Dan,
What would this VC1810 do?
How can I read this paper that Duke wrote?
Thank you,
Kurt

 

Barry, We actually discussed the thought of a slipping harmonic balancer but as it was also brand new we ruled out the possibility of that being related.
Can I check this by pulling the distributor cap and watching for the rotor to point to number one plug and then the timing marks should be on the scale. Is that correct??
Kurt

 

I'd NEVER debate Duke (or JohnZ, or Lars) when it comes to ANYTHING on these cars. I'm the first to admit I'm still trying to learn myself.

My understanding is that Kurt doesn't have a VC1810 can, he still has the original can on his car. The VC1810 can is a replacement can that is very close in specs to the original one.
The differences between a VC1810 and a VC1765 is the amount of vacuum it supplys for the vacuum advance. I could see the VC1810 not being a perfect replacement but the actual original can being wrong??
Also, I thought I read an earlier post where Kurt test drove the car with the vacuum can unplugged and it still pinged so I'd imagine that should eliminate the can from being the source of the trouble because of too much vacuum advance. Even with the can hooked up he is supposably only getting 50* total timing plus vacuum advance and that shouldn't be too much.
Remember, the vacuum advance is in full at idle (assuming it's hooked to a full manifold vacuum port) but as you are accelerating and the motor is under load the vacuum advance is not operating so it's not really part of the equation anymore.
He is getting pinging under acceleration but that's when the vacuum can does not come into play so while I may be way off base here it would be my guess that the vacuum can isn't the issue and that the problem lies elsewhere

 

yes, partially
rotor should be pointing to the #1 plug lead on the cap, index on the balancer should be lined up with mark on the index plate but you need to confirm that all this is happening with the #1 cylinder piston in the actual TDC postion. You can do this with a tool called a piston stop or just remove the #1 sparkplug, place your finger over the sparkplug hole and have someone crank the bump the motor over (pull the coil lead off the distributor first so the motor won't fire up).
When the #1 piston reached TDC it will cause a compression that will blow your finger off the sparkplug hole.
once you know the piston is at TDC than check that all the other items are properly lined up - rotor pointing at the #1 plug position and the balancer mark lines up with the index plate mark

 

We did a test on the vacuum can today and it holds a nice steady 20 pounds or whatever it si called. I think that rules out that the diaphragm is leaking. Also, it is getting its vacuum from the plug at the base of the carburetor as the preformed metal lines line up perfectly with that location and I am assuming that is what you guys refer to as ported vacuum. Paul tested vacuum on the manifold also and said it was excellent. That's what is so puzzling, everything checks out exactly as it should but the car runs lousy, idles great, but runs lousy on the street. Right now it is getting to be a pretty nice looking and expensive portable radio on wheels.

Lars, help,,,,,,,,,,
Kurt

 

With some of the cheap import parts we get I wouldn't rule that out. I finally concluded my new balancer was off from the beginning or slipped right away when I tried timing my car with a light. I finally gave up and did it the old fashioned way, by ear. There are two things you go by when timing this way. The most obvious is pinging. The second thing is if your starter seems to "kick back" when starting the car. Both things indicate your timing is advanced too far. I would retard the timing until the pinging is gone and the starter sounds right. I can't say whether or not timing is the root of your problem but you don't want to drive your car too much if it's pinging.

 

To all:
Not that this will help this problem, but this is what Duke has written on vacuum cans in a previous post. Using his recommendations as a starting point, this will at least eliminate 1 variable. It wouldn't hurt to make a copy of this for future reference as it may not be posted again (read his last sentence).

The basic rule for vacuum can selection (henceforth referred to as THE RULE):

THE VACUUM ADVANCE CONTROL SHOULD PROVIDE FULL ADVANCE AT NOT LESS THAN 2" LESS THAN PREVAILING IDLE VACUUM AT NORMAL IDLE SPEED WITH APPROXIMATELY 24-32 DEGREES TOTAL IDLE TIMING.

This is a system engineering rule of thumb, and total idle timing should be in the upper half of the range for "big" (high overlap) cams and the lower half for "mild" (low overlap) cams. With a 16 degree vacuum can this is achieved with 8-12 degrees of initial timing for mild to medium cams and 12-16 degrees for medium to big cams. Based on overlap, the "300HP cam" is "mild", 327/350 and all BB cams, except L-88/ZL-1, are "medium", and all SB mechanical lifter cams are "big". L-88/ZL-1 cams are "REAL big".

Idle vacuum in neutral is an inverse function of effective overlap, and the range on C1/C2 engines is the very high overlap 30-30 cam, which only pulls 10"@900 to the low overlap base engine SB cams (which were also used on some optional engines) that pull about 18"@500. All others are in between, except L-88/ZL-1, which are pure racing engines that were never intended for street use so they were not equipped with vacuum cans. In all cases, typical idle vacuum is effected by both idle speed and total idle timing. Higher idle speed increases vacuum and, up to a point, so does increasing total idle timing, which is why high overlap cams need both higher idle speed and higher total idle timing.

Initial timing should also be established to keep maximum WOT timing in the 34-40 degree range for SBs and 36-42 degree range for BBs, and WOT detonation may dictate the lower end of these ranges depending on compression ratio, cam, and operating conditions such as ambient air temperature and altitude. Higher ambient temperatures promote detonation as do low altitudes where average air density is higher.

Higher overlap increases exhaust gas dilution at idle and cruise, which slows flame propagation speed, which increases the timing requirement. Insufficient total timing at idle and low speed cruise increases EGT, which will cause more heat to be absorbed by the cooling system, which can result in high operating temperatures and, in extreme cases, overheating to the boilover point, even if all cooling system components are within their original performance range.

Using THE RULE, one of the following three NAPA/Echlin vacuum cans should be appropriate for all C1/C2 OE engines, including those converted from ported to full time vacuum advance and C1 engines that are converted from the non-vacuum advance dual-point distributor to a single point vacuum advance distributor.

Vacuum cans for modified engines (such as cams that alter OE idle vacuum characteristics) should be selected using THE RULE.

My system engineering "best fit" for all OE engines is also listed including those not originally equipped with vacuum cans, but a few "best fit" vacuum cans (396/425, 427/435 and '63 327/340,360) are significantly different than OE, due to either a poor match to engine idle vacuum characteristics i.e. don't meet THE RULE ('63 327/340) or will not meet THE RULE when converted from ported to full time vacuum advance (396/425, 427/435, '63 327/360). My "best fit" for 327/350 is also different than OE, which I discuss below.

VC680 (stamped "B1") 0@8". 16@16" (283/220, 230, 245, 250, 275; 327/250, 300)
VC1765 (stamped "B20") 0@6", 16@12" (327/350, 396/425, 427/390, 400, 425, 435)
VC1810 (stamped "B28") 0@4", 16@8" (283/270, 290, 315; 327/340, 360, 365, 375, L-88/ZL-1)

These same Dana Controls manufactured vacuum cans are also marketed under other brand names/part numbers such as Delco, Borg Warner, and others, but I don't have a complete part number cross reference list. The alphanumeric code stamped on the mounting bracket (B1, B20, or B28) is the code that denotes the specifications regardless of the brand name/part number.

If you're at the ragged edge of THE RULE a small increase in idle speed - on the order of 50-100 revs - will usually achieve the 2" difference since vacuum increases with increasing idle speed, and IMO some OEM recommended idle speeds, especially on SHP/FI engines, are unrealistically low - mechanical lifter cam engines should be idled in the 800-1000 range, and add at least 100 revs with FI. OE vacuum cans with Powerglide may not achieve the required 2" margin idling in Drive, in which case the next more aggressive can should be installed.

The correct total vacuum advance for most pre-emission Corvette engines is about 16 degrees. Any ported vacuum signal lines (such as SHP big blocks and '63 327/360) should be converted to full vacuum advance, and on some of these applications, a new can (B28 for 327/360 and B20 for 427/435) must be selected to comply with THE RULE. The OE '63 327/340 has full time vacuum advance, but the OE vacuum can does not comply with THE RULE, so it should be replaced with B28.

A "more aggressive" than necessary vacuum can (significantly more than 2" difference between idle vacuum and full vacuum advance) is okay (but not necessarily "ideal") as long as there is no detonation. Too aggressive vacuum advance may cause transient detonation, such as on upshifts or part throttle acceleration. One choice is to reduce initial timing, which may reduce total WOT timing below optimum. Another is to install a less aggressive vacuum can as long as it meets THE RULE. The "best choice" is to install the "best fit" vacuum can.

Using THE RULE, one of the three above mentioned vacuum cans should provide full advance in the range to at 2"-4" less than typical idle vacuum, which is the "best fit" range.

For example, 327/350 was originally equipped with a Delco can (stamped "236-16" - last three digits of the "long" GMPD number and maximum crank advance as are all other OE Delco cans) equivalent to the current B28 replacement can, which is more aggressive than necessary. Since 327/350 pulls enough idle vacuum (14"-15"@750-800) to keep B20 pulled to the stop at idle with 2"-3" margin, it is the "best fit".

A "not sufficiently aggressive" vacuum can - one that does not keep the plunger pulled to the limit at idle to "lock-in" maximum vacuum advance can cause high coolant temperatures due to insufficient total idle timing, and variation in total idle timing due to an "unlocked, dithering diaphragm" can lead to idle instability, poor idle quality, and even stalling! Engine run-on at shutdown is also a symptom of too little total idle timing, which heats up combustion chamber surfaces and causes preigntion that can also lead to detonation during normal operation. My '63 327/340 suffered from these problems for several years until I realized that the OE 15.5" can (essentially equivalent to the B1 can) was not suitable to an engine that only pulled 12" at idle, so I replaced it with a Delco 236-16 8" can (equivalent to current B28), which solved my idle quality/stability/run-on/detonation problems, and THE RULE for vacuum can selection fell out as sure as E=mc**2 fell out of Einstein's Special Theory of Relativity.

One other issue. Some think that ported vacuum advance is "correct", but it is NOT on pre-emission controlled engines.

Ported vacuum advance is an emission control technique to increase EGT, which promotes oxidation reaction in the exhaust, but it also increases operating temperatures, increases the tendency to detonate and run-on at shutdown, and increases fuel consumption. With a handful of exceptions, all GM pre-emission engines equipped with vacuum advance used full time manifold vacuum.

For some inexplicable reason, the '63 FI engine used ported vacuum advance- the first year vacuum advance was used on Duntov-cammed engines. Maybe GM thought that idle quality (always a problem on FI engines) would be better with ported vacuum advance, but it wasn't, and the '64-'64 FI engines got full manifold vacuum advance.

L-72/71 have ported vacuum advance to meet CA emissions since there was only one version of this engine for all 50 states.

If your experience with Corvette engines does not go back to pre-emission engines, then all you've ever seen is ported vacuum advance on emission controlled engines.

I am never going to talk about vacuum cans again. EVER!

Duke

 

I replaced my 80 L82 with a ZZ4 and found that with the distributor supplied with the ZZ4 engine I had detonation at part throttle (slight accelleration). I didn't wnat to affect the total mechnical advance to I worked with my local parts house and found a vacuum can that pulled in less advance.

After installing the can the detonation was gone. I run nothing but high octane unleaded premium.

The vacumm cans listed in the response could help the situation.

Items not to over look is a small vacuum leak or carb too lean which would cause detonation

 

Great thread....have been working on my distributor. So far no problems here..

THINKING OUT LOUD HERE.....
What about a combination of higher temps (with the 190-200* thermostat) and a lean fuel condition? I believe I read that Lars had done this carb, so the carb should be good. But if a manifold gasket was old or used or leaking???? Any other vacuum leak....this would cause a lean condition. Plug all the lines to test

To check for a lean condition ...PULL SOME PLUGS...ck them out. This will tell what's going in in the chamber.

and finally time the engine by "ear" JUST IN CASE the damper is in question of being defective.

JUST MY 2 cents Best of luck
Mark

 

These idoits are all talking rubbish - leave the timing alone - select favourite music and turn your boombox up full - problem solved.

 

Addressing the possibility of the "new" damper timing marks being misaligned, here is my experience. I replaced my original damper with a GM replacement from a name-brand supplier. On my short block, I determined absolute top-dead-center of number one with a dial indicator. Damper TDC "Zero" was actually 4 degrees advanced. It never hurts to check.

 

I sure thought this sounded like too much vacuum advance, but obviously I was wrong. This is a new rebuild, just installed? I thought I read that it ran fine and just started this, then read it had been at a machine shop for rebuild. If it is a fresh rebuild, did the shop mill heads and or use higher compression pistons, therefore raising the compression? I can't think of anything that would cause pinging without too much advance other than too much compression. Good luck, i'll be

 

Kevinator,
It is a new rebuild and ran fine sitting in the garage. My original block was sent to machine shop and new cam crank pistons, bearings etc were fitted using factory GM parts for L79 motor. Engine had 5-6 hours running time sitting in garage in car up on blocks, never driven until just this past week. That is when I noticed the pinging. Machine shop was under strict instructions to build factory stock engine. I supplied the factory correct cam in new unopened box from GM and the crank, etc, they ordered the bearings and cleaned and balanced etc, and assembled. They also checked the distributor and the Chevy guru at the shop came highly recommended by several local car enthusiasts.
That's about the extent of what I know.

 

Just for fun...let's run a good cranking compression test to see how much we're dealing with. It will give us an idea of how close the cam timing is to being correct and if there are any unsurmountable items were' fighting..like goofy cam degreeing, wrong pistons etc.


JIM

 

I will need to buy a compression gauge first as I do not have such a tool and I am really not to keen on taking my car to a shop and letting them screw with it. I have pretty much done everything on this car since 1972 when I learned this lesson the hard way. Things get chipped, scratched, dented, broken, and no one knows how it happened.
Its hard to find a shop that gives a damn these days. I used to work for a Chevrolet dealership and I can tell you horror stories of what I saw and had to deal with as office manager. Customers cars used for lunch runs, parts removed and forgot to replace, work charged for an not performed, parts substituted for expediency with wrong parts.
Sorry, I am jaded but it is based on actual a experience. I'm not going down that road again.
I will have to get a gauge and learn to do the compression test.
Kurt

 

oh yeah, that's a good response.

do you have ANY idea what detonation can do to a motor and how quickly it can destroy pistons?

 

Kurt
there is a BIG difference between running as motor in a garage and on the road. In a garage the motor isn't under load and than issues like detonation, vacuum issues, and other things will show up. A lot of things won't show up unless the motor is under load.

I was with a good friend of mine today who knows these older cars inside and out, has been doing his own work on all his Corvettes since the 60's and who's judgment I'd take over pretty much any mechanic out there.
He agrees that with what has been found so far thru the info in this thread that it's pointing to the balancer.
i'd again suggest you check that the balancer was installed and degree'd correctly. It could easily be off by 20* or so and that would easily explain the problems you are experiencing.

Comfirm that when the #1 piston is at true TDC that the index on the balancer is correctly lined up to the index mark on the index plate and that the distributor rotor is popinting to the #1 sparkplug wire on the distributor cap.

 

I am going to try to contact the machine shop that built the engine tomorrow and take the car over there and let them look at it if I can reach the specific guy who built it. It is a two man shop so hope to have him look at it. A few e-mails have suggested that perhaps the cam was installed one tooth off. I assume only way to check that would be to rip the front of the motor off and look if the timing marks on the crank and cam line up. That's something I really want to do now that the car is completely detailed and I haven't been able to drive it in over 10 years. At this rate I will be dead before this car is driveable. I'm not exactly 21.
This has ruined my whole weekend and I am extremely pissed.
Kurt

 

Barry,
The balancer is slotted for a keyway on the crank snout. It can only go on one way. It was a brand new in the box factory correct finned balancer from Paragon. ($386.70) Other things I have been told is possibly the carb, the cam, or as you say the balancer "slipped" on the rubber but it hasn't been run long enough to do that.
Kurt

 

well, let us know what the shop finds when they get a look at it