startup and high rpm idle

[JAM]

Sent email. I had a horrible time with mine because it was all worn out on shafts. Converted old mechanical unit you see in pic.Had to machine everything. I figured out a few tricks hope they work for you

[Yves]

Engine started to kick back on the starter after I got the idle in a better state today and with the engine warm. Still on fixed advance and with the Vref disconnect, meaning that the ecu is not controlling timing yet.

[billr]

Is your cranking advance still at 10 deg? Reduce it to 0, that will reduce the tendency to kick back at the starter.

[JAM]

I am not sure what you have control over on the ign side but consider this:
while cranking: the output ignition advance will be at your 'cranking advance' settings until it hits the rpm threshold you set as "cranking rpm" in startup menu. Looking at your log while you are still cranking it jumps out of the cranking setting and advances the timing before it is really running on its own. in my applications i have found that the "cranking rpm" should be high enough to where you have disengaged the starter AND/OR the rpmdot is (relatively) hi (meaning the engine is really taking off to run by itself not just hitting a few cylinders)

some of this will get better as you get the fuel side of your startup fine tuned

[JAM]

another thought:
to visualize why timing is less at cranking and why it can kick back if too advanced, remember we are talking about timing NOT degreeing, (we just measure in degrees) so the engine cranks over really slow, there is tons of TIME for the mixture to burn... 10 degrees is a lot more TIME before top dead center while cranking than it is at say 5000 rpm. so if you start the flame with too much TIME BTDC the cylinder pressure will get higher before it hits TDC and want to push the piston down before it gets to the top of stroke.

at least thats what i think is happening somebody may tell me i am wrong

[Yves]

I think you are right.

I'm using the GM HEI. As said I had the 5V Vref cable still disconnected to prevent the computer from advancing too much while the tables were not roughed in for idle. Since the module in the distributor doesn't get a 5v signal it doesn't signal the ECU to commence adjusting timing. So in essence it keeps running at the fixed angle I set the distributor at with the timing light. (much like an old style HEI's initial ignition timing is set).

I'm going to work on this a little more and will set the cranking rpm to 500 or so.

[billr]

Have you checked ignition timing with a strobe light; do you know that the "cranking advance" that you set is truly happening?

Edit: Why set cranking speed up to 500? Most SBC engines don't crank that fast, more like 200-250 rpm, and at 500 you run the risk of dipping back into the cranking settings (cranking advance, cranking fuel PW) if the idle "sags" a little. That is assuming you are trying for an idle in the 700-900 rpm range.

[JAM]

His idle is 900-1000. The log I looked at you could see it jumping out of the 300 rpm cranking setting when it would hit on a cylinder but not really run. Once all startup tuning is refined he may be able to drop it down but in my experience these bigger cam engines just don't start as smooth as the tame cam ones, you may have a hit or 2 before it really fires. I hade the same problem on my engine with ITB and I set mine up a bit and it smoothed out. For what it's worth...

[BigLou240sx]

Edit: Why set cranking speed up to 500? Most SBC engines don't crank that fast, more like 200-250 rpm, and at 500 you run the risk of dipping back into the cranking settings (cranking advance, cranking fuel PW) if the idle "sags" a little. That is assuming you are trying for an idle in the 700-900 rpm range.

My understanding is that the cranking settings are not used after the engine is running, regardless of a drop in rpm

http://www.msextra.com/forums/viewtopic ... 01&t=50085

[billr]

Oops, James M. indicated you are right, and he should know! Still, I see no advantage in using "cranking" settings past when the engine has fired and is running. I figure you are better off getting some ignition advance and ASE/WUE factors going. Of course, that is partly because I like to use 0 deg. for cranking advance, to avoid "kick-back" against the starter...

[BigLou240sx]

Oops, James M. indicated you are right, and he should know! Still, I see no advantage in using "cranking" settings past when the engine has fired and is running. I figure you are better off getting some ignition advance and ASE/WUE factors going. Of course, that is partly because I like to use 0 deg. for cranking advance, to avoid "kick-back" against the starter...

I agree with you on no advantage to using cranking settings once the engine has fired - I'm guessing it all boils down to setting the cranking speed "right" in that it should be a speed where there is no chance of the engine weakly cranking, firing and falling back under your intended run speed

Haven't tried 0 degrees cranking yet, but had to drop my cranking advance down to 5 BTDC to avoid kick back on some particularly strained starts over the winter to charge the battery. Good to know cranking advance works lower!