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I was reading through the Gas Compromise thread this morning which is talking mostly about timing.
This made me think of some of the timing issues that I have had with my truck. Since I've owned the truck I have had back fire, dieseling, and general rough idle problems.
When we went to stick the timing light on my truck we noticed it had two scales for timing. One on the drivers side timing cover and another on the passenger side.
Driver side scale
Passenger side scale
Our cure for this was to set the timing by ear.
Problems were mostly solved. The truck runs good. It does not backfire or diesel anymore. Normally it idles at 1000 RPMS sometimes it has a rough idle where for what ever reason it wants to bounce around at 500-700 RPMS.
The second part to this discussion is related to gearing. I commented on a thread some while ago about what my normal readings were while driving down the road. My truck in 4th gear going about 55 normally reads about 2100 2200 rpms. Someone commented saying that it was high and I should check my gearing. I do have the 3.08 gears in my truck.
With all this said I do not have any real issues with it any more. I frequently haul heavy loads and pull trailers without a problem. And will run 55 while doing it without breaking a sweat. My thoughts on it right now is if its not broke don't fix it.
Im looking for opinions on my rpms at idle and at speed (whats normal). As well as advice on my timing Im sure that if my rpms are high its related to timing. Just somethings I have been thinking about for the past couple of days.
I would verify which scale is correct, and the best way to do that is with a piston stop or something like a straw stuck in the plug hole. Once you know which scale is which then determine what your current timing is, but make sure you get the idle to less than 1000 or the mechanical advance might be creeping in. And, make sure the vacuum advance is disconnected.
As for MPG, yours is poor for a six. And that plus your RPM at 55 really makes me question the rear gearing. When you checked the gearing did you put both rear wheels on jack stands and turn the drive shaft 10 turns while counting the wheel rotations - and making sure both wheels rotated exactly the same?
I don't know what stock tire size is on an '84 F-150, or what transmission you have. But assuming you don't have an overdrive, you'd be at 2100 - 2200 rpm at 55 mph with 3.08 gears if your tires are 26" - 27" dia. that seems a little small, but not too ridiculous. And if you have an auto trans with a non-lockup torque converter you'd turn the engine a little faster too, so that could push the tire size up to around 28" for that engine speed. That would seem even more reasonable.
I don't know what stock tire size is on an '84 F-150, or what transmission you have. But assuming you don't have an overdrive, you'd be at 2100 - 2200 rpm at 55 mph with 3.08 gears if your tires are 26" - 27" dia. that seems a little small, but not too ridiculous. And if you have an auto trans with a non-lockup torque converter you'd turn the engine a little faster too, so that could push the tire size up to around 28" for that engine speed. That would seem even more reasonable.
My truck has 3.08 rear gears with 215/75 X 15 [original size] tires. However, my truck has OD and apparently, yours does not. I do not normally drive @ 55 mph in 3rd [1:1 ratio] which is the same as your 4th gear, but I believe it is @ ~2000-2050 RPMs. At 2000 RPMs in OD, mine is doing 70 mph.
Regarding the accuracy of the speedometer, go to a nearby highway or interstate with mileage markers. Drive at least 10 miles [the more the better] and as you pass the 1st marker, note your odometer reading [tenths]. At the end of 10 miles, while staying in the same lane, note the odometer reading. If it reads 10 additional miles, the speedometer is right on. If 10.5 miles [as an example], it is reading 5% fast. If 9.8 miles, it is reading 2% slow.
The speedo's are off on many of these trucks as there are only a few speedo gears available but many tire sizes. And the speedometer is different than the odometer. On Rusty one is off and the other isn't. And with only one gear to change you have to choose one or the other to get right.
Yes, you should be able to use GPS but I've not done it so can't say it'll work for sure.
By the way, ask David what his MPG is with his truck. You'll see why I say yours is low.
Have you checked to see if your computer is throwing codes? I'll bet it is. And, if so you may well have the advance locked into limp-home mode that would normally be 10 degrees BTDC, although your timing adjustment may have moved that.
Yes, Amazing numbers. His thread is what got me thinking about my mpgs.
He has a duraspark conversion which i am considering. Also I tried to look but couldn't find if his truck was 4wd or not
Regarding gas mileage, most of my driving is local. However, Friday the 12th, I will be driving to meet some special forum members which live approximately 575 miles away. This will provide ample highway only driving. The mileage will be recorded and reported, later. While there [OK], some who are much brighter than I will make every attempt to enhance the performance in the hope of improving the current mileage. The return trip will provide a direct comparison and the results will be compared. I am looking forward to the weekend!
Make sure that where you are located, emission inspection is not required before you consider the DS conversion.
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