Testing manifold vacuum

[MtnClimber]

Never done it before. Just wondering what reading would be optimal for me at 300ft above sea level idling at around 660-700. I'm attempting this vacuum setting/timing thing. Right now I'm getting a reading between 16 and 18...

 

[fieldsken1]

you kept turning the dizzy till you get the highest vacuum reading. its that easy so if 16 to 18 is it than thats it. this well explain everything.www.centuryperformance.com/tuning-with-a-vacuum-gauge-spg-148.html

 

[MtnClimber]

Good read. I'll have to go over and over with it to catch it all.

 

[Mr. Toad]

You should also tune your idle mixture for max vacuum.

 

[Splangy]

and set your idle to the proper rpm.

 

[MtnClimber]

I got my RPM as close to 650 as I can get it. The idle wavers so it's hard to keep steady, plus I have an ignition miss. I'm still looking things over and trying to find out which screw is what and how to do things step by step without blowin the engine.

Oh, and I mistakenly mistyped the wrong RPM before and I edited that. I meant 660-700, not 560-600...

 

[zags]

You need to fix the miss before anything else. That will throw everything off.

 

[MtnClimber]

I was hoping to set the carb a little better in hopes that the miss would improve or go away. I have new plugs (old ones were very carbed over- all black) all elect. connections were cleaned, wires snug.

BTW, looking at a tear down sheet for my Rochester, there is no 'Mixture screw'. Is there another name for it? What should I be looking for? The needle screws on the flange that have the springs that go into each venturi are the ones I thought, but they are called 'Idle Adjusting Needles'. Are those it?

 

[edwjmcgrath]

There are two idle adjustments. Mixture and speed.
The speed screw is mounted so that it touches up against a cam that adjusts how far the throttle is open.
If you operate the throttle by hand, you will see that the end of the screw comes off the cam as the speed goes up.
The idle mixture screw goes into the base of the carb and has a needle valve on the end of it and usually has a spring around it. As you back the screw out, it opens up a little port that allows more fuel to go in at idle.

You should be able to tell what they are by looking at them.

 

[MtnClimber]

That's what I figured they were, but my sheet calls them the 'Idle adjusting screws'. I already knew about the 'Idle speed set screw'.
I took some info that I found on google specifically for the Rochester 2 Barrel. Had it warmed up, set idle speed, played with the mixture screws set idle, backed out one screw at top RPM increase, set idle, backed out the other, set idle and repeated. Then I checked the timing. It's more stable(8*), but still floats around a bit. Runs smoother and there is NO hint of any hesitation when the throttle is jabbed. I can just punch it and it'll rev up no problem....sitting in the driveway in neutral. I'll try it later on the road to see if it will still hesitate under load. Crossing 8 fingers!

 

[pb4ugo]

Idle adjusting screws/idle mixture screws per your description would be the samething. 2 screws @ the base of the carb. What I do: adjust idle rpm to spec. Then slowly turn 1 idle mix screw in until it effects idle then back out til smooth or best idle. Then back that screw out about a 1/4 or 1/2 turn. Now do the same thing w/the other screw. Now re-adjust the idle to spec &* you should be golden. Or adjust w/a vacum gage as previously stated, but the adjustments will be the same way,your just watching the gauge. Idle mixture adjustment will only effect idle, and has no effect on how it acelerates.

A shaking vacum needle, usaully means something is not right w/the valve train or a weak cyldr. Make sure all plugs are gapped the same & at proper spec. Have you adjusted your valves. Proper adjustment will help smooth the idle out too.

 

[MtnClimber]

Idle mixture adjustment will only effect idle, and has no effect on how it acelerates.

A shaking vacum needle, usaully means something is not right w/the valve train or a weak cyldr. Make sure all plugs are gapped the same & at proper spec. Have you adjusted your valves. Proper adjustment will help smooth the idle out too.

I was having a stumble/ hesitation/stall at medium to heavy throttle application before. Now I don't have any issues. Would it be that the change in fuel mixture at idle changed the mixture just enough to improve the throttle-on mixture transition? I was thinking that the jets of fuel weren't creating the right mix in time and I was leaning out to a stall. Is this a possibility, or just a coincidence that the engine is revving better now?

I did notice that the characteristic of the needle fit the description of an ignition problem or a valve train issue. I do have a tapping noise and I do need to adjust the valves. I plan on doing that soon. I already have the gasket for the valve cover, so I'll do that as soon as I get other stuff done, ie. fuel pump changed, oil change, and perhaps a new coil, points/distributer and wires.

 

[zags]

Actually, the idle circuts account for about 20% of the fuel mixture at wide open throttle, so they can have a significant effect on acceleration and cruisng speed performance.

 

[pb4ugo]

Adjust your valves, timing, then adjust the carb. Make sure the dist is operating correctly.


EDIT
Have you checked or replaced the pcv valve?

 

[MtnClimber]

I haven't dealt with the PCV yet. In fact, I wouldn't be surprised if it wasn't even there. This truck is so jerry-rigged is embarrassing, thanks to a PO.