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Unread 02-14-2014, 03:04 PM   #1
Jamesromeos
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304 timing and vacuum advice

Looking for the Ideal vacuum pressure at the ideal timing for a 304 with a holley 450 4bbl carb and 4bbl intake. I do not know if the cam is stock or not, I have read 5-8* BTDC is ideal, but I don't know. I rebuilt the carburetor but am still getting a carb backfire and do not know if I have the air fuel set properly. It will idle all day fine, but as soon as I rev it to move it I get a bog and a loud fart out of the carb before stalling. I know I rebuilt the carb right, done many in my day. I have little experience with a vacuum gauge, but don't know what kind of pressure I am supposed to be looking for and at what rpm, timing, etc. should be set at.

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Unread 02-15-2014, 12:37 AM   #2
Jamesromeos
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????
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Unread 02-15-2014, 06:42 AM   #3
Matt1981CJ7
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Without knowing what cam you have, it's hard say what vac pressure you should have. 18-22 psi is normal with a stock cam at or near sea level, slightly lower at higher elevations.

6-10 BTDC initial timing should be about right.

Matt
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Unread 02-15-2014, 07:34 AM   #4
swatson454
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Assuming your timing is as Matt has suggested and the cam doesn't warrant more than that and there are no vacuum leaks, it's looking like there may be something amiss in the carb. Here's what Holley writes for possibilities:

Idle adjustment
Idle passages (dirty or plugged)- how many turns out from seated are your idle mixture screws?
Choke adjustment- I'm guessing the choke is pulled off
Metering jets (loose or plugged)
Power valve (loose or sticking)
Pump discharge holes (dirty or plugged)- are you getting a squirt of fuel from the pump discharge?
Pump diaphragm (worn, cut)
Pump ball checks (dirty, sticking)
Choke diaphragm adjustment (vacuum leak)
Secondary linkage adjustment
Venturi cluster (loose or dirty)

Hopefully that helps.


Shawn
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Unread 02-15-2014, 08:03 AM   #5
Jamesromeos
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The choke is still on the jeep, I am looking for a manual choke bracket, however I do manually operate the choke until it warms up, and then kick it down before I move it. I rebuilt the carb and cleaned the whole thing out, set the float to factory specs, and popped it back on to the truck. I have checked for vacuum leaks, none found. the air fuel mixture screws should be set at 2.5 turns? is that 2.5 full 360 turns as you are looking at the screws? I will put up a pic a bit later to show my choke cable and bracket issues, maybe someone has a suggestion on how to modify or build a bracket that will work for the choke. I am going to check the timing again today as well. I had it set at 5BTDC
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Unread 02-15-2014, 08:42 AM   #6
Matt1981CJ7
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Be sure to disconnect the vac advance and plug the vac source before setting timing. Matt
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Unread 02-17-2014, 08:00 AM   #7
Jamesromeos
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a combination of timing slightly off and air fuel and idle adjustments have got this thing running pretty good now.
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Unread 02-17-2014, 08:06 AM   #8
swatson454
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Good deal


Shawn
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