68 RS Convertible

iTrader: (1)

6.0 iron block .030 over
mahle flat top pistons with valve reliefs
crank is out of 07 vette with 58 tooth reluctor wheel
eagle h beam rods
823 cast L 92's milled .025
TSP 233/239 595/603 cam
Comp 7.40 push rods
GMPP intake/ Holley 750HP
MSD 6012 running the ignition

I'm having issues with what I believe to be the timing. This is my first LS build so forgive me if I'm completly off somewhere. Motor starts fine and idles great. I have tried all 6 timing pills, plus tried my own timing curve but the ignition is still cutting out on me. At about 3000 RPM I will, for a split second loose ignition, with a huge backfire out the exhaust, sounds like a gunshot (thats while driving and accelerating). When this happens I just instinctivily pull out of the gas and it then goes back to running fine. If I hold it at any RPM, say 2500 RPM, with the car parked, you can hear the ignition cut out for an instant and come right back, very rythmic, tach also goes to zero when ignition cuts out. If I am driving it only cuts out under acceleration between 2800 and 3000 RPM, big backfire, then comes right back. Have not had the motor above three grand while driving. Also plugs from #2 and #8 cyclinders are running very, very rich. I'm not seeing or feeling a skip though. All the other plugs look great. Running 74's in the primary and 80's in the secondary, 35 squirter primary, 31 secondary. I may be a bit fat in my jetting, but I only have about ten miles on the motor so I'm still trying to see what it likes. I called MSD they told me to zero out the MAP advance which I did, same problem. I have run as little as 17 degree's of timing, number 2 pill timing curve minus the MAP advance. I have run as much as 32 degrees, again no MAP advance. And again I have tried all the timing pills provided, plus my own timing curve, with and without MAP advance. Anyone have similar issues or ideas? I brought it to the machine shop that did my machine work and he thought it may have something to do with the rev limiter. I had it set at 7200 for my own timing curves but dont know what MSD sets it at for the timing pills. My next step is to max out the rev limiter and see what it does. I am also planning on calling MSD back on Monday for more help.

Shawn MacAnanny

iTrader: (12)

You know that the timing curves dont work unless you remove the pills right?

I am running 72 primaries, 76 secondaries on a vacuum secondary carb. 50cc accelerator pump up front with a #35 squirter. Is your carburetor new? Have you been monitoring the total timing live with the cable hooked up? Whats your manifold vacuum? What power valve are you running?

forcd ind

iTrader: (7)

a couple things-as far as the plugs, the best way to see how they are burning is to kill the motor after a full blast, which you cant now-the intake will show the plugs diff. at idle, do to mixing, so dont worry to much about that now-just a guess, but there is a wire that if you hooked +12 to, is factory set @ 3k, it kills the engine-it is for a 2 step(i use a 6010, on mine it is the blue wire)
look at the left side of your screen, the data section-my guess is you may have used that wire on +12, disc it if not running a 2 step retard, or maybe
just raise the min rpm to 8k or something, incase it is wire or x'ed up in the box itself
if you didnt use that wire, at least try and raise the 3k min to something higher and see what happens
i have found the LS engines want a big shot of fuel off idle, i had to use the green pump cam(on a single plane intake) and use a pv with a # not to far below idle vacume
i run a procharger on mine, and use the 2 bar map sensor, but you can run the rpm timing ok-just dont use both graphs, map and timing
let me know what happens
oh, something else we found on the dyno, when you set the rev limit, it actually starts to activate 2-300 rpm before the set amount, so take that into consideration when setting a limit-good luck, hope that cures it(you did use the correct crank sensor, 58, right, lol)

S10xGN

iTrader: (11)

Make sure your ground and power connections are good (paint and corrosion-free) and try a direct connection to the battery if not already done.

Russ

HELLBENTfabrication

you may have intermittent or dead coils.
make sure you have good connections from your MSD and everything is hooked up right.....swap the coils from the 2 and 8 cyl with lets say 1 and 7 and then run the engine if those cylinders start running fat replace the coils

68 RS Convertible

iTrader: (1)

thanks for all the input. Shawn the carb is not new I ran it on my gen I 350, but have changed it for this set up. I am pulling 10.5 inches by my vacuum gauge and have I believe 5.5 power valves both front an back. I don't have a laptop so I run a long cable to my computer in the house (yeah yeah) to make all my changes so short of seeing it at an idle I don't really get to monitor anything(havn't sent the wife out there yet!!). MSD felt I had too much total timing that is why they suggested zeroing out the MAP advance, but that did not help the problem. I have swapped the number 2 coil with the number 4 to see if that is, well one of the problems. I'm going to also play with the rev limit to see if that helps. I have checked and double checked my connections and ground so am convinced they are good. I am running the grey crank sensor and a 4x timing gear with the cam sensor in the front cover. I'll post any progress, thanks a bunch for the ideas and help!!!

68 RS Convertible

iTrader: (1)

upped both the rev limiter and the rev launch, both set to the max of 12,500 RPM, no change. I swapped the 2 and 4 coils and ran it for a bit today, but have not got out to see what they look like yet. gonna try again with MSD tomorrow, keep you posted.

68 RS Convertible

iTrader: (1)

Finally got some time to work on the car today and seem to have corrected my ignition problem. I bit the bullet and bought a laptop and when I hooked it up I noticed my battery was only showing between 6.5 to 7.0 volts through the pro data MSD program. My gauge was showing close to 14 volts and my battery tester was showing the same, so I thought the box was wrong/bad. I was drawing my power from my external selonoid, the pole that is not used, it is a switched power source. Well apparently it does not put out 12 volts. So I ran the power to my fuse box and bingo. Now I just have to wait for the weather to break to see how it runs. Thanks for all the input.

rojs234

iTrader: (5)

I've been watching this thread to see the results. Really glad to hear you found something. I'm betting you've solved it! Seems like it's always some little weird thing that screws a guy up. Good luck!

forcd ind

iTrader: (7)

cool-you prob have salted streets from snow up there by now
def would have caused a problem with low voltage

marco_chevy

what is the best timing curve for a 6.0L , L92 heads #823 whit GMPP carb intake ? do you use one of the 6 preset or you make Your own programation ? I have a similar set-up.. please let me know. I will start my motor this week-end... i have compcam 54-469-11.

marco_chevy

nobody can tell mw what is the best timing curve for a 6.0L whit L92 head, carb set-up....!

LSNotch

iTrader: (1)

I'll say start around 16*-18* initial, 3k advance to 32*-34*. Hope this helps! Watch for detonation!

marco_chevy

I have exactly the same problem.. motor cut at about 3000 rpm, tach go to 0, and if i dont pull out of the gaz, i can hear a very big back fire... when i see tach go to 0, i pull off the gaz pedal, wait about 2 sec, and all is good, put i'm not able to go faster... tomorow morning, i will take a wire direct on the fuse box, and connect the pink wire to it.... for have a good 12v switched power! hope have good result.

87olds442

with the 6012 box ive found that the "curve 3" chip made the best et and mph at the track on my 6.2 with l92 heads and carb and thats only with a peak timing of 19 degrees at 3500. are you really running that much timing with yours?

marco_chevy

hey 87ols442, do you run a vac advance whit the MAP sensor ? and the pills 3 is -2 degre i think?

87olds442

no i dont have the vac advance hooked up the #3 pills timing is
0 rpm- 0 degrees
1000rpm- 4 degrees
2000rpm 18 deg.
3500rpm-19 deg
5000rpm- 14deg
6000rpm 17 deg

and my car seemed to like this better than the 4 or 5 chip . ive even called msd and asked why such a big difference in timing with this box and the 6010(24 reluctor wheel) and he couldnt tell me . just said that MAYBE r&d found something and left it at that lol

marco_chevy

If you dont use vac advance, whit MAP sensor not connecter, you dont have 0 at 1000 rpm... whit 6012, you always have 20 deg advance whit the map sensor not connecter... chek in your installation sheet. it's about 20de 0-5, 10deg 5-10 psi et 0 a 15 psi or something like that.. If MAP sensor is not conected, it's always 20 deg advance.. so add + 20 degre at all your number! I think...

I connect my computer on the msd, and whit the pill 4, i have 22 deg at idle.. For have 0 VAC advance, you must transfert your own timing curve! Let me know if i'n wrong!

3pedals

iTrader: (3)

wow I better check mine.

marco_chevy

Why MSD did not connect the knok sensor on this 6010 or 6012 timing box ? Whit this option, it will be easier to put more timing!

I wrote to MSD and they did not want to give me a timing curve for my set-up, they told me i must go on a dyno!