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stumbling f100 under acceleration

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F100-1977
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stumbling f100 under acceleration

Post #1 by F100-1977 » Sun Mar 24, 2013 9:19 pm

Hi all!

Okay, my truck is a 1977 f100 with a manual 3 speed on the tree. I keep it stock, my goal is to get optimal mpg and that truck is quite strong enough to me. My problem is:
When I try to accelerate, it almost dies, then pick up speed pretty well. It idles fine, runs fine, and on neutral, accelerates fine. It's only on the road, when I put into any gear. the worst is after engine braking, if I depress the gas pedal suddenly, it stumbles real hard and makes a loud clicking or detonating noise.
It does it since I did some repair - replaced both exhaust and intake manifold with secondhand, kitted and cleaned the carb (carter YF)
the only adjustment that may have changed is the float level because by accident I bended it and had to reset it the best I could (by the rebuild instruction book). The acceleration pump seems to work fine, I can see gas pouring into the carb when I move the throttle.
So, what's the next guess?

Thank you!

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First Fox
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Re: stumbling f100 under acceleration

Post #2 by First Fox » Tue Mar 26, 2013 2:52 pm

Was there an EGR system on the original intake system? Is there one on the second hand parts you installed?

The stumbling and detonation in your case is likely a lean condition and an EGR could cause it. The fuel level in the carb is also important and can cause a lean condition if it is not set correctly. It could also be cause by a vacuum leak from a warped or improperly installed intake manifold. If you say it idles fine though, an intake leak is not likely.

Another area to look for is the distributor vacuum advance. Did you reconnect it to a ported source, or to the intake? Do you remember where it was connected before?

Also, even though you see gasoline discharged when you move the throttle, does it seem to be a proper amount of pump shot? It should be a long steady stream. I am not familiar with that carb, but the accelerator pump linkage is usually adjustable somehow and needs to come in early enough to do its job properly. Perhaps someone that is familiar with these carbs can chime in and tell you a procedure to set it.
My garage:

1962 Falcon. 170/Auto.
1965 Fairlane. Twin turbocharged 289/4 speed.
1965 Thunderbird. 390/Auto.
1980 Fairmont. Turbocharged 200, 260 Comp cam/T5 and '93 Mustang steering/suspension.
1981 F-100. 300/4 speed OD. I use this primarily to haul my cars home after I modify them and they break.
1987 Thunderbird Turbocoupe. 2.3/T5. Porche designed 16 valve, twin cam cylinder head.


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F100-1977
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Re: stumbling f100 under acceleration

Post #3 by F100-1977 » Wed Mar 27, 2013 8:39 am

Hmmm, Thank you Fox

If the EGR is the round thing attached to the vacuum lines, well I did reinstall the one that was on the truck. As for the float level, I tried a few different setting, and it change nothing except that I must change the adjustment on the mixture screw every time. Right now it is as close I think to the factory setting, and the mix screw is out 1 3/4 turn, which seems fine. The distributor is connected (new line) to a port on the carb with 'dist' cast on it. The other port 'EGR' goes to a check in the air bowl, then to those round thing connected at the base of the carb, there are two of it. I have two lines from the manifold, a big one to the valve cover, and another to somewhere at the air bowl intake. The accelerator pump was also my guess so I double checked it, and even bended the link from the throttle shaft to the accelerator arm so it delivers sooner, with no change. BUT, I don't know how good should be the stream, how early, and how to adjust it anyway. My link bending was a guess...
I must say, I made these repair at first because the truck was backfiring badly as soon as I left the gas pedal on the road, and I didn't knew if the cause was the cracked manifolds, or the carb itself so I changed both. Maybe I should have go one at a time...

I have an appointment with a carb guy around here but only next week, and I try to find something on my own, so I learn how it works.
if one question remains it would be about the accelerator pump, how to know if I have a good quality stream?
And another question on the back of my mind, can it be the timing?

BIG 6 farmer
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Re: stumbling f100 under acceleration

Post #4 by BIG 6 farmer » Wed Mar 27, 2013 8:32 pm

Maybe the Dura spark Ign. module ? Use to change a lot of those in the Shop. Can cause Drive ability problems. I wont use one anymore.
83 F 150 SB 4x4 300 six NP 4speed - - 1950 IHC L162 (1&1/2 ton?) - 87 & 88 T-Bird Turbo Coupes - 2000 Triumph Tiger , 76 Honda GL 1000 , & other toys and parts (& junk) -

F100-1977
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Re: stumbling f100 under acceleration

Post #5 by F100-1977 » Wed Mar 27, 2013 9:20 pm

Hi farmer,

the ignition is the next thing in order, for now I run on brand new spark plugs (the cheapest, champion I think...). I want to make sure my trouble originates from a repair I've done wrong before I mess something else... I sure will want to time the ignition, I already bought a vacuum gauge so I must learn to use it now. I don't know what brand is the ignition module. If like everything else, it must be the original made by ford in 1977. When I bought this truck four years ago I think even the air filter was original...

One question just hit my mind: Does anyone knows the exact quantity of gasoline the accelerator pump should deliver in one full stroke? I could easily pick it up and measure.

Thank you!

Frank

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Re: stumbling f100 under acceleration

Post #6 by Harte3 » Wed Mar 27, 2013 9:46 pm

"the ignition is the next thing in order..." JMO, that should be the first order of business. Carbs get 95% of the blame when they may only be 5% of the problem. Trying to cancel other system problems with a carb adjustment usually just doesn't work.
'83 F150 300, 0.030 over, Offy DP, Holley 4160/1848-1 465 cfm, Comp Cam 260H. P/P head, EFI exhaust manifolds, Walker Y Pipe, Super Cat, Turbo muffler, Recurved DSII, Mallory HyFire 6a, ACCEL Super Stock Coil, Taylor 8mm Wires, EFI plugs.

BIG 6 farmer
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Location: On a dirt road near Washington Nebr.

Re: stumbling f100 under acceleration

Post #7 by BIG 6 farmer » Wed Mar 27, 2013 10:44 pm

Go on www. gofastforless. Tells ya how to wire up a GM 4 pin Ign. Module. Its easy, inexpensive. Your Truck will start & run better, maybe use less fuel. You have to have your Ignition system & timing right before you go any further. Champion Spark Plugs should work fine. Dont worry about how much of an Accel pump shot you have , as long as you have one. :nod:
83 F 150 SB 4x4 300 six NP 4speed - - 1950 IHC L162 (1&1/2 ton?) - 87 & 88 T-Bird Turbo Coupes - 2000 Triumph Tiger , 76 Honda GL 1000 , & other toys and parts (& junk) -

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First Fox
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Re: stumbling f100 under acceleration

Post #8 by First Fox » Thu Mar 28, 2013 1:28 pm

Maybe I misread your original post, but I was under the impression that this problem began after you swapped the intake, exhaust manifold, and had the carburetor apart?

If that is indeed what happened, then it is REALLY unlikely to be ignition related.

As far as your accelerator pump shot, the amount IS important indeed. If the check ball and weight were left out when the carb was apart, you would see a small pump shot but most of the fuel would be returned to the float bowl and this could cause the stumble you are describing.

Being as you had things apart and the problem started after that, it would probably be best to look there for a boo boo before tearing into something unrelated.
My garage:

1962 Falcon. 170/Auto.
1965 Fairlane. Twin turbocharged 289/4 speed.
1965 Thunderbird. 390/Auto.
1980 Fairmont. Turbocharged 200, 260 Comp cam/T5 and '93 Mustang steering/suspension.
1981 F-100. 300/4 speed OD. I use this primarily to haul my cars home after I modify them and they break.
1987 Thunderbird Turbocoupe. 2.3/T5. Porche designed 16 valve, twin cam cylinder head.


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THE FRENCHTOWN FLYER
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Re: stumbling f100 under acceleration

Post #9 by THE FRENCHTOWN FLYER » Thu Mar 28, 2013 1:50 pm

First Fox wrote:...the amount IS important indeed. If the check ball and weight were left out when the carb was apart, you would see a small pump shot but most of the fuel would be returned to the float bowl and this could cause the stumble you are describing.




I wrote a reply suggesting that too, but somehow it got lost in the interweb ether. Also, check the three long screws that hold the carb main body to the throttle base plate. I've had them come loose and cause lean spots in the fuel curve.

I don't know the spec on the delivery volume, but Holley uses two sizes of pumps on their 2V carbs - 30cc or 50cc. That figure is for ten complete accellerator diaphragm strokes, or 3 and 5 cc per stroke respectively. So ballpark if you have 10 - 20 ccs for ten strokes you should be close.
FORD 300 INLINE SIX - THE BEST KEPT SECRET IN DRAG RACING

F100-1977
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Re: stumbling f100 under acceleration

Post #10 by F100-1977 » Wed Sep 04, 2013 8:47 pm

Hi again all!

Ok I know its an old topic, but it's mine!

I partly solved my problem this spring when I bought a bicycle (honda cm400t 1981) to drive at my job but now automn is coming and I'm still scratching my head about the truck. However I still use my truck so here are some additions to the 'facts' I gathered. The stumble problem is ONLY when I depress the gas pedal from Idle, so this means to me that the problem is not the accelerator pump. AND, as other remarked (first fox) the problem is not the ignition because I did no change there, it must be somewhere in the carb itself. The problem is a lean condition when I move the throttle from idle, first there is a flat spot, where the engine almost dies, then there's this clicking sound when I'm giving it a lot of gas or when the engine is loaded. It idles real fine and if I richen the idle mix the engine runs even worse.

I put it apart many times to try to blow any passage that may have blocked and it changes nothing.

any hints?

Thank you again!

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Re: stumbling f100 under acceleration

Post #11 by blmhawkeye » Thu Sep 05, 2013 11:07 am

im in the same boat as you are with ur flat spot. what we determined is that my carb wasnt giving its accelerator shot soon enough to enrichen the mixture quick enough. We also beleive I have a massive vacuum leak somewhere as well. Does it make any kind of odd noise at the venturi when you take it off idle when you have it in the driveway with the hood up?

F100-1977
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Re: stumbling f100 under acceleration

Post #12 by F100-1977 » Thu Sep 05, 2013 7:53 pm

Hi hawkeye,

I don't hear any vacuum noise, I changed all my vacuum hoses already anyway. As for the accelerator pump, how could it disadjust that way? I mean I didn't have that flat spot before I try to 'repair' my carb. And I still would have the clicking noise when I hold the gas steady and the truck is needing power...

First fox talked about my EGR, how to see if it works?

I'm really beginning to think 'new carb', I then would either have solved the problem, or know what is NOT the problem...

thank you all!

blmhawkeye
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Re: stumbling f100 under acceleration

Post #13 by blmhawkeye » Fri Sep 06, 2013 1:57 am

I wont be much help with the egr system as I dont have one on my pickup.

the noise we got was when the air cleaner was off and we would move the linkage to rev it up it made a noise like a flap was fluttering over the carburetor. The other thing is that my carb leaks so much its gotten gunked up so the accelerator pump will only dribble in fuel where it drips into the carb. a new carb might do ya good, just keep the old one since it still will run with it.

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