First-time lean idle drop...help! (1 Viewer)

[BackFromTheDead]

I'm attempting to do the lean drop for the first time, and just have some questions about the process. I didn't find too much ont he actual process of doing this, so I just pieced together what I could. Please confirm this is right or wrong...thanks!

1. Remove air cleaner, and take out cap of the idle mixture screw
2. Hook up tach
3. Turn idle mixture screw clockwise (in) until the engine RPMs don't increase any more
4. Turn idle speed screw counterclockwise (out) until RPMs decrease to 690
5. Turn idle mixture screw until counterclockwise (out) until RPMs decrease further to 650

Wasn't sure if this was the right method, or if I confused myself. I also read the instructions at the bottom of this page from MAF, which confused me even more:

http://www.man-a-fre.com/technical_info/weber.htm

Thanks!

 

[Tinker]

You've got it backwards unless the idle mixture screw is left hand thread.

Do you have a Weber or stock Aisin?

 

[NocalFJ60]

I stick a small stubby in on the idle mixture screw and then put on the air cleaner and set it with everything back together. Put on some leather gloves cause it gets hot close to the screw on idle mixture. Make sure the engine is warm and the cold air flap is down so you are getting cold air. I use a vacuum gauge hooked up to the intake and watch the tach on my timing light. You unscrew the idle mixture and watch the vacuum increase to peak and then screw in just a hair. Then lower the idle speed down to 650. It take a few interations of each before it is good.

The procedure is in the FSM. I can double check later.

 

[2mbb]

This is the procedure from the emissions manual, [with my comments in brackets]

Initial conditions:

• air cleaner installed [I remove mine from the carb but flip it over on the passenger side of the engine so the vacuum tubes on the passenger side are still connected]
• Normal operating coolant tempeature
• Choke fully open
• all accessories switched off
• all vacuum lines connected [I have the three that connected to the air cleaner above the carb disconnected but not plugged]
• ignition timing set correctly
• transmission in N
• fuel level should be about level with the dot (center) of the sight glass
• [start with the idle mixture screw turned out (counter clockwise) two full turns from seated]

start the engine

using a long screwdriver set to the maximum speed by turning the idle mixture adjusting screw [this can be either leaning out (clockwise) or richening (counter clockwise) the fuel mixture. I usually start with leaning out the mixture and turn the screw in 1/4 turn increments. If the engine drops, then I turn the screw the other way. As I reach the maximum idle speed, I reduce to 1/8 turn increments. By going in 1/4 or 1/8 turn steps its pretty easy to get back to your last condition if you have overshot the maximum speed.]

set to the idle mixture speed [690 rpm] buy turing the idle speed adjusting screw

before moving to the next step continue the adjustments [mixture and speed] until the maximum speed will not rise any further no matter how much the idle mixture adjusting screw is adjusted.

set the idle speed [650 rpm] by screwing in the idle mixture adjusting screw

 

[BackFromTheDead]

Thanks for your responses, guys. I have a stock Aisin carb. I think I got confused with all the "turn mixture screw to set idle speed, turn idle screw to set idle speed", etc. etc. I'll give it a whirl. Any other tips you have from experience are much appreciated!

 

[BackFromTheDead]

Well, I finally did my first lean drop, and it seems to be working fine...the manifold vacuum at idle went from ~15 to about ~19, and it idles smoother.

I am a little concerned, however, about how I did the lean drop. The digital tach that I hooked up would not stay at a constant reading at all throughout the whole process. When I was turning the idle mixture screw out to try and find the max RPMs, I had to "guess" when it was at max, since it would fluctuate between 850 and 900. When I was turning the idle speed down to 690, I had to guess again, as it would fluctuate between 670 and 710. And finally, I had to guess when turning the idle mixture in to get the RPMs down to 650, as it fluctuate between 630 and 670. The engine didn't sound like its RPMs were fluctuating, so I'm not sure what the problem is.

Because of all this guessing, I'm worried my carb is too lean, or maybe too rich...not sure.

Any ideas, or anyone have similar trouble with their tach?

 

[NocalFJ60]

Yeah my tach jumps around too about that much. Doesn't need to be real precise. It is only changing how rich or lean it runs at idle. If you get a smog test, you can see how clean it is running at idle.

The point is that where you left the idle speed the mixture screw is right where the vacuum starts to go down. The idle speed +/- 20 doesn't matter.

 

[BackFromTheDead]

Thanks for the response, NoCal.

So, with the vacuum gauge hooked up, would I do the following?

1. Turn the idle mixture screw out until the vacuum stops increasing

2. Turn idle mixture screw in until it justs starts to decrease below max

3. Turn the idle speed screw until the RPMs decrease to 690

4. Turn the idle mixture screw until RPMs decrease further to 650?

Thanks!

 

[NocalFJ60]

2mbb instructions above are from he FSM and is what I did the last time. I just use the vacuum gauge too but it is not required. I think when I did it the first time it said to use a vacuum gauge but reading over 2mbb's procedure above it does not sound like it is required.

 

[BackFromTheDead]

I don't think it's required either, but judging from my first experience doing the lean drop, it would probably help a whole helluva lot, since I can't get a constant reading from the digital tach.

Thanks again!