low rpm hesitation and bucking
#1
09-22-2015, 01:32 AM
low rpm hesitation and bucking
I have been working to get my new to me F150 300 running well and am having issues. I know these engines are supposed to have good torque at 1500 rpm but what about off idle to just below 1500 rpm?
Here is what I have 84 F150 manual 4x4 with carb
78 yf carb and dist (w/ vac advance)
just rebuilt carb (does not have tag on it so best I can do is go by markings 6-2228)
hei ignition
efi exhaust manifolds
no cats or emissions equipment
computer has been removed
timing is set to 18hg
idle smooth and running between 650 and 700
Starts easily, idles well and revs smooth in neutral. Pulls hard above 1500rpm and sounds good... then when shifting to 2nd 3rd and 4th bucks if it is not above 1400 rpm. Have tried more and less timing, idle mixture is at 1.5 turns out and I have also tried changing that. Final adjustment is the metering rod and I used the recommended just touching and another full round down to set that.
Any help or direction is much appreciated.
Here is what I have 84 F150 manual 4x4 with carb
78 yf carb and dist (w/ vac advance)
just rebuilt carb (does not have tag on it so best I can do is go by markings 6-2228)
hei ignition
efi exhaust manifolds
no cats or emissions equipment
computer has been removed
timing is set to 18hg
idle smooth and running between 650 and 700
Starts easily, idles well and revs smooth in neutral. Pulls hard above 1500rpm and sounds good... then when shifting to 2nd 3rd and 4th bucks if it is not above 1400 rpm. Have tried more and less timing, idle mixture is at 1.5 turns out and I have also tried changing that. Final adjustment is the metering rod and I used the recommended just touching and another full round down to set that.
Any help or direction is much appreciated.
#2
09-22-2015, 09:06 AM
Lead Driver
#3
09-22-2015, 10:53 AM
#5
09-22-2015, 10:56 AM
Lead Driver
#6
09-22-2015, 11:05 AM
Lead Driver
That is a small leak. Many plug the inlet where the metal tube from the exhaust manifold was mounted to the choke housing, but in theory, since the carburetor was calibrated with this vacuum leak in mind, the mixture will be a little richer when the opening is plugged. I do not think this is your problem, but an easy fix is to convert to a manual choke and take a piece of 1/4" vacuum tubing and connect the opening on the choke housing with the port at the top of the carburetor so clean air can be drawn into the choke housing.
#7
09-22-2015, 11:08 AM
Lead Driver
Most feel that the addition of heat to the bottom of the manifold is best, but The Frenchtown Flyer [former/retired Ford Engine Development Engineer] does not think it is necessary.
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#9
09-22-2015, 12:16 PM
Planning to add heat, just have not gotten to that point yet. Is the idea that this could be the cause of my issue?
#10
09-22-2015, 01:04 PM
Lead Driver
Without adequate heat at the bottom of the intake manifold, at low RPMs, when the actual air flow in the manifold is low, fuel droplets condense on the relatively cool metal and puddle. Liquid fuel does not burn... it is the vapor. At higher RPMs, the intake velocity increases, causing turbulence and better fuel atomization.
This is more pronounced when the temperatures are cooler, but even in the Summer, when first starting, one can have a problem. After running for awhile and under hood temperatures rise, the intake manifold will be heated by the heat radiating from the exhaust manifolds, since they are located underneath.
When you checked the ported vacuum source at the carburetor, did it measure 0" @ idle and then rise as the throttle plate was opened? The reason I ask is that my Carter YF came from a 1970 F350. At idle speed [~700 RPMs], the vacuum is 5" Hg and never gets any higher than ~ 11" Hg. I believe this is called venturi vacuum. This was used due to having a points based distributor, so I have been told. There was no other vacuum port and the ignition would not advance properly when under high vacuum conditions [cruise], so I had to go with manifold vacuum.
This is more pronounced when the temperatures are cooler, but even in the Summer, when first starting, one can have a problem. After running for awhile and under hood temperatures rise, the intake manifold will be heated by the heat radiating from the exhaust manifolds, since they are located underneath.
When you checked the ported vacuum source at the carburetor, did it measure 0" @ idle and then rise as the throttle plate was opened? The reason I ask is that my Carter YF came from a 1970 F350. At idle speed [~700 RPMs], the vacuum is 5" Hg and never gets any higher than ~ 11" Hg. I believe this is called venturi vacuum. This was used due to having a points based distributor, so I have been told. There was no other vacuum port and the ignition would not advance properly when under high vacuum conditions [cruise], so I had to go with manifold vacuum.
#11
09-22-2015, 01:05 PM
Fleet Mechanic
I would be surprised if it was, during the summer months. An easier fix might be to try a heat stove to duct hot exhaust manifold heated air into the intake, but I doubt it.
If you get to the end of your rope with no solution try a new camshaft. Yours may have base circle runout on one or more of the lobes. RARE
If you get to the end of your rope with no solution try a new camshaft. Yours may have base circle runout on one or more of the lobes. RARE
#12
09-22-2015, 01:23 PM
When you checked the ported vacuum source at the carburetor, did it measure 0" @ idle and then rise as the throttle plate was opened? The reason I ask is that my Carter YF came from a 1970 F350. At idle speed [~700 RPMs], the vacuum is 5" Hg and never gets any higher than ~ 11" Hg. I believe this is called venturi vacuum. This was used due to having a points based distributor, so I have been told. There was no other vacuum port and the ignition would not advance properly when under high vacuum conditions [cruise], so I had to go with manifold vacuum.
Also, thanks for the info Frenchtown Flyer. Had not considered the cam and I just replace the Timing Gears about a month ago...
#13
09-22-2015, 04:07 PM
Went back through all connections, the intake and all gaskets with starter fluid no leaks, so I thought I would try tuning again and followed these instructions:
Did about 3 rounds using this procedure and it seems to be running better than it has since I have owned it. I will Drive it a few days, check the plugs and go from there.
I am surprised that in 4th (granny low tranny) at 1000-1200 it doesn't pull harder. I do a good bit of 45mph driving and in fourth she is at @1100 rpm but in 3rd she is at 2k so I choose 4th. The good news is she's no longer bucking, it just seems she needs something right in that spot.
Thanks to all for helping and reading, what an amazing resource!
I'm assuming the stock 1bbl?
One of the best tuning tools you can get is a vacuum gauge. They're not super expensive and they can tell you dozens of things about your engine that you just can't tell without one. I can't stress their value enough, even while driving.
Hook the vacuum gauge up to DIRECT manifold vacuum. If you have an unused port on the intake itself, that's the best spot, but a port below the throttle plates on the carb works too.
At idle, you should have about 18 - 21hg of vacuum with a good, healthy engine.
From there, while it's idling and all warmed up, turn your idle mixture screw outwards until the vacuum needle doesn't increase anymore. Then, slooowly turn it in until you get the maximum vacuum reading possible. Normally, as you turn it in, the vacuum will slowly climb, and then suddenly drop dramatically. Once it does that, back it slowly out and find that sweet spot. That's your best idle.
Next, set the timing on the distributor with your vacuum gauge. Loosen the distributor and advance it until the vacuum stops increasing (it'll start to run rather rough), then retard the timing slowly until the vacuum drops to about 1 1/2hg below the maximum reading you were able to get.
Next, set your idle RPMs to what they should be (about 700).
Repeat all the steps again to see if any changes were necessary. Continue until no more adjustments can be made. That'll set your engine really nice.
Now, this is taking into consideration that there aren't any other problems with your engine. But, the biggest one I'd assume would be vacuum leaks. Get one or two cans of the cheapest carb cleaner you can find. Then, with the vacuum gauge in hand, spray around all of your carb/intake/head connections. Areas to pay extra attention to are where the carb bolts to the intake, all six runners of where the intake bolts to the head, and the throttle shafts on the carb. Also, spray around your vacuum trees and lines. If, with any of these, you see the vacuum needle waver and stumble, you've found a vacuum leak. The leak will need to be addressed before you can really do anything else.
Hope that gets you started in the right direction.
One of the best tuning tools you can get is a vacuum gauge. They're not super expensive and they can tell you dozens of things about your engine that you just can't tell without one. I can't stress their value enough, even while driving.
Hook the vacuum gauge up to DIRECT manifold vacuum. If you have an unused port on the intake itself, that's the best spot, but a port below the throttle plates on the carb works too.
At idle, you should have about 18 - 21hg of vacuum with a good, healthy engine.
From there, while it's idling and all warmed up, turn your idle mixture screw outwards until the vacuum needle doesn't increase anymore. Then, slooowly turn it in until you get the maximum vacuum reading possible. Normally, as you turn it in, the vacuum will slowly climb, and then suddenly drop dramatically. Once it does that, back it slowly out and find that sweet spot. That's your best idle.
Next, set the timing on the distributor with your vacuum gauge. Loosen the distributor and advance it until the vacuum stops increasing (it'll start to run rather rough), then retard the timing slowly until the vacuum drops to about 1 1/2hg below the maximum reading you were able to get.
Next, set your idle RPMs to what they should be (about 700).
Repeat all the steps again to see if any changes were necessary. Continue until no more adjustments can be made. That'll set your engine really nice.
Now, this is taking into consideration that there aren't any other problems with your engine. But, the biggest one I'd assume would be vacuum leaks. Get one or two cans of the cheapest carb cleaner you can find. Then, with the vacuum gauge in hand, spray around all of your carb/intake/head connections. Areas to pay extra attention to are where the carb bolts to the intake, all six runners of where the intake bolts to the head, and the throttle shafts on the carb. Also, spray around your vacuum trees and lines. If, with any of these, you see the vacuum needle waver and stumble, you've found a vacuum leak. The leak will need to be addressed before you can really do anything else.
Hope that gets you started in the right direction.
I am surprised that in 4th (granny low tranny) at 1000-1200 it doesn't pull harder. I do a good bit of 45mph driving and in fourth she is at @1100 rpm but in 3rd she is at 2k so I choose 4th. The good news is she's no longer bucking, it just seems she needs something right in that spot.
Thanks to all for helping and reading, what an amazing resource!
#14
09-24-2015, 02:04 PM
Carter metering rod clarification
So, I was reading on another thread about adjusting the metering rod, it said to hold down the accelerator pump and screw down the screw until the metering rod bottoms out and then down another turn. Is this correct?
I had gone by a video I believed was telling me to hold down the accelerator pump and adjust the screw until it barely touches the plate below it (not sure actual name of what is below the screw) and then down another turn.
Can anyone verify the correct setup of the metering rod?
I had gone by a video I believed was telling me to hold down the accelerator pump and adjust the screw until it barely touches the plate below it (not sure actual name of what is below the screw) and then down another turn.
Can anyone verify the correct setup of the metering rod?
#15
09-24-2015, 05:47 PM
Tach was not reading correctly, all settings wrong
Found that the tach was setup for 8 cylinder not 6 and everything was set incorrectly. Idle was set extremely high...
After resetting the idle (this tach and my tester tach both are inaccurate going as low as 700), timing and idle mixture things seem to be more responsive. Not sure that everything is perfect, but it is better. When using the vac meter to do adjustments it is not very stable, bounces around about +-2" so I am wondering if something is still not exactly correct. But I will keep working on it.
After resetting the idle (this tach and my tester tach both are inaccurate going as low as 700), timing and idle mixture things seem to be more responsive. Not sure that everything is perfect, but it is better. When using the vac meter to do adjustments it is not very stable, bounces around about +-2" so I am wondering if something is still not exactly correct. But I will keep working on it.