Distributor adjustment question

[steves66]

Now that I have made my beast a little quiter I can actually hear other things and I have noticed that I have some detonation when I throttle it in high gear without down shifting. I can barely hear it but I can tell its there. I have tried bringing my timing down all the way to 10* btdc, different gas, octane booster, nothing seems to work. I thought my advance was coming in too soon so I went to adjust the screw in the vacuum canister and I noticed it was turned in all the way clockwise. According to the instructions that came with the electronic conversion kit I bought from Mancini, the screw should be turned clockwise to reduce the vacuum advance. Any help on this would be appreciated. My build is a 440 stroker, electronic ignition upgrade, MSD 6AL box and I have the vacuum advance plugged off.

[firefighter3931]

Steve, plug off the vac advance and tune w/o it. I allways disable them....hate the stupid things ! That combo will want ~18* of base timing and max 35* total. Eliminate the vac advance and your problems will be solved. 


Ron

[steves66]

Steve, plug off the vac advance and tune w/o it. I allways disable them....hate the stupid things ! That combo will want ~18* of base timing and max 35* total. Eliminate the vac advance and your problems will be solved. 


Ron

Thanks Ron. So I am assuming that with the vacuum can unplugged the screw adjustment don't matter.

[firefighter3931]

Steve, plug off the vac advance and tune w/o it. I allways disable them....hate the stupid things ! That combo will want ~18* of base timing and max 35* total. Eliminate the vac advance and your problems will be solved. 


Ron

Thanks Ron. So I am assuming that with the vacuum can unplugged the screw adjustment don't matter.

It shouldn't but check with the timing light to make sure. From the sounds of things, the vac advance was pulling too much timing into the engine under load. The problem with Vacuum advance on big cammed engines is that the cannisters are designed for a stock build that makes at least 15in of vacuum. Our street/strip bruisers don't make anywhere near that and then the tuning problems start. For that reason i disable them. 


Ron


Ps. How did the race bullet "resonater" modification work out ? I imagine some of the "bark" is gone out of the big dog. 

[grouseman]

With it screwed all the way clockwise, the adjusting plate is all the way forward so there is very little compression on the vacuum spring.  In other words, you are getting lots of advance even with very little vacuum. 

Keep turning it COUNTERclockwise, which will decrease the sensitivity to vacuum so that less advance comes in for any given vacuum level.  I.E. keep turning until you've eliminated the rattling.  That's about all there is to it. 

[steves66]

Steve, plug off the vac advance and tune w/o it. I allways disable them....hate the stupid things ! That combo will want ~18* of base timing and max 35* total. Eliminate the vac advance and your problems will be solved. 


Ron

Thanks Ron. So I am assuming that with the vacuum can unplugged the screw adjustment don't matter.

It shouldn't but check with the timing light to make sure. From the sounds of things, the vac advance was pulling too much timing into the engine under load. The problem with Vacuum advance on big cammed engines is that the cannisters are designed for a stock build that makes at least 15in of vacuum. Our street/strip bruisers don't make anywhere near that and then the tuning problems start. For that reason i disable them. 


Ron


Ps. How did the race bullet "resonater" modification work out ? I imagine some of the "bark" is gone out of the big dog. 

Yes Ron, some of the bark is gone out of the beast, but the bite has not suffered as far as I can tell. Now if I can get this detonation solved I will be ready for some track time next spring! FWIW the vacuum advance has been plugged for a long time now, that is my concern.

[firefighter3931]

Steve, a few questions ;

(1) what plugs are you using ?
(2) Have you pulled a plug and looked for color ? Tan to light brown is what you should be seeing
(3) Where is the total timing coming in at ? Map out the ignition curve in 200rpm increments from idle to wherever it stops and post it here. It should have at least 16* at idle and ~35-36 total.
(4) has the carb been re-jetted at all ?
(5) is the motor running hot ?
(6) run a cranking compression test to see how much cylinder pressure the motor is making

Ok, here are my thoughts ;

It's not a compression issue....you have perfect quench and only 10.5:1 with aluminum heads and a healthy cam
Possibly, the ignition curve is too agressive as in the timing coming in too soon or too much total timing....Or it's running lean causing it to predetonate. Reading the plugs for color should point you in the right direction assuming the ignition curve is good. The plug heat range will also come into play as well. The NGK bcp(r)-6es is what it should have in there.


Ron

[steves66]

Ron, the plugs are the NGK bcpr6es and the few that I pulled out are tan.  I haven't mapped out the ignition curve yet, but I will. Right now I have 18* at idle and 36* @2800. The plugs look good to me so I haven't jetted up the carb. The motor will run hot only when I throttle it, but when I let out of the throttle and return to cruise she will cool back down. That leads me to believe its in the timing somewhere, but I had the timing all over the place with the same results. I wlll map out the ignition curve and see if its coming in too soon.

[firefighter3931]

Steve, how hot is the engine temp under hard throttle conditions & what's the normal run temp ? Has this been an ongoing issue or something that recently started happening ? Do they use "wintergrade fuel" in your area ? Have you tried to spike the fuel with a few gallons of 110 to see if it helps ? Can you run a compression test to see how much cylinder pressure it's making ?

Just trying to brainstorm here a little. 


Ron

[Todd Wilson]

Dont feel bad I got a stock 318 that does the same thing. Cant get rid of it no way no how. Not even Chryco could do it!!!!!!!!     




Todd

[Chryco Psycho]

you could be lean @ WOT , try increasing secondary jetting to keep it cool & decrease ping

[steves66]

This has been an ongoing issue since the rebuild. I still don't have that many miles on the engine, maybe around 400. Normal driving temp is around 190-200 and under hard acceleration she will climb to around 220. I don't know about the winter fuel here, but I wiil try and find out. I will also do a compression test. It supposed to be nice here this weekend.
Chryco you may be on to something because the plug readings I took was primaries only done with normal driving staying out of the secondaries. I need to check them after a wide open throttle run then pull them and look at them.

[firefighter3931]

Chryco you may be on to something because the plug readings I took was primaries only done with normal driving staying out of the secondaries. I need to check them after a wide open throttle run then pull them and look at them.

Yep, it sounds like a secondary jetting issue. I doubt it is timing related because the detonation occurs even with the timing backed way off. If you had a chassis dyno available or even a wideband O2 meter available a lot of these questions would be quickly answered. Keep at it Steve....you'll get it worked out ! 


Ron


Ps. I assumed that you had done a WOT plug reading.....my bad !