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1978 long bed


rharm78

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Hello, I have a 1978 longbed that was my grandpas work truck. It has factory ac and an automatic. Its my daily driver to highschool in riverside, Cali. Im just curious, is there a way to tell if you have the manual trans. rear end or the automatic trans. rear end with out taking apart the rear end? Also any good mods to help mpg? I average 20-25, city. The truck is basicly stock so nothing to out there because I would like to keep it that way. Thanks!

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Okay so question #2( im sorry I start thinking about all of this stuff only at night when I reflect on driving the truck and how it was running today), is there a vintage air option for the "factory" in dash ac? I have the blower and everything that goes in/under the dash but nothing for under the hood so what would be my best option for getting ice cold ac in my truck once more? Thanks in advance!!!!!!!!!!!!!!!!! :rofl: :w00t:

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  • 3 weeks later...

Octane rating, compression and spark timing are interrelated to each other.

 

The big issue is to avoid spark knock from the timing being too far advanced.  An engine that is knocking is loosing power, and basically trying to destroy itself.   If an engine is knocking, many times retarding the timing slightly will eliminate it.  Or switching to a higher octane fuel will eliminate it.

 

A modern fuel injected engine will monitor itself for spark knock, and if it senses it happening, it will automatically retard the timing.

 

Our Datsuns do not do that.  You set the timing.  By setting the timing, you are basically deciding what fuel to use.  You can use more spark advance, with higher octane fuel, and you may get a very slight increase in fuel economy, but each gallon of fuel costs more.

 

The mileage you mentioned is not really too far out of line.  Old Datsun engines were pretty efficient, but again compared to a modern computer fuel injected engine, not so much.  The automatic transmissions in old Datsun trucks was not the most fuel efficient choice, either.

 

Get a timing light, and check the initial timing at idle.  You need to find out the exact way the timing is set.  This truck may require you set, or check the timing in gear, with the emergency brake set.  This makes the truck want to roll.  Be careful.  if your truck requires being in gear, I strongly recommend you have a friend sit in the driver seat, with their foot on the brake, and well as the hand brake on, and the wheels blocked, so the truck cannot roll, with you in front of it, checking the timing.

 

What should  the timing be set at?  I do not know. First look for a decal somewhere in the engine compartment with tune up information.  then look at the engine serial number, and compare it to the engine number on another under hood tag.  If you find the same number on the engine, and on the tag,  Use the information on the under hood tag.  If the engine number, and tag engine number are different, then use the service information you find here, or from other sources.

 

Another method for deciding what timing to set it distributor at is this.  Advance the timing very slightly, and drive the truck and see if it knocks under hard acceleration.   If no knock, advance it a little more, and do the acceleration test again.   When you find the point it knocks even slightly, retard the timing about 2 degrees.

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Thanks DanielC, I tried running it on 87 and it runs just as good as it did on 91. Ive noticed no change in performance and no unusual knocking or anything like that so I think I will go ahead and keep running it. The timing is set to the tag under the hood, because its the original hood and engine. But like I said I haven't noticed anything different. Thanks!

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So, how bad is the speedometer error on other's 620's? Because when my speedo says 50 the light up speed reader on the side of the road says 45. Also in the morning when its cold the speedo is extremely jumpy, almost as if the grease in the cable got really thick and is keeping it from spinning. Is the speedo cable bad or what do you guys think the problem is. Could it be me not knowing what gears I have in the rear end with a automatic trans? Thanks in advance!!!!!!!!!

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  • 6 months later...

The easy way to find the diff ratio is to jack up one side of the rear axle, mark the tire and the driveshaft for position reference.  Turn the tire 2 revolutions (other tire on the ground).    The driveshaft revolutions will be your differential ratio.  4.1 turns is 4:11, etc...

A lot of folks on here tell you different ways to test it, but this is the fastest and most accurate.  

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