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 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. 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.

 

 

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

 

this is what i like to do.
1. after rebuild make sure the float is correct and clean fuel filter.and timming is set to your likings
2.preset idle mixture screws to 1 1/2 turns out from seat.hook up vacuum gauge to manifold vacuum.
3.start car and let run til the engine reaches operating temp.
4.have a friend or myself have wife sit in the car, put in gear.
5.with air cleaner off turn the idle down to the lowest point with out stalling.
6.adjust your idle mixture screws about a 1/4 turn at a time going from one side to another give it a few seconds between each turn to allow the engine time to change not only will you see the difference in the vacuum gauge you will hear it as well if you turn them in too far. you want to go out until there is no more change after 1/2 turn out.
7. at this point put your air claener back on and make one more adjustment to your idle mixture screws. in or out to see if you gain anymore vacuumif i dont see a change with in 1/4 of a turn after i put the air cleaner on then i leave them alone the setting before the air cleaner was put on.
8. take air cleaner off adjust idle to about 700 to 750 or to your likings
9. reinstall air cleaner and remove vacuum gauge and enjoy

 

dont forget about your wife who has been sitting in the car the whole time asking are you done yet.and put your car back into park to set your idle not while its still in gear.