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Vacuum reading question
https://slantsix.org/forum/viewtopic.php?t=30537		 Page 1 of 1
Author:  Reed [ Wed Aug 13, 2008 8:57 am ]
Post subject:  Vacuum reading question
The vacuum guage on my brother's van reads a steady 20-21 inches at idle and stays solid during driving until it gets down below 5 inches on acceleration. If the van is accelerating and the vacuum is below five inches the needle will flutter in about a five inch range. It is especially pronounced on hard acceleration where the vacuum would normally be a solid zero. Is this bad? Burned valve? It is weird becase in all nonhard-acceleration conditions the needle moves smoothly with no bounce. What's up?
Author:  68barracuda [ Wed Aug 13, 2008 11:48 am ]
Post subject: 
With how much does it flutter?
Author:  Joshie225 [ Wed Aug 13, 2008 12:02 pm ]
Post subject: 
Reed,

Where exactly is the gauge connected?
Author:  Reed [ Wed Aug 13, 2008 12:54 pm ]
Post subject: 
The guage is connected to the vacuum tap on the #6 runner. Not ideal, I know, but the Holley 2280 has fewer "extra" non-ported vacuum taps than does the Carter BBD.

The needle starts to flutter a little bit below 5 inches, but if I accelerate hard the guage drops to zero then begins fluttering between 0 and 5 inches.
Author:  Joshie225 [ Wed Aug 13, 2008 1:02 pm ]
Post subject: 
Venturi Effect! When the air moves fast across the bottom of that fitting it pulls a vacuum. :D
Author:  Reed [ Wed Aug 13, 2008 1:33 pm ]
Post subject: 
Cool. I was worried it had a bad valve or something was messed up internally. I might try a different vacuum tap just to put my mind at ease.

Now I just need the VC-208 distributor advance pod to get here and I can finished dialing in the distributor. I discovered that the pod on the dist right now doesn't give any vacuum advance below 10 inches and it only adjusts to higher levels of vacuum. :? This really sucks considering that moderate acceleration and cruising on the van occurs at 5-10 inches.
Author:  rock [ Wed Aug 13, 2008 4:17 pm ]
Post subject:  Hey, Reed, think a out your vac adv.coming in from 15 -22
Reed,

I was "driving" at this question in a pretty recent thread that Rob, Ted and Emsvitil among others chimed in on. I know they experiment like I do with endless recurving so I followed their advice with good success.

I used to think about the vac advance coming in where you are planning...til Rob's recent analogy about starting out, cruising at 15, nice girl comes along, you mash down a tad to acccerlerate, vac to maybe 10 then back to 15 - 17, and vac at 15 - 22 for steady cruise. I took off my VC 208 and put on a 8.5 can 2 1/2 turns out...great curve no ping, but I used some of Rob's springs(THANKS ROB) and got it too tight on vac advance. With just a little less tension on secondary spring I think I will have it. Read that tip about having the vac begin about 2 above your power valve. I have a 10.5 PV so my can adustment got the vac to come in at 13. All I need now is the total curve pulled down to the right a tad and extended out a tad in range to maybe 4500.

As other say here, just my .02 worth.
rock
'64d100
Author:  Wizard [ Wed Aug 13, 2008 4:18 pm ]
Post subject: 
How much this vacuum gauge reading flutter with a 4 cylinder engines?

Wanted to know too because I tried that while ago.

Cheers, Wizard
Author:  steponmebbbboom [ Wed Aug 13, 2008 4:28 pm ]
Post subject:  Re: Hey, Reed, think a out your vac adv.coming in from 15 -2
Quote:
til Rob's recent analogy about starting out, cruising at 15, nice girl comes along, you mash down a tad to acccerlerate,

rock
'64d100
what, are you guys scared of nice looking girls? I hit the brake, but that's just me.
Author:  Reed [ Wed Aug 13, 2008 4:35 pm ]
Post subject: 
Thansk rock. Problem is, the Holley 2280 and Carter BBDs don't have a "power valve" (at least in the sense of the typical Holley carb, or at least that I can see and figure out how to measure).

I am still fine tuning the dist curve but I need to get vacuum advance below 10 inches. The can I have now starts at 10 and the adjustment only makes it go up from there. I printed out that discussion from the other thread about the chick in the barracuda and will be using that as my guide.

The problem is the van cruises at about 10 inches and any significant acceleration dips it down to about 5 inches. Most of the time driving around town it has no vacuum advance and I can really tell.
Author:  emsvitil [ Wed Aug 13, 2008 6:23 pm ]
Post subject: 
BBD,BBS have a needle for the power valve (as do the 4bbl carters,edelbrocks)

There's a little piston with a spring against it that vacuum pulls against.

Probable need an A/F meter to figure out at what vacuum the transistion occurs.
Author:  rock [ Thu Aug 14, 2008 6:41 am ]
Post subject:  Reed 9and Ed) thanks and some more thinking
Reed,

Thanks, and for sure I didn't even think you were a 2bbl guy...because it seems folks asking about advance curves are 4bbl guys (like me) and recurvers. (I admit I would like to play with a 2bbl some but am married to a 4bbl now.) I gues I am like the guy who buys those 1,000 piece puzzles of a sky with a few clouds.

Ed's input re. the needles on those carbs is one to file in my "Arcane Technical" book though. Two weeks ago I finally bit the bullet and bought a good wideband A/F gage....the NGK Powerdex. Why would I spend almost $300 for something I won't even leave in once I get my my A/F mix right?

Because with it I CAN get it right and find out even more about my curve and its vacuum, as Ed is suggesting. Tuninig with this gage is so fast and so easy and accurate that I am glad to have done it. As an analogy any of us can time by ear...but put a good timing lignt and osciloscope on your ignition and see if your ear isn't "off" a tad. I don't think a wideband system should be the first thing one buys when tuning, where a good dialback light is as important as having several dizzies and spring sets, but as you get closer to the elusive perfect curve, seeing the prize in reach will likely impel you to spend "just" $300 more instead of spending weeks and burning gas and still "not getting it".

rock
'64d100
Author:  Reed [ Thu Aug 14, 2008 8:56 am ]
Post subject: 
Since this motor is in a van, I am building for low RPM torque and mileage, not high RPM horsepower and speed. Hence the two barrel carbs, and hence the switch to a Holley 2280 from a perfectly functioning BBD.

I have an Autometer A/F guage, but the motor doesn't have an O2 sensor to plug it into. Plus, for the amount of money I am being paid to do all this ($0) I am just not willing to go all out on the fine tuning. If I can get it pretty good with my dial-back timing light, vacuum guage, and tachometer, I will be happy.

Now, on my 84 Ford e-150, I DO have a Holley 4360 Economaster carb sitting on a 351W Hight Output motor. When I get around to swapping that motor into my 89 E-150 and ditching the PITA fuel injected 302 I will be installing the A/F guage, a dual port intake, and spending the time to get the timing curve dialed in good. But that is a task for another day....
Author:  Aggressive Ted [ Fri Aug 15, 2008 8:55 pm ]
Post subject: 
Reed,

Put a "T" in you vacuum pull off line for the choke.
Let us know what reading you get there.......

Then you can start to do some good tuning. I can't believe this engine is running that low of vacuum. Sounds like the timing is not high enough for the load.
Author:  Reed [ Fri Aug 15, 2008 11:56 pm ]
Post subject: 
Ted- I will switch the vacuum tap and post the results. The more I drive this van the more I think that the timing just isn't set up right. Immediately off idle feels good but it stops pulling hard pretty quickly. I know my vacuum advance pod doesn't kick in until 10 inches. I cordered a VC-208 pod from Napaonline but it hasn't shown up yet. :x

I think I need to swap to a lighter spring. I am going to pull the distributor and take it apart tomorrow. I can't remember how I set it up the last time I was in there. Also, I know that the last time I worked on this motor I discovered that the vibration dampened had slipped and was reading 8 degrees retarded. Hopefully getting alighter spring and a vacuum can that will come on at 5 inches will really wake the motor up.

I am now running a Holley 2280 off of a v-8 which seems to be set up right and running really good. No hesitation or bog, smooth idle. I also get no pinging, so I think I just am not getting enough mechanical and vacuum advance.
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