74 620 with L20b

[MikeyGee87325]

Hokay so...

 

My buddy acquired this truck a bit back, clutch had detonated (now fixed).

It goes to portland, the head makes all kinds of noise, couldn't adjust the valves to kill the chatter.

Soon after it became apparent the truck had 2 dead cylinders (2&3)

with spark and fuel issues alleviated, and soon burning coolant we called it a head gasket

we pulled the head it was cracked on all 4 cylinders between the intake and exhaust ports

new head installed, ignition timing in, head and crank timing in, still has dead 2&3

 

my question is: what is the likelihood of those cylinders losing compression through the rings?

Is this a common occurrence for a L20b with xxx,xxx miles on it?

Is there a way to check this without re-removing the head?

 

Brainstorm with me if you could please.

[datsunaholic]

Dead how? Not firing, or no compression? If it has no compression, then if it's not valve lash being too tight you aren't fixing it without pulling the head. Wet/dry compression test might tell if it's the rings or not.

 

But if it has compression but is still dead, are you sure you don't have #2 and #3 spark plug wires reversed? 1-3-4-2, COUNTER clockwise. Can't count how many times I've seen that screwed up.

[Eagle_Adam]

do a compression test on 2 and 3

[Dawa]

sounds like the motor is bad luck. cursed, even.

do yourselves a huge favor and throw it in the ocean

[hobbes_the_cat]

First, double check the firing order. I've reversed a wire many times. Was your new head resurfaced or was it just a used head? All the warped heads I have seen were warped the same way, high in the center. Maybe the "new" one is warped too?

[MikeyGee87325]

Quadruple checked the wires, everything is in it's right place.

Moved the spark plugs to different cylinders, 2 and 3 are still not alive.

 

Compression test:

Cyl 1: 95 psi

Cyl 2: 100 psi

Cyl 3: 90 psi

Cyl 4: 100 psi

 

I know the cylinders are supposed to test between 115 and 180 psi, but also test within 5 (ish) psi of each other.

I'm out of the 115-180 psi range, but within 5 psi of the other cylinders.

 

As a side note, while the head was off the bottom end of the motor was spun over, by hand, a few times to inspect the cylinder walls, I did not notice anything abnormal at this time.

 

Also, both the warped and 'new' head were at the machine shop. There, the 'new' head was tested and rebuilt.

[datzenmike]

Still not understanding 2&3 'still not alive'. What are you using to compare them to 1&4? How are they different?

 

That's very very low compression readings.

 

... the head makes all kinds of noise, couldn't adjust the valves to kill the chatter....

 

How is this possible? Valves can always be adjusted unless the valve has worn into the seat and sunk so low there is no adjustment, but this is too tight, not loose. What were you setting the valve clearance at? If too tight you might get the low compression numbers you report.

[MikeyGee87325]

I have what seems to be no spark on cylinder 2 and 3. Ignition system is good, spark is getting to the cylinders. The carburetor is supplying fuel. Compression is listed above.

 

The noisy head was before the new head was installed.

 

Also, the valve clearances are supposed be set hot, however, for the purpose of heating it up we have adjusted (cold) intake to: .008 in. exhaust to: .010 in.

[banzai510(hainz)]

hot or cold open the valve cover

when the lobe is pointing UP and the rocker wiggles a little bit then you know the valve is closing.

 

ck for the lash pad installed. soemtimes they fall out.

if still no firing make sure you got the fire order correct. other wise I think you got a hole soemwhere if you dont have compression. cracked valve? Piston?

 

 

cam timming set riight??????????

[datsunaholic]

I have what seems to be no spark on cylinder 2 and 3. Ignition system is good, spark is getting to the cylinders.

 

That's a contradiction- either you are, or are not, getting spark.

[Pacific coast Datsun]

Watch the Hainz video a few times.

L motors are easy to work on.