histoy

I've got an unusual issue with my 327 - 300 hp - PG engine. I just got this engine rebuilt to original specs including an OEM 300 hp cam. It idles like a dream down to 500 rpm, but picks up an intermittent miss on both banks above 1,000 rpm while in park. I'm running 30 degrees dwell and 8 degrees initial timing. The centrifugal advance adds another 14" as I increase the speed. While in drive at 650 rpm it has 17" of manifold vacuum. When I slip it into park the vacuum increases to 19". I'm running a B1 vacuum advance and it's adding 20 degrees of advance at 900 rpm in park. My problem is that the added vacuum advance is causing the miss. If I disconnect the hose and plug the line the miss is eliminated. As soon as I hook up the hose to the vacuum advance the miss comes back. I'm running with the intake crossover blocked, so I can tell there's an intermittent miss on both banks of cylinders.

I've read a post about problems another member was having, and SWC Duke suggested he use a B22 which pulls full vacuum at 15" for a manual trans. He also suggested a different number for PG that pulls full vacuum at 12". I'm not sure what I should purchase because I have higher vacuum and higher advance with the B1.

Currently I plugged the vacuum line and increased the initial timing to 12 degrees to help compensate for no vacuum advance. This is simply putting a bandaid on the problem. I really want to get a proper functioning advance that will eliminate the miss, improve fuel economy, and keep the engine running cooler. I'd appreciate your advice...Brian

Nowhere Man

Follow the two inch rule on the advance can to the engine vacuum

vark_wso

Brian
I would lose that B-1 at once & replace it with a B-20 or B-26. Also, recommend you consider installing one of Lars' Vacuum Advance Corrector (VAC) devices along with the new can. My '66 300/powerglide performs flawlessly with that combination; centrifugal all-in at 2800-3000 rpm, centrifugal + initial at 2800-3000 = 36*. The new B-20 or 26 can with the VAC in place will result in about another 12* for a sweet 48* at highway cruise.

https://www.corvetteforum.com/forums...m-advance.html

cardo0

U really have most the information u need Brian. That B1 can is still pulling to much adv as the rpm increases and your miss is some detonation occurring. It's not that harmful at low rpm but u do want to fix it. For testing vac adv I use a "Mighty Vac" hand pump connected to the vac can and observe what vac the can starts to pull in and then what vac it's full in. I use that to match my engine vac at ideal and then make sure that vac drops out by 'bout 3,000rpm. So from your info I would want full movement of that tab on the can by idle vac reading of 19" and start to move out as vac decreases. So at your high idle vac of 17" the tab should have moved out some. Others suggest a 2" vac rule/margin i don't see why if have accurate reading and already having excessive adv problems.
Something else to try is a ported vac source on the carb - unless that's what your connected to now. Port vac source doesn't add adv until the throttle blades of carb are above the idle/shut position. This leaves out the vac adv at idle which can be helpful if you are using a high initial adv to start with.

I really think you are close enough to fix this for yourself soon. Good luck and let us know what works.

Frankie the Fink

As much as I respect Lars, you shouldn't need a special 'fliz' to get the vac advance adjusted. And the vac advance can pulling too much advance as RPM increases ?

Your centrifugal weights adjust advance as RPM changes; vacuum advance adjusts advance based on load.

You can statically measure how much advance the can will provide 'all in'; but you can't measure what it actually does rolling down the road at cruise speeds from revving the engine in a stopped car. Hence the 2 inch rule as a general guideline.

I wound up putting an Accel adjustable vac advance can on my '61 - not everybody wants to do that.

Get the right vac can on there and your issues should clear up.

Donald #31176

Since your car has a PG trans you should measure the vacuum @ idle (600 RPM) while the the trans is in Drive. If the vacuum is 14" HG or below then a B20 or B26 VAC is appropriate. Above 16" Hg use a B22 VAC. In either case be sure the VAC is connected to full manifold vacuum and there is no vacuum leak. If you remove the distributor this is a good time to optimize the curve and shim up the drive gear endplay .

65tripleblack

The issue of the misfire is puzzling. Detonation...................no. It's possible that you have a "flat spot" in the carburetor. This means that the engine is idling at a spot where the air/fuel mix is momentarily lean.
Try racing the engine slightly and see if the idle smoothes out. Try decreasing the idle slightly for the same result. You might have to fiddle with the idle mixture screws to move the "flat spot" out of your idle range.

SWCDuke

The B1 is a boat anchor as it may require 18-19" to pull to the limit, so it doesn't pass the "Two-Inch Rule" on a base cam engine. Plus, it should only provide 16 crank degrees, so it is out of spec.

I recommend the 15" B22 for manual transmissions idling in neutral, which yields about 18". Since automatics idling in Drive generally pull less vacuum the 12" B26 may be a better choice, but I've never received any reliable data from someone with a 300 HP/PG. For '66-'67 all 300 HP configuration used a 12" VAC, and I figured that may have been to be compatible with automatics.

A 300 HP/PG should idle smooth and steady at 500 in Drive.

You report 17" at 650 in Drive, but that's not enough to pull a B1 to the limit.

I suggest you use a Mity Vac to pump down the VAC to 19-20", so it's locked at the limit. Then go through the idle speed/mixture adjusting procedure in Drive with a target of 500.

Once you've got it dialed in, use measured idle vacuum (with the B1 still pumped down as much as possible) and use the Two-Inch Rule to select either the 12" B22 or the 15" B26, and please report the data, here.

Another way to do this test is to disconnect the VAC and plug the signal line, then set initial timing at about 24 and go through the idle speed/mixture procedure and record vacuum idling in Drive at 500.

This may or may not solve the idle in Park miss, but it's best to get the spark advance map dialed in first.

Also, you didn't mention the model year, so please do so, report the centrifugal curve per the CSM/AMA specs and then measure it with a dial back light and report the results.

Duke

63 340HP

The miss can be caused by something as simple as the insulation on the wire to the points being worn down and shorting to the distributor body or plate when the point plate is rotated back to full advance. Check the wire. Replace the wire if there are any insulation cracks or it looks like it has been pinched.

The miss can also be promotes by high tensions wire cross fire between plug wires as they are positioned in the wire loom under the shielding tin. Parallel wires can crossfire if the insulation is breaking down. Crossing the plug wires as they snake behind the block can provide a temporary fix, until you can install new wires.

A no load detonation miss will be most noticeable on deceleration and fast idle when the vacuum is highest and combined vacuum and centrifugal advance is greatest. This is rare. If the fuel is so poor that you experience this. it will usually effect multiple cylinders (for fun try running on white gas or Coleman fuel to experience no load detonation, just not on the Sting Ray). The fix is to reduce total advance at the no load condition.

If you experience no load detonation then installing a vacuum advance limit plate can help. It truncates the peak advance, at the peak vacuum (and has no effect on the full load centrifugal advance that should be 34 to 38 degrees). The B1 can that is normally a 16 degree advance at 19 inches can be limited to a 10 degree advance all in at 12 inches (you have to test with a vacuum source to see what you get).

My guess it the problem is a short, rather than advance. It can also be a lean condition combined with poor fuel quality and heat.

Good luck.

davekp78

What is the "two inch rule"?

histoy

Duke,

We have a '64 Corvette with a newly overhauled engine. I just went out and checked the vacuum at 500 in drive. The engine runs steady at 500 in drive and the vacuum was steady just on the underside of the 17" line of vacuum on the face of the gauge. When I increase the idle speed to 650 the vacuum is steady at just over the 17" line. My wife prefers a higher idle so she doesn't have to worry about the engine stalling at a stoplight in drive. Engaging the AC drops the idle speed 100 rpm when the compressor kicks in, so it runs at 550 with the AC compressor running. It will idle fine at 500, but she feels more comfortable at 550. No worries for her. BTW, when the car was put it park the vacuum increased to 21 inches.

While working on it earlier this afternoon, I started thinking that maybe the problem was in the distributor points, the small ground wire to the plate, or the lead to the coil. I decided to replace the points and when I removed them I found the wire from the points to the coil had worn a bare spot. I thought that I had found the problem. I installed a new set of points and made a new wire to the coil. The small ground wire was fine. I was disappointed when these changes made no difference.

Two other things worth noting as long as fuel has been mentioned. The carb was sent out and professionally restored as part of the engine restoration. When I received the car it had a very slight intermittent miss above 1,000 rpm, but idled smoothly at 650. After driving it for a few days I felt that the engine was running rich at idle, so I readjusted the idle mixture screws to get the best idle setting. I was surprised at how many turns they went it, but I failed to count them. I had expected to turn them in a turn and a half. I currently have the idle mixture screws set at 1 1/4 turns, and the engine runs smoothly in park and at 650 in drive. Taking them to 1 1/2 turns has no measurable impact on the idle speed or steadiness.

plaidside

You state you have 8 degrees initial and the vacuum advance is adding another 20, so that is 28 total at idle, but you develop a miss.
I run my cars 12-14 degrees initial and my vacuum advance adds another 16 degrees at idle giving me 28-30 total at idle and I have no issues with my four cars.
I agree I would change the can using a B20 which is a NAPA VC1765 or Standard Ignition VC181. This will bring in all the vacuum advance between 11-13 hg and 16 degrees crank.
I believe you have another problem. I would install your dwell meter and timing light and with the engine running apply full vacuum to the chamber and watch the dwell and timing mark.
Does the dwell change? Is the timing mark jumping around?
Joe

Nowhere Man

The vacuum advance has 2 more HG then engine vacuum at idle SWCDUKE written a great article on this

histoy

plaidside,

My vacuum advance adds 20 degrees at 900 rpm and above in park. In drive at 650 rpm it adds 16 degrees. I've had both the timing light and dwell meter on the engine several times in the last 2 days. The dwell remains steady at 30 degrees regardless of speed, and the timing doesn't jump around, it just moves as the advances change it. The vacuum gauge needle also holds steady in park or drive.

I have an intermittent miss at 900 with the vacuum advance hooked up, but as I increase the speed from 1000 to 1500 rpm the miss becomes more frequent on both banks. When I disconnect the advance the miss goes away. With the advance hooked up the engine runs fine at higher rpm and has a lot of power when I get into the throttle.

Today I tried retarding the initial timing from 8 degrees BTDC to TDC. When I hooked up the vacuum advance the engine still missed above 1000 rpm. That would be like taking away 8 degrees of vacuum advance, but the problem remains. If the problem is there with the equivalent of 12 degrees total vacuum advance, I'm confused how adding 16 degrees advance at anywhere from 11" to 15" of vacuum is going to help me. I'm thinking that I have an off idle carb problem that is being exaggerated by additional timing, but I don't know why.

cardo0

Well for idle speeds u want to run a little rich. Not a lot rich just a little. Then as rpm and load increase u want to be stoichiometric/14.7 a/F. A cheap narrow band can tell u this and w/o a cat converter u can just clamp the sensor in the tailpipe temporarily. I believe u can find n/b O2 for like $50. Just an idea that could eliminate a possible problem. Now u shouldn't use a n/b for WOT but your not looking there right now.

Hope this can help.

Frankie the Fink

Just to recap, the OP has an intermittent miss at high idle on his parked car with vacuum advance, the miss abates when the vacuum advance is removed. The assumption is that the extra advance at high idle is causing the miss ? One suggestion is that the miss is detonation due to the advanced timing.

I'm not sure that is a good diagnosis. Detonation occurs most often under a load as in climbing a hill with the accelerator pedal held steady; if it occurred in a parked car the timing would have to be off the charts IMO.

And I certainly don't advocate ported vacuum unless you want to set your tune up back to the age of emission control and poor running.

In any event, I'd ensure the harmonic balancer (if two piece) hasn't slipped and get the right vacuum can. When that's done and this miss is still there (which I have a feeling it will be) then the OP can move on to electrical/carb areas.

What does the vacuum advance actually do ? It moves the distributor breaker plate. I'd be looking in the distributor while operating the vacuum on the can. You can do this 'old school' by sucking hard on the vac can hose (no jokes please) repeatedly.

Look for sticky plate movement, frayed wires, loose connections, general sloppy operation. Wouldn't hurt to check out the centrifugal weights/springs while you're dinking around in there...

Just for grins I'd swap out the points and condensor.

histoy

Thanks for your feedback Frankie. Saturday I removed the vac adv and the plate moved back and forth freely. I didn't try the sucking method however. Yesterday I installed new points and condenser and I replaced the lead from the coil to the points because the insulation was worn off in 1 area. The small ground wire inside the dist is neither broken nor frayed.

Frankie the Fink

Funny because I have a similar problem on my 250hp '63 original motor. With the vac advance not hooked up I have a rock steady gorgeous, idle at 650 RPM (its a manual tranny but I've added A/C so I have the idle a little high so things are good when the compressor engages). With the vac advance hooked up the car 'drops shots' at idle. A slight, occasional 'popping' from both exhausts. (Distributor rebuilt by Lars 10 months ago).

Not enough to make me tear things apart but mildly annoying and I've yet to get to the bottom of it. I rebuilt the WCFB carb, checked for vacuum leaks everywhere and run the Breakerless SE points conversion (but the car had the same problem with points).

Maybe if you figure out your issue I'll get a freebie fix...

Avispa

The Accel vacuum can kit Frankie mentioned comes with a backstop plate that attaches to the vac advance can above the vac advance pin slot and below the breaker plate. I looked for the plate by itself but had no luck. The backstop plate limits the "no vacuum" position of the vacuum advance, it's adjustable and you can dial in the right amount of total vacuum advance in 2 degree steps. You can use the vacuum canister you have now and dial in a good total vacuum advance, say 10-14 degrees. I have one of those on the rat. 16 initial timing, 36 all in centrifugal at 2200 rpm and 12 total vacuum advance. Works perfectly.

Frankie the Fink

This is the adjustable vac advance - I use it on the '61, not on the '63 as that car is judged (or has been):

http://www.jegs.com/i/Accel/110/3103...oductId=747725