Why does is smoke black if I time the injection too advanced?

[Christian9112]

I can understand white smoke for the retarded timing. Diesel doesn't have enough time to burn and just goes out to the exhaust. How does advanced timing make it black? If it's firing to early, why is it black and called unburnt fuel? I've tried researching it but I can't find anything.

 

[Thewespaul]

This is a very complicated question that I could rattle for hours on, basically:

White smoke= fuel not getting enough heat to combust.

Black smoke= fuel not getting enough oxygen to fully combust.

 

[The_Josh_Bear]

What he said ^^^^.

Essentially you have too much fuel for the amount of air in the cylinder. It's more "incomplete burn" than un-burnt.
This is not really a symptom of advancing the timing that I know of. I've had pretty advanced timing and it didn't "cause" black smoke. Too much fuel caused it.

 

[Macrobb]

It's caused by "too much fuel" locally - I believe that it happens due to the firing happening when the piston is still coming up, which is pushing air into the pre-cup. Fuel is injected into the pre-cup, and it burns. Normal timing has the air swirling out of the pre-cup as it's burning, mixing with the remaining air outside the pre-cup.
If it's too advanced, that swirl doesn't happen until after the burn has happened, and so it burns rich.

What this means is that with little enough fuel, it won't smoke even with advanced timing... it's just with correct timing you can burn /more/ fuel cleanly.

 

[Christian9112]

Ok, if I understand correctly, the air fuel mixture in the pre cup auto ignites to early but since it ignites in the pre cup without the proper blending, not enough oxygen is able to burn the fuel and just decompose into soot? I haven't properly timed my engine. I only have a pyro. Would to much advance timing hurt the engine even if I keep it below 1200? It's an N/A 7.3

 

[Macrobb]

Play with it. Just retard it a bit at a time till you see blue smoke, then advance it a hair.

 

[Christian9112]

Right now it is a dimes width advanced from the factory lines. I have new reman injectors but the IP is old. Yeah I know. I don't have enough money for the ip

 

[Christian9112]

It doesn't smoke bad. Nothing on idle. Black smoke on sudden step on the throttle. Also smoke on hills. I am currently playing with the fuel pressure. I put a black holley pump that puts 12 psi from factory. I put a voltage regulator on it to regulate the pressure. It runs okish at 8 psi. Runs ok with %80 wmo at 12 psi

 

[Macrobb]

Again, play with it. See what works best. Don't go with "eh it should be advanced", just play. with. it.
Adjust, drive. With a worn out pump, that's the best thing you can do. You can often get it to run pretty well.

Heck, I've ended up running a number of worn out pumps a whole gear tooth(6-ish degrees) off, just to make it run well again. A lot of conventional wisdom goes out the window when you are talking about a worn pump.
edit: Also, get fuel pressure gauge in cab and check that. 8 psi is fine, if it's 8 psi from idle to WOT. 8 at idle and 0 at WOT(which is likely) won't net you good performance...

 

[Christian9112]

I have the fuel pressure gauge in the cab. I read on here that it does change timing. Everytime I see the pressure drop, I increase the voltage to 12v. What are the symptoms of a good timing adjustment? No smoke at all?

 

[The_Josh_Bear]

Proper timing will net the most MPGs for a given driving style, and also the best performance when you step on it.

How much is your fuel pressure dropping? And what do you mean by "I increase the voltage to 12v"? Is your pump set up on a potentiometer or PWM?

 

[Thewespaul]

Yes fuel pressure changes timing, but only about 1-2* per 5 psi. You do not want to exceed 10 psi on a stock pump or you can seize the head. These pumps were designed for 5-6 psi inlet pressure and will not work properly with double that without modifications. Putting fresh injectors on a worn pump will work fine for awhile, but expect to start having hot start issues come summer. I’d start saving for a new pump now.

 

[genscripter]

I have the fuel pressure gauge in the cab. I read on here that it does change timing. Everytime I see the pressure drop, I increase the voltage to 12v. What are the symptoms of a good timing adjustment? No smoke at all?

Some testing of mine:

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[bbjordan]

^^^ love this kind of empirical testing! Thank you genscripter.

 

[genscripter]

^^^ love this kind of empirical testing! Thank you genscripter.

That reaction was a genuine reaction. I honestly didn't know what to expect, and when it was about 4 degrees difference, I was genuinely surprised.