351w stumbles

[Madgyver]

Ive noticed that the secondary air valve is not opening. i need to understand how this thing works. the vaccumn diaphram prevent it for opening.

 

[WheelHorse]

Could it be a spark issue?
Like in the dizzy, vacuum advance?

Check the vac advance with a hand vacuum pump... slowly start to increase vacuum at idle and you should see your timing start to move up 18-25 degrees.

What are you running for a Dizzy? Motorcraft duraspark, P&C or aftermarket?

 

[Rustytruck]

If the secondary air valve is not opening the acceleration will drop off at around 50-55 mph.

If the carb is working properly but needs secondary air valve adjustment it is a fairly easy adjustment.

On the passengers side of the carb top there is an Allen head adjustment screw. You don't see it because it screws up from under neath the top of the carb. It is attached to the end of the shaft of the secondary air valve.

You loosen the screw and slide it around the shaft and then tighten the screw. It is a very fine adjustment. The air valve is balanced by this adjustment. You are actually adjusting spring tension that controls the secondary air valve tip open. The suction of air from the engine through the carb sucks this valve open. Too much spring tension and the valve wont open when you need it. Too little spring tension and the air valve opens too soon making the engine run too rich when not needed.

It is one of the reason that this same carb can be used on very small engines to very large engines. Take your extra Quadrajet and flip it over and you can see how it operates.

One other thing to check with the Quadrajet is on the drivers side throttle linkage. Make sure that when you open the throttle up that the linkage on the carb is operating all the way. The secondary air valve has a lockout in the linkage so that it only opens on acceleration. This is adjusted differently on different models on various engine sizes.

Just make sure that it is operational on your set up while you at your acceleration point. It is important that this lockout work properly for acceleration and for deceleration. It must close and lock out the secondary air valve on deceleration.

Lets say your climbing a hill with good throttle and you get to the top and because there is a steep down hill run on the other side, as you crest the top and go over you drastically let off the gas to make the down hill run. If the secondary lockout isnt working properly as soon as you let off the gas and close down the primary side of the carb you will get high vacuum through the carb. If the secondary side is not locked out on deceleration the high engine vacuum would open the secondary air valve and add fuel. Not something you would want.

 

[jagbucket]

another vote for spark issues. give it a tune up just rule it out specifically cap rotor plugs and wires

 

[Madgyver]

Load or no load, spark issues would be present across the rpm range.

 

[Madgyver]

I've swapped 2 reman qjets and both had the same issue under the same condition.
Im starting to think it is the carter electric fuel pump. It has been in there since 06.

 

[Madgyver]

I think we found the problem, won't know till test drive tomorrow on the open road.
I looked over the electric pump and noticed another fuel filter that I installed on the suction side of the pump. It was installed in 06 so I just forgot about it. I had Taz open it up and sure enough it was clogged with debris from the tank. Cleaned it and ran pump to dump fuel in a bottle and it flowed so much better. Also flushed out the fuel line from the tank.

 

[HoldMyPocket]

I think we found the problem, won't know till test drive tomorrow on the open road.
I looked over the electric pump and noticed another fuel filter that I installed on the suction side of the pump. It was installed in 06 so I just forgot about it. I had Taz open it up and sure enough it was clogged with debris from the tank. Cleaned it and ran pump to dump fuel in a bottle and it flowed so much better. Also flushed out the fuel line from the tank.

*Bump* I'm curious if you ever resolved this. I'm having similar issues and I'm starting to think it's a failing mechanical fuel pump. I've replaced all of my ignition, rebuilt the carb, changed the power valve, forced the secondaries to open with the screw trick (Holley 4160), can't get over 50-60 mph without stumbling if at all.

 

[.94 OR]

I'll throw an idea out there.
I had an '86 F150 5.0 that had something similar and I couldn't figure it out. Took it to the Ford garage while I was off for work for the week. Came back, they couldn't figure it out either. Being EFI they could at least put a pressure gauge on it and it met the specs. I lived about 200 miles south of the shop and was running down the highway in the rain and it was doing it real bad. Popped the hood and found that the coil top was cracked and since there was rain water being pushed in the coil would siphon off some of the juice as it grounded. I could watch it short out on the intake manifold. It ran, but had no top end when it needed maximum spark.
Just a thought.

 

[bronco italiano]

EPA took the poi out the fuel!!!

 

[Madgyver]

*Bump* I'm curious if you ever resolved this. I'm having similar issues and I'm starting to think it's a failing mechanical fuel pump. I've replaced all of my ignition, rebuilt the carb, changed the power valve, forced the secondaries to open with the screw trick (Holley 4160), can't get over 50-60 mph without stumbling if at all.

It was resolved that time and recently happened again last week. clear all fuel lines and fuel filters. with compressed air blow gun with a rubber tip, blow out fuel line to the tank, remove gas cap and you will hear it blow through. Aftermarket tanks has pick up tube in lowest corner. bad gas gunk can build up there.
Next time tank comes out I'll do a complete clean out and flushing.

 

[hsach]

I am on my 3rd mech fuel pump in the last 3 years on my 78 bronco. The first 2 had the diaphragm fail, you could easily blow through the pumps when I removed them, no resistance at all. After the first failed, I changed the tank and lines because they were old/original. Cleaned the new tank before install and the new pump lasted around 6 months. Since then, I have only run it with ethanol free gas. I can't remember the brand of the first failed pump, but it was most likely a Carter reman from Summit. The next 2 were Carters from Summit. Same part number but they were physically different. Inside diameter of fittings were the same, but the outside required different sized wrenches and were slightly different angles. Not happy with the quality of remanufactured fuel pumps so I will have to do something different if this 3rd Carter fails.

 

[73azbronco]

*Bump* I'm curious if you ever resolved this. I'm having similar issues and I'm starting to think it's a failing mechanical fuel pump. I've replaced all of my ignition, rebuilt the carb, changed the power valve, forced the secondaries to open with the screw trick (Holley 4160), can't get over 50-60 mph without stumbling if at all.

step one on running diagnosis, check fuel pressure, fuel flow, it's easiest.

 

[73azbronco]

I am on my 3rd mech fuel pump in the last 3 years on my 78 bronco. The first 2 had the diaphragm fail, you could easily blow through the pumps when I removed them, no resistance at all. After the first failed, I changed the tank and lines because they were old/original. Cleaned the new tank before install and the new pump lasted around 6 months. Since then, I have only run it with ethanol free gas. I can't remember the brand of the first failed pump, but it was most likely a Carter reman from Summit. The next 2 were Carters from Summit. Same part number but they were physically different. Inside diameter of fittings were the same, but the outside required different sized wrenches and were slightly different angles. Not happy with the quality of remanufactured fuel pumps so I will have to do something different if this 3rd Carter fails.

carter electric pump, magically delicious.

 

[bigmuddy]

This reminds me of something stupid I did one time.

I moved from Colorado to Missouri and left my bronco at a buddies house, flew back out later to make the 13 hr drive home.
My wife and I look over the bronco grab the truck and head east, make it over in Nebraska and all of a sudden everytime I drove up a slight grade or a large hill it would slow down and cut out, then going down hill it ran like a top and never missed a beat. I stopped repeatedly trying to figure it out! Kept happening more and more until we just weren't making any time!
Finally pulled over and looked over every inch of the rig including the fuel line. Ended finally realizing in my haste to get it running to bring it home I had used zip ties to hold the metal fuel line to the frame. Ended up the fuel line was sliding backwards and the 4 inches of rubber fuel line was pinching off at the tank, I frankly couldn't believe it! Swapped the 4 inches out for new line, attached the metal line to the frame and drove all the way home without another hiccup! Still feel stupid, almost 30 years later!!!

 

[73azbronco]

Nah, when I was 16, on a family trip in the folks buick estate wagon, all 45 feet of it with a 454, kept driving slower and slower about half an hour after filling up, this was when the station attendant did it, also checked oil. This was epic Lucile Ball Long Long trailer stuff. We finally pulled over only doing about 5 mph. Popped the hood, dad, bless his heart was a dog looking at a clock when it came to mechanical stuff. I looked around, not fully understanding what to look for at that age, and there it was, a little bit of the red rag the guy used to check oil, sticking out of the air cleaner snout. Pulled it, did 80mph for rest of way home.