Performance under load issue
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Cross-Country
Joined: Sep 2014
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Performance under load issue
Hello all! I've got a new issue with my truck and was hoping for some help. It's a 75 F-100 shortbed with a 351M motor. Edlebrock intake and 4 barrel carb. Its the 750 cfm 4217 model. Also installed is a Holley fuel pressure regulator. This truck is weekend cruiser. Yesterday while driving, I got into it some and all of a sudden it basically fell on its face running rough. Ran perfectly prior. I was able to get it home, but any acceleration other than idle and it ran terrible. Almost floored it just to get to highway speed. I swapped out the inline fuel filter hoping for an easy fix but that's not it. I also pulled each of the vacuum lines to the carb and checked leaks by looking at it and by mouth. The carb/lines etc are all about 1.5 years old, but it doesn't get driven but on weekends. Any thoughts and help on this would be greatly appreciated!
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Joined: Jul 2013
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From: Isla de Muerta
Are you using Ethanol gas...if so, how old is the gas?
Disconnect the fuel feed line at the carb and get a sample of the gas, use a mason jar.
Check the gas after it's had the chance to sit for 5 min or so...look for ethanol/water
phase separation at the bottom of the jar.
Untreated E10 gas doesn't live much past 60 days before it goes bad.
Disconnect the fuel feed line at the carb and get a sample of the gas, use a mason jar.
Check the gas after it's had the chance to sit for 5 min or so...look for ethanol/water
phase separation at the bottom of the jar.
Untreated E10 gas doesn't live much past 60 days before it goes bad.
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Cross-Country
Joined: Sep 2014
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I don't think I've got any ethanol gas in it, I try to avoid it like the plague. However I couldn't be for certain. It ran really well for over an hour and then BAM, running like junk. I don't have a fuel pressure gauge on it, I checked it when I installed the regulator but not since. The distributor should be tightened down. I haven't adjusted it recently, but that's definitely something worth trying. I didn't notice any glaring issues with spark plug wires.
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Joined: Oct 2004
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From: Richmond
Just gonna chime in because I just went thru this over the weekend. I'm just getting my dad's truck roadworthy. I had replaced the fuel tank, fuel lines, fuel pump and rebuilt the carburetor. One thing I hadn't checked was the Wix, screw-on filter on the front of the carburetor. It had a lot of junk in it, from old ethanol. If you have one, check it. It was the last piece of the puzzle for me and I was good to go. I had the same problem you did. Several test runs with no issue, then BAM. Good luck.
Postmaster
Joined: Mar 2011
Posts: 2,698
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From: Taxachusetts
Another thing to check:
Yank the distributor cap off.....try to move the breaker plate by hand. Does it move freely?
Then, apply suction to the vacuum can's nipple....observe the vacuum can's arm that moves the breaker plate. Does it move freely?
Vacuum advance is load related so in your case (as you stated in the title) this could be an issue.
Yank the distributor cap off.....try to move the breaker plate by hand. Does it move freely?
Then, apply suction to the vacuum can's nipple....observe the vacuum can's arm that moves the breaker plate. Does it move freely?
Vacuum advance is load related so in your case (as you stated in the title) this could be an issue.
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Cross-Country
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Thanks Filthy Beast, I will definitely check this as well. I intend on jumping into it this afternoon after work
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Fleet Mechanic
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From: Nevada
Yep
There seems to be a perfect dynamic needed between the dizzy, carb and your timing.... I have played with mine for the past year and it can be frustrating 
If you have an old dizzy laying around, throw it in, start fresh with timing and see how the truck acts, can help narrow down your checklist.... I also ran into a bad refurb dizzy that drove me nuts... When in doubt you can never go wrong with a fresh dizzy, timing tuned from true TDC and a fresh tune on the carb..... or backing out of your current carb tune and starting fresh.... Also a vacuum pressure test is easy and cheap.....
If you have an old dizzy laying around, throw it in, start fresh with timing and see how the truck acts, can help narrow down your checklist.... I also ran into a bad refurb dizzy that drove me nuts... When in doubt you can never go wrong with a fresh dizzy, timing tuned from true TDC and a fresh tune on the carb..... or backing out of your current carb tune and starting fresh.... Also a vacuum pressure test is easy and cheap.....
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Joined: Nov 2003
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From: Southern Oregon
Will I think everyone beat me to what I had to say. But I did notice that your running a 750 carb. Is this a really high performance engine? If not, I would stick to 600 cfm carb and I think the motor would like it a whole bunch more.
What carb you running?
What carb you running?
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Cross-Country
Joined: Sep 2014
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Well I swapped out all the fuel lines the carb, to the fuel pump, and to the steel line that goes to the tank, I also swapped out all the vacuum lines on top of the engine as well. No difference. I opened the distributor cap and was able to see the advance thing move with suction on the line, although it was only by mouth so I don't know how much it was supposed to move. Could something be wrong with the sending unit? I think I've read where there is a small filter on the bottom, could that be it. It idles completely fine and revs fine when in park. It only falls on it's face when in drive and accelerated.
The carb is a Edelbrock 750, I've had good luck with it so far. I do have a fuel pressure regulator mounted to help keep the pressure down from the stock fuel pump
The carb is a Edelbrock 750, I've had good luck with it so far. I do have a fuel pressure regulator mounted to help keep the pressure down from the stock fuel pump
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Cross-Country
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Thought I'd post an update on the issues I was having. I appreciate all who chimed in with ideas/solutions! I replaced all the fuel and vacuum lines, but that didn't fix it. I then replaced the vacuum advance on the distributor, but no fix there either. Then I replaced the entire distributor with cap and rotor, but still no change. After that I changed out the fuel pump, but nothing changed. After that I removed the carb, and replaced the accelerator pump and BOOM that fixed it right up! That $12.99 part cost me about $150.00 in other parts but it runs like a champ now. Thought it would be good to post my findings to maybe help the next fella down the road.
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