Manifold vac reading

[Langdon]

I finally put a vac gauge on the engine after almost a year of owning it. 20 in. at idle, which Chiltons says is normal. Question is, the needle flutters about 3-5in. in a regular 'vibration.' Not knowing a lot about engines, is this normal, or should it be rock-steady?
TIA,

[Veepster]

flutterring is OK.....remember that when a motor is running it does not have a continuous flow of vacuum...it has 8 separate and distinct pulses and at low RPM there is gaps between those pulses, therefore your guage flutters a bit..

------------------
Peace.............BartG

the Green Flash!
360ci
Custom 4v TBI
Edelbrock Performer Manifold,
TFI Upgrade,
TH400, QT with low
4" skyjacker suspension lift, 3" body lift Rancho 9000's
33x12.50 BFG KO's 8" American Racing Baja Rims
Thorley headers, 3" exhaust, Dyno Max
Infiniti power leather seats
Custom billet shift knob by Millerluck
50%Luxury Car, 50%Tractor
http://www.teamgodspeed.com

[mtn goat]

Wow, I did the same thing last sunday...but I have only owned mine for 10 months http://www.ifsja.org/ubb/biggrin.gif...I reading at idle was 20 and almost dead on..only fluctuated +/- .5 in. That makes me happy http://www.ifsja.org/ubb/smile.gif but I have the 258 maybe your engine is different. I think 5 in fluctuation is excessive....but what do I know....



------------------
'82 Cherokee WIDE TRACK
258/T176?/NP208/D44/20
big orange fender..."She's the toughest hunk of junk in the galaxy"

[jeepbob]

Actually, the fluttering is a sign of a problem. If it is a rapid flutter the cause is probably a burned valve. If it is a slow flutter the idle circut on the carb is out of adjustment. The needle on the gauge should remain steady. There is a good article in this months Chevy High Performance magazine on how to adjust your carb using a low rpm tach and a vac gauge. I have used a vac gauge as a diagnostic and tuning tool for 30 years and it is good to see it coming back into "style" as a tool.
BTW a vac gauge is important in getting your carb set up for your motor, which is, I think, why my holley will sit on a 40 degree incline and run without loading up or stalling while Jeepguzzi and I try to decide if my frt end is working (it wasn't).

------------------
65 wag. 360/edelbrock rb4/t400/20 t-case/4:10 d27/d44 broken power lok/onboard air/onboard 110v power(no inverter)/1999 Lincoln t.c.leather buckets/Lincoln ctr console/winch/33x12.50/tunes/water proof door pads
soon to have new motor/d44frt/d60r(4:10)welded diff/custom bumpers
see ya in da mud

[Langdon]

So jeepbob, you're saying that the needle should be very steady? The motor runs fine...I suppose a compression check would reveal any probs, huh?
Thanks for the replies,

[Veepster]

hhmmmm I think we need to clarify.......It all depends on the camshaft....with a stock cam I am with JeepBob......BUT if you use a 'big' cam the vacuum signal is seriously impacted....and you will never get a steady signal at idle.

------------------
Peace.............BartG

the Green Flash!
360ci
Custom 4v TBI
Edelbrock Performer Manifold,
TFI Upgrade,
TH400, QT with low
4" skyjacker suspension lift, 3" body lift Rancho 9000's
33x12.50 BFG KO's 8" American Racing Baja Rims
Thorley headers, 3" exhaust, Dyno Max
Infiniti power leather seats
Custom billet shift knob by Millerluck
50%Luxury Car, 50%Tractor
http://www.teamgodspeed.com

[jeepbob]

Right the stock the needle is steady with a big cam it will flutter. A compression test is ok but I prefer a cylinder leakdown test as this will let you know which valve is bad and/or the condition of the rings.

------------------
65 wag. 360/edelbrock rb4/t400/20 t-case/4:10 d27/d44 broken power lok/onboard air/onboard 110v power(no inverter)/1999 Lincoln t.c.leather buckets/Lincoln ctr console/winch/33x12.50/tunes/water proof door pads
soon to have new motor/d44frt/d60r(4:10)welded diff/custom bumpers
see ya in da mud

[Langdon]

Well upon further review...the flutter is only about 1-2 inHg. I did a compression test,results good; all the plugs looked good, too. So I think it was just my inexperience showing...
Thanks for the replies!

[1jeep4me]

Try a leak down test for burnt valves

[<Tom 400 CFI>]

I agree W/ Jeepbob and 1Jeep4me. There should be no flutter at all, and since you already did an inconclusive compression test, I would folow that up with a leak down test.

[jeepbob]

I would try tuning the carb as it is prolly just a little out of adj.