Surging at idle

1

1969mach1351

Member

Apr 18, 2003

285

0

16

Sacramento, CA

Apr 1, 2005

#1

• Apr 1, 2005

• #1

My engine has almost always had a little surge in the idle since I put in this engine. I have changed carb gaskets and just recently changed the manifold and it still does it. Could there be a vacuum leak in the carb? It’s a 750 Demon that is about 4 years old. It runs perfect when the engine is really cold and really hot. Also if the air temp is above 80* there is no surge. I’ve plugged all other vacuum lines and it still does it. Do I just need to rebuild the carb?

 

69Rcode_Mach1

Active Member

Apr 20, 2004

1,473

1

37

Salt Lake City, Utah

Apr 1, 2005

#2

• Apr 1, 2005

• #2

Buy a vacuum guage they are only $20 and will help you determine the problem. If it's not a vacuum leak then atleast we know that's not the problem.

 

krash kendall

Active Member

Nov 19, 2004

1,258

0

36

Aldergrove, B.C. Canada

Apr 1, 2005

#3

• Apr 1, 2005

• #3

I have the exact problem at similar conditions. When idling at at stoplight in gear it runs fine with an occasional momentary burp as if one cylinder fired with a little more power every3-5 seconds. I've vacuum checked it and ignition checked it and just can't find a solution. It isn't causing any problems what-so-ever, I just expect perfection.

 

Pbum5

Member

Oct 27, 2004

515

0

17

Minneapolis, MN

Apr 1, 2005

#4

• Apr 1, 2005

• #4

If it is a vacuum leak
Try spraying some carb cleaner around the carb, intake, etc while it’s running and it might help you find the vacuum leak.

 

F

ForceFed70

That's why they call it "dope"

Founding Member

Dec 6, 1999

4,818

1

69

BC Canada

Apr 1, 2005

#5

• Apr 1, 2005

• #5

I've seen such a problem on a vehicle where the power valve was opening due to a drop in vacum. Once the additional fuel was dumped in... the idle and vacum picked up for a second and so the power valve closed. Only to have the idle and vacum drop and cause the power valve to open again.

Could be your power valve, could be a vacum leak.. could also be the wrong power valve to run with your cam.

 

1

1969mach1351

Member

Apr 18, 2003

285

0

16

Sacramento, CA

Apr 1, 2005

#6

• Apr 1, 2005

• #6

My vacuum is about 14 and steady, I don't think it was surging tho last I checked it. My next test was going to be spray carb cleaner around, when i buy some more.

 

R

Rapid

New Member

Mar 31, 2005

297

1

0

Columbia, SC

Apr 2, 2005

#7

• Apr 2, 2005

• #7

Do you have the right fuel pressure to the carb?

 

84convertablegt

New Member

Nov 6, 2004

589

0

0

Columbus, OH

Apr 2, 2005

#8

• Apr 2, 2005

• #8

heres a weird story. i had my engine recently rebuilt. they installed a ford factory 4v spacer with the pcv hookup. well i had a crazy surge going on. first thing i did was hook up a vacuum gauge, and it read a steady 15 lbs, no surge. i was very confused. then by just feeling around i managed to stick my pinky finger under the spacer and barely got it to fit down into the intake where the carb mates up. i couldnt believe it. i bought a jegs phenolic carb spacer and a new gasket and the problem was solved. the moral of the story, the vacuum gauge didnt tell me squat! weird isnt it!

 

2nd Mustang

Founding Member

Feb 24, 2002

2,488

0

46

Southern California

Apr 2, 2005

#9

• Apr 2, 2005

• #9

84convertablegt said:

the moral of the story, the vacuum gauge didnt tell me squat! weird isnt it!

Click to expand...

The vacuum gauge can help, but is not always the only thing to help trouble shoot. I had many carb related problems, even some minor vacuum leaks, but the vacuum gauge always read a steady 14 inches at idle, even with a brand new carb, it still reads 14 inches due to my cam.

 

1

1969mach1351

Member

Apr 18, 2003

285

0

16

Sacramento, CA

Apr 2, 2005

#10

• Apr 2, 2005

• #10

I have the holley blue electric fuel pump with the regulator. I think its set at 6 PSI. Thats about right isn't it?