Top end issues

[RocksRcool]

Hi all,

I picked up an 85 nissan 720 4wd kingcab last week, it was barely running. Ive changed oil an filter, new spark plugs (ngk of course), fixed a leak in the carb fuel line, tightened down the valve cover (it was super loose), re done all my vaccum lines, most were missing, threw on a new distributor cap and rotor, set my timing via the sticker under the hood. Im idling fine (800-900), BUT, im still lacking a bunch of power an she just wont get above 50-55mph...whats going on? Could it be the vaccum advance?

Any help would be appreciated for a new 720 enthusiast

[datzenmike]

Check that you have spark from both coils. Dual plugs shorten the burn time so ignition timing can be run around 3-5 degrees. If one set of plugs are not firing the timing would normally like to be around 12 degrees. If it would like 12 degrees with a single plug and you are running 3 degrees it will lack serious power. The most likely cause of the exhaust side plugs not firing is the first fuse on the far left side of the fuse box being blown. Just replace it.

 

Check and set the valve lash. 0.012"

 

Won't be vacuum advance as it only affects part throttle operation.

 

When warmed up is your choke still on? Take the top of the air filter off and see if the choke plate is part open or fully open. The choke plate should be vertical, fully open.

[RocksRcool]

Well i have spark from both coils, all my spark lines seem to be fine, ran a compression test from cylinder 1 to 4: 137, 140, 137, 125( tested each cylinder 3 times to give myself an average). Guessing cylinder 4 was cooled off more by the time i got to testing it, or its a lil more worn then the rest. I removed the catalytic converter, no noticeable change, de-smogged it as well. Same result of no change. Im going to get a carb rebuild kit, noticed a lil wetness comeing from a seal. I bought it from a feller who barely spoke english, he used it as a farm truck, so i have no clue if he drove it up from cali or maybe down from denver.. i wonder if the carb is jetted totally wrong for the altitude im at.

[RocksRcool]

Oh an the choke plates are verticle as should be

[xsdg]

Do you have a timing light?  That'll make it pretty clear if you're getting advance or not.  Also, if you have access to a vacuum gauge, what's it reading at idle?  Ideally, you want to read around 20 in-Hg of vacuum at idle.

 

If you can take a picture of the engine bay, that'll help us figure out what exactly is on the truck, and consequently what might be going wrong.  There are a bunch of different varieties of these beasts.

[datzenmike]

Need timing confirmed and the valve lash set or checked. You're getting close.

[720inOlyWa]

This wont chase down the gremlins you seek, but i would always re-torque the head on a 720 I didn’t know much about. And then do it again, once a year or so, as I chased all the other issues (like the ones you are experiencing)..

[datzenmike]

The Z24 is notorious for blowing the gasket every 100K. The Nissan recommended 'fix' is preventative.  Every tune up (I figure once a year is fine) with the engine COLD loosen a head bolt and then torque to 60 ft lbs. Only when it is tight, move on to the next bolt. Any order you like.

 

This keeps a consistent clamping force on the gasket as it ages.

[RocksRcool]

So i finnally got a chance to do my valve lash, set it while hot to .013. Before i go farther, timing is set and advances fine with throttle. Did a carb rebuild on my weber 32/36 dgev, my jetting seemed screwed up, fuel jets were both 35's, air jets were 170 primary, 120 seconday, main idle jet was 55 an secondary 50. Ive order a jetting kit. Im at 2400ft of elevation.

 

So when i was setting valve lash i noticed 5-6 of my head bolts were not tight, so i went to tighten them but their stripped, not the bolt itself, but whatever their mounting into, they bite a lil bit but never fully tighten down, def not to 60lbs. How screwed am i?

Also, i still have no top end, an am missing a bunch of acceleration. The engine is quieter which is good.

[Draker]

If they aren't grabbing, then the block itself is messed up. You could pull the head and install some heli-coils. Time sets would be a better option.. but much more expensive.

[datzenmike]

Block threads pulled out would seem to indicate excessive tightening coupled with over heating. Aluminum expands twice as much as steel.

 

They should only be tightened when cold for proper torque anyway. Take one of them all the way out and inspect. If the block threads are pulled out you'll see them on the bolts.

 

 

The Z24 is problematic for head gasket failures due to not properly holding the bolts. I've heard several reasons from, the metallurgy was flawed to the block not being tapped deep enough. The 'cure' was a preventative measure: re torque the head bolts every tune up when cold. Loosen fully, one bolt at a time, and torque to 60 ft lbs. Only then, move to the next bolt. Any order you like.