2150 Mixture Screws

[TenSeven]

After a ton of help from all you guys, my 2150A equipped 2.8 is has been running well, thanks!

One thing that occurred to me is that back when I was adjusting the mixture screws the engine wouldn't idle smoothly enough at the time for me to accurately adjust the idle mixture at anything less than 950rpm, which now I'm thinking, if I was adjusting the idle mixture screws then it should have been done at IDLE rpm, duh!

Anyhoo, I'm going to go back and adjust correctly this time.

Question, do you adjust both screws at the same time for highest rpm, or do you adjust one at a time for highest rpm then move over to the other for adjustment?

 

[TenSeven]

Or, does it really matter?

I thought that each mixture jet affects only one side, or one bank of cylinders, maybe incrementally adjusting both of them at the same time would make sense since while in operation, both sides, or cylinder banks, are firing.

 

[RonD]

If the intake manifold has a separator plate then yes, 1 barrel of the carb supplies just 1 bank of V engine, but most intakes are open, so both barrels supply either bank of V engine.

You should use a vacuum gauge to set best idle mix, so attach that to intake manifold vacuum, not carb vacuum

You should adjust the idle mix screws with engine warmed up and choke fully opened.
With engine off turn each screw in until it bottoms out, don't crank it hard or you will damage the needle, just turn it until it stops, count the turns in until it stops, should have been 1.5 to 2.5 turns.
Unscrew each to 1.5 turns

Start warmed up engine and let it idle
Adjust mixture screws for highest vacuum
Also lower the idle screw if needed for desired idle RPM
And readjust mixture screws for highest vacuum.


Once you are done, test it by creating a small vacuum leak, i.e. unhook vacuum gauge
If RPMs climb up alot higher then your setting was too Rich
If RPMs barely change then your adjustment is right on, but I like to run a little Rich so with vacuum leak open I might turn each screw slightly to increase RPMs just a tad

Spark advance also plays a part in idle RPMs , so you need to set base spark timing first.

 

[TenSeven]

Thank you Ron.

I didn't maintain idle speed last time I messed around with it.

I've got a vacuum gauge so I'll use that. Before, when I was first trying to just get the engine to run decent, it would idle erratically so the vacuum gauge needle was hard to read, it was easier at the time to just use the tach setting on the strobe. But now, mainly after getting the timing right and getting a metering block on there, it runs smooth enough I should be able to use the vacuum gauge.

 

[TenSeven]

Worked great Ron. Unplugged the vacuum gauge, no change. Thanks again.