Need help with my Dads 1981 720

[powderfinger]

Simply repeating myself from other threads where people asked the same question regarding larger tires. My answer is always the same, just don't do it if you plan on driving it on the highway at all. You will hate it. Your tires look like they may already be a bit bigger than stock tires. I wouldn't go any bigger or you will be cussing at every hill you get to.

[datzenmike]

The '84, for example, uses a 205/75R14 and these are just over 26" tall and run on a 3.545 ratio differential. (a ridiculously low number for a truck) If you go to a 29" tall tire the effective differential ratio that the engine has to turn is 3.17!! That's in the basement. In addition, the speedometer is out 11%, meaning the engine thinks it's going 50 MPH but is struggling to maintain 55.5 MPH into the wind or up hill or passing. Wind resistance increases with the cube of the velocity so to double your speed you need 8 times the power! 5.5 MPH doesn't seem like much but it comes at a cost of performance and mileage.

[ElliotV]

4 hours ago, datzenmike said:

The '84, for example, uses a 205/75R14 and these are just over 26" tall and run on a 3.545 ratio differential. (a ridiculously low number for a truck) If you go to a 29" tall tire the effective differential ratio that the engine has to turn is 3.17!! That's in the basement. In addition, the speedometer is out 11%, meaning the engine thinks it's going 50 MPH but is struggling to maintain 55.5 MPH into the wind or up hill or passing. Wind resistance increases with the cube of the velocity so to double your speed you need 8 times the power! 5.5 MPH doesn't seem like much but it comes at a cost of performance and mileage.

Most 4x4 trucks came with 4.11 for the later ones or I think 4.56 in the 83 and earlier although my 83 with a h190 rear has 3.889 gears. Those would suck to put 31s on. 

[datzenmike]

4x4

'80-'82 were 4.375. '83 and up were 4.11. There was an optional 3.889 for a few months in '83.

[ElliotV]

5 hours ago, datzenmike said:

4x4

'80-'82 were 4.375. '83 and up were 4.11. There was an optional 3.889 for a few months in '83.

That's what it was 4.375. I knew it was shorter than 4.11. this 83 I have had the z24 the later tailgate with the utility style bed with the hooks and reverse lights on the roll pan and the 84+ style taillights. I was surprised when I saw the 3.889 on the tag cause I had not heard of that ratio in a 4x4 model. 

[datzenmike]

I've also struggled with this 3.889 mentioned in the 4x4. Used 01 '83 to 06 '83.  Then changed to the 4.11.

[ElliotV]

9 hours ago, datzenmike said:

I've also struggled with this 3.889 mentioned in the 4x4. Used 01 '83 to 06 '83.  Then changed to the 4.11.

I'm sure it did a little bit better for mileage on the highway but I don't think I would want higher gears than 4.11 when mine is running right I just let the clutch out in 4 low and can walk right next to the truck. 

[datzenmike]

It might have better mileage on the highway buy very little. I bet the extremely slow pick up wasn't what the driver's wanted in town, not to mention the dismal off road crawl.

[Jared55b69]

On 1/12/2025 at 10:06 AM, datzenmike said:

Not really any upgrades as the Z series of engines have built in impediments to performance increases. What you can do is get it running in top shape. 

 

Fast idle is likely the choke is stuck. Once warmed up take the top off the air filter and look. Choke plate should be vertical an fully open like this...

 

 

Not partly or fully closed like this...

 

 

Use only NGK spark plugs, never use fram oil filters, run Shell Rotella T4 oil in 10w30 as this has the required ZDDP levels that are no longer available for our older gas engines. Set valve clearances to 0.014" on a hot engine, timing around 5 degrees.

 

 

 Post a few engine pictures. May be easier to point out what could and what you shouldn't remove. Hose removal won't change anything if expecting this to improve thngs. In a lot of cases it can cause problems.

It’s not letting me post pics for some reason! Any help would be appreciated please 

[ElliotV]

your pictures need to be less than 4mb to post them on here. the other option is to upload them to a app called imgur on your phone then post the link to those pictures here. 

[Thomas Perkins]

This site is funny about posting pictures.Some times it will and some times it wont.Will say your pics are too big,but you have just sent one with your phone,then it won't let you send one.Sometimes what I do when it says it is too big.I then send it to my Facebook page,then save to phone,then send it here.I have used Imgur,but not in a long time.I do it this way when it says too big.Or I start another message and send it from that one.My computer works good with Imgur,but my phone doesn't always.

 

[Jared55b69]

On 1/12/2025 at 10:06 AM, datzenmike said:

Not really any upgrades as the Z series of engines have built in impediments to performance increases. What you can do is get it running in top shape. 

 

Fast idle is likely the choke is stuck. Once warmed up take the top off the air filter and look. Choke plate should be vertical an fully open like this...

 

 

Not partly or fully closed like this...

 

 

Use only NGK spark plugs, never use fram oil filters, run Shell Rotella T4 oil in 10w30 as this has the required ZDDP levels that are no longer available for our older gas engines. Set valve clearances to 0.014" on a hot engine, timing around 5 degrees.

 

 

 Post a few engine pictures. May be easier to point out what could and what you shouldn't remove. Hose removal won't change anything if expecting this to improve thngs. In a lot of cases it can cause problems.

How do you post pictures? lol 

[datzenmike]

I have no idea on a hand held device, never had one. At home I got an imgur picture hosting site account, it's free*. You upload your picture(s) to the site and when you want to use one, you copy the information supplied there, bring it here and add to your post. I copy the Direct Link provided but to be honest I just experimented till I got something that worked.

 

*Do NOT use Photobucket .