ford390gashog, what exactly do you mean? I don't know what you mean by "set the timing and carb".

And how do you have yours set?

 

Advancing your timing by ear is a darn good way to burn holes in the tops of your pistons. There's a reason there's marks.

 

26 years setting by ear and no holes yet. The marks work fine if you have a new motor but if have an older motor and your timing chain has stretched a bit or any of the rest of the internal parts have any slop your marks are going to loose their accuracy.
But I do agree, this may not be a good thing for everyone to do but it has worked very well for me on a lot of vehicles and a lot of miles.

 

Well, I haven't had that luck, , but I guess stick with whatever works for you.
Unfortunately, you don't always hear preignition.

 

quam,
timing at top dead center? Did you add the time for the flame front travel and maximum pressure time?

 

The books and studies I have been looking over all indicate the best time to reach the point of peak pressure (PPP) is 14 to 16 degrees after top dead center - that applies the most leverage on the downward traveling piston.

It is sort of a balancing act - the pressure builds as the flame progresses and the gases expand, while at the same time your combustion chamber volume is changing as your piston moves through top dead center. You want to start your flame front in time to reach PPP at 14 degrees, but not so early as to cause too much pressure in the combustion chamber before the piston goes over center and starts to move down. The time to reach PPP is constant, but the time to reach 14 degrees ATDC changes as the RPM changes.

It gets very complicated - setting your initial timing to 6 or 8 degrees works well for a closed throttle at an idle, the mechanical advance takes care of adjusting the timing as the rpms increase, and the vacuum advance takes care of the timing changes needed for the changed fuel/air mix when you open the throttle. You need all three to be working correctly in order to get the best performance from your fuel.

Preignition is something else - something causes the fuel/air mix to explode before you reach your initial timing mark. This can get very destructive very fast and is to be avoided at all costs. The one time I experanced it I thought I had thrown a rod out the side of the block. Spark knock, or pinging, happens when the remaining fuel/air mix spontaneously explodes before the flame front reaches it - The spark fires the mix but the burn is not continuous but instead violently explodes. This is much less destructive and your engine can tolerate a fair amount of spark knock.

The new computer controls advance the timing until spark knock is detected, and then retards the timing a bit. The computer is in a continuous loop, always advancing the timing and listening for a knock, then retarding the timing until the knock goes away. When the knock is gone it will advance the timing until it hears it again - like I said, a big loop always searching for the most advanced setting that does not knock.

For myself, I time the engine such that I hear some pinging on a slight hill or when gently accelerating at highway speeds, but such that the pinging stops when I open the throttle and the vacuum advance retards the spark.

Frankly, I think you can get more mileage gain from a computer-controlled ignition than from adding EFI.

 

Beemer, I don't know what you're asking.

WillyB, what is your timing set at?

What I did this weekend is warm up the truck, then at idle I advanced the timing til it started to misfire. While I was advancing the timing the throttle increased. I took this to mean the truck was being more efficient. I test drove it and it pinged when I floored the gas. I then backed off until no more ping. In all I probably increased the setting from 14 BTDC to 20.

I'd still like to hear from some of you guys as to what your timing is set to.

 

I have mine set at 8 now - I was running it at 12, but between the hot weather, poor gas, and the load I am pulling in our mountains, I dropped back to 8 to avoid excessive pinging. My 390 is bone stock, those with aftermarket cams will run more advance.

Also, you should tune your engine where you want it to run - you can make it run very good at an idle, but run real poor at speed. I like mine to ping a bit under light loads, but not to excess. When I am loaded or pulling hard I do not want to hear any pinging - the lower vacuum caused by the wider throttle opening should make the vacuum advance retard the spark enough to avoid ping.

Actually, the vacuum advance should have been named the vacuum retarder - the high vacuum at idle and light loads causes the most advance of the spark, but any change in the throttle will drop the vacuum and retard the spark.

There is, or was, a technical article about timing your engine using a vacuum gage. Basically, you adjust the carb for a steady vacuum reading at idle, and then adjust the timing for the best vacuum. I have seen this work - but those still running points need to be careful that their dwell is set within the limit.

Good luck, and let us know what you come up with.

 

With my timing at around 20 BTDC, I did a highway road trip this weekend and helped a buddy haul a load of 15 large 3 in x 12 in x 14 ft pine boards he had milled by some rancher up in the mountains. The load was heavy. Total trip was about 160 miles. Drove around another day and filled up this morning. I got 17 mpg. I've never gotten that type of mileage with this truck and I've had it on the road for about a year now.

I'm really not sure what I'm doing when I advance my dist to 20 but the truck doesn't ping and it has plenty of power climbing hills. It doesn't have off the line get-up-and-go but it doesn't loose power in the climbs. And I've now gotten the best mileage ever. I'll keep an eye on it.

 

20 degrees sounds like an awful lot. I'm wondering if maybe your dampner hasn't spun a little? Anybody think this is a plausible idea?

 

Read the third post again, it has been mentioned, and I have reason to believe it may be true.

 

Sure seems to me like there is something wrong with your setup - it is almost like the mechnical advance is not working, or not working very good. If so, there is more mileage you can get.

Easy enough to check - just watch your marks with your timing light. At an idle the timing should be as you set it, and then speed your engine up 500 rpm at a time. You should get somewhere between 25 and 30 degrees additional advance by the time you get to 2500 rpm - and it should not increase after that. You should see something like an additional 7 degrees at 1000 rpm. 15 degrees at 1500, 22 degrees at 2000, and between 25 and 30 degrees at 2500. ( This is in addition to the advance you set originally - for me it works out to 38 dgrees at 2500 rpm.)

You should have the vacuum advance disconnected with the hose plugged for this test. If you don't get something like I discribed above then you need to work on the springs and weights of your mechnical advance until you do.

This is called recurving your distributer - one article I read claimed to get an additional 30 horsepower at the rear wheels by making this adjustment.

 

My dist is new. My old one had a broken spring and the idle would jump all over the place. And yes, with the timing light I do see the advance change when upping the rpms.

 

Nice going Qman, I wish I could that kind of mileage out of mine. 12 is about all I can squeeze out of it.

 

Check your volts going to the plus side of your coil if its not 12 volts you have a resistor wire and need to replace it to get the volts up!