Flat cam lobe?

[high winder]

so i have a 78 B210.. it started running a touch rough, mild backfire every once in awhile.. like maybe 1X a week. hit the the driveway and it acted like i blew the head gasket.

never overheated but was blowing smoke and i thought it smelled like coolant. i pulled the head to replace the gasket but couldn't find where the gasket was leaking but i thought nothing of it.

replaced gasket, set valves etc. started it up and it was popping. shut it off and i didn't see anything out of the ordinary. i decided to check the lash again to make sure timing etc. was all correct.

when i removed the valve cover the #4 pushrod was out. i thought well maybe i set the rocker on the edge and didn't get in the center of the  cup. i reset everything again and triple checked that all the pushrods were in proper position,  started it back up and popping again, i kept in running for a second and it was definately a  valve popping. i again removed the valve cover and the #4 pushrod was out.

i took all the pushrods, head and rocker assembly to the machine shop to have em all gone through beyond what i could do and it all checked out.

 machinist said to see it cam was flat. he asked what oil i ran and if i put a zinc additive in it. anyways i've been running a full synthentic with STP oil treatment but i'm thinking i put a flat spot on the cam.

 last night i reinstalled the head with old gasket and put it all back together, rotated the engine by hand and sure enough the pushrod popped out.

am i seeing signs of a bad cam or something else more sinister??

[datzenmike]

Is the push rod bent?

 

When the engine is set for setting the valves on that cylinder, are both the valve springs up at the same level? or is one lower than the other?

 

Assemble the push rod to the rocker, set the lash and watch it as you turn the engine with the starter. Does it move the same amount as the others?

 

When setting the valve lash on the bad one are you running out of adjustment screw???

 

 

 

 

 

 

 

Today's oils had been slowly phasing out the zinc anti scuff additive ZDDP. I don't know if synthetic oils use them or not but regular 'dyno' oild are down t about 1/2 their former levels because newer engines don't need it and it fouls the new catalytic converters. ZDDP is needed in our old engines. 

 

I don't know if a synthetic will do the same job as an oil with lots if ZDDP in it. I'm also not a fan of additives as the oil you choose should do the job you want it to do.

 

What I do is run a diesel oil that is still using the same ZDDP anti scuff additive package as was used in normal oils back in the 60s and 70s. Shell Rotella T4 in 15w40 although you can also get it in 10w30 for year round use. I only run it in the summer. Chevron Delo 400 is also good and there are others like Castrol GTX. I don't know what the STP additive is but makes more sense to just run an oil with it already at the correct level.

[high winder]

Pushrods all checked out fine for trueness and length, he pulled that valve and polished it in the event it was sticking.

i’m going to reassemble tonight, put a different pushrod in and see if and when it pops out.. I’m thinking either the valve is slightly hanging up or the rocker is hanging up.. 

I would think that even with a worn cam there still should be tension throughout the cycle, it just won’t allow the valve to open as much..

once again I’m leaning towards something hanging up when under tension, everything seems to be moving freely when it’s unbolted

[datzenmike]

When setting the lash is the bad one running out of adjustment screw???

 

 

 

Well if he had the valve out then the seat has not popped out.

 

[high winder]

No the adjustment screw does not run out of threads... can’t tell any difference in length from the others as far as that goes..

I’ll look the rocker assembly over again and see if I can find anything..

I’m kind of baffled not finding anything that jumps out and says I’m the culprit

[datzenmike]

Yeah, sorry forget that. That was to do with a seat that had fallen out.

 

Did you watch the rocker when the engine was turned by the starter?  

[high winder]

Not yet, just getting off work then try and figure it out

[high winder]

So it’s appears that something has caused the rocker arm on the #4 valve to hang up or I have a jacked up lifter..

with all the lash set at .014 (cold) when the pushrod comes up it’s pushing the valve open and continues until the valve closes, the pushrod continues down as the cam is indexing to the next cylinder which is where the disconnect happens... if I hold my finger slightly on the rocker arm it will stay in contact with the pushrod through the cycle.

however on that rocker arm the adjustment screw sets about 2mm higher than the rest. I have no reason too think that’s abnormal but I could be way off base.

 I removed the rocker assembly to inspect for some kind of a bur, something to see what might be causing it and found nothing. All the oil journals were clear, rocker arm moves freely etc. I placed a straight edge on it and it appeared flat and straight. However I bolted it back on without the pushrods installed and the #5 rocker arm is very stiff and #4 seems to have no issues but the problem is in #4.

 I measured the best I could with just the pushrods installed with #1 TDC and then rotated them all through the cycle and measured the exhaust stroke and the intake stroke and they all seemed to measure the same..

Am I still missing something somewhere... these are pretty basic motors but I be damaged if I can figure it out

Edited May 13, 2020 by high winder

[Tom1200]

If the adjustor screw is higher (more thread sticking up above the adjuster locknut) then when the lifter bottoms out on the cam lobe the 2mm gap may be enough to let the push rod fall out. If the rocker arm is stiff on the shaft, either the shaft or the rocker is galled. As for number 4 rocker, have you checked the rocker to see if it's bent? Compare it to the other rockers.

 

If it's a cam lobe typically it won't have as much lift. I've never heard of it but it's physically possible a lifter is so worn it drops lower but you would expect to the lash adjust srew to be turned really far down into the rocker.

 

 

 

 

[high winder]

It has to be in the cam or lifter... pulling the motor in a few..

when I hand turn the motor the valve is barely being pushed open, maybe 1/4”, as the cam is indexing to the next cycle the pushrod falls back in the hole and comes out of contact with the pushrod...

 I checked all the rockers and they appear to all be straight. When I tighten it onto the head #5 rocker arm gets a little tight but stays functioning like it should..

I’m hoping it’s just in the lifter because I can’t find a cam anywhere. I’m assuming I’ll have to send it off for a weld and regrind.

 I’m kind of baffled why it’s only the one pushrod, guess I’ll find out soon enough

[datzenmike]

The compressed valve spring should push the rocker arm down on the push rod as it goes down the other side of the cam lobe.  Rocker must be seized.

[Rat-a-tat-Dat]

Process of elimination... swap #4 rocker arm with any another one and check for any difference. 

[high winder]

4 hours ago, datzenmike said:

The compressed valve spring should push the rocker arm down on the push rod as it goes down the other side of the cam lobe.  Rocker must be seized.

It’s does but the pushrod continues down toward the lifter, the valve spring can only push it so far, on the stroke where it’s supposed to open the valve it only opens it maybe a 1/4”.. the rocker arm moves freely in both directions but the pushrod falls away when the rocker arm has returned from the now uncompressed spring if that makes sense

[datzenmike]

You said that you set the valve clearance and why you were asked if you ran out of adjustment. Valve clearance is set OFF of the cam lobe. If clearance was set, the spring should return the rocker arm and push rod all the way back to the set position even it it only moves 1/4"

[high winder]

Correct, when the lash is first set on #4, 6,7 8 it is all good, when you rotate the motor from this point it pushed the valve open and then the next stroke it closes, when the valve closes the rocker arm does push the rod back down but as soon as the cam turns for the next stroke the pushrod begins to drift down and out of contact with the rocker..the valve is closed now so there’s nothing to keep the rocker in contact with the pushrod.

this is what is leading me too believe that on the downward side of the lobe something is not right...

 if I’m a complete idiot and continue to set the lash wrong I would think it would be doing it on other cylinders as well..

I wish I had some way and I would post a video

[datzenmike]

Then there must be a divot on the base circle on the cam where you are setting the lash. Probably right at the end/bottom of the lobe where the lifter slides down and hits it.

[high winder]

That’s exactly what I’m thinking.. 

it seams it’s kind of an oddity. Atleast for me anyways, I’ve had cams go flat, hydraulic lifters collapse, blown a few rods right out the side of the block but I can’t recall having a cam go bad in one specific spot.. live and learn I reckon

[datzenmike]

305s were notorious for eating cams and lifters. I had an '82 GP given to me because it didn't run right. Half the lobes were worn down some completely and the lifters cupped. $200 for RV cam and new lifters and boom nice fee car, ran really well too. GM used to make good cams and lifters then they started making them cheap and relied on hi tech engineered oils to save them. Which would have worked but GM owners knew from past experience that skipping an oil change or extending them was no big deal, do it all the time. Friend had a 7 year old  '79 Monte Carlo same thing.

[high winder]

pulled the cam and that lobe is dang near round, lifter looked good with no signs of wear any different than the other 7.

I suppose that valve must have never been adjusted correctly..

I cant seem to find a new cam for it so I guess ill have to send it off for a weld and regrind

 

I appreciate the comments.. help me sort stuff out in my pee sized brain

[datzenmike]

Try...  https://ratsun.net/classifieds/category/9-datsun-parts/

 

There are lots of A owners that might be able to spare you one, and that got to be cheaper that what you plan.

[high winder]

Will do