BB tune up specs help, spark knock

[veltex]

Ok, I need some fresh ideas to kick start my brain, I am having some spark knock at mid rpms, my info: 396 unsure of comp ratio (saw piston tops a while back though, sort of a pop up but flattened off with just a cut for intake), head guy said the chambers looked like 122s? truck heads for sure though, very even but average compression 130 + or - only one or two. New cam, lifters, intake, carb, guides, springs, seals, been running pretty darn good lately, the package works, uses a little oil, and a lot of gas. Most recent change is HEI, that is when the mid knock was noticed, the changes were Petronix Hei flame thrower distributer, wires, and a regap of 45, thought it was fuel (still could be) been trying better gas, Chevron super around here is only 91, and in winter I think has stuff in it. OK, so yesterday I figure adjustable vacuum advance is to much, back it down a little at a time, not much of a change, dissconnect it totally and put a plug on, makes knock better but doesnt eliminate, truck still runs great without it. I road test timing from 10 to 4 advanced, 8 is where it is now, backed down the vacuum advance from 16 to 8, only seem to get about 16 out of the dist with the middle springs, total is around 30 maybe a little more, isnt 30 total a little low for a mild BB? I dont rev this old girl way up, dont have a tack but would be surprised if when I stomp on it, it makes it above 4k. I am running Autolite platinums at 45, have a new set of AC 45xls, will try just in case thats it, dont hear it at idle or wot, just mid rpm, in any gear if you hold it 1st ,2nd or in drive at about 50 to 65. Very happy with the performance so far of the new dist, it advances great and makes the truck feel torquee, just cant deal with this noise and eventually damage. Even though the total advance # seems low, if I didnt know the #s and the spark noise wasnt there, I wouldnt touch a thing. Sorry for the long post, any ideas appreciated.

[veltex]

anyone?

[dwaite72lnghrn]

Are you running ported vacuum or manifold vacuum. Do you have a vacuum gauge while you are running to see how many inches you are pulling when it starts to ping and is the vacuum steady throughout the rpm range.

[veltex]

Dwaite72, ported vacuum, no in dash guage, test guage reads a steady 15 at idle , with steady rev climbs some maybe to 20 bounces up at let off then right back down to 15, from what I can tell its fine that way- it does have an edelbrock cam (mild). It wont ping running/testing in the driveway. I have run it with the vac advance unhooked and plugged, it still does it. ?. keep brain storming for me, thanks

[miket]

Originally posted by veltex
Ok, I need some fresh ideas to kick start my brain, I am having some spark knock at mid rpms, my info: 396 unsure of comp ratio (saw piston tops a while back though, sort of a pop up but flattened off with just a cut for intake), head guy said the chambers looked like 122s? truck heads for sure though, very even but average compression 130 + or - only one or two. New cam, lifters, intake, carb, guides, springs, seals, been running pretty darn good lately, the package works, uses a little oil, and a lot of gas. Most recent change is HEI, that is when the mid knock was noticed, the changes were Petronix Hei flame thrower distributer, wires, and a regap of 45, thought it was fuel (still could be) been trying better gas, Chevron super around here is only 91, and in winter I think has stuff in it. OK, so yesterday I figure adjustable vacuum advance is to much, back it down a little at a time, not much of a change, dissconnect it totally and put a plug on, makes knock better but doesnt eliminate, truck still runs great without it. I road test timing from 10 to 4 advanced, 8 is where it is now, backed down the vacuum advance from 16 to 8, only seem to get about 16 out of the dist with the middle springs, total is around 30 maybe a little more, isnt 30 total a little low for a mild BB? I dont rev this old girl way up, dont have a tack but would be surprised if when I stomp on it, it makes it above 4k. I am running Autolite platinums at 45, have a new set of AC 45xls, will try just in case thats it, dont hear it at idle or wot, just mid rpm, in any gear if you hold it 1st ,2nd or in drive at about 50 to 65. Very happy with the performance so far of the new dist, it advances great and makes the truck feel torquee, just cant deal with this noise and eventually damage. Even though the total advance # seems low, if I didnt know the #s and the spark noise wasnt there, I wouldnt touch a thing. Sorry for the long post, any ideas appreciated.

Just my two cents...I do not like platinum plugs. From my experience and reading others, the plat plugs run hot and thats what keeps them clean.

I changed them in my dads 90 suburban....we went back to non plat plugs and it descreased pinging and he picked up some noticeable power.

MSD says not to run them with a high output ignition system as well.

I think they could possibly causing your slight ping problem. Try bringing the gap to 35 and see what happens.

[dwaite72lnghrn]

Did the distrib come with tuning specks or mechanical advance springs. When you are in the drive way and rev up the engine does the mechanical advance move the timing up smooth(vacuum plugged) and stay high with the rpms? how much mechanical advance does it look like? Is it only at cruising you hear it and when you get into it after you here it does it go away? this could tell us if you are running lean at cruise and when you stomp on it it, dump the accelerator pump and richen it up it goes away. How does the temp look when you cruise does it seem to get hotter(lean) or does it stay the same.

[gangforward]

I'm with Miket and in fact I hate Autolites in Big Blocks. I'd suggest you try another set of plugs. Personally I run good old AC Delco's in mine, and NGK's in my race car. I have never had much luck with autolites.

[veltex]

OK, changed plugs to AC45 gapped at 45, still does it, gapped at 35, still does it, tried two other stes of plugs, it aint the plugs. Dwaite, dist came with springs and specs(I have the middle set in, also adjustable vac advance, dist advances properly I think it runs it up about 18, could get more with the lighter set, but that may worsen the problem. Starving for fuel at cruise could be it, when you kick it down it runs great without pinging (at least that I can hear, its loud), and parked revs cant make it ping, it has to be under a load. I have tried a couple brands of gas. How do I make the carb(edelbrock thunderer) fatter in the middle? Am I missing something, will octane boost give me an idea if its gas related? help

[miket]

Originally posted by veltex
OK, changed plugs to AC45 gapped at 45, still does it, gapped at 35, still does it, tried two other stes of plugs, it aint the plugs. Dwaite, dist came with springs and specs(I have the middle set in, also adjustable vac advance, dist advances properly I think it runs it up about 18, could get more with the lighter set, but that may worsen the problem. Starving for fuel at cruise could be it, when you kick it down it runs great without pinging (at least that I can hear, its loud), and parked revs cant make it ping, it has to be under a load. I have tried a couple brands of gas. How do I make the carb(edelbrock thunderer) fatter in the middle? Am I missing something, will octane boost give me an idea if its gas related? help

Based on the changes you made the only things I can think of are the advance of the new distributor. Maybe you have a failing fuel pump and with the new advance curve (or changes from the old) you might be running lean at a point in which the dist is advancing timing...how about an open vac line? just thinking out loud.

[dwaite72lnghrn]

If you are cruising/vacuum high you will be running the leanest the carb can run. I am not sure on the thunderer how to adjust this but on the 1406/1411 you get a tuning kit that has the meter rods and jets. You start by looking in the tunning manual and pick the next step of richness and add the meter rod in the list. This normally is 4% which would probably fix it. The springs are for how fast you go into rich mode which is not a problem at cruise only at lower vacuum. With the new Distrib you might be running more effeciently and burning everything which could cause a lean condition. I think the tunning kit for the 1411 was about $45. The Thunderer I think adjust the same way. you could also get just the rod you need seperately.
By stepping on it or dropping down a gear and the problem goes away I would say it isn't advance You would be reving higher and more advance the problem would get worse. By stepping on the gas and the problem goes away I would say its lean and getting more gas as the pedal gets pressed down.

[veltex]

thanks guys, I dug out the manual that came with the carb, will work down the trouble shoot list, I feel confident that all the experimenting with the spark components eliminates that (I hope). too lean , dirty low speed circuit, fuel filter, air leak, and ignition system is what they suggest, I will start on those. Since this started right after the dist, lean may it, unless the other problems just happened right then.

[veltex]

Oh yeah, maybe if it is fuel I can bump the advance up to normal after its solved, what is normal advance #s? anyone else running something similar? Initial, mechanical, vacuum. thanks

[1-PU70]

Since it started with the new dist. and no other changes , I would think the advance is comeing in to fast. Go for a stronger spring before changing things that have worked before.

[miket]

Originally posted by veltex
Oh yeah, maybe if it is fuel I can bump the advance up to normal after its solved, what is normal advance #s? anyone else running something similar? Initial, mechanical, vacuum. thanks

Sorry for the process of elimination but unless you are working on a newer computer controlled car there aren't a whole lot of options. Good luck....let us know how it turns out!

[Bill72BB]

Here are the HEI advance specs GMC JiM posted awhile back.

When I installed an H.E.I unit in my '71 GMC ( 402 engine) ; I had a distributor rebuild shop set the advance curves as per the GM factory manual. The specs are :- 0 degrees @ 930 RPM; 2 @ 1260 RPM; 16 @ 2400 RPM and 30 @ 4400 RPM. Vacuum advance specs are 0 degrees @ 8" hg and 20 degrees @ 17" h.g. Initial ignition timing is 8 degrees advance @ 600 RPM ( idle speed).
... Above specs are from GM 1971 Chevrolet Truck Service Manual.
... Another GM reference is the 'GM/AC/Delco Tune-up Specs Manual # SD-100. These specs call for a 10-13 degree centrifugal advance @ 2000 RPM and a total advance ( vacuum & centrifugal ) of 28 to 35 @ 2000 RPM...... When I installed my unit about 10 years ago; I used an adjustable timing lite and set the timing to 35 degrees @ 2000.........No problems and I am well satisfied with the engine performance.............

[dwaite72lnghrn]

I will have to copy those spec's for future use.
I don't think that is his problem tho, he said he ran it with the vacuum disconnected(he never would of been at full advance) and still ran into the same issue at cruise. The only thing other than "lean" I can think of is mistaking valve clatter for spark nock/ping.

[veltex]

Bill, I copied down the specs too, I think it might be lean at cruise like Dwaite suggested, I wish I had the parts here today, but this may have to wait shipping time from tommorow, unless Napa carries what I need. I dont think its valve clatter, although they seem a little busy in there, and I do have an exhuast leak on passenger side, I am no wizard but think it is spark knock, like they say about some rocks in a coffe can, responsive to very slight up and down of the throttle on cruise in any gear, like when you power down the road get going the speed limit on level ground and ease up and maintain speed, when you are on and off the throttle in short bursts you cant hear it, like in downtown. It may take me a few days, I do appreciate every ones help, I will keep track of this post and let you know how I do, maybe someone will have a problem I can make a suggestion to. thanks

[Jerdog]

veltex, have you had one set of plugs in long enough to get any kind of a plug reading? Also exhaust leaks can drive you nuts. Sounds like pinging or a loose lifter when it's really just an exhaust leak. Been down that road before!

[veltex]

OK, changed to richer metering rods, then different rod springs, then fuel filter, one at a time, no change, the only thing left on my trouble shooting chart for mid throttle pinging, is dirt in low speed circuit (which I have no idea how to get to), and an air leak. I did replace all my vacuum lines inside the cab, they were old and brittle, and hooked canister back up, that is where I will start first, by unhooking can, then maybe the plastic brake booster elbow, pcv? Can any of the vacuum pods or vacuum canister or heater air valve have enough problems to cause a lean condition but be unnoticable at idle?

[miket]

Are you sure it is a "ping" in the true sense of the word? Sure it isn't something else making a noise that sounds like a ping? Just checking because it sounds like you are covering alot of the bases with no effect.


Are you losing coolant? Do you have a vac leak? Does it idle smooth? Any hesitation?

[veltex]

Mike, man I dont know, it wouldnt be the first time I chased my tail around one of my cars or trucks. Only had it 3+ months and have been fixing it nonstop since, it is a great truck, but I am getting tired. Coolant is fine, oil isnt milky, no hesitations to speek of, idles pretty smooth (it idle better in Sept. before winter temps, but still pretty good on winter gas), vac leak is where I am headed next. I am fighting one exhuast leak, this is different, I can make the exhuast leak worse in the driveway, and pinpoint it, the pinging I cant. I wonder if my vacuum pods or canister have bad diaphrams? or the valve on heater control? or the brake booster? I am thinking of plugging everything off to eliminate them, and then concentrate on gasket leaks, I know everything is tight, I hope its not a vac leak in the valley, how do you find these? Colder plugs maybe? time for another beer!

[miket]

Common sense tells me something happened when you changed the HEI since your pinging started then. I am thinking your pinging might be an exhasut header/manifold leak...or something else...

check the brake booster by starting the truck and depress the pedal once or twice just to make sure they are working correctly....shut the truck off....give it a few minutes and remove the vac line at the booster....you should here a suction noice as it should be a closed system and vac should have built up. If not, you have a leaky booster.

I had been trying to track down a similar pinging prob in my dads 90 'burb. Come to find out he had a small intake manifold leak...not saying this is the prob.....just exhausting the possibilites. You may have an intake manifold leak....thats possible....you can tell by spraying carb cleaner around the area and see if the motor seems to stumble....be careful though.

yeah plug everything and take of the exhaust manifold leaks and then see what your result is.

[71sub]

For what it's worth ... everything that's been said here I did, minus re-calibrating my Edelbrock carb. This is my setup :

350, Performer RPM intake, Edelbrock 600 cfm vacuum 2nds, stock HEI, Accel coil, Hookers

What I found out was the hot lead coming from the fuse block to the HEI was ONLY 18 gauge ... and when you switch to HEI, it needs to be at least 12 gauge. Also, I change my plugs to AC Delco @ .045 and ran new MSD 8.5mm wires. Finally, I set timing to 10 BTDC w/ vacuum advance disconnected and plugged and tuned carb with vacuum gauge insuring highest vacuum (17) within 6-800 idle RPM range.

From everthing I read, vacuum should be between 16-20 Hg .. if it's steady at 15, it means retarded advance.

I now have strong acceleration in the complete RPM range w/o pinging in the mid-RPM and no popping in the exhaust pipes on de-acceleration and idling.

I just finished doing this yesterday PM

My $.02

Ev

[veltex]

man I am still having a hell of a time with this, I have been ultra busy at work and been driving another truck, but thought I found the slightest intake leak (I think now it was the starting fluid evaporating and getting sucked in the breather and then thru pcv then making engine studder), so, re-did intake gaskets with Felpro 1212, thought that it might have been that I used Mr. gasket instead of Fel pro. No change, still mid throttle pinging, will change carb base gaskets today incase its that. Fattened up the carb with new rods, springs,no change. Plugs, no change. Timing up and down, no change. All vac lines seem good, fuel filter new. I may try my old Q-jet and see what happens. I have a question about performers, but will try new post to get help, what is the "low speed circuit" how do I get to it to clean it, and also can some one check there Performer carb w/electric choke and see if there is a small hole sucking air just under and to the front of the housing, facing directly sideways?

[Bill72BB]

Noticed you are running a new 1806 which is a 650 cfm carb ,I'm not familiar with the new Thunder series but when I ran a 1406 on BB ,I changed the jetting to richen up the mix. when I get to the garage ( thats where my book is ) I'll post the size I used .