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I am at a bit of a loss and hoping the more specific inline 6 forum can help.
First off, here is some basic information on my truck...
1978 F150
300 I6
Offy Dual Plane Intake
4bbl Holley 390 cfm w/ Vacuum Secondaries (Rebuilt)
New Plugs, Cap, and Rotor (Wires looked okay visually)
Manual NP435 w/ New Clutch
No vacuum leaks that I can find with the spray method
Motor vacuum at a steady 17 inches on my gauge
Here are my two major issues:
1) When the truck is warm, sitting at idle, and I quickly snap the throttle it falls on its face. If I hold the throttle open after it stumbles it will either cough a little and catch up with itself, or through a backfire out through the carb. If I apply the throttle slowly it revs up just fine and runs great. I have adjusted the mixture screws to the highest vacuum already (0.5 turns out was the best vacuum reading).
2) When driving the truck, it accelerates smoothly in 2nd gear (I start in second), accelerates through 3rd well, but as soon as I shift into 4th and give it some gas it stumbles and shudders. I can let off the throttle and apply it very slowly and this helps some, but it will still stumble a little. If I go to WOT in either 3rd or 4th it stumbles hard. My thought was it may not be getting enough gas? The nozzle in the Holley is only a #25... I'm not sure about the jetting.
Some more notes:
I have gone through the carb pretty extensively as far as tuning goes (accelerator pump lever, mixture screws, idle screw, and float levels), but have not made any jet changes or nozzle changes since I rebuilt it this month. This truck is new to me and has always stumbled like this, however it was much worse before the carb rebuild and tuning attempts.
Another thing to note, I was checking my timing advance with the timing light while the vacuum advance was plugged in and noticed that it takes a moment for the advance to kick in. While the truck was idling I would snap the throttle while looking at my timing mark. It would stumble as usual, and the timing mark wouldn't move for about a half second, then it would advance at the same time the motor evened back out. If I applied the throttle slowly the timing mark would advance normally and the truck would rev great. Is there something that can cause my vacuum advance to be delayed?
I hope this is enough information, I know there may not be a perfect answer, but any ideas would be greatly appreciated! If you want to see my build progress, my project thread is titled: 1978 F-150 Ranger Project - Dapper Dan. Thanks!
Distributor troubles can definitely cause a stumble, sounds a little like the breaker plate is binding. However it should still accelerate fine and perform well without stumble, even with the vacuum advance disconnected and plugged at the carb port. First make sure that the mechanical advance itself inside the distributor is operating smoothly with vacuum advance disconnected and plugged. Put it through the paces in neutral all the way up to a few thousand RPM with the timing light, ignition advance should respond quickly to a high level and the timing marks on the damper should return to base timing when throttle is let off. Look for "scatter" or erratic movements of the marks, they should stay focused and sharp.
Look for "scatter" or erratic movements of the marks, they should stay focused and sharp.
What is the cause of this scatter? I have noticed that the mark will disappear sometimes from the timing light ( like the plug didn't fire or something), or flash 15-20 degrees off of the base timing mark. Could this just be my timing light? The truck seems to idle fine when I am observing these characteristics.
Hm, not sure on that, it could be the timing light itself I guess. If a plug is misfiring, it should cause uneven idle or shudder. Scatter could be a worn or stretched timing chain (does a 6 have a chain?) or real bad wear in the distributor.
I am at a bit of a loss and hoping the more specific inline 6 forum can help.
1) When the truck is warm, sitting at idle, and I quickly snap the throttle it falls on its face. If I hold the throttle open after it stumbles it will either cough a little and catch up with itself, or through a backfire out through the carb. If I apply the throttle slowly it revs up just fine and runs great. I have adjusted the mixture screws to the highest vacuum already (0.5 turns out was the best vacuum reading).
2) When driving the truck, it accelerates smoothly in 2nd gear (I start in second), accelerates through 3rd well, but as soon as I shift into 4th and give it some gas it stumbles and shudders. I can let off the throttle and apply it very slowly and this helps some, but it will still stumble a little. If I go to WOT in either 3rd or 4th it stumbles hard. My thought was it may not be getting enough gas? The nozzle in the Holley is only a #25... I'm not sure about the jetting.
This is a somewhat common problem with some 390s. You are on the right track with the pump squirter. Go real big - like a 38 or 41. It worked for us. No other changes should be needed since you have done due diligence on setting the rest to spec.
I have not looked into what makes the 390 do this. Maybe somebody else has?
I'm not sure on this one either, but had similar issues. It needed a BIG pump shot.
Not to dissuade you, but I finally gave up on the 390cfm after fighting with it for about 2 years and threw on a 600cfm and it was like someone yanked a thorn out of my side. Gas mileage improved by about 150% too. Granted, I've read lots of success stories with the 390 as well, so I might have just had a bad one (happens).
A 25 pump squirter isn't big enough for any Holley carb I've ever used. I have had a 390cfm, 465cfm, 600cfm as well as a 450cfm QuickFuel and the 25 was notoriously small. I've usually had to go with a 35 to 40 in size.
I've never liked the 390 cfm 4V for the 300. No matter what anyone tells me, I feel it's too small for the 300. I ran one on a 2.3L SOHC engine I built for a Mustang I use to own.
Thanks for all the feedback guys, it’s a huge help! You all really know your stuff. Last night I was able to figure out part of my problem. I was working in the dark trying to set my timing, when I noticed a little sparkle under the hood. When I saw this I turned all my lights off and found that the terminals on my coil were arcing to the spark plug wire that leads to the distributor. Along with that, several of the wires were arcing to each other. It’s a miracle I didn’t shock myself.
After replacing the coil and wires my hesitation when accelerating went away. It’s nice to have the truck running smoothly.
However, it still falls on its face when I snap the throttle wide open. Like everyone said, a larger nozzle is probably needed. Part of me is thinking of buying a new Holley Street Warrior and dropping that on instead. My Holley 390 has a manual choke that doesn’t operate correctly, the lever mechanism is very hard to move, causing the choke cable to be ineffective. I have to move it by hand under the hood. The Street Warrior is a 600 cfm and has an electric choke. Anyone ever run one of these? It seems affordable at my local parts store... $320.
Unrelated, and maybe a mute point, but I noticed you have an aftermarket coil. Please make sure you read the installation instructions for that coil carefully (if it came with any?). I was reading a thread recently on the older truck forums... can't remember if it was the bumpsides or dentsides... anyway, the owner had an engine fire at a stop light and it was determined to be caused by an aftermarket coil. The coil was mounted horizontally (like yours), but the manufacturer specifically warned that it must be mounted vertically. Mounting horizontally caused it to overheat, the fluid inside to expand and blow the "cap" off, and then set fire to his engine bay. He did not have a 300 though (some V8, can't recall).
As I said, might be specific to his engine, year, etc etc. I had never known this could happen, and may not apply to "us" in any way, but you don't forget something like that after seeing it! Maybe some of the electrical gurus here can put my mind at ease
12-12-2018, 02:00 PM
