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Discussion Starter · #1 ·
Hi all. I will apologize up front for this post being a little on the long side. I have been trying to tune my 302 (Stock build. All original GM parts. Cam is the Crane repo of the original. Compression is still 11.1 and the engine was bored 0.020. I did change out the points for a Pertronix module on the 480 distributor). I have been having an issue with vacuum at idle. I only seem to be able to get around 7" Hg at around 800-900 rpm and I would expect to be around 14-15 according to the service manual. I am measuring vacuum at the fitting on the back of the manifold that supplies the brake booster and RS headlights. I am connecting the headlight vacuum connection. Since the vacuum is at 7, the vacuum advance is not pulling in at idle. Also a bit worried that this low vacuum is also right on the threshold of holding the power valves closed as they are 6.5's

I have checked the valves 3 times using both the factory sequence as well as John Z's process and everything is good. Compression on all cylinders is around 160-170. I have sprayed starting fluid all over the engine and cannot hear any change in RPM.

I am running about 10 degrees initial timing at 800 rpm. The distributor and carb were both rebuilt by Jerry McNeish. The engine only has around 100 miles since it was built.

The engine runs and idles fine in the garage but most of the time when I drive the car, the idle will jump up to about 1200 rpms and won't come down. I have replicated this in the garage where, every now and then, I seem to pick up vacuum and the vacuum advance kicks in and, of course, the engine starts to increase speed to about 1200 rpm. If I drop the idle back down when this is happening, the rpms drop, I lose vacuum (and advance) and the engine will stall.

When I rev the engine about to around 1200-1300 rpms, I am seeing around 12-14 "hg vacuum.

Any ideas what would cause this behavior? I searched this forum and found a post with similar symptoms from several years ago but there was no real resolution.

The car runs great on the road. It just gets this jump in idle where the idle will stick about 1200 rpm. It rarely happens in the garage no matter how long I let it sit at idle and rev it with the car in neutral but I can get it to happen every now and then. Almost seems like being under load is playing a role in this.

I have disconnected every vacuum connection to the carb/manifold and added back one at a time and no sign of any changes.

PCV has good vacuum. I can feel it in the air cleaner breather grommet on the passenger side valve cover. I really enjoy driving the car except for this completely unpredictable idle.

What do the 302 guru's think?
 

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I had a low vacuum problem with my stock DZ302 also. I also used the same valve setting procedure that you mention from JohnZ and set them to .026” lash cold. My vacuum was also around 7-8” at idle and my compression was around 160.

I then realized that my stamped steel rockers from Howards Cams actually are 1.5 to 1 rather than the typical stock 1.37 to 1. I adjusted my valves again but this time to .032” (since it was cold) and my vacuum went up to 10” and my compression to 185. The engine idles much better now and runs much stronger.

I don’t know what the valve lash is supposed to be for your cam, but if you have true 1.5 rocker arms you may want to set it to .002” greater than it’s supposed to be when using that valve adjustment procedure.
 

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I had low vacuum issues with my 302 as well. Mine was around 7-8” and now is typically around 10-11” after I made a bunch of changes. I’d be surprised if anyone gets 14-15” at idle out of a 302 with a stock-like cam. A few things you might check:
  • Connect distributor vacuum advance to full manifold vacuum (don’t use ported vacuum). You may need to put a stop on it to limit its total addition to timing.
  • Don’t know what vacuum advance cannister you are running, but sounds like it comes in too late. I’ve attached an article about advance cannisters below. There is quite a range in cannisters as to when they first start to apply advance and where they stop out. You might want to change your cannister. I changed mine and added a stop (but I’m running an MSD distributor). You could put on a Crane vac advance unit that has the ability to control the rate at which the vacuum is added.
  • My 302 doesn’t like to idle at 800 and wants more like 1000 or so.
  • With vacuum connected to full manifold vacuum your 10 deg of initial timing is ok, but at idle you want 20-22 deg of total advance, i.e., vacuum advance should add 10-12 deg at idle and no centrifugal at idle.
  • With your low idle, power valves numbers should be reduced to about ½ of your idle vacuum, 6.5 too large. I wonder if your power valve is leaking randomly.
 

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Be sure to run the correct Z28 or LS6 PCV valve. You might pick up a little vacuum.
 

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I had low vacuum issues with my 302 as well. Mine was around 7-8” and now is typically around 10-11” after I made a bunch of changes. I’d be surprised if anyone gets 14-15” at idle out of a 302 with a stock-like cam. A few things you might check:
  • Connect distributor vacuum advance to full manifold vacuum (don’t use ported vacuum). You may need to put a stop on it to limit its total addition to timing.
  • Don’t know what vacuum advance cannister you are running, but sounds like it comes in too late. I’ve attached an article about advance cannisters below. There is quite a range in cannisters as to when they first start to apply advance and where they stop out. You might want to change your cannister. I changed mine and added a stop (but I’m running an MSD distributor). You could put on a Crane vac advance unit that has the ability to control the rate at which the vacuum is added.
  • My 302 doesn’t like to idle at 800 and wants more like 1000 or so.
  • With vacuum connected to full manifold vacuum your 10 deg of initial timing is ok, but at idle you want 20-22 deg of total advance, i.e., vacuum advance should add 10-12 deg at idle and no centrifugal at idle.
  • With your low idle, power valves numbers should be reduced to about ½ of your idle vacuum, 6.5 too large. I wonder if your power valve is leaking randomly.

OP

I was going to post similar to the above regarding your low vacuum. The attached to the above thread is the same document I attached to a thread I had about the Crane adjustable VA kit (no longer available) that also included the stop detent plate to limit the amount of VA can produce. David Ray has published info about doing this modification to VA dizzys

See if you can see the # stamped on your VA can and compare to the attached chart to see what you have. With more initial timing (20-24) with a VA can+ base timing your vacuum should come up, ideally to about 14ish. You also may need to raise idle RPM more than 800-850 to around 900+ to increase vacuum also

Once timing is sorted (read may need, you should, install an adjustable VA and use some form of VA stop plate) you may likely need to adjust carb power valve springs to another size but get your VA/timing sorted before you make, if needed, any carb changes
 

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Discussion Starter · #6 ·
Thanks everyone. Looks like I need to turn the heat on in the garage on Saturday and reset the valves. I am almost certain the after market rocker arms are 1.5's so I am probably too tight on lash. Pretty sure the cam card called for 0.030/0.030 so I set lash using John's procedure at 0.028 cold. I will get that sorted to see if vacuum improves and then go after the VA can. Easy enough to pull the distributor to check. I will post back with whatever I find. Appreciate you guys posting the links to the VA stuff as well.
 

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A few points of learning I had with the same car/engine...all stock. I used the Crane re-pop cam. I set the lash according to the JohnZ(RIP) method, but John does say that this only applied to GM OEM cams. One can not conclude that the Crane has the same .020" clearance ramps. What this means is that you can actually set the valves at .030" and pick up a better idle and vacuum. Jerry McNeish said to set at .030" and it made a difference. I did not tell him about my Crane cam. I suspect it relates to the minor differences in the cam grind...my supposition. It did help. Also, I have 10.5-11.5" of vacuum, at 900RPM. I run 14 degrees initial and use a Crane VA stop, so I only get 35-36 total advance, still a bit much, but I don't run this car very hard anyway. One thing to look at as stated, is you MUST run vacuum advance from the manifold, not the ported connection. It does matter. One thing that may be causing the idle to jump up and stay up is merely the advance can link being held in place. Not so much the can, but the points plate may be sticking. You mentioned a Pertronix conversion....double check for free movement only by the vacuum advance. Make sure the plate moves freely. You can check this manually when the car is off. Make sure it moves freely when you pull a vacuum with a piece of hose stuck in your mouth. Release the vacuum and it should snap back quickly. Sticky VA cans will add advance just by sticking in place, only to drop away for no reason. This did happen to me. Overall, I get a fair idle, no stalling, and a fat stinky idle. That may be another problem I have yet to solve.

Best of luck and post your success for all of us to learn from.
 

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Overall, I get a fair idle, no stalling, and a fat stinky idle. That may be another problem I have yet to solve.
Not to get off topic from the OP's question, but a comment to ChuckD about fat stinky idle. I had the same problem due to way too much exposure of the transfer slot in my carb. To get the idle up to where I needed it (prior to making the necessary changes) I had to turn up the idle rpm screw thereby tipping the primary throttle blades too far and exposing more of the idle transfer slots than the recommended 0.02". Fix this and your fat idle will probably disappear or at least be lessened dramatically.
 

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Not sure what the JohnZ method is to adjust valves but I would only use the EOIC method.

 

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Discussion Starter · #10 ·
John is one of the Camaro experts here and at CRG. He worked at GM and has published a wealth of info for Camaros and Corvettes particularly on valve train, timing, and vacuum advance. He published instructions for setting valve lash on the 30-30 cams that is based on the original cam drawings for the factory cams. Lash is still set on the cam base circle using his method but you set lash at 0.028 cold. However, as was pointed out to me above, he cautions that this is only good for a factory cam. I used it on a repo so I am going to revert back to the sequence in the factory service manual. Will post some results tomorrow.
 

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The sequence in the factory manual will not work if the cam is very much different from stock. EOIC is a fool proof method.
 
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Discussion Starter · #12 ·
Well...resetting the valve lash did not change anything. Still have about 7" of vacuum at idle. Can't find a part number on the VA can other than a B1. Tested it with a hand vacuum pump and it starts to pull in at 7" but I did not checknto see how much advance it produces. Also confirmed that there is nothing obstructing movement.
I did figure out that I probably have the wrong PCV. I have an AC 2507 and I think that is a somewhat generic valve for most small blocks.

I am going to pull the carb and go over it again. I seem to remember having good vacuum before it was rebuilt.

Noticed the initial timing is jumping around a bit from 10 to 12...no VA connected.
 

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Still have about 7" of vacuum at idle. Can't find a part number on the VA can other than a B1.

Noticed the initial timing is jumping around a bit from 10 to 12...no VA connected.
This is the spec for the B1 can, its what came with the 69, 302 Camaro

Part # VC680
ID: B1
Starts # hg: 8"
Max Adv (Distr. Degrees @ “Hg.): 8 @ 16-18

1 degree of dizzy Advance = 2 at crank

When you installed the Pertronix...did you measure the up/down play in dizzy shaft/gear? Their kit includes shims to address that. If there is too much play the dizzy will "lift" slightly making timing variable by a few degrees
 

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Well...resetting the valve lash did not change anything. Still have about 7" of vacuum at idle. Can't find a part number on the VA can other than a B1. Tested it with a hand vacuum pump and it starts to pull in at 7" but I did not checknto see how much advance it produces. Also confirmed that there is nothing obstructing movement.
I did figure out that I probably have the wrong PCV. I have an AC 2507 and I think that is a somewhat generic valve for most small blocks.

I am going to pull the carb and go over it again. I seem to remember having good vacuum before it was rebuilt.

Noticed the initial timing is jumping around a bit from 10 to 12...no VA connected.
I picked up about 1” vacuum going to the Z28/LS6 PCV when I was tuning my big block.
 
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