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Top speed on highway for 720


matrophy

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My 1986 720 with 724z with 5 speed and stock wheels which I've owned for 18 months has always had a top speed of roughly 65-70 mph. It seems to want to stop accelerating at around 60 - 65. A buddy of mine with a similar 720 but with smaller wheels says he routinely cruises at 75 mph and had gotten his to 90 mph. Mine seems to idle OK and gets around 25mpg so I am wondering if I should expect more high speed out if it. Could it be something like rear end gear ratio?

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They are slow. 720 was fitted with 3 different engines. Different gear ratios. 
 

I had a 720 4x4 5 speed.. z24. It was so damn slow. Ran great though. I got about 16-18mpg.

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To compare the two, you’d need to make sure they had the same setup front to back. Gear ratio would make a difference.

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 My 86 720 2WD Z24 Weber 32/36 5 speed 370 gears cruises at 85 on a flat road and has plenty of power to pass on the freeway at 85.  Do not know what top speed is. 

 

Make sure you are getting full throttle on your truck.  I have had to adjust the linkage when I first got the truck and added the Weber.

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The stock tire for a 2wd is 195/75R14, these are 25.5" tall or diameter. If yours came with taller ones you are reading low. Just for example a 30" tire would be reading 60 but you would be going 70.

 

Your friend with the smaller tires? His speedometer will read higher but will be going slower.

 

I suggest pacing with a friend who has a newer car and see what you are actually doing for speed. But on the face of it you should be capable of over 70 I would think.

 

 

In no particular order...

 

Look down the secondary barrel with a flashlight. (engine off) Have someone step down fully on the gas pedal and push up on the secondary diaphragm. Is the secondary butterfly free to fully open in the vertical position? While there check the primary too.

 

Next time the engine is fully warmed up take the top off the air filter and look. Is the choke flap fully open and vertical?

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Take one intake side plug wire off and put an old plug in the end and place on a grounded surface. Have someone turn the engine over with the starter or start it. Do you see a spark jumping the gap? If yes replace the wire and do one of the exhaust side plugs. You should see sparks on both sides.

 

 

Take the valve cover off and set the valve lash to 0.012" on a hot engine.

 

Check that the ignition timing is 50 advance.

 

Could the catalytic converter be part plugged or is it or any of the exhaust pipe crushed?

 

 

 

Here is something non related but should be addressed by every Z24 owner. The Z24 tends to blow the head gasket every 100K. To reduce the chance of this happening the correct thing to do is re-torque the head bolts every tune up or say once a year. The procedure is to do this on a COLD engine. Loosen ONE bolt and torque to 60 ft lbs, then move onto the next bolt. Make sure that only ONE bolt at a time is loose and you can do them in any order you like. The sooner the better you do this.

 

 

 

 

 

 

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My choke is stuck open but I'll check the butterflies. Hadn't thought about checking both coils. Right now I'm searching for a speedo needle so I can't do the pacing with another car and I bet my buddy with the small tires is running slower than he thinks (hehe). The valve lash adjustment is on my quarantine-time-killer list so I'll check the head bolts and timing while I'm at it. Is there a way to check if cat converter is plugged?

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Run the engine on both trucks and feel the exhaust pulse.  If yours is way weaker its possible.  

 

Remember its a 35 year old truck.  I've owned like 5 driving 4x4 720s and had them get 25 mpg but (literally) take 5 miles to reach 65 mph.  I've also had them get 14 mpg on 31x10.5s but do 80 with a weber.

 

Basic health is next step after understanding what you have (gears/tire size/etc).  Compression test, quality spark and fuel supply,no vacuum leaks, etc. Are things you want work through.

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Keep in mind that they like going 60/65mph, to go faster you will need to put your foot into the pedal, but you mileage will go down.

You say you got a top speed of 65/70mph, but your not floored are you?

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5 minutes ago, wayno said:

Keep in mind that they like going 60/65mph, to go faster you will need to put your foot into the pedal, but you mileage will go down.

You say you got a top speed of 65/70mph, but your not floored are you?

Not floored but as I do step on the gas it seems like it doesn't want to accelerate as much as it would at a lower speed/gear. I realize that in 5th gear it's going to be less responsive but it seems like very little response at high speed after about 60 mph on a level road. I have a compression tester but I am not sure if sparkplug threads on the head are metric or standard because I am pretty sure the compression tester is one I bought when I had American cars.

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28 minutes ago, bottomwatcher said:

How many Miles on the engine? They do get tired. Also how old is your fuel filter? The classic sign of a clogged fuel filter is loosing top speed.

 Changed fuel filter maybe 1000 miles ago and I am just about to reach 69000 miles. That was a bitch of a job btw.

I'm going to do a compression check just to see what I am working with.

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I have an 86 2wd 720 long bed with standard tires, a weber and a newly rebuilt engine with .020 overbore and I run 60 MPH at 2400 RPM in 5th gear on the tach. I could not come near that with the truck before the engine rebuild.

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Keep in mind the Z series was built to be low emissions and as such many things did not have to be included in the build that promoted performance. For one it wasn't intended to run at 80 MPH but at highway speeds. It doesn't have to breathe well above say 3,500. The carburetor is somewhat small for a 2.4 liter engine. The intake and exhaust ports are low and close to the block and even though the valves are tilted there is a sharp bend into the combustion chamber. Again, good enough for breathing at highway revs but chokes itself when run higher. You can't add too much valve lift or they will hit each other during overlap. Compression is a terrible 8.25 but it runs on crappy gas and lowers emissions. Think low emissions and this is what it's good at.

 

To it's credit it makes about 130 ft lbs of torque at a low 2,800 RPMs ... again, perfect for pulling a heavy truck at highway speeds. It has a close to HEMI shaped combustion chamber which are very efficient burning. It has dual plugs which reduce emissions by reducing the ignition timing which lowers nitrous oxide production but also make the head very detonation resistant. It's also a cross flow head which keeps the intake ports away from the hot exhaust ports and the need to flow more coolant around them.

 

matrophy:

Look for slightly lower compressions on any two adjacent cylinders. This would indicate that the head gasket is perhaps in the early stages of blowing. I don't expect it yet but re-torquing may help prevent this. As long as all cylinders are close to the same reading you should be ok.

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Thanks. I'm starting to wonder if maybe I'm expecting more than the truck is capable of. I really have no need for traveling any faster than 65 mph and I am still getting used to having an ultra economy truck rather one with all the bells and whistles like I've owned before. I plan on following your suggestions and giving the engine a once over - carburetor doing what it is supposed to do, compression, valve lash and re-torqueing head bolts - to get an idea where we are. I am lucky that it is such a relatively low-mileage vehicle and I want to maintain it to run as long as possible.

 

A little off the subject, but there is a book called The Reckoning by David Halberstam that is very interesting. It's mostly about hubris in the auto industry in Detroit in the 60's and 70's but there is a lot of information about the post-war auto industry in Japan and the necessity for them to develop rugged, cheap and efficient autos.They have had decades more experience with this style of car than us Americans have had. 

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Our old engines need the protection afforded by the old oil they used to make 35 years ago. Today's engines are running on half the level of ZDDP, a zinc anti scuff oil additive than they used to. It's not needed today on mostly roller cams and it's harmful to the catalytic converters so oil makers are slowly phasing it out. Diesel engines run on high levels of ZDDP so I just switched to Shell Rotella T4. Chevron Delo 400 is also good as are other oils like Castrol GTX. There are others but you have to know what you want. There are ZDDP additives but why not just buy oil with the correct levels?

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85 4x4 stock wheels and tire size. Perfectly flat freeway speed 70- 75 on a good day.

50-55 on a 4% grade Completely stock engine, also been using Shell Rotella T4 for a few years now. 

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My 85 720KC  4x4 only gets into 5th going downhill or cruizing on frwy in the valley,but has gone 85+pulling a 4x8 trailer w a roadracer in the trailer, or atleast that's what the CHP told me

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I have owned 5 720's a diesel rwd, a 2.2l gas rwd, a 2.0 gas 4x4, a 2.4l gas rwd, and now a 2.4l 4x4. The diesel was one slow ass chugger but got great gas mileage and I loved her, top speed was 65 going downhill with a tailwind. the 2.2 rwd was a scooter and performed great for its age and a stock carb, top speed 80+ the 2.0l 4x4 was fairly tired when I got it and the carb was a joke, put a weber on it and got some more life out of it even it though it burned more oil than gas, top speed 70ish. My current daily is a 2.4l rwd with a FRESH rebuild,a weber, straight pipe, no emissions. If it had wings it would fly! top speed unknown... My new 4x4 is a project... Great trucks!

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I put a Z24 with a Weber into a 1970 Datsun 521 I built, it accelerated to 60mph just fine(3.7 gears in the rear I believe), after that I had to start putting my foot into it to get any more speed.

I built a LZ23 for my work truck, that truck weighed 3400lbs at the time, the acceleration getting on thew freeway was about the same, but after 60mph my work truck would walk away from that 1970 Datsun 521 I built, it pulled all the way thru 5000rpms, but that LZ23 has a modified head(equivalent to a 219 head) and dual SUs.

I have a 1969 Datsun 521 kingcab I made with a stock SD25 diesel engine/inline injection pump in it, I drove it that way for years, it was fine on level ground and would do 75mph all day long getting 27/28mpg, but when it came to a hill or a pass I was floored and lucky to be able to maintain 60mph, about 3 years ago I figured out how to install a turbocharger on it and not have any issues(I got lucky that I found that particular turbo), I now can drive 75mph all day long and get slightly over 30mpg and i don't even move my foot when I come to a hill and it goes right up that hill(shows more boost going up the hill), but I have to keep my foot in the same place going down the other side of the hill(shows less boost going down hill), there is more involved but I won't get into that but if I could drive 55mph I would likely get almost 40mpg if not more, when I had a truck with an SD22 it drove me crazy as they could not get out of their own way, so I sold it when I finished my 521 kingcab with an SD25 diesel engine.

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Matrophy, you are in Corvallis and we are just north of Millersburg. My wife used to work at McNary Dining Hall at OSU for years, so we are about 25 miles away. Go east on Hwy 34 to I-5 and then north to exit 238. When you go down the exit ramp (cloverleaf style) you will come to a stop sign, which will be on Old 99E. Turn right and go back under I-5 and towards Albany. About 1/2+ miles and there will be a electronic speed sign. The speed is 50 MPH there, and that is why the speed sign is there (used to be 55 and lots of problems with speeders there, maybe I'm one of them?). Anyhow, that speed sign seems to be accurate as it agrees with three different vehicles I have. 

 

I realize the sign is a bit of driving for you, but you need a small trip after being cooped up for a while. There may be other speed signs closer that I don't know about, but I am very familiar with that one.

 

Don

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3 hours ago, datzenmike said:

Our old engines need the protection afforded by the old oil they used to make 35 years ago. Today's engines are running on half the level of ZDDP, a zinc anti scuff oil additive than they used to. It's not needed today on mostly roller cams and it's harmful to the catalytic converters so oil makers are slowly phasing it out. Diesel engines run on high levels of ZDDP so I just switched to Shell Rotella T4. Chevron Delo 400 is also good as are other oils like Castrol GTX. There are others but you have to know what you want. There are ZDDP additives but why not just buy oil with the correct levels?

What ppm of zinc do you look for in a high-zinc oil? I fell down th einternet rabbit holes reading about high-zinc oil...

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5 minutes ago, 620slodat said:

Matrophy, you are in Corvallis and we are just north of Millersburg. My wife used to work at McNary Dining Hall at OSU for years, so we are about 25 miles away. Go east on Hwy 34 to I-5 and then north to exit 238. When you go down the exit ramp (cloverleaf style) you will come to a stop sign, which will be on Old 99E. Turn right and go back under I-5 and towards Albany. About 1/2+ miles and there will be a electronic speed sign. The speed is 50 MPH there, and that is why the speed sign is there (used to be 55 and lots of problems with speeders there, maybe I'm one of them?). Anyhow, that speed sign seems to be accurate as it agrees with three different vehicles I have. 

 

I realize the sign is a bit of driving for you, but you need a small trip after being cooped up for a while. There may be other speed signs closer that I don't know about, but I am very familiar with that one.

 

Don

That is awesome. I lost the needle on my speedo and I have been trying to think of a way to calibrate it by driving and trying to reattach the needle to match whatever speed I am going at that time. I know about the cell phone mph part but this sounds like more fun.

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I think it's down around 500-650 now so over 1,000 and 1,200 would be great. What ever  Rotella T4 is.....

 

This lobe wear is probably from years of using the shitty oil they make nowadays. Rockers were gouges also. This shit doesn't heal, once fucked it stays fucked. Protect you cam and rockers and don't believe what the oil companies tell you that today's oils are reverse compatible. I'll keep using the higher original levels thank-you.

AlI8XOe.jpg

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