So about a year and a half ago I had ATK build me a 408 LS engine for my 84 Chevy that's just a toy on the weekends and Offroad at the sand dunes. The issue I'm having is I'm getting occasional spark knock under load and randomly it wants to run on when you turn the engine off. I snapped a pic of the timing table on the msd. Not running any map sensor currently. I contacted ATK about it and they thought maybe it was fuel related, so we tried running the best 93 octane we could get, no changes. ATK is the one who set up this map and dyno tested it when the engine was built. I have put about 150 miles on the truck so far with this config.

The questions I have are: should the map curve table be all 0 since I am not running a map sensor? Secondly, if I did want to plug in a map sensor, what changes would I need to make to the software to recognize that and make sure its set up correctly?

Any thoughts?

408 from a 6.0 iron truck block
10.5:1 compression
L92 heads
Custom cam by Straub

 

Should be zeroed if not using or it will still add in timing as some have found out. Be sure to save the file so you can put it back in when/if you install the sensor.

And do NOT use the MSD pills if you have them.

 

Ok, zerod out the map table, no more spark knock now. Yay! Question on the idle timing, running about 850-900 rpm idle. Seen mixed reviews on what to run. Any suggestions?

 

Depends on combo....23x/24x and 950 +/- 50 @ 10.5:1 comp....and dialed in highest vacuum reading tuning with AFR gauge also.

 

Truck sat overnight and started right up this morning but the idle quality is really poor now and throttle response off idle sucks. I think the 12 degrees for idle and low RPM timing isn't enough. I have seen other folks running 28 degrees from 800rpm up to about 2500rpm. Does this seem like too much? Definitely idled better and had better throttle response from idle before when the timing was much higher

 

I believe my idle timing is around 20 degrees and it has great throttle response. I'll have to check tonight

 

My curve looks like
0rpm=16*
then to 34* at 800rpm to red line
I've also played with the idle timing some....25* from 600 - 1500 and straight to 35*at 1600 to help with run on.

 

Running without a map means the unit applies full vac advance all the time. Not sure which box you are running but the 6010 box for the 24x reluctor adds a bunch more vac advance than the 6012. If you are running that box and just zero out the map curve than your whole timing curve is low.

Why not just run a MAP? Pretty simple to do.

Also remember that plugging in a pill wipes out your custom curve and adds the vac advance again.

 

What size stall do you have? It doesn't sound like you drive it a lot so a regular non MAP tune sounds fine IMO. Your lower timing is very conservative. Also a nice thing is to set the Max timing setting to avoid it going over a preset number you are comfortable with.

0 rpm 14 degrees for startup
next dot at 1000 RPM move it to 18 or 20.
3rd dot 2000 RPM go as high as you're comfortable...maybe even 36 degrees.... Just have it back down to your chosen 32 degree's before your stall "locks". This will give it more part throttle but the timing would be down before the real load is placed on the engine.

Seems like that's all you'd need for a play toy not drag racing IMO.

 

still having an issue with run on when the truck get turned off. Have adjusted the timing down so low at idle that it idles like crap and barely wants to run. Any suggestions to clear up the run on?

 

Do you let it idle any before shut down, or is it a quick blip of the gas before shut down ?

 

I always let it idle down before I try and turn it off. It idles about 800rpm or so.

 

What heat range plug are you running?

 

I'll have to look at the build sheet on the engine. ATK built this and I am running it exactly as it came to me out of the crate.

 

Maybe you have some carbon build up in the cylinders. That can definitely cause both issues. Running too rich will make this worse. Like someone asked are your plugs getting black soot on them?

Higher octane will almost certainly cure both issues.

High RPM knock can be made worse by running too lean at RPM, while the run on will be a bigger issue if the idle circuits are rich.

I assume no wideband measurements? A good wideband tune might make both problems go away.

Cutting off fuel flow at idle will cure the run on. Once had a 1979 vehicle with a solenoid valve built right into the carb that cut off the idle fuel passages when the power was off. Not really an option I understand, just shows what a problem that sort of thing was back in the 70's. One way to get the same effect is to fully open the throttles when the dieseling starts. That will stop gas from being sucked out of the idle circuit.

Turning off the motor with the trans still in gear is another way to minimize dieseling

Your compression isn't high enough to need water injection.

 

Need to also check and make sure that the fuel transfer slots in the carb are not uncovered when it's idling. If so there is unmetered fuel dumping in at idle causing the diesel/run on issues.