The indefatigable 720 engine problems

[None_zero]

So I have this snowballing problem which all began with me misdiagnosing my original condition which turned out to be bad ground to body connection. Since that time a myriad assortment of adjustments and cleanings and wirings and such have taken place but here is where I am. The truck starts but requires that I gas it until the engine gets warm. And then if I take my foot off the clutch it drops off performance and sometimes idles down until it dies I have decent power in 1st and second gear but 3rd and 4th don't do as well and if I take my foot all the way off the clutch the engine cuts down a little and runs roughly going up a hill of any noticable grade is almost impossible. Timing at idle is set to 5-7 degrees advanced rpms at between 750 and 900 but the idle spikes while coming to a stop to about 1500 rpms but sometimes as high as 2k (it spikes worse if I am stopping and also turning and worse still if I'm doing those things on a downward slope. Any idea where I should be looking ?

[Chuckthebull]

I have not had a Z car in a lot of years but it was fuel injected..is this carb or FI?
I know they made a both version in the 720.. 
If FI,,check fuel pressure and fuel filter .. if rust is slowing down fuel rate this might account for it.. 
Also if its FI,, clean the MAS air flow sensor (they make a dedicated spray for this..)

if its a carburetor you might have to clean the orifices in the carb.(jets etc) .idle circuit might be ok but fast running circuit is clogged or both are....(that why you get no power on acceleration... also if its a carb the float might be leaking ..thus the high revs..
too much gas is as bad as not enough..air to fuel ratio is important. lack of either will bog down the engine.
Check those things... and the accelerator pump if it has one..(usually do) might want to juist rebuild the carb with a kit

https://www.autozone.com/fuel-delivery/carburetor-kit/nissandatsun/720

[None_zero]

@Chuckthebullit's natural aspirated and rebuild the carb is next on the agenda for me. I cleaned the maf sensor with crc qd electronics cleaner figured that would be safe enough I have cleaned around on the accessible parts of the carb I even removed the choke paddle and whatever I thought I could get away with and not have to replace gaskets and such. You reminded me just now though that I have an aftermarket fuel pump installed on it right now with a questionable flow rate I checked the old pump and it is still good so I'm going to put it back on there and replace the filter again. I've been fighting that gas tank rust transfer issue for a long time. I've even dropped the tank and cleaned it with chemicals and a chain that I dropped in to scour it with. So fuel filter is my go too when it starts running crappy. 

[None_zero]

Also while it's down I decided to pull the exhaust manifold and have a look since I'm building so much carbon on everything I wanted to see the state of things inside there and I fear the news is not good. I found this that was done to plug an open port. looks pretty janky and likely accounts for the damage I found to the section that ports the exhaust for the rearmost two cylinders

[None_zero]

This is the state of the manifold mating surface. I'm aware that this is far from ideal but if I have to continue using this will I even be able to with a new gasket? You'll see the two ports closest to the weird plug it had are pocked and it surface isn't perfectly level across the mating surface 

[None_zero]

Here is the side that looks better 

[None_zero]

And the next three photos are with a straight edge laid on it to show the level of the surface 

[None_zero]

[None_zero]

[None_zero]

Soooo... The ex-wife took those last ones while I held the straight edge I don't think she quite knew what I was after but maybe they show it well enough.. how f'd is my b on this??

[None_zero]

Not sure why it keeps wanting to insert that same message the caption that goes with this photo was more along the lines of I plan to patch this little hole also if the manifold can be used without causing damage or failure to function.

Edited April 16, 2022 by None_zero

[None_zero]

The other issue I have is knowing which carb rebuild kit I need, it seems the tag has been removed from my carb.. and while it's almost identical to the one listed on rock auto for the z24 from autoline it's not exactly identical maybe one of you guys can verify that it's the same carb... picture too follow 

[None_zero]

Here is the one I've got..the little plastic half moon on the top was there but it broke a while back and fell out.

[None_zero]

2 hours ago, None_zero said:

*this little hole*

 

[datzenmike]

Exhaust pipe?????? Not going to cause any problems. Patch or tighten a hose clamp over it for now,

[MGTS]

I was having a similar issue. The main relay behind the battery had gone bad. It controls the auto choke and the fuel cut solenoid. Got another one from the scrap yard and it's been running fine since

[None_zero]

On 4/21/2022 at 2:30 PM, MGTS said:

I was having a similar issue. The main relay behind the battery had gone bad. It controls the auto choke and the fuel cut solenoid. Got another one from the scrap yard and it's been running fine since

Thanks for posting I was thinking about that relay the other day but I forgot about it and haven't done anything with it yet I'll add it to my check list because mine was cut out and spliced back in a while back (not by me) well I did splice it back in,, anyway the wires were super short and I never have any luck getting wires out of and back into those relay harness cubes whatever their called.. and so I used the exceedingly short wire stubs that were there and after getting it all tied together I covered over it all with hot glue to prevent the wires contacting their neighboring wires or grounding to the body or anything. Which worked great for a year or two but it looks like it's a bit worse for wear now. As it happens I keep at least one spare on hand for every relay in the truck not to mention a spare serpentine belt fuel filter and fuel pump... I hate getting stuck somewhere with a vehicle that is throwing a tantrum and refusing to go, if you can't tell. I was going to just update the thread, @datzenmikeI'm pretty sure that bolt and stack of washers plugging the unused port on the back of the exhaust manifold is responsible for that issue I mentioned about how the first three sparkplugs we're black as sin but the two in the rearmost cylinder were two toned appearing to be running very rich and very lean at the same time. Am I wrong to say that the loss of pressure from the exhaust manifold that close to the piston would cause it to build excess heat in the cylinder and when combined with what can only be described as an ungodly amount of carbon thoughout the system but most notably in this case in the area around the cylinder could be the source of the excessive dieseling I've been experience and having one or two combustion chambers getting significantly hotter than the other two would make achievement of a smooth idle nearly impossible and also make finding the sweet spot between rich and lean equally difficult? I have this problem in my brain where once I imagine a way something might be failing I have to take it off and look so I can understand what if anything it contributed to the problem, thus my removal of the exhaust manifold, which i have cleaned up some and replaced the bolt/washer plug with a weld to close it off, I basically ground the threads off of a giant bolt I picked up walking along some railroads one day and tapped it into the home and welded it in place, I cut the head of it off and ground it down to a rounded nub I also welded a patch onto the exhaust hole and coated everything  except the mating surface in a clear ceramic coating rated up to 2k degree temps before replacing the manifold gaskets and reinstalling. The last time I drove it, it was running more and more roughly and in the last stretch before I got home it produced a sound from somewhere over by the exhaust side, that had me afraid I wouldn't get home, truth be told though I think I heard it once or maybe twice and I didn't hear it well enough to say for sure it didn't come from the hub or caliper on that side but I I have been working on it every since that fellow Hit my ass end and ran me into the truck in front of me. Yesterday I got the front bumper straightend back out and lowered back where it belongs. More kt I have a couple little things still to square off before bimbresdy to put it on the road but while it's been sitting there I replaced the dzzy cap and rotor and the spark plugs for new air filter and I'm trying to clean the little filter for the crankcase breather, probably should just replace can't be more than a coulen bucks for that 

[None_zero]

You may recall I had the same problem with the water pump thinking there might be something wrong and even though you assured that wasn't the source of the trouble I was having back then. I still had to take it off and just have a look. Once I saw it myself I immediately understood how you could say with such confidence that the waterpump was a non factor. But I'm hard headed like that, I think the most common string of 10 words (hope I counted them correctly lol) I I've heard repeated over and over in life are "why do you have to learn everything the hard way"

[datzenmike]

 

This is down stream from the engine and can't affect what's upstream of it. It's just one of the air injection inlets to the exhaust. The exhaust is a series of pressure and vacuum pulses that is piped up to the air filter. The air filter has a one way valve that only allows air into the pipe from the air filter on the vacuum pulses. Air is sucked into and mixes with the hot exhaust adding oxygen so that the catalytic converter can burn any emissions. The engine will run weather this is connected or not.

 

Carbon is from an over rich condition probably from the normal running of the choke. The combustion chambers and the spark plugs will self clean by themselves normally if the engine is allowed to warm up and if driven far enough. Assuming the choke is working and shuts off.

[None_zero]

On 4/26/2022 at 11:32 PM, datzenmike said:

 

This is down stream from the engine and can't affect what's upstream of it. It's just one of the air injection inlets to the exhaust. The exhaust is a series of pressure and vacuum pulses that is piped up to the air filter. The air filter has a one way valve that only allows air into the pipe from the air filter on the vacuum pulses. Air is sucked into and mixes with the hot exhaust adding oxygen so that the catalytic converter can burn any emissions. The engine will run weather this is connected or not.

 

Carbon is from an over rich condition probably from the normal running of the choke. The combustion chambers and the spark plugs will self clean by themselves normally if the engine is allowed to warm up and if driven far enough. Assuming the choke is working and shuts off.

Oh I get it now, my thinking was that if you had a component leaking pressure that close to the combustion you'd have compression loss from it. But not only is the exhaust manifold not pressurized in anyway It also has the closed valve between it and the combustion. So it's moot. I'm slow sometimes but I can learn. This probably seems pretty freaking random 10 months later... I was just reading back over some of the other posts about the 720 and ran across this and a light came on 

Edited March 2, 2023 by None_zero