The Ford Barn

19Fordy · 11-26-2016, 10:41 PM

I have a 390 Holley #8007 on my stock 8BA. I notice that when I adjust the idle mixture screw on the driver side of the carb the vacuum reading and idle changes. When I adjust the idle mixture screw on the passenger side of the carb the idle changes very little and the vacuum doesn't change. I just rebuilt the carb. Jets are #49 and Power Valve is 6.5. Vacuum reading is 16. Prior to rebuilding the carb the vacuum reading was 18.
What could cause this? Thanks.

51 MERC-CT · 11-27-2016, 06:16 AM

Possibly the secondaries are out of adjustment. As I understand it they are part of the idle circuit.
Went thru some of the same gyrations with the 390 I had until I found the solution to all the anomalies (AFR, idling,etc.) by installing an Edelbrock 1403.
Just curious, what is the lowest idle RPM you can get?

big job · 11-27-2016, 08:29 AM

As 51merc said make sure the secondary's are closed because even cracked open
a little bit the vac will be divided in two between primary and secondary's. This is
common for one idle screw to work or both don't do anything-providing all is clean.
To work properly the primary works as a two barrel . Just tweek that U shape link with
needle nose while holding the throttle plate closed and check the vac actuator for
bottoming out and being free and make sure there's a spring on the diaphragm (secondary) vac actuator.. Best I can say without 'it' on my bench.....sam

19Fordy · 11-27-2016, 08:57 AM

51MERC: I don't have an RPM meter, but I can get it to idle really low RPM"s.

Thank you all for your help. JIM

scicala · 11-27-2016, 11:52 AM

Seat both idle mixture screws (engine off) and then turn them both out 1 1/2 turns. Then warm up engine and adjust both screws for maximum vacuum making sure both screws are the same amount of turns out. Then trim idle speed screw to what is comfortable for you and check mixture screws again for max vacuum at idle. If that fails, then look elsewhere.

Sal

19Fordy · 11-27-2016, 12:42 PM

Sponsored Links (Register now to hide all advertisements)

scicala: Have done exactly what you have said to no avail.

51 MERC-CT · 11-27-2016, 01:48 PM

Quote:

Originally Posted by 19Fordy

51MERC: I don't have an RPM meter, but I can get it to idle really low RPM"s.

Thank you all for your help. JIM

Haven't seen one of those RPM meters in years.

11-26-2016, 10:41 PM	#1
19Fordy Senior Member Join Date: May 2010 Location: Coral Springs FL Posts: 10,944	HOLLEY 390 Idle Mixture Screw Adjustment I have a 390 Holley #8007 on my stock 8BA. I notice that when I adjust the idle mixture screw on the driver side of the carb the vacuum reading and idle changes. When I adjust the idle mixture screw on the passenger side of the carb the idle changes very little and the vacuum doesn't change. I just rebuilt the carb. Jets are #49 and Power Valve is 6.5. Vacuum reading is 16. Prior to rebuilding the carb the vacuum reading was 18. What could cause this? Thanks.

11-27-2016, 06:16 AM	#2
51 MERC-CT Senior Member Join Date: May 2010 Location: East Hartford, Ct Posts: 5,898	Re: HOLLEY 390 Idle Mixture Screw Adjustment Possibly the secondaries are out of adjustment. As I understand it they are part of the idle circuit. Went thru some of the same gyrations with the 390 I had until I found the solution to all the anomalies (AFR, idling,etc.) by installing an Edelbrock 1403. Just curious, what is the lowest idle RPM you can get? __________________ DON'T RECALL DOING SOMETHING FOR MYSELF BASED ON SOMEONE ELSE'S LIKES OR DISLIKES Last edited by 51 MERC-CT; 11-27-2016 at 06:33 AM.

11-27-2016, 08:29 AM	#3
big job Senior Member Join Date: Sep 2013 Location: Dighton, Mass Posts: 1,230	Re: HOLLEY 390 Idle Mixture Screw Adjustment As 51merc said make sure the secondary's are closed because even cracked open a little bit the vac will be divided in two between primary and secondary's. This is common for one idle screw to work or both don't do anything-providing all is clean. To work properly the primary works as a two barrel . Just tweek that U shape link with needle nose while holding the throttle plate closed and check the vac actuator for bottoming out and being free and make sure there's a spring on the diaphragm (secondary) vac actuator.. Best I can say without 'it' on my bench.....sam

11-27-2016, 11:52 AM	#5
scicala Senior Member Join Date: Jun 2010 Location: Detroit suburb, MI Posts: 3,706	Re: HOLLEY 390 Idle Mixture Screw Adjustment Seat both idle mixture screws (engine off) and then turn them both out 1 1/2 turns. Then warm up engine and adjust both screws for maximum vacuum making sure both screws are the same amount of turns out. Then trim idle speed screw to what is comfortable for you and check mixture screws again for max vacuum at idle. If that fails, then look elsewhere. Sal

11-27-2016, 12:42 PM	#6
19Fordy Senior Member Join Date: May 2010 Location: Coral Springs FL Posts: 10,944	Re: HOLLEY 390 Idle Mixture Screw Adjustment Sponsored Links (Register now to hide all advertisements) scicala: Have done exactly what you have said to no avail.

11-27-2016, 08:57 AM	#4
19Fordy Senior Member Join Date: May 2010 Location: Coral Springs FL Posts: 10,944	Re: HOLLEY 390 Idle Mixture Screw Adjustment 51MERC: I don't have an RPM meter, but I can get it to idle really low RPM"s. Thank you all for your help. JIM