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Red6a1

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Discussion starter · #1 ·
The most I can get out is 10in while in park. I tried fine tuning the fuel mix screws on my Holley 4 bbl part number 0-80457S. The engine has a 350 cam, pistons 30 over and 202 heads. Idle is set to 650rpm and 8 advanced. The engine runs great, but I feel I should get more vacuum and it is really rich still.

I did a smoke test into the carb port and I couldn't see any smoke coming from the intake or hoses. I talked to Holley and one tech told me to change the jets to smaller ones as it getting to much fuel. Another tech told me the jets have nothing to do with it when at idle and I needed to change my power valve from a 6.5 to a 4.5 to reduce the fuel going in.

I see a lot of conflicting information. So looking for some advance to fine tune her.
 
#2 ·
The tech who said do not mess with the jets is correct. At idle the main jets are not in use.
Power valve should be less than half of your measured manifold vacuum, or else you risk it opening and enriching your fuel to air mixture.

You didn't mention your cam, but since you have this problem and are using an aftermarket carb I assume it isn't still stock. That desire to emulate your hero's rough idle is a costing you actual performance.

Big Dave
 
#4 ·
Larger Dave and COPO are spot on with their suggestions. If your distributor has a vacuum advance is it connected to ported vacuum or full manifold vacuum? If connected to ported, connect it to full manifold vacuum instead and plug off the nipple for the ported vacuum. This will have several benefits, one of which is to increase your idle speed and you can reduce the rpms with the curb idle screw and thereby close down the primary throttle blades to return to the idle speed you had when it was connected to the ported vacuum. If your engine is running rich it may be because your low vacuum is requiring you to open the primary throttle blades too far exposing too much of the transition slot. You mentioned 650 idle rpm which is pretty low, so you may be ok with the setting of your primary throttle blades and this may not be an issue. Just something to consider.
 
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#5 ·
PLEASE, do not connect the vacuum advance to full manifold vacuum UNTIL you verify the vacuum canister only delivers 8 to 10 crankshaft degrees of added timing. Doing so can give far too many added degrees of timing you really do not want.

Engines like between 20 and 24 degrees of IDLE timing, but, jacking the INITIAL to that spec will cause serious starting trouble.

A simple home made, or Crane 99619-1 stand alone degree stop would help immensely to stop down the vacuum advance degrees of timing for full manifold vacuum use, and they are not expensive, nor hard to install.

If you would like a FREE complete install and pictures package on how to do this operation right, please send a real world email and ask for them.

gmvacuumadvancemodifications@gmail.com

You have nothing to lose but time, in getting this part of it right, and, I would suspect the throttle plates are open too far to have the idle transfer slots in the correct position. Doing the timing right will help get it a lot closer. Getting the throttle plates closed down will help to raise the idle vacuum.

BTW, the ONLY adjustable vacuum advance worth using is the Crane, as they work down to 4 in/hg vacuum, others simply do not. On any vacuum advance, the Crane stand alone plate is the easiest way to go, but, DO NOT MOUNT IT THE WAY CRANE SAYS TO, DO IT MY REVISED WAY.

Before everybody starts yelling and whining that I am wrong, please realize, I do this for a living, fix mess ups on ignition systems and curves, EVERY DAY.
 
#6 ·
dave ray said:
PLEASE, do not connect the vacuum advance to full manifold vacuum UNTIL you verify the vacuum canister only delivers 8 to 10 crankshaft degrees of added timing. Doing so can give far too many added degrees of timing you really do not want.

Engines like between 20 and 24 degrees of IDLE timing, but, jacking the INITIAL to that spec will cause serious starting trouble.

A simple home made, or Crane 99619-1 stand alone degree stop would help immensely to stop down the vacuum advance degrees of timing for full manifold vacuum use, and they are not expensive, nor hard to install.

If you would like a FREE complete install and pictures package on how to do this operation right, please send a real world email and ask for them.

gmvacuumadvancemodifications@gmail.com

You have nothing to lose but time, in getting this part of it right, and, I would suspect the throttle plates are open too far to have the idle transfer slots in the correct position. Doing the timing right will help get it a lot closer. Getting the throttle plates closed down will help to raise the idle vacuum.

BTW, the ONLY adjustable vacuum advance worth using is the Crane, as they work down to 4 in/hg vacuum, others simply do not. On any vacuum advance, the Crane stand alone plate is the easiest way to go, but, DO NOT MOUNT IT THE WAY CRANE SAYS TO, DO IT MY REVISED WAY.

Before everybody starts yelling and whining that I am wrong, please realize, I do this for a living, fix mess ups on ignition systems and curves, EVERY DAY.
Click to expand...
Yes, good point about limiting the VAC. I tried a couple of different VAC's and finally ended up using the Crane With a limiter as Dave suggests to stop mine down to 10 deg max advance. I didn't like the way the stop supplied by Crane fit in my MSD dist so I made one like Dave suggests as shown in the thumbnail. I should have pointed that out in my earlier post suggesting use of a manifold vacuum source instead of ported. Thanks for pointing this out Dave.
 

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#7 ·
I have used a MSD Pro-Billet Street Distributors part number 8361 that ships with a mechanical stop for vacuum advance as well as including an adjustable vacuum can on the distributor. I don't like long duration cams desiring to build my power lower in the RPM range and add gears to the tranny to cover the distance I am going to race. (Hey I still have my CDL and don't mind shifting twenty times or more between strop lights).

Seriously with an OD tranny you have an extra gear why not use it.

Big Dave
 
#8 ·
The thumbnail shows one person's version of the "home made degrees stop" that I outline in my vacuum advance modification instructions. I developed that home made stop method when Limeworks Street Rods, in Whittier California, had issues with EVERY ZZ4 engine timing plot they installed into their complete build hot rods. Seems GMPP set the curves on a dyno, NOT the right place to do anything for real world fine tuning. They were literally forced to do so, because those ZZ crate engines were vended outright, and no GMPP control was in effect for them to NOT "sneak" their way into California emissions controlled vehicles.

So, I took a distributor, sped up the already correct mechanical 22 degrees of timing, from the GMPP start/limit points of 1,300 start, 5,500 limit, to a real world 850 start, 3,200 limit. The reason the curve was so slow, was to allow the vacuum advance to be used on their recommended PORTED vacuum, giving two acceleration curves, and NO light to no load assistance.

Once the vacuum advance was limited to 8 crankshaft degrees (pull pin travel of .086 to .090), and the vacuum advance sourced to full manifold vacuum, the engines ran smoother, accelerated nicer, no pinging, and ran cooler, just as they should have in the first place. their 11 initial degrees gave curves of 11 initial, 22 mechanical, for 33 total, 11 initial, 8 vacuum for 19 IDLE, and, these same modifications work when done to all ZZ engines with stock large HEI in place, and replacement distributors as well. Carbureted ZZ engines with stock GMPP ZZ HEI have the SAME curves as all other carbureted ZZ distributors.

The Crane stand alone plate, 99619-1, is a lot better to do, gives active adjustment, easier to mount, all that is in the instructions and pictures package I alluded to above.
 
Discussion starter · #10 ·
I advanced to what I believe is 12. My vacuum rose to 12" while in park. I think this is the max I can go now as I can't adjust my curb idle screw anymore to lower the rpm. In park I'm at 700rpm. Of course when in Drive my vacuum drops to around 8". So should I go ahead and install the 4.5 power valve? Attached is a picture of my timing marks. No numbers but my assumption is each tic mark is 2.
 

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#11 ·
OP, please outline the ignition system you have, distributor, initial, mechanical, and vacuum advance degrees, what vacuum advance sourcing, so we can get that right, BEFORE you attack the carb.

Yes, do the 4.5 PV, it can be changed later if needed.
 
#12 ·
So sorry if I missed it (I am getting old, so perhaps these old eyes just didn't see it) but I am not sure what camshaft you have, just from this statement:

"The engine has a 350 cam..."

Is it a GM camshaft that originally came in the 350 horse 327? From which year?

OR..... is it the camshaft that GM used in the L48 350?

Big difference. Not trying to argue semantics, but Dave and Dave and everyone else here will need to know for certain before giving you base lines to start tuning.

If you have the casting no and or the part number that would be a good place to start.
 
Discussion starter · #13 ·
No it's not the original cam. I bought a rebuild kit from the machine shop and he referred to it as a 350 cam that's stock. The distributor and VA is original. I did remove the points and installed the pertronix ignitor.
 
#16 ·
The most important fact on the cam card is the intake valve closing point in degrees ABDC. That is where the capture is complete for the fuel/air mix that makes compression. Anything before that, some of that volume of compressible mixture is blown back up the inlet system, destroying vacuum.

Anyone ever see a low vacuum engine have low vacuum @ idle, then, as the rpms were raised to, say, 1,200, get significantly better? Well, the cam is the same, the intake closing point is the same, how come the engine makes more vacuum there? Well it is directly related to 'real world time. At idle, there are a set number of intake closing events, so, the REAL WORLD TIME between them is fairly long, and this allows more mixture to be blown back up the inlet port before the intake valve closes.

Now, INCREASE the RPM's, and what have you done differently? More RPM's give more intake valve closing events, CLOSER TOGETHER, which is LESS REAL WORLD TIME for the valve closing to allow mixture to get forced back up the inlet port, past the valve.

The shorter the time allowed for compressible mixture to be blown back into the manifold creates two things, more compression, and less mixture reversion that kills vacuum.

One day, we will have to go over Dynamic Compression Ratio, it changes as all this intake valve closing, RPMS, and time come into play and change.

I would still like to hear the timing specs and distributor type, because that engine would really like to idle at or near between 20 and 22 deg/BTDC.

Also, you might check the carb for the secs throttle plates being too far open, some rebuilder's do that, and it isn't correct.

The right way would be to make sure the secs plates just stop from full closed, just stop from binding in their throttle bores, NO MORE. On a Holley, for example, there is a throttle plate stop screw threaded into the secs base from the bottom of the plate. Unscrew it until the secs plates bind in the bores, then, tighten screw to get 1/8th turn after just contacting linkage on carb side, that is secs set right.
 
#17 ·
Dave, fantastic explanation above. Thanks for putting that into words i think most people can understand. Since you brought up dynamic compression, it made me think about advancing the camshaft. Will the same cam advanced raise the vac level of the engine at the same given idle rpm?
 
#18 ·
Yes advancing the cam increases bottom end power were you feel it.

This is why almost all cams are marked so that if installed "straight up" it is four degrees advanced from the cam card.

You can advance it even more when you are degreeing the cam while you are doing your initial set up. Which also includes measuring and modifying if required valve relief depth in your pistons to verify you have enough piston to valve clearance on both the intake and the exhaust valves. Retarding the cam yields higher peak power numbers at the expense of street manners.

Big Dave
 
#19 ·
From what seems like centuries of my personal engine building mayhem, I always advance a cam when I install one that is to be driven by a chain. For a hyvo link type chain, 3 degrees, roller type, 2 degrees, because I have never seen a timing chain that didn't stretch.

For a gear drive, no more than 1 degree, but usually not needed. There are massive differences in gear drives, there are ones that rotate the cam reversed from the crank, 2 gears, an idler set with 3 gears, and the worst gear drives on the planet, Pete Junkson "dual idler" types with 4 gears and link bars. If you go for a gear drive for anything, Milodon and other 3 gear idler types that use an added plate to support the idler gear IS the way to go.

If you really get into the way an engine works, you will come to the realization that although it is a machine, it is somewhat active, and...alive. So many factors that all interact with each other, that if only one facet is altered, the other parts see the change and react.

DCR is important because of what the engine sees as compressible mixture changes as RPM's change, up to the point where the engine "hits" the power band. When you feel the cam "come in", what you are actually feeling is the time of operation coming to the point the later intake valve closing time has little to no effect on reversion of mixture, and the mixture capture efficiency is at its greatest, no power loss form mixture getting pushed back out of the cylinder, into the inlet manifold.

Engines are like little human beings, to a point, only you don't have to tuck them into bed at night.
 
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dave ray
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