Jump to content

Datsun 720 burning excessive oil burning


Hum0ldt_720

Recommended Posts

I have an 82 Datsun 720 (z22) and just recently it started burning oil. At first I thought it was leaking from drain plug, so I changed drain plug gasket from the home made gasket paper one to the one made for oil drain plugs. It went through a full quart the first time, didn't pay attention to how long it had been since I change my oil last ( wasn't very long though). This most recent time, I changed my oil and about two weeks later I have to top of oil with over a quart. Dip stick said it was below the low mark. Doesn't seem to have any smoke coming from tailpipe, maybe a few puffs when I start it up in the morning, but I'm wondering what may cause this, or where I should start to figure it out. Don't know if any of this is a factor, but I have a rebuild hitachi carb( rebuilt my americancarburetors.com), edelbrock highflow airfilter that replaced the original bulky blue air air filter, did a vac delete including get exhaust manifold tubes plugged, and flow master exhaust. Other problems I'm not sure about which might help someone help me is that it idles at about 1500 rpms( lowest idle adjustment screw will adjust it), minor dieseling at shut off. Dieseling have been improved since I got the truck after running fuel cleaner with quality fuel every time I fill up. Truck also seems to make popping sounds when I down shift to slow down to stop at stop light, in every gear. I don't know what the owner had done to it before I owned it, but The two things I know is I had to restore it from the dead after it sat for years and that the old man use to race it I guess. Any help or direction is much appreciated

  • Like 1
Link to comment
  • Replies 13
  • Created
  • Last Reply

Top Posters In This Topic

Top Posters In This Topic

Dry compression test. Record readings.

 

Wet compression test. Compare readings.

 

 

 

If those numbers are extremely different, wet test should/will read higher, then likely culprit is rings.

 

 

 

So CE you say smokes on start though, I'm leaning towards valve guide seals.

 

 

Do compression tests first.

  • Like 3
Link to comment

How many miles on this engine since new / rebuild? What do the plugs look like? I am leaning toward thoroughly tired engine, but that compression test should tell the tale.

  • Like 1
Link to comment

Visually inspect for leaks. If the engine is covered in dirt and oil, park over cardboard and inspect above where ever there are drips. Oil tends to migrate so the source, of the leak is usually higher and farther forward. A quart in 2 weeks will leave a hell of a mess, it should be obvious like the valve cover gasket.

 

 

The exhaust popping and the 'run on' after shut off is likely from the over fast idle.

 

Fast idle: Some will say a vacuum leak, but I've never seen a fast idle caused by this. A vacuum leak would cause a lean idle mixture and a lowering of RPMs.

 

1/ The secondary throttle plate is not closed. Use a flash light and look down the secondary, the barrel closest to the valve cover. It should be closed tight. Hold the throttle half open and push up on the vacuum diaphragm rod that pulls it open. See if it's stuck. Push it closed gently with a pencil?

 

Looking up from below... secondary is on the left. The round thing (marked K3 OSI) is the vacuum operated secondary diaphragm. It pulls the secondary open. It may just be sticky and need to be manually opened and closed. There is an interlock so be sure to hold the throttle at least 1/2 open. 

carb03_zpszrnd5i38.jpg

 

 

 

2/ BCDD. This is the round thing on the valve cover side of the carb. It adds gas and air during deceleration and is activated by high intake vacuum and turned off when this drops at or near idle. If set too sensitive it won't shut off and is a supply of air and gas that raises the idle.

 

There is an adjustment in the center. Turn counter clockwise to decrease sensitivity. Yours may not be exactly like this one...

IMG_0677.jpg

 

3/ Throttle cable too tight and not allowing the throttle to close.

  • Like 1
Link to comment

Back in Black: wil do compression test soon. What do you mean wet and dry test? Have never done a compression test on any of my vehicles so it's new ground for me.

 

720inoylwa: it have 125000 miles on it. Idk if that's a lot for these motors, I figured since they were fairly decent engines that they would be on their last leg at least over 200k, but the old man I got it from use to race it I guess, and when I got it, it looked like it had been neglected for its whole like.

 

King rat: I will inspect it right now. This is the first carb engine I've owned, so still trying to learn about it. Had no idea the bcdd valve was adjustable. When I adjust it, I assume truck should be running, and how much do I turn it at a time, and will it show idle drop instantly or should I wait for a minute for the carb to adjust before turning more.

 

Thank you all for the information and help. I hope to have it running like a raped ape soon and will post my results soon

  • Like 1
Link to comment

I would just give it a couple turns counter clockwise. If this sets the sensitivity too weak it simply will stop working and you'll never notice. It may also get rid of the fast idle. I had this once. Pull up to stops and the idle was high. Tap the gas and it reset or if you shut it off and restart it. The BCDD was hanging and not resetting itself. It's only supposed to work when decelerating like out on the highway or down shifting. If the sensitivity is set too sensitive it turns on all the time and doesn't shut off properly.  

 

I've also had the secondary stick ever so slightly open. Pushed up on the diaphragm to open it to see if something was caught in it and it closed properly. Just stuck. Fixed the problem.

  • Like 1
Link to comment

A wet compression test means put a drop of oil in the cylinder, it will temporarily act like a good ring for the commpression test.

 

Side note: do the dry test first.

 

What you're doing is testing how much pressure a cylinder can hold. If wet test gets performed it seals the rings enough to hold pressure temporarily.

 

 

If burning oil/rings stuck, the pressure built will sleep past the pistons in to the crankcase.

 

What happens when running...oil travels up past rings and gets burnt during ignition stroke.

 

 

A compression test is never a 100% diagnosis. It should only be to determine next phase of diagnosis. Compression testing should always be one of the first tests performed; never the last.

  • Like 1
Link to comment

I've owned vehicles that burned a quart per tank of gas. That much is visible behind you as a haze of bluish smoke. Plugs will be wet with oil if the rings. Take the plugs out and look at them. If clean then it's not 'burning' it.

  • Like 1
Link to comment
  • 2 weeks later...

The parts truck I got had been sitting for a year and when I swapped it over, put a battery air filter and changed the oil and spark plugs it ran like a champ. Never thought about rebuilding and selling the z22. Didn't think it was a hot ticket item, but I might. And blue tinted window? What's up with the blue tint. I don't think any of the windows are tinted, but it has the fake wood on the dash. Lol rolling around in style now

Link to comment

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.