JB

Just can't get the popcorn rattle to go away. Got the timing set at 2degrees BTDC, running Chevron 93octane + 2 bottles of octane booster + lead substitute, weights & springs on distributor seem to work fine, & I've run two bottles of Techron through to get rid of any carbon. Car pulls like a champ, no rattling, until it reaches close to operating temp (which is always between 190-200). Then if I put my foot in it at all, or even if I go up a reasonably steep hill, it sounds like one of the those "popcorn popper" toys. Gettin' real old. Any other suggestions?

JB

mrvette

I postednon another slightly related thread, about what's going on...

in short, you got too much dynamic compression...not necessarily the static calculated by math type compression, but that combined with the performance cams can yeild too much running pressures...

gotta drop something, or maybe check your engine temps for accuracy...


I suggest your running a mismatch on your cam and static compression...

GENE

NHvette

JB,

It sounds like it may be a localized overheating - perhaps some buildup
in the water jacket surrounding a cylinder or head. Maybe a good coolant
flush. If the radiator is good - then maybe one of the coolant system
cleaners that some folks have used.

As a last resort: lower the temp of the thermostat - assuming the system can
pull the temp lower than 190. Go with 160.

Although I don't fully understand the relationship between static
compression and the cam == dynamic compression, I tend to believe
that it is not overheating. Overheating would give a sharp "pinging".
You describe a lower toned popping - possibly coming from the exhaust area ?

Gene - do you mean that because a longer duration cam will allow the
intake mixture to keep piling into the cylinder - after the piston has reach
BDC - you get more air-fuel charge than you bargained for ?
Would "degreeing" the cam help to tweak this out ?


:seeya


[Modified by Daves_rusty_75, 5:48 PM 9/3/2002]


[Modified by Daves_rusty_75, 5:48 PM 9/3/2002]

burners

What is your timing curve? Perhaps the slots in the dist. are worn to the point that they are allowing more advance than they should. You may also want to delay the advance a bit with stiffer springs. Does it ping during part throttle? Can you jamb the gas pedal to the floor and have the pining go away? If that's the case, it's vacuum advance. Get an adjustable advance cannister from Jeg's of Summit.

flynhi

Take it to Leonard at AutoTek in south Austin and have him recurve the distributor.
This is a reach but cold air from cowl might help with pinging - it will certainly help with HP!


[Modified by flynhi, 6:17 PM 9/3/2002]

JB

I'm pretty sure it's got the stock cam. If I jam the pedal to the floor it rattles like mad. Not sure about the advance curve--I'll go look up Lars paper ('less someone wants to give me the quick & dirty on how to figure it out here). Might try the stiffer springs, though the fact is, when I disconnect the vacuum advance entirely, it still rattles. Where can I get a set of those springs, BTW?

I don't much like the idea of using a cooler thermostat--everything I've read seems to say you need your engine to be running at least 190 for your oil to work right & to keep acids & water from building up. It could be that the stock temp gauge is reading wrong, though I'd think I'd hear boiling or the cap would be releasing steam if it were getting really hot. It's kind of annoying that I can't look directly into the radiator and see the condition of the cores & such.

Any other thoughts?

JB


[Modified by JB, 5:18 PM 9/3/2002]

daily_driver

JB, get an adjustable timing light so u can map out your timing curve......

sb69coupe

Go to http://www.corvettefaq.com and get Lars' paper on timing. Sounds like you've got too much mechanical advance at higher RPM. You need to map out the advance curve and the vacuum advance to understand what your total timing is.

One thing you may want to try in the interim is to buy some race gas and blend up some higher octane fuel. See if that will eliminate the ping.

Shannon

Maurice

There are some very differing opinions on octane boost in a can out there, you should try do a search of all of the forums for "octane boost" in the archives and do a litte reading on some fo them. I think that the concensus is that they don't do very much. I haven't had any luck at all using them in my 427. I use a couple of gallons of 110 octane with each fillup and I have solved my problems, when I want to do a little more spirited driving I just use a little more of the higher octane stuff. Good luck reading up on the older topics and I am sure that some of the folks here have some outstanding advice.

mrvette

Lemme see now, been a while, it's...advance the cam for more lower end torque....he disconnects the vac adv. and still pings...with 2* initial...that means his dist would have to be totally screwed to do what he's indicating...that means he has iron heads and high compression...maybe a lean enough mixture to compound the problem....mild cams tend to raise compression as they are tuned for more lo end torque....hi compression and radical cams go better together, but if the cam is advanced too much it could cause problems....

I have had the same problems on other cars, another reason DPFI and I are such good friends....seems that today's gas just will NOT run well on a carbe'd hi enough compression engine....ping all to hell, especially with iron heads.....
I"m just quoting bitter experiences over the last 30 years of this garbage gas and pinging, and hotrodding, and cams and heads, and carbs, and such junk.....
overheating is certainly one cause/effect of this problem...and compounds the sh ituation evermore......overheating defined as anything over 180f...

GENE

Langadorf

Assuming you have a stock cam and stock compression, the first thing I'd do would be to verify timing w/ a timing light. You want to be sure it's where you think it is. If you can't do that right away for whatever reason, put in heavier springs to see if that helps any.

Next I'd check your spark plugs - make sure they are the proper heat range for you motor.

Then check your carburetor - make sure no jets are clogged and your power valve, idle etc. are in proper working order.

If the problem isn't solved any of these steps, I'd suspect a carbon buildup. Even running techron through your system won't clean out old hardened carbon buildup. When I pulled my heads off I had to scrub for hours to get the chambers and pistons clean looking again. I couldn't believe how much carbon buildup there was from just a couple years. Short of removing the heads, you may try the water/seafoam trick that has been discussed multiple times recently to see if that helps out. What you do is get your car warmed up and at a high idle (3000 RPM) SLOWLY pour a cup full of water down main barrels of your carb.

PRNDL

1. Just to see if it makes a difference I would put in some race gas. Available at your local dragstrip. Can you get aviation fuel at the airport? I would run the tank down to 1/8 or less and throw in a few gallons.

2. I got some distributor springs with a Mr Gasket recurve kit. Pretty cheap and easily available. Didnt use the weights or bushing that came with.

3. My L36 produces gobs, tons, mucho gusto more power at 180 degrees vs 210. I thought the factory 195 stats was more an emissions issue vs a wear and tear issue. I have seen the "graph". To me it looks like 180 - 185 is a fair tradeoff and you don't get into a lot more wear unless you run at 160, but that is just my guess - no experience to base this on.

My engine is original and no pinging but I dont know what has been done to it internally. MJ

JB

Just to sum up: yep, iron heads and original 10.25:1 compression. I checked the timing today with a timing light before going on a 20 mile run, still at 2degrees, still pinging. Plugs are the stock heat range. I'm gonna go buy a new timing light that's adjustable tomorrow and do the timing curve thing, though it seems to me from reading the papers at corvettefaq.com that that's more for making power than curing ping.

Now, about the race gas/airport gas. How the heck do you do that? Just drive up to the local civil-aviation type airport and start looking for someplace with pumps? Do race tracks generally sell gas to anyone who drives up? Can I put it right in the car at the airport/track, or do I need to go there with some cans?

JB

Brettmc

High compression and short duration cams do not mix well. You can get away with high compression with a long duration cam because some cylinder pressure is drained off on the intake stroke by the radical intake valve timing. Sounds like it's a distributor problem, like the advance is coming on too fast. But then again, if you have a lot of static compression and a short duration cam I bet timing will not fix it. If you could give more info on compression and cam timing it would help. As stated above, your only fix might be av/pump gas mix.

theandies

Lars paper indicates that initial timing at idle is nothing more that a check you can do periodically after you get your timing curve where you want it. 2 defrees seems very low to me....I'd start with at least 8 degrees. You need to be looking at total timing. Connect your timing light and disconnect your vacuum advance. Rev the engine while observing your timing marks with the light. I suspect you are way over advanced. 36 degrees BTDC is a good place to start. If you still have ping then back off in 2 degrees increments until the ping goes away. Springs are not the answer....all they do is regulate how fast your total timing comes in on the RPM curve. After you get the ping out you can adjust the springs on how fast your timing comes in. Are you running points or an HEI distributor? You need to get your total advance (again without vacuum advance connected) to a combination of initial (in your case 2 degrees) + total mechanical (which should be around 34 if you use your 2 degrees initial) that will give you 36 total. Make sure when you connect your vacuum advance back up that you use a ported source. This will keep your vacuum advance from working at idle and also at full throttle application. I have an MSD distributor and to adjust total mechanical advance all I have to do is change a stop bushing that limits the total amount the advance weights will move. I don't know how to accomplish this with a stock style points or HEI distributor. To sum up......check the total advance with vacuum advance disconnected (and plugged) while revving your engine by hand. You may be surprised what you find. Call Lars if all else fails, he helped me a bunch tuning my ZZ4 to the MSD system.
John

PRNDL

Not sure about the airports, but dragstrips, yes. I pulled into Englishtown for a Corvette Challenge event and there was a guy leaving in a beautiful 70 LT1 convertible. I asked him why he was leaving - the Corvette Challenge hadn't even started. He just stopped by to get gas for his LT1 ! :lol: They have pumps just like your regular gas station - goes right into your gas tank. Not sure if you would have to pay an entrance fee + the very high price at the pump. MJ

hudman

JB-

Hows it going?

Read about your pinging. As you know, we have very similar cars. As you also know through various direct emails, I have had my entire engine tuned up by a very good vette mechanic here in town. One of his goals, among others, was to get rid of the pinging. Long story short, he made it way better, but could not get rid of it entirely (and he tried everything). He did a lot of the stuff that was mentioned here. When it is warmed up, it would ping with any throttle at all.

Long story short, do not waste your time and money on octane boost. It does very little. A lot of folks on the forum talk about Kemco Oil's Lead Supreme 130 (www.kemcooil.com). You have to order this stuff direct and it cost $120 for 10 cans. 1 can to the fill up is really supposed to do wonders. I may be mistaken, but I think Chuck runs it in his L88. You may want to ask him if he likes it. I have never used it.

I run the high octane stuff as suggested by Maurice and MNJack. I have been told to stay away from the aviation gas though for some reason. I have a friend that used to have a really hopped up '70 440 GTX. To this day it is still the highest torque car I have been. He could not keep tires on that car. Anyway, that car sure did love av gas. Both the small county airport and the big international airport (at the private plane section) would allow us to pull the car onto the tarmac and fill it up. There are no pumps. You would pump it straight from a truck. We also would fill up a few cans as well. As an aside, the small airport let us do quarter mile runs on the runway after it closed. Boy those were the good old days.

Locally, I have several choices, but have really only experimented with one of them. A local BP station with C-store sells "racing gas". It is basically 100 octane fuel (unleaded). The pump is right there on the island with all the others. This is a normal cstore in a very high traffic area of town. Not some place in the middle of nowhere. You can pull right up and pump as much as you want for $3.35/gallon. I have usually been mixing @50/50 with 93 premium. Under extreme heat, I still have an occaision ping under full throttle, but very rarely. I have run a heavier mixture, and boy does the car love it. No pinging whatsoever. But it is expensive. Bottom line, in my car, nailing the proper mixture will surely eliminate pinging.

The local small track will sell you 110 octane but will not let you pump directly into your car. It is leaded. You have to pump into a can. It costs @$4.50/gallon here. I have never done that because I do not want to have to put it in a can and then transfer to the tank.

Finally, a car lot/muscle car restorer here in town will sell 5 gallon cans or 55 gallon drums of 100 or 110. This is designed for racing applications as well. It is packaged by some small no name oil company, and because it comes prepackaged it is just more expensive. I have never priced the drum, but the 5 gallon can is $27 for the 100 and $39 or so for the 110.

I would suggest finding some of this high octane stuff and mixing it. You really have to experiment to get the optimal mixture, but it is worth it. The car seems to love it, and boy does it smell good. It certainly fixed my pinging ills, and it is much easier solution than doing all of the above. You may end up there, but I would start with finding some high octane fuel.

Octane boosters are a marketing joke.

Good luck


[Modified by hudman, 4:38 AM 9/4/2002]

JB

Thanks for the advice, guys. Hudman, I wish to heck there was a BP station around here. To tell you the truth, I don't think I've ever seen one in Texas, though maybe I just ain't lookin' in the right places.

theandies--the reason for the 2degrees initial advance is that the factory advance for the '69 L36 is only 4degrees. I've got to admit I don't understand the "set the curve first" thing--does that mean that once you've got the total advance in at the right rpm, whatever initial advance is the result is the right initial advance? Or am I just missing it altogether? I do have the original non-hei distributor, BTW.

JB

hudman

JB-

Ask around and see if there is a gas station selling that stuff. It is certainly easier to find a place that you can pump directly into the car than having to go to the track and pump into a can. This place is certainly not convenient to me, but as you know, we always need a destination. I use this place as a cruise destination.

I do not think that BP sells this stuff all over. It is probably just the desire of the franchisee to sell it. He probably has a hot rod!

I was actually told that this place had it by some guys I know that race Legends cars on the weekend. In fact, the first time I went I just assumed that they would have some special pump on the side and was looking on the side of the place where they usually have the Kerosene pumps etc. and could not find it. I asked and sure enough, it was right on the normal pump island. It was an old timey single pump between 2 of the big digital multiple dispenser card reader pumps. I would have never found it if I had not been looking for it.

Maybe you got a local gas dealer selling the stuff under a different flag like Exxon or Chevron or something.

Ask around. Somebody will know where to get it.

Maurice

JB, I don't know jack about timing and how it all works either, but an adjustable timing light makes it crystal clear! I picked up mine at Checker auto for just over $100 and it is the "fancy" electronic one that also shows RPMs. The advantage of an adjustable timing light with the RPM gauge is to see where your advance is coming in at and how fast it comes in. In english that means you have your engine at idle, and you can adjust your timing light to see what it is set at, you rev it +500 rpms, readjust your timing light and see what it is set at, repeat until your timing quits advancing and you have a picture of what your timing curve looks like. From there, you can put on lighter or heavier springs to make the timing come in sooner or later and you will notice more or less power at different rpms. This doesn't help your pinging at all, but, knowing what all your setting are will help with the performance once you get the pinging worked out. Also, you will see what your total advance is now, and you back it off the top end for the time being and that might help with the pinging. I hope someone else can chime in if I am off base, but that is how I've done it. Please also remember that I am a complete rookie and take no resposibilty of any actions as a result of this post :D