o-ring on the accel transfer tube made it very tight. Also I was worried about the amount of space left between the new power valve and the throttle body; seems very tight clearance.
Reinstalled carb, cranked and it fired up but ran very rough with billows of black smoke out the exhaust. Adjusted the choke but no diff. Attempted to adjust the front float but the valve was in a bind. So I've pulled the carb AGAIN trying to figure out what's going on. The original power valve has stamped K 1 5 7 or K 7 5 1. Where should I start looking first? Oh, I thought this block had some BB's somewhere but now I don't see them; have two more left in the kit, one larger than the other. Assuming these are blowout protection, but where do they go? Any help is appreciated.
Reinstalled carb, cranked and it fired up but ran very rough with billows of black smoke out the exhaust. Adjusted the choke but no diff. Attempted to adjust the front float but the valve was in a bind. So I've pulled the carb AGAIN trying to figure out what's going on. The original power valve has stamped K 1 5 7 or K 7 5 1. Where should I start looking first? Oh, I thought this block had some BB's somewhere but now I don't see them; have two more left in the kit, one larger than the other. Assuming these are blowout protection, but where do they go? Any help is appreciated.
Posting Guru
Joined: Oct 2010
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From: Missouri
See that brass tube over the power valve? That is the accelerator pump transfer tube and it runs right through the vacuum chamber for the the PV. If the o-rings are old and flat then the vacuum will draw fuel right into the intake. The same thing happened to me the first time I replace a power valve in a 4180.
Also make sure your power valve gasket is seated and centered correctly. Single/Dual stage power valves use different gaskets. The single stage is round, the dual stage has three little ears pointing inward to keep the gasket centered.
If everything checks out okay then test the power valve, you can wash it with soap/water and let it dry. Then suck on the end and see if it holds a vacuum. Or you can just put a spark plug boot on the end and do it that way.
Also make sure your power valve gasket is seated and centered correctly. Single/Dual stage power valves use different gaskets. The single stage is round, the dual stage has three little ears pointing inward to keep the gasket centered.
If everything checks out okay then test the power valve, you can wash it with soap/water and let it dry. Then suck on the end and see if it holds a vacuum. Or you can just put a spark plug boot on the end and do it that way.
Laughing Gas
Joined: Feb 2010
Posts: 781
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From: Sanford, NC
It definitely needs to rebuilt with a good rebuild kit, either Holley or Motorcraft. It also
needs to be taken apart and thoughly cleaned before being reassembled. Set all of
the settings to the factory settings and adjust when you have it running. You might
want to try a single stage 6.5 power valve or even a 5.0 power valve to see if it makes
the truck run better and get better gas mileage.
needs to be taken apart and thoughly cleaned before being reassembled. Set all of
the settings to the factory settings and adjust when you have it running. You might
want to try a single stage 6.5 power valve or even a 5.0 power valve to see if it makes
the truck run better and get better gas mileage.
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From: Virginia
You may need a new carb. The kit has a diagram, and has a picture where all the "BB's" go. If you can't figure out where they all go, then it's new carb time. They are usually check *****, and some of them are important for the accelerator pump operation.
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That's a new o-ring on the tube, and I used the correct gasket ring the the PV. The old PV is still working. My kit didn't have any other new parts except gaskets to fit several models, just the bowl valves, PV, accel pump, secondary diaphram, and a few other diaphrams for other carbs I guess. The only two items I replaced the second go'round were the PV and secondary diaphram, and readjust the accel pump after it loosened up. Assuming everything else is kosher, is the two-stage PV just too much fuel causing the black smoke?
I have followed instructions on my rebuild kit....so I assume it has been rebuilt, at least to a degree. Just trying to figure out the change from lean to rich by just replacing the PV.
I have followed instructions on my rebuild kit....so I assume it has been rebuilt, at least to a degree. Just trying to figure out the change from lean to rich by just replacing the PV.