Jetting & Power Valves & Vacuum - OH MY

dream66 · Dec 23, 2014

Everything was near perfect with the former RACE engine combo.

Changed the short block from 408 to 433 cu.in.
No change to Heads, intake, carb, distributor or Compression ratio, or fuel (93 Exxon pump gas)

Big change on cam/lifters/springs from Hyd roller to Solid Roller

Former Cam = 232/242@.050 // .578/.598 lift // 112 LSA (Lunati shelf)

Current Cam = 245/253@.050 // .722/.722 lift // 108 LSA (Bullet spec)

ISSUES:

Transition to VACUUM secondaries - installed stiffer spring - 90% improved...may step up one spring stiffer

VERY Rich exhaust odor throughout RPM and vehicle speed - Need advice

Summit Steet/Strip 750 CFM vac secondary (which I have had excellent performance from on 2 other engines)

GM HEI distributor (New) and no changes to weights or springs
Timing = 16 @700 RPM...36 @ 3,000 RPM (all in mech & NO vacuum)

Idle Vacuum = 5"
1500RPM Vac = 10"
Other RPM vacuum = Unknown

Current Power valves = 6.5 Primary // 9.5 Secondary (Stock)
Current Jets............ = 77 Primary // 79 Secondary (Stock)

- Pretty sure I need to drop down to a Primary 2.5 PV but no clue on the secondary PV

- Float levels half way up the viewing window = Okay

- Jetting...Haven't checked the spark plugs yet

Before I continue with changes I would like some advice on troubleshooting sequence. Which comes first...chicken or egg?

jeff swisher · Dec 23, 2014

Your secondary also needs to be less than idle vacuum.
Some say divide idle vacuum in park by 2 and add 1 to 2 numbers and this is what PV you need.

Your fuel level seems high at 1/2 way up the sight glass?

I usually set my Holley at trickle out if you shake the car back and forth.
We all like something different though.

bradley67 · Dec 23, 2014

Try lowering your PV to a 2.5 or 3.0 front and rear.
Bring float down so gas shows just above window. I'm guessing this is a street tune.
Square jets.
Make only one change at a time and test after each change.
Your probably getting some reversion at 700 rpm idle.

dream66 · Dec 23, 2014

bradley67 said:
Try lowering your PV to a 2.5 or 3.0 front and rear.
Bring float down so gas shows just above window. I'm guessing this is a street tune.
Square jets.
Make only one change at a time and test after each change.
Your probably getting some reversion at 700 rpm idle.

Race Tune - I only drive it on the street for testing

I'm aware that Holley recommends bottom of the sight glass but Summit recommends 1/2 way up - not that I won't experiment

??? What is reversion at idle ??? (sorry, I read constantly on this stuff but I am by no means an experienced expert)

dream66 · Dec 23, 2014

jeff swisher said:
Your secondary also needs to be less than idle vacuum.

That's good to know - I couldn't find secondary PV info on any carb tune site...till your post.

jeff swisher · Dec 23, 2014

You get pulses in the intake and exhaust runners you slow the speed down to 700rpm and the pulsing gets slowed down and this pushes ex gas or combustion back up the intake creating what bradley is possibly referring to

jeff swisher · Dec 23, 2014

With a secondary power valve if you lift off the throttle and vacuum comes above the PV number and you still have secondaries part way open it will go leaner.

I assume you know this.
And the powervalve number should be for a sort of reference.
just because it stated a 2.5 or a 6.5 PV does not mean it will happen at that exact number.. some may start to open at sooner or later than the number on them.
Yes that is a bad deal.

I would bet your smell that you speak of is rich smell.. but Lean will also smell.
My carb is at 15.5 AFR at cruise and smells rich to me.

dream66 · Dec 23, 2014

jeff swisher said:
With a secondary power valve if you lift off the throttle and vacuum comes above the PV number and you still have secondaries part way open it will go leaner.

I assume you know this.
And the powervalve number should be for a sort of reference.
just because it stated a 2.5 or a 6.5 PV does not mean it will happen at that exact number.. some may start to open at sooner or later than the number on them.
Yes that is a bad deal.

I would bet your smell that you speak of is rich smell.. but Lean will also smell.
My carb is at 15.5 AFR at cruise and smells rich to me.

Nope...didn't know that. Thanks

Interesting. I need to find a shop with a ??? Wide band tester ???

dream66 · Dec 23, 2014

Back to the 700 RPM idle. I don't need to idle at that low RPM but I use that as a baseline for BASE TIMING and idle mixture settings based on recommendations from other respected members here. You know the names.

I'll probably idle up to 900-1100 for track operation.

ddeennis · Dec 24, 2014

I just wonder why folks still think the PV has anything to do with the idle circuit, I could write a book about this, but in short, the PV has no connection at all with the idle circuit, If anyone of you would take the time and look at a blown up diagram of the carb you will
see what I know and some others and you would stop this "PV has to be 1/2 of this and that and this and what not".

The power valve is strictly connected to the main jets, when the main jets are drawn on then the power valve channel restrictors will ADD fuel to WITH the main jets in(according to the number of the power valve vacuum). The power valve is there to work with the main jets, why do you think you have to jet up when you take the power valve out.

The engine can idle all day long with 3" of vacuum and that PV that is stamped 6.5 will not add anymore fuel to the idle, it (power valve)will sit open and expose the power valve channel restrictors (pvcr), now the engine will need enough air flow thru the ventura to SUCK the fuel from the bowl thru the channels and dump into airstream thru the carb- so you will need about 2500 rpms or so enough rpm to start to draw on the jets.

Again the power valve is to assist on the jetting, the power valve you want to open as soon as possible to avoid dead spots, if your sitting at the starting line with the engine loaded and you have 10" of vacuum and you floor it on launch why do you want to wait for the extra fuel to assist your jets until your vacuum reaches about 2.5", you want that extra fuel now, so having a 8.5" would be good even if your car idles with 6" of vacuum.

Like I said I can write a book, your issues stem from the large overlap of the cam, more then likely your at the point you need to crank the timing way up and get some extra timing dialed in thru full port vacuum so your idling with about 45 degree of timing at 1,000 rpms, this will take the load off the carb some.

Which now your getting deeper into tuning which is another book.

I might be coming off as an azz, but I just want folks on this board to be able to give solid true advise for lack of better words.

You don't have to believe what I write, just look into it and look at diagrams and do a little research.

I too used to believe what has been posted above about power valves, but did the research and learned a little more and just trying to pass that on.

magicrat56 · Dec 24, 2014

x-2 think about it.

dream66 · Dec 24, 2014

Dennis, you make a lot of informative posts and I for one appreciate the info you provide.

On this thread, I didn't interpret anyone's posts about the PV effecting idle, but your point is well taken.

Like I said I can write a book, your issues stem from the large overlap of the cam, more then likely your at the point you need to crank the timing way up and get some extra timing dialed in thru full port vacuum so your idling with about 45 degree of timing at 1,000 rpms, this will take the load off the carb some.

Thanks for that advice. I suspected the cam was a contributor to my symptoms and wanted to hear confirmation of that from someone such as yourself.

Back to the PV sizing. I can only parrot the information I've read from the carb manufacturers and have much to learn. Your post has taken me yet another step forward in understanding the carb circuits. :beers:

dream66 · Dec 24, 2014

magicrat56 said:
x-2 think about it.

The thinking about it is easy. It should be obvious by virtue of my post that I am thinking.

The base knowledge and circuit corelations are what I'm missing. Do you have any reliable knowledge to pass on?

dream66 · Dec 24, 2014

ddeennis said:
Like I said I can write a book, your issues stem from the large overlap of the cam, more then likely your at the point you need to crank the timing way up and get some extra timing dialed in thru full port vacuum so your idling with about 45 degree of timing at 1,000 rpms, this will take the load off the carb some.

Which now your getting deeper into tuning which is another book.

Deeper into tuning is exactly where I'm heading. All of my prior experience is with much milder builds on which I had enough knowledge to get fairly close.

I'm paranoid about detonation on 93 pump gas which I may not hear due to the solid lifter noise and exhaust drone and I'm increasing the advance in 2* steps. (I haven't experienced detonation or pinging noise since the 70's)

45* timing with Vacuum at 1,000 RPM sounds way high to me. Everything I've read recommends about 38* all in mechanical with max 14* vacuum at steady 3,000 RPM.

jeff swisher · Dec 24, 2014

Sorry about my ignorance on holley PV maybe why I do not care for most holley's.

But I guarantee on a carter afb or a motorcraft 2V or a Qjet the Powervalve circuit does in fact have a direct effect on idle.

Yes I have had bad power valves on motorcraft 2V carbs and it will blacken the plugs in seconds at idle and soon it will not run.

The AFB type is a spring under a cup(power piston) basically and if you have less vacuum than is needed to keep that sucker pulled down it will be up or bouncing and idle will be crap and Jumpy RPM at idle.

I do know one 4.5" vacuum engine that had poor idle with a 4779 that had 6.5 in it and I stuck 2.5 in it and it cleared up a little.
Big cam 253@.050 on a 106LSA 210afr heads and 9.0 compression 355"
Holley literature on the subject states poor idle.
Or is this not Holley literature.

http://forums.holley.com/entry.php?430-Holley-Power-Valve-Tuning

magicrat56 · Dec 24, 2014

Cory no way am i slighting you ; it took me until just this year to really stop and think about what Dennis and others have said here. You want the power valve to open up "early" under a load.that way its sometimes possible to take some jet away from it when you get p/v openings timed right.best gauge to get inside the car is a vacuum gauge so you'll take all the guess work out. My 454 has responded great by going from a 6.5 to 9.5. Even had a 10.5 in it but it seemed just a little to fast.My vacuum at idle is 13-14,vac on the road 19-20 in.when you get your baseline think i'd play with the timing.Merry Christmas! Did'nt mean to come off wrong.

1Super69 · Dec 24, 2014

ddeennis said:
I just wonder why folks still think the PV has anything to do with the idle circuit, I could write a book about this, but in short, the PV has no connection at all with the idle circuit, If anyone of you would take the time and look at a blown up diagram of the carb you will
see what I know and some others and you would stop this "PV has to be 1/2 of this and that and this and what not".

The power valve is strictly connected to the main jets, when the main jets are drawn on then the power valve channel restrictors will ADD fuel to WITH the main jets in(according to the number of the power valve vacuum). The power valve is there to work with the main jets, why do you think you have to jet up when you take the power valve out.

The engine can idle all day long with 3" of vacuum and that PV that is stamped 6.5 will not add anymore fuel to the idle, it (power valve)will sit open and expose the power valve channel restrictors (pvcr), now the engine will need enough air flow thru the ventura to SUCK the fuel from the bowl thru the channels and dump into airstream thru the carb- so you will need about 2500 rpms or so enough rpm to start to draw on the jets.

Again the power valve is to assist on the jetting, the power valve you want to open as soon as possible to avoid dead spots, if your sitting at the starting line with the engine loaded and you have 10" of vacuum and you floor it on launch why do you want to wait for the extra fuel to assist your jets until your vacuum reaches about 2.5", you want that extra fuel now, so having a 8.5" would be good even if your car idles with 6" of vacuum.

Like I said I can write a book, your issues stem from the large overlap of the cam, more then likely your at the point you need to crank the timing way up and get some extra timing dialed in thru full port vacuum so your idling with about 45 degree of timing at 1,000 rpms, this will take the load off the carb some.

Which now your getting deeper into tuning which is another book.

I might be coming off as an azz, but I just want folks on this board to be able to give solid true advise for lack of better words.

You don't have to believe what I write, just look into it and look at diagrams and do a little research.

I too used to believe what has been posted above about power valves, but did the research and learned a little more and just trying to pass that on.

PV's are referenced to 1/2 the idle vac reading so they do not open as soon as you crack the throttle and cause a rich condition before middle circuit comes on in relationship to butterfly relationship.

PV's will effect idle quality if it is open or blown out at idle because it allows flow thru the valve due to engine idle vac being lower than what it takes to close the circuit off. Like adding approx 8 sizes to your idle enrichment if completely blown out or open.

So set idle speed where it needs to be , adjust carb to achieve highest vac reading with vac gauge without off idle stumble , take vac reading divide in half... install PV accordingly and begin base tunes from there.

Buy your self an AFR gauge for sure.

Hollow65 · Dec 24, 2014

I was taught it's cruising vacuum you reference. Like Denis says, I haven't seen them effect idle. And I have blown some. Idles great but does the lean pop when blown. My cruise vacuum is 21, I run 10.5 pvs.

dream66 · Dec 24, 2014

I hope I don't sound un-appreciative but my concerns are not with the idle unless I'm missing something in the dialogue.

I'm attempting to tune out the apparent over rich mixture at other than idle conditions through a combination of timing and carburetor adjustments to extent it is possible with the somewhat radical cam for this size BBC and compression etc.

Wolfplace · Dec 24, 2014

1Super69 said:
PV's are referenced to 1/2 the idle vac reading so they do not open as soon as you crack the throttle and cause a rich condition before middle circuit comes on in relationship to butterfly relationship.

PV's will effect idle quality if it is open or blown out at idle because it allows flow thru the valve due to engine idle vac being lower than what it takes to close the circuit off. Like adding approx 8 sizes to your idle enrichment if completely blown out or open.

So set idle speed where it needs to be , adjust carb to achieve highest vac reading with vac gauge without off idle stumble , take vac reading divide in half... install PV accordingly and begin base tunes from there.

Buy your self an AFR gauge for sure.

Not really correct,,, the power valve does not add fuel to the idle circuit.
Blown power valve yes it is a direct leak from the bowl into the intake through the vacuum channel,,,, stupid rich
But open has no effect on idle or just off idle.
The only time they will add fuel is when there is enough air passing through the carb to activate the main circuit & the vacuum is lower than the PV opening
Other words if you cruise at 9" & the P/V is 6.5 for instance it is not adding fuel,, if you were for some reason cruising at 6" you will be rich but this should not happen until you put a load on the engine or open the throttle enough to drop vacuum far enough

Cory
As far as being rich all the time with the bigger cam you most likely have the throttle blades open too far & are on the transition circuit or circuits.
You do not want this idling at 700 RPM, might sound really cool but it is too low for your combo
900-950 is where I would start my tuning
Your cam is going to want a lot of advance as already stated. If you have vacuum it will have to go to the intake not ported & you will need a can that works at or below your idle vacuum in gear if automatic or you will be chasing your tail with inconsistent idle
Personally I would lock the distributor & set it at 36-38° without vacuum until you have a reasonably clean tune then you can go back. put in a curve & add the vacuum if you wish

Jetting & Power Valves & Vacuum - OH MY

dream66

jeff swisher

bradley67

dream66

dream66

jeff swisher

jeff swisher

dream66

dream66

ddeennis

magicrat56

dream66

dream66

dream66

jeff swisher

magicrat56

1Super69

Hollow65

dream66

Wolfplace

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