I put a Carter AFB (600cfm) carb on my 390 and the instructions say nothing about how to tune the idle mixture, do I need a vacuum gauge?

 

>I put a Carter AFB (600cfm) carb
>on my 390 and the instructions
>say nothing about how to tune
>the idle mixture, do I need
>a vacuum gauge?

Dave, Here is info from an early post of mine.

>>>>
1 . "Tuning"
Posted by jbhf250 on Dec-22-99 at 11:54 PM (EST)

First and formost!! You need a Good Vacuum Gage and Tach do do this properly.
You Write>>>
I have a 2150 2bbl carb on my '75 390 (or is it 360?, I have yet to do the stick-through-the-plug-hole-test). I have heard of many ways to tune the carb. My question is this: What would be the standard procedure for tuning a carb after rebuild, including setting the timing, idle speed (my valve cover sticker is long gone), etc. My most important question would be: where do you start the mixture screws?

>>>I would start a 1 1/2-2 turns out each side for now.
REMEMBER WHAT YOU TURN THESE AS YOU GO SO THAT EACH SIDE WILL BE THE SAME!!<<<

I understand that you would turn them in or out an equal number of turns, but what fraction of a turn do you go on each one before turning the other one the same number of turns; 1/16, 1/8, 1/4? I guess you watch for the fastest idle when turning the mixture screws, then back off the idle screw to set the idle.<<<

Yes in general. look for the Highest Vacuum reading AT the highest RPM. adjust the idle mixture screws several times to obtain this.
If the RPM increases Reduce it to close the throtle plate to stay on the Idle circuit.


>>>I have been looking for a while now and have not found a good through procedure. I guess I'm looking for an experienced opinion..<<<

The Vaccum gauge is the KEY to let you know that the mixture is Even on both sides. DON~T over compensate. When it is Close let the engine come up to Operating Temp!!
Check the Base Timing setting at the crank. 6,8,10 or whatever, with the vacuum line removed from the dist. Then reconnect it!!

Look for a vacuum reading in the 13-18 range at IDLE speed. When the engine is `Hot` Fine tune the idle jets. Close the idle jets Down 1/4-1/2 turn at a time OR until the vacuum just starts to drop!
If you see the Vacuum drop SLIGHTLY... STOP! Its getting lean... Do/check Both Sides.
Set the Idle speed for what you want 550-750??

Now having said that. When the engine is Dead Cold you may find that Opening UP the Idle jets 1/4 to 1/2 turn will Improve cold weather starts!
But It will idle on the rich side and Faster when HOT!!. I would not settle for any Idle Less than 12 inch of vacuumn. If the engine is in good shape. Double check for leaks. Gaskets OK!! etc....

Just for ME... I set the Idle Jets with the engine running at as Low a speed as Possible! to get them EVEN Edlebrock 4bl.. But thats My thing!

Without being there this may be greek.. but it works for me.. Iam sure there are as many `tricks` as there people doing there own tuning thing. You will `develope` a feel and pattern for what you get. But like I said at the start the Vacuum gauge is my `Bible` thing for setting Carbs.
Give it a try. Let me know if you run into something you need help with. With the holidays here this week I may now read things here as often as I have..



__JOHN__Õ¿ö
72 F-250




__JOHN__Õ¿ö
72 F-250