What tests to do with vacumm gauge?

[chevyc1068]

What tests should I do to help me find the culprit of a dead spot, bogging, hesitation, whatever you want to call it, in acceleration. I still haven't found out why it is bogging. I got a vacuum/fuel pressure gauge for Christmas and need to know what tests to do and how to perform them.

[Fred T]

Usually they come with instructions, but I need a magnifying glass to read em. Go to http://www.centuryperformance.com/vacuum.asp

[rcatalano]

Everything I ever read about bogging or hesitation on acceleration points to a lean condition in the motor. Most of the time, the fix is to adjust the accelerator pump squirt by either changing the pump cam profile or the squirter size (for Holley carbs). I'm not sure how this adjustment is made for Quadrajet or Edelbrock carbs.

It could also be that there isn't enough advance in the timing. You might try adding a little more timing first and see how it responds. A little more detail about your motor configuration would be helpfull....

Rick

[jimmydean]

I use a vacuum gauge to set my timing because my high speed redneck chrome timing cover lacks a timing mark. Best way to set it anyway IMO.

[chevyc1068]

How do you change the pump cam profile on a Rochester 2BBL. Or do you think that the vacuum advance is not working or the timing advance needs to be advanced a little further?

Yes it did come with instructions but I want to know which ones to do to look for a vacuum condition that will lead to hesitation.

For most of the charts I've seen on the internet of gauge readings are they all just hooked up to a manifold vacuum at idle and then you watch from there? Like the link that Fred T has, is that just manifold vacuum at idle or something different?

[neonlarry]

This does not relate to using a vacuum guage but it might help narrow down what is causing your problem. Mind that this is describing a Q-Jet but the principle is the same: From-Corvette Action Center
How can you tell if an off-idle stumble is caused by a lean or a rich condition?
A carb running rich, as well as a carb running lean, can cause an off-idle stumble or hesitation upon acceleration. To narrow it down, tap the roll pin out of the accelerator pump lever by using a small pin punch or a small finish nail. I actually use a small, broken drill bit that's just the right size. Using a hammer, gently tap the roll pin in towards the choke air horn wall. Don't jam the pin right up against the wall: Leave just a little bit of a gap so you can get a screwdriver blade in between the wall and the pin to pry it back again. With the pin tapped out, remove the accelerator pump lever. I like to do this with the engine running so I won't have any trouble starting the engine without the accelerator pump. Now, rev the engine a little with the throttle. Notice if the engine seems quicker and more responsive, or if the hesitation & stumble is worse. If the engine actually feels more responsive with the accelerator pump disconnected, you have a rich condition. If the hesitation is worse than before, you have a lean condition. If there is no change whatsoever, you have a non-functional accelerator pump.

To verify a suspected lean condition after this test, simply hold your cupped hand lightly over the choke air horn area with the engine running at idle, restricting the air flow. If the idle speed and idle quality momentarily increases, you have a verified lean condition. You need to select a jet/rod combination that will give you a little more Cruise Metering Area. Make these changes in less than 10% increments using the Figures provided in this paper.

[Green Machine]

Seriously, the best cure for the old 2bbl is to replace it and the intake with ANY 4bbl setup. I have fought those carbs many times before, and you can make them better, but they still like to bog. One problem is that the accel pump shaft wears out the casting and gets sloppy, I have made bushings to put in there and it helps, people talk about junk carbs, but those 2bbl's either the smaller one or even the larger on 350's and 400's define the term "bog".

[chevyc1068]

Neonlarry what paper are you referring to about the figures? But anyway that sounds like a very good test.

Ok new question. When I was testing with the gauge I did what the instructions said to do for a cranking engine test, its supposed to be an engine condition test, with the throttle plates all the way closed and the PCV valve from the carb plugged up and the air cleaner removed with gauge connected to manifold vacuum. I cranked it over and the reading was from 8 to 12Hg the needle just moved back and forth fairly quickly. Then I did one with one with everything hooked up and motor was idling all I did was hook it to manifold vacuum and the reading was not perfectly steady it read about 16.5 to 17.5, is that normal or should it be perfectly steady? Also when I would accelerate and hit the dead spot when it sounds like a vacuum, the needle would drop all the way to 0 and would stay there till either the engine died or you let up on the accelerator.

Can anyone give me some info on what this means, the instructions weren't very explanatory??

[neonlarry]

Here is a link to the whole paper:
http://corvetteactioncenter.com/kb/q....php?qstId=444

[1978steven]

I'm by no means an expert on this and maybe someone else will chime in and help out.

But with the info you gave in your last post and the link that Fred T posted "http://www.centuryperformance.com/vacuum.asp" You might have some carb adjusting to do or maybe some valve work to be done. If you look over that post it goes over some examples you could compare your results to. I guess depending on the range of movement of the vacuum gauge you could moreless get a general idea or area of the culpret.

1 Steady Needle Normal reading (usually 17-22" Hg. in stock engines) Race engines vary "a lot" and in most cases will be considerably less.

2 Intermittent Fluctuation at Idle/Ignition miss, sticking valves, lifter bleeding off (hydraulic), or just a BIG camshaft

3 Low, though Steady Reading/Late timing, low compression, sticking throttle valve, carb or manifold vacuum leak (remember most race engines with a big cam and a tight centerline and high overlap will be naturally low ... you must decide your baseline vacuum reading)

4 Drifting Needle/Improper carb setting or minor vacuum leak

5 Fluctuating Needle as RPM Increases/Ignition miss, blown head gasket, leaking valve or weak or broken valve spring

6 Steady, but Needle Drops Regularly/Burnt valve or incorrect valve setting (too tight), "needle will fall when bad valve operates"

7 Gradual Drop at Idle/Clogged exhaust, excessive backpressure (extreme cases engine will die at idle)

8 Excessive Vibration that Steadies as RPM Increases/Worn valve guides