engine stumbles at 55 mph
Thread Starter
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Laughing Gas
Joined: Jan 2004
Posts: 913
Likes: 2
From: Pensacola, FL
engine stumbles at 55 mph
i am having a weird kind of problem.
if i am accelerating hard(no, not me
), when i get up to about 55, still under hard acceleration, it acts like it is running out of gas, i mean surging bad, almost stumbling over its own feet if you know what i mean, that is the best way i can describe it.
if i accelerate normally, like a sane person, it still does it, but not until i get up to a higher speed, about 65. i replaced everything in the ignition system, thinking maybe i was running out of spark (needed replacing anyway), and it starts up like a champ now, but still have the other problem.
how likely is it that i am sucking a fuel line shut somewhere and starving the motor for gas under high demand conditions? or, should i be looking somewhere else.
'73 f250, 460, c6, edelbrock 750 with manual secondaries.
i replaced the vacuum line going to the shift modulator on the tranny, and what few vacuum lines were on the motor, so i am down to the fuel thing if i cant figure anything else out.
thanks for any and all help.
jamie
if i am accelerating hard(no, not me
), when i get up to about 55, still under hard acceleration, it acts like it is running out of gas, i mean surging bad, almost stumbling over its own feet if you know what i mean, that is the best way i can describe it.if i accelerate normally, like a sane person, it still does it, but not until i get up to a higher speed, about 65. i replaced everything in the ignition system, thinking maybe i was running out of spark (needed replacing anyway), and it starts up like a champ now, but still have the other problem.
how likely is it that i am sucking a fuel line shut somewhere and starving the motor for gas under high demand conditions? or, should i be looking somewhere else.
'73 f250, 460, c6, edelbrock 750 with manual secondaries.
i replaced the vacuum line going to the shift modulator on the tranny, and what few vacuum lines were on the motor, so i am down to the fuel thing if i cant figure anything else out.
thanks for any and all help.
jamie
Posting Legend
Joined: Sep 2001
Posts: 30,255
Likes: 37
Make sure all of your rubber fuel lines have been replaced. The hoses get hard and leak or allow air to be sucked in but they don't collapse. A leaking gas line can cause a bad fire.
Otherwise check your fuel filter first then pump delivery.
Otherwise check your fuel filter first then pump delivery.
Elder User
Joined: Jan 2004
Posts: 561
Likes: 0
From: houston
I had a 79 w/460, same thing. I was convinced it was a fuel delivery problem. The stock pump just can't feed that monster at high RPM. I considered installing a high volume pump but I was too cheap. Other than that it never gave me any problems for 180,000mi. I did however run into the same type of problem on a BB Chevy. It had duel valve springs and the inner spring on one cylinder broke. Everything was fine on normal driving. However at about 4500RPM the single spring couldn't control the valve and it would start to float. You could check that out but I still think your problem is fuel starvation. Good luck.
Junior User
Joined: Apr 2002
Posts: 63
Likes: 2
From: Port Orchard
Another thought for the stumble
Howdy. It's possible your secondary jets are too small for the load your putting on them under hard acceleration.
There should be plenty of fuel in the float bowl to satisfy the demand (assuming your floats are set correctly), but under hard acceleration if there's not enough fuel making it past the jets to match the demand, the engine will become fuel starved.
Increase the size of the secondary main jets by two sizes and see if that helps. Recommend you don't change too many things at once or you'll never know what the real problem (or fix) was.
Also suggest if the engine is behaving well under normal conditions you go with the existing primary main jets. No sense creating an over rich mixture for normal loads.
Good luck.... let us know how it goes.
John
There should be plenty of fuel in the float bowl to satisfy the demand (assuming your floats are set correctly), but under hard acceleration if there's not enough fuel making it past the jets to match the demand, the engine will become fuel starved.
Increase the size of the secondary main jets by two sizes and see if that helps. Recommend you don't change too many things at once or you'll never know what the real problem (or fix) was.
Also suggest if the engine is behaving well under normal conditions you go with the existing primary main jets. No sense creating an over rich mixture for normal loads.
Good luck.... let us know how it goes.
John
Thread Starter
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Laughing Gas
Joined: Jan 2004
Posts: 913
Likes: 2
From: Pensacola, FL
well, i dont think it is the jets, because it didnt use to do this..........so, i will try new fuel lines and a new filter and if that doesnt work, troubleshoot the carb from there.
thanks for the help so far guys, i will let you know how it goes
thanks for the help so far guys, i will let you know how it goes
New User
Joined: Jan 2008
Posts: 0
Likes: 24
How old is the vacuum advance on that distributor.
I know if they corrode & seize up they will cause all sorts of problems on the top end.
Maybe test it while you're under there
I know if they corrode & seize up they will cause all sorts of problems on the top end.
Maybe test it while you're under there
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Thread Starter
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Laughing Gas
Joined: Jan 2004
Posts: 913
Likes: 2
From: Pensacola, FL
ahhh, good idea, how can i test that, i know if i suck on the hose going to the advance diaphragm(sp) the breaker plate moves, but i dont know if it is moving enough????
feel like i am getting closer.
btw, i thought i had changed all of the rubber lines, but missed one. took it for a test drive and same thing, which is dangerous on california freeways. will change the last one and if Mil1ion or somebody else checks back in with me will take it for another test drive.
thanks guys
feel like i am getting closer.
btw, i thought i had changed all of the rubber lines, but missed one. took it for a test drive and same thing, which is dangerous on california freeways. will change the last one and if Mil1ion or somebody else checks back in with me will take it for another test drive.
thanks guys
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New User
Joined: Jan 2008
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Click this link
http://www.edelbrock.com/automotive/
Go to bottom left (Black column)
click Techinical information
click : Installation Tips
scroll down until you see this
Edelbrock Performer Series Carburetors
click:the underlined word owner's manual
scroll down until you see this
CALIBRATING THE POWER MODE STAGING
then click Float adjustment
http://www.edelbrock.com/automotive/
Go to bottom left (Black column)
click Techinical information
click : Installation Tips
scroll down until you see this
Edelbrock Performer Series Carburetors
click:the underlined word owner's manual
scroll down until you see this
CALIBRATING THE POWER MODE STAGING
then click Float adjustment
Last edited by Mil1ion; Jun 4, 2005 at 10:44 PM.
Thread Starter
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Laughing Gas
Joined: Jan 2004
Posts: 913
Likes: 2
From: Pensacola, FL
yeah, actually i think i have seen you around, and there is a guy in somis that has another 73-79 too
thanks dennis for the link, i will try to do this tomorrow before i go to work
thanks dennis for the link, i will try to do this tomorrow before i go to work
Last edited by kermit; Jun 4, 2005 at 09:31 PM.
Senior User
Joined: May 2002
Posts: 215
Likes: 0
From: Catlin USA
Since it is an Edelbrock carb, I thought I would throw this in - Have you ever had it backfire through the carb? May or may not be related, but mine on my 78's 400 had a similiar problem last winter. Ended up being a sticking accelerator pump on the carb. If I didn't let it warm up for several minutes when it was cold, it would backfire through the carb. Well, eventually it started running as you describe yours to be - cutting out around 45-55 mph - had to let off the accelerator and ease it up to speed. Sucked getting onto the highway.......
Posting Legend
Joined: Sep 2001
Posts: 30,255
Likes: 37
Put a pressure gauge on the fuel line and watch it while the engine idles and while you are driving. You can also follow the directions in your service manual and test fuel pump volume. It is kind of a mess, but if you install a T in the line you can use it for the pressure test also.