Truck running rough any suggestions what it might be

[Sergio146]

I have a question for anyone who knows what this might be. For an insight  I did the timing kit for an l20b engine (w58 head) and want to know if this could be caused if the water passage gasket was torn or if it is something else such as a head gasket. Compression is 175psi all 4 cylinders, electronic matchbox distributor, ngk bpr6ey plugs. Truck can hold an idle but shakes a lot with it being around 700 rpm with 18.5 in Hg vacuum. The plugs are burning very rich and the idle mixture screw is turned 2 turns out (hitachi dch340 carburetor). The bubbles form when done a leak down on both cylinders 2 and 3 but nothing on 1 and 4. Engine does not overheat. This picture illustrates how the bubbles form when air is introduced on cylinder #2.

[datzenmike]

It will run rich when cold and the choke is on. Spark plugs are self cleaning so you would have to take for a drive for them to clean themselves. If warmed up and still rich choke may be stuck on or the idle mixture is set too far out. I wouldn't worry about this just yet. Needs to be running properly.

 

Leak down test's compressed air in 2 and 3 causes bubbles in radiator pretty much confirms air is getting into the cooling system. The combustion chamber is the only place under pressure so has to be the head gasket. 2 and 3 is the middle of the head where any warp is maximized. 

 

You used a new head gasket?

You cleaned the head and the block surface till it was spotless?

You tightened the head bolts is a specific pattern? starting in the middle and working progressively outwards to the front and back?

Tightened to about 60 ft lbs?

 

If yes to the above then the head is likely warped.

 

 

Not suggesting you do this. If warped have a shop mill it properly....

Here is an old warped head I have. I used a certified straight edge and placed it diagonally across the head in an X pattern and tried to fit a feeler gauge under it. I managed a 0.015"between 2 and 3. The allowable limit is 0.004" so almost 4 times too much. I got a large sheet of glass and glued 180 emery paper on it. I pulled and pushed it lengthwise across the paper 20 times. cleared any aluminum dust and switched ends.

 

 

 

You can clearly see the low spot between the #2 and #3 combustion chambers.

 

As the low spot gets smaller there is more surface to sane so it takes longer and longer but eventually there was only the spot between 2 and 3...

 

Aaaaaaaand eventually all gone.

 

If severely over heated the aluminum will lift or pucker up away from the block. This may have been done by a previous owner and nothing happened and removing the head gasket made it impossible to seal with a new one.

 

If you check straightness with a straight edge and it is warped, take the rockers off but keep in order. Try spinning the cam by hand. If you can turn it then the head warp is not affecting it.

[Sergio146]

This engine looks like it has had a head gasket replaced previously and the cam towers have a spacer as well. I don't know much about the engine other than it just recently happened but if there is no other choice, then I'll have to see how the head gasket looks like. The one that it currently has on it by looking at it, it seems like the fel pro brand was used. What I could do also is retorque the head bolts on a cold engine to see if one of them is out of spec.

[datzenmike]

If it's leaking air into the coolant tightening the bolts won't help.

 

When the old gasket comes off you'll be able to see where it's leaking.

[Sergio146]

I'll then have to remove the head to further investigate the problem I guess. I would keep you posted on what I find. 

[datzenmike]

Yeah, I'm afraid so.

[Sergio146]

Just a thought, can this l20b with water passages be compatible with a U67 head? I'm asking since I have this head but not sure if I should swap it or fix up the head that it currently has on it (W58) if it needs work?

[datzenmike]

The U67 was used on all '75-'77 L20Bs. Just drill two holes for the coolant flow through the intake like the W58. I did this to my U67 head in order to use a later L20B intake from a W58.

 

U67. Note there are no coolant holes below the two pair of intake ports and the exhaust ports are square.

 

 

W58. In addition to the two coolant holes below the two pair of intake ports the exhaust ports are round

 

 

[Sergio146]

This is what the block looks like after the head has been removed. And this is the head gasket. I don't see anything obvious other than a slight black spot missing on the #3 cylinder but that's pretty much it. 

[Sergio146]

[datzenmike]

Not seeing the middle picture of the top of gasket. Don't forget this is a very slight leak that didn't show up on a compression test

[Joecar]

My truck has a problem similar to this as well, runs really rich and you can smell it with a Weber, also not sure if this has anything to do with it but I changed my oil a week ago and put about 150 ish miles on it and it’s already pitch black

[datzenmike]

Joe, yes but this is Sergio146's thread. My answer to you might confuse him. Better if you had a project thread for your 720 and all your topics would be under one roof. This way if looking for something in your posts it will be easier to keep track of one.

 

To answer your question when a carbureted engine is started up cold the automatic choke will come on. The choke forces a rich condition for smoother starts and running when engine is cold and turns off when engine warms up. Now if you drive short distances you choke will be ON more than off and it will carbon up. It's possible the choke is stuck in the on position or perhaps the choke heater isn't working. Start a new topic and let me know.

[Sergio146]

I have installed the head and have driven it. Now the issue that it has now is that as I drive it for 5 or 10 min, it runs rine but after a while, I lose power and it idles erratically. My suspicion could be that the ignition coil is not the right one since the spark plugs seems to be showing that brown tan color but possibly a weak or inconsistent spark could be causing the other plugs to not fire as they should. As a side note, this coil has a 1.6ohm reading and this came off of another datsun 720 that had the z22 engine with dual ignition coils.

[banzai510(hainz)]

did you put a timming light on this.

Cam timing is good?

 

I think a Z22 coil be the same or close to a 79 coil.

 

is your choke staying ON after warm up?

 

I dont see a timing wedge on the chain !!!!!!!!!!!!!!! so maybe the tensioner is popped out

[Sergio146]

Furthermore, this is the distributor that I'm working with and I'm trying to figure out whether the coil would be appropriate for it or not.

[Sergio146]

Hainz. I've already driven the truck and ensured that the tensioner was not out and double checked on my chain position (its on #2) and timing is sent at 12° BTDC. Choke does stay on after it is warmed up.

[banzai510(hainz)]

the later model Nissan  coil dont use a ballast resisitor so should be close to the same.

[banzai510(hainz)]

3 minutes ago, Sergio146 said:

Choke does stay on after it is warmed up

Ok I mean ON by the choke flap still being closed. This will cause a over rich condition, Usualy means the 12volt wire is not hooked up to turn the flap to open.

 

also at 12deg BTDC make sure you still have movement on the timing plate and it not cranked to one side and it runs out of time meaning a tooth off on the oil pump. But if the pump wasnt removed then it will not change that part

Edited October 27, 2024 by banzai510(hainz)

[Sergio146]

Oh I see. It has to be something then since this only happens when the engine is warming up and it could maintain an idle but not when it's under load.

[Sergio146]

My bad, choke is off after it is warmed up. On at initial startup.

[Sergio146]

For those that wanted to know about the head gasket. Here are some pictures, but like I said, it doesn't look bad.

[datzenmike]

Really hard to see a problem until driven long enough for the gasket to erode away. Does look a little rusty between 2/3 compared to 1/2 and 3/4. Also the head may be warped upwards above the gasket. Did you check head for flatness yet????

 

 

 

 Best check the head for flatness, fix that if needed and replace the head gasket before worrying about how it runs. Blown gasket could very well be the cause of poor running now.

 

Yes, there doesn't seem to be anything holding the timing chain.

[banzai510(hainz)]

I dont see anything obvious. with the gaskets

Edited October 27, 2024 by banzai510(hainz)

[Sergio146]

I'll be checking for flatness later next week when I obtain my mechanics square. Then I'll know what's wrong.