[Garrettsoler]

Hi everyone,

I have a brand new rebuilt 350 engine that I've been trying to turn on with no luck, I've narrowed it down to a compression issue. First I compression tested every cylinder and was getting 145-150psi in 6 of the cylinders and about 120psi in two others(5 and 8 to be specific). I ran a leak down test today and ran 60 psi in, and about 50psi or so was coming out(piston at tdc). I suspected I had overtightened the valves so I loosened them and then set them to the point where the pushrod has no up/down movement and then 1/2 turn more, and immediately when I do this the gauge on the leak down tester drops below 10 psi and I can hear a difference in the hissing air escaping. However, after a few minutes, the gauge will go back up to 50ish psi. If I tighten the valve another half a turn it drops back down to 10 and does the same exact thing. Could someone please help?? I'm so confused, I am now overtightening the valve and opening it but it seems the pressure keeps dropping? Doesn't make sense to me.

PS everything on the heads(rockers,lifters, pushrods) are new and I had the heads (and block) brought to the machine shop where the guy said he had fixed them up. Seems to me like something is wrong with the valves?

 

[ericnova72]

What you are describing is normal correct operation.

The pushrod cup and piston in the hydraulic lifter have an oil chamber beneath them. You adjust to where there is no preload on them while the plunger is at the top of it's bore and the chamber of oil is full.
Then you add the required running preload and something has to move....oil is not compressible so the plunger can't move and the result is the valve gets pushed open a bit and gives your drop in cylinder psi.
After a few minutes, the oil in the lifter chamber bleeds out of the internal clearances and the valvespring pressure closes the valve back up.
If the motor was running when you did the adjustment, the engine would chug through a cylinder miss for a second or two until the plunger equalized to the setting and the miss would stop.

This is normal hydraulic lifter operation, doing it's job to self adjust the running valvetrain.

Once you find the zero preload point for each lifter, then adjust each for your required preload setting, then leave them alone., they are set.
As you rotate the crankshaft to set each cylinder, you give time for the previously set lifters to bleed off a little oil, just as hey are designed to do.
Don't adjust them again after that, if you do you just make them too tight.

 

[RWENUTS]

That amount of compression difference won’t keep it from starting.
likely you’re timed 180 degrees off.

 

[BogiesAnnex1]

First we need to be sure you’re adjusting lifters going through the firing order with the piston of each cylinder at TDC firing. This insures accuracy where other flat rate processes can get you into trouble especially as the cam gets bigger.

The way you're seeing the lifter behave is normal. when you sink the adjusting nut the load on the plunger wants to move it down faster than it can bleed oil so the valve gets pushed open till the oil bleeds around the plunger that pushes on the push rod allowing the valve to seat again. Totally normal.

I’m inline with the thought that the distributor is not timed. Two basic problems usually are the ones that crop up.

First is the distributor is 180 out as mentioned. The crank makes two revolutions to complete firing all cylinders. That means number 1 and number 6 share TDC timing marks at different times so it’s easy to be out.

Second is the distributor gear as it unwinds from the cam gear pulls the oil pump drive with it such that reinstalling gets things at least a tooth off sometimes two.

There’s several ways to do this the factory set up is the vacuum advance can points to cylinder number six while the number one distributor terminal points at cylinder number 1. At this point hopefully the rotor is also pointing at number one. If it’s pointing at a different terminal you can call that number one and wire the firing order from there. Both the crankshaft and the distributor shaft turn clockwise, pretty tricky of those Chevy engineers.

Bogie

 

[Garrettsoler]

Thanks for all the replies. So I am pretty sure the distributor timing is set correctly - before installing the motor into the truck I used a piston stop tool, marked(using the timing tab 0 point as my reference) when the piston hit the stoptool, then marked again when it hit going the opposite direction and found the center point between them as tdc. I set this at the timing tab 0 and called this tdc. I had originally set the distributor 180 off and to fix it and put the engine at tdc compression stroke I watched the valve movement of cylinder 1 and when the exhaust valve began to open I rotated the crank counterclockwise back to tdc (so now both valves remained closed) and then went 8 degrees before tdc to give time for spark to travel to the plugs, and then set the distributor down, meshed with the cam gear and oil pump driveshaft, pointed it towards number 1/3 cylinders, and then set the distributor cap on top with the rotor exactly pointing to what I'm calling cylinder 1. Then I followed the normal gmc firing order 18436572. What didn't make sense to me was even after this I was still getting backfire in the carb (poofs of flame coming up) so this is what led me to believe it was a compression issue.
To perform the leak down test today all I did was verified cylinder 1 was at tdc compression, and then went through the cylinders via the firing order, using a borescope in the spark plug hole to verify that I was at tdc of the particular cylinder. Then I attached the leak down tester and performed the test, and reset the valve lash and got what I was describing in my first post.

Btw, almost every part (distributor cap, rotor, ignition coil, spark plugs, spark plug cables, carburetor, timing chain, cam) are brand new.

 

[curtis73]

What you are describing is normal correct operation.

The pushrod cup and piston in the hydraulic lifter have an oil chamber beneath them. You adjust to where there is no preload on them while the plunger is at the top of it's bore and the chamber of oil is full.
Then you add the required running preload and something has to move....oil is not compressible so the plunger can't move and the result is the valve gets pushed open a bit and gives your drop in cylinder psi.
After a few minutes, the oil in the lifter chamber bleeds out of the internal clearances and the valvespring pressure closes the valve back up.
If the motor was running when you did the adjustment, the engine would chug through a cylinder miss for a second or two until the plunger equalized to the setting and the miss would stop.

This is normal hydraulic lifter operation, doing it's job to self adjust the running valvetrain.

Once you find the zero preload point for each lifter, then adjust each for your required preload setting, then leave them alone., they are set.
As you rotate the crankshaft to set each cylinder, you give time for the previously set lifters to bleed off a little oil, just as hey are designed to do.
Don't adjust them again after that, if you do you just make them too tight.

Just to echo this... I sometimes use the running method to do a final setting because it really gives you a perspective into the lifters. A good, new lifter you back it off until you get ticking, then tighten it until no clicking (zero lash) then do your 1/2 or 3/4 turn preload, but only do it in 1/4 turn increments. You'll go 1/4 turn and have a misfire for a bit until it stabilizes, then go another. The real perspective is.... how long does it take for them to stabilize. Some might take 30 seconds, others might take almost no time at all.

The flip side of the coin is if you don't have oil in the lifters yet. Since it's hard to get your fingers on the pushrod between the rocker and the head, it's not difficult to blow past the preload and bottom the plunger without knowing it. Not saying you did that, but it's a possibility. In that case your zero lash is actually bottomed out and additional turns will just hold the valve open.

 

[eric32]

If your valve lash is setup good and you have not verified exactly how much timing you have at idle etc with a timing light then you can be close to start off with but you might still not have enough. Last year on my new 350 build everything was setup to start wise and I don't know what size camshaft you have in your engine but the bigger the camshaft the more timing it will take to start it up and run.

Mine is not big at all at only 268/272 220/[email protected] 510/510 lift 114 lsa and when we first went to fire it up and to adjust the timing the distributor was put in correct but it was still not enough to start off wise and wow I had a big old backfire pop out through my carb for a few reasons but timing not being enough was one of them.

Adjusted it some more then it fired up without anymore issues for running wise until I had time to get my timing light out and check my timing and setup to what it needs and it was still off even by adjusting it by ear and still needed quite a bit more of a turn so without knowing exactly what it is your at timing wise it can be a tricky thing to start off with a shot in the dark starting wise even if it turns over fast and sounds good as mine did it can still not be enough. I also had a carb issue as well but not to where it would cause a backfire problem.

Hope you get it sorted out and glad for the others who posted as I have been through that before setting valve lash and thought something was wrong when it wasn't when I was new to setting lash.

 

[Garrettsoler]

What you are describing is normal correct operation.

The pushrod cup and piston in the hydraulic lifter have an oil chamber beneath them. You adjust to where there is no preload on them while the plunger is at the top of it's bore and the chamber of oil is full.
Then you add the required running preload and something has to move....oil is not compressible so the plunger can't move and the result is the valve gets pushed open a bit and gives your drop in cylinder psi.
After a few minutes, the oil in the lifter chamber bleeds out of the internal clearances and the valvespring pressure closes the valve back up.
If the motor was running when you did the adjustment, the engine would chug through a cylinder miss for a second or two until the plunger equalized to the setting and the miss would stop.

This is normal hydraulic lifter operation, doing it's job to self adjust the running valvetrain.

Once you find the zero preload point for each lifter, then adjust each for your required preload setting, then leave them alone., they are set.
As you rotate the crankshaft to set each cylinder, you give time for the previously set lifters to bleed off a little oil, just as hey are designed to do.
Don't adjust them again after that, if you do you just make them too tight.

Thanks for this great explanation. So is what I am doing (putting the piston at tdc compression, adjusting the valve lash until the pushrod no longer moves up/down and the leak down tester drops to 0%) correct? The one thing that is confusing me still is that as I am TIGHTENING the rocker the leakage drops to 0, shouldn't it be 0 already if the rocker arm is not tightened at all? I thought the valve remained normally closed and adding preload was just to make it to where it's ready to be opened once engaged by the camshaft/lifter.

 

[johnsongrass1]

Put the leak down guage down for a minute. It's confusing you.

What happens here is the rocker will eventually bottom out the plunger in the lifter, or the oil in the lifter is making you think it's at 0 lash but when the oil drains from the lifter, the settings are then too loose, so you tighten and begin opening the valve. Once the valve begins opening, air begins escaping out of the valve. Left alone, spring pressure might cause the lifter bleed down, might not, and the valve closes again.
That's no good, You want the fine line between the push rod not wiggling, and the valve remaining closed. You only have a 1/2 or less of the nut to get in the right range of pre-load.
Your guage is confusing you because by the time air is escaping, it's WAY too tight and the lifter plunger is moving and you get three things to have to manage. Put down the gauge a minute.

A lot of guys will do this running, loosen till it clacks, then tighten till it stops. That's messy, hot, smokey, and still leave a lot of room for error as the running engine makes it harder to hear the clacking. I find it better to wiggle the push rod up/down, tighten just untill it stops, tighten slightly more and leave it.

Another way of making this idiot proof, is lashing with a .010 feeler guage as if it were a solid lifter, then tighten the nut 1/2 turn exactly and leave it alone.

 

[BogiesAnnex1]

Preload is just positioning the plunger inside the lifter body at a location it will hold unless a gap appears in the valve train. If a gap appears oil pressure in the cavity under the plunger will push the plunger up till it meets resistance greater than the engine oil pressure, then the inertia valve under the plunger closes making the lifter hydraulically solid. This is the basis of pump up where the plunger relieved of load moves up to close any gaps. If the gaps are larger than the length of preload the valve is held off its seat till the lifter bleeds back to a position where the valve is seated.

The plunger has an upper and lower travel limit. The upper is set by the retaining ring the lower by a seat machined in the outer housing that stops the bottom travel of the plunger piston before crushing the inertia valve mounted on the bottom outside of the plunger. I should note that there also is a spring in the oil cavity below the plunger that always applies a mild upward force on the plunger, this is what will initially close any gaps forming in the push rod link to the rocker. It’s sudden upward push opens the inertia valve against its wristwatch strength spring allowing oil to enter the chamber which in turn shuts the inertia valve trapping that oil volume in the bottom cavity below the plunger. The plunger can be set to operate anywhere in its range of travel. The “preload” simply is a means of placing the plunger somewhere in its travel distance to where the oil trapped underneath is not too much in quantity and the plunger is not so close to the top of its travel that it risks pushing the wire retainer out if it pumps up. Preload, therefore, is nothing more than using the engine valve closed on its seat to push the plunger to some position in its allowable range of travel. For a normal not high leak down racer lifter not to stay quiet either it’s at or a bit past it’s upper travel limit and is ticking because there is a gap in the valve train it can’t close because it’s resting on it’s retainer clip or inside the inertia valve is leaking to where it can’t trap oil at some degree of functionality in the cavity under the plunger. So while we use the term “preload” when setting up the lifter it really is not an accurate explanation of what’s going on, not inaccurate either just not complete. You’re really setting a zero lash condition where the spring pushing the plunger up from wherever you adjusted the number of turns from first arriving at zero lash against the force of the valve spring on the rocker. You are maintaining this point of zero lash that is not changing as the plunger is moved into the lifter body so in terms of a preload nothing has changed. What has been accomplished is a preposition that the lifter will maintain unless a gap forms in the system such as happens mostly in the higher RPM range when the valve spring looses control of the valve resulting in gaps forming in the valve train linkage which the plunger will close. This is the “pumped-up” state. It will eventually bleed off depending on how fast oil trapped in the bottom cavity escapes. The oil trapped in the cavity of the lifter body under the plunger is nothing more than the “incompressible” connection between the lifter body and the plunger. “Incompressible” being more hydraulic theory than reality, I’ll get to that later.

Performance lifters use deliberate oil leaks to prevent pump up or speed recovery from it they also tend to use less plunger travel. As a result they tend to tick with aggressive lobe shapes because the lobe on the back side drops out from under the lifter faster than it can close any gaps forming in the valve train linkage. So there is a tick when the lifter arrives on the lobe heel and the valve spring removes any linkage gaps. This is heard mostly at low RPM as at high RPM it’s simply a time function where the lifter is coming back on the lift side of the lobe lobe faster than the gap coming off the lobe can form.

Racer tricks to get around either pump up or elasticity in the trapped oil consist of running the plunger against its retainer or collapsing it.

In the first case you can get around pump-up by running the preload/preset position on the plunger to where it rides on the retainer. The common wire retainer is unsuitable for this as the plunger can push the round wire out of its groove in the lifter body spilling the pushrod cup and plunger assembly into the engine. A solution to this is to replace the wire retainer with a TruArc of suitable diameter to solidly engage the retainer groove and installed sharp edged side up so the pressure imposed by the plunger drives that sharp side edge into the lip of the groove. The opposite side of a TruArc being rounded presents the same problem as the wire retainer so it’s BigTime important to pay attention to what you’re doing. While this stops pump up the large amount of oil in the cavity under the plunger has a certain amount of entrained air so this introduces a little elasticity to where the valve is not precisely following the lobe in terms of duration or lift, this brings us to the other functional racer option.

In the second case the plunger spring is removed as well as the inertia valve, then the plunger is completely collapsed to the bottom of its travel. This requires a longer push rod as now the rocker tip is not addressing the stem properly. In this condition pump-up is knocked out because the plunger cannot cycle at all so there is equity of oil pressure in the cavity and that of the oil pump. pp, thus nothing to drive the plunger to close snd gaps forming in the valve train. Remember at this point the plunger is registered against its machined stop inside the body cavity of the lifter. So the motion imparted to the plunger is directly that of the cam lobe in duration and lift as any air induced elasticity of the oil is inconsequential as there is no longer a hydraulic connection between the lifter body and plunger it is strictly a structural connection, the lifter is functioning as if it were a solid lifter. You still want a TruArc in there to stop push rod cup or plunger from coming out should a top end failure results in the push rod coming out of the lifter.

Sorry for the length of read, how hydraulic lifters work is rather complicated requiring a lot of words at the moment this is as distilled as I can figure out how to describe how they work with some having some meaning in the content. These devices are mostly not well understood.

edit addenda:

What you are doing by adjusting the lifter preload while pressure testing is forcing a situation identical to pump up. If you did this on a running engine you would get the same type stumble as if the engine in this case cylinder being adjusted as you would see it the lifters pumped up and held the valves off their seats.

Bogie

 

[alwill923]

Use the EOIC method to adjust the valves. This makes sure you are on the base circle.

 

[Garrettsoler]

Put the leak down guage down for a minute. It's confusing you.

What happens here is the rocker will eventually bottom out the plunger in the lifter, or the oil in the lifter is making you think it's at 0 lash but when the oil drains from the lifter, the settings are then too loose, so you tighten and begin opening the valve. Once the valve begins opening, air begins escaping out of the valve. Left alone, spring pressure might cause the lifter bleed down, might not, and the valve closes again.
That's no good, You want the fine line between the push rod not wiggling, and the valve remaining closed. You only have a 1/2 or less of the nut to get in the right range of pre-load.
Your guage is confusing you because by the time air is escaping, it's WAY too tight and the lifter plunger is moving and you get three things to have to manage. Put down the gauge a minute.

A lot of guys will do this running, loosen till it clacks, then tighten till it stops. That's messy, hot, smokey, and still leave a lot of room for error as the running engine makes it harder to hear the clacking. I find it better to wiggle the push rod up/down, tighten just untill it stops, tighten slightly more and leave it.

Another way of making this idiot proof, is lashing with a .010 feeler guage as if it were a solid lifter, then tighten the nut 1/2 turn exactly and leave it alone.

Ok thank you. I think the issue too is just having a tough time figuring out if the pushrod still has movement or if I'm just compressing the lifter as I tighten it down more. I don't know how far exactly I need to go until it "has no up/down movement", it seems like there is always just a little unless I keep going, and then It feels like I'm overtightening. I can try a feeler gauge too.

 

[95 Truck]

This all started with erratic compression, so something drew your attention to checking compression, what was reason for compression check?

is this a freshly rebuilt engine?

 

[johnsongrass1]

Ok thank you. I think the issue too is just having a tough time figuring out if the pushrod still has movement or if I'm just compressing the lifter as I tighten it down more. I don't know how far exactly I need to go until it "has no up/down movement", it seems like there is always just a little unless I keep going, and then It feels like I'm overtightening. I can try a feeler gauge too.

It's a feel thing. A very delicate feel thing.
Engine building and machine tooling is kinda strange in the way that your hands are expected to deal with daily hot metal, sharp edges, wear and tear, callouses, burns, cuts, abrasions and heavy chunks of metal, chips and expected to be strong, durable and still expected to have the accuracy and touch of a brain surgeon.

Those damn factory swedge rocker nuts make it even harder because of the twisting force required to move the nut clouds the ability to feel the gentle click of the lash.
A feeler guage helps because you have a barrier of the sorts, the light drag of a feeler gauge as a reference point to how much more to tighten. I frankly throw the nuts away and replace everything with poly locks just for this reason. I can spin the poly nut with 1 finger and much more easy feel when it's just hit the 0.000 point.

 

[Garrettsoler]

Hey guys, got an update with good news/bad news:
So I think some of the valves were indeed overtightened, I went through and reset them all (basically set the piston to tdc compression and then loosened valves all the way and tightened until there was no up/down movement of pushrod and 1/2 turn more) and I get no more flames or smoke out of the carb. However, when I go to turn the engine on, it just about turns on and as soon as I let off the ignition the engine just dies. Also, the choke light comes on (carb/choke isn't connected to anything, I've read this can also be for low oil pressure? I don't think the oil pressure is low though I was getting 60 psi when I primed the engine before installation). Anyone have any ideas??

 

[Garrettsoler]

Also I should mention, I tried advancing/retarding the distributor, got flames again out of the carb so figured I advanced it too far. Went back and tried a few more times with retarded timing and still nothing (it was originally set to 8 degrees before tdc.) On my last attempt I pumped the gas instead of just holding it all the way down to try to feed more gas into the engine, and got flame out of the carb again

 

[ericnova72]

Also I should mention, I tried advancing/retarding the distributor, got flames again out of the carb so figured I advanced it too far. Went back and tried a few more times with retarded timing and still nothing (it was originally set to 8 degrees before tdc.) On my last attempt I pumped the gas instead of just holding it all the way down to try to feed more gas into the engine, and got flame out of the carb again

Now that you have the valvetrain all sorted out.... it sounds like you also have the distributor installed 180° out of time.
Spits and sputters, almost wants to try to start, backfires or stalls, and relatively unresponsive to ignition timing changes indicate for this.

Describe, in detail, how you initially aligned and installed the distributor to coincide with the #1 cylinder at TDC on the firing stroke???.

 

[Garrettsoler]

Now that you have the valvetrain all sorted out.... it sounds like you also have the distributor installed 180° out of time.
Spits and sputters, almost wants to try to start, backfires or stalls, and relatively unresponsive to ignition timing changes indicate for this.

Describe, in detail, how you initially aligned and installed the distributor to coincide with the #1 cylinder at TDC on the firing stroke???.

Sure so I should preface by saying I accidentally stripped the crank bolt, so ive had to turn the engine over by the flywheel. So initially to set tdc I had a friend put their finger over spark plug #1 and rotated the engine until they told me they felt air pushing. Then I rotated the engine over until the mark I have on my dampener (which I set using a piston stop tool) aligned with the mark on the timing cover tab. Then I set the engine 12 degrees before tdc and put the distributor in. Now like I said I was spinning the engine over by the flywheel, which is very slow, and so when all this started and I suspected timing I figured my friend might've felt tdc exhaust not tdc compression. To check, What I did instead was take the valve cover off, set my phone to record the valves of cylinder 1 as I went underneath and rotated the flywheel (clockwise just to be specific) and watched for the exhaust valve to open. Once I saw this I rotated the engine counterclockwise back to tdc and when I took the distributor cap off the distributor was pointing at 180° off from the #1 cylinder spark plug wire, so I figured that this must mean it was 180° off. I also confirmed by still recording the valves as I rotated the piston up to tdc, and the valves did not move at all (signaling to me that they were fully closed). I took the distributor out and reput it back in now pointing to cylinder 1 and set the normal gm firing order (18436572). I used a magic marker on the distributor housing to ensure that the rotor was pointing directly at what I was calling the #1 cylinder spark plug wire. Once I tried starting it after all this it seemed much more like it wanted to start.

 

[ericnova72]

OK, sounds like you've got it timed close.

 

[Garrettsoler]

I will test each spark plug tomorrow to see if there's spark (should be, new spark plugs/wires/distributor cap/rotor/ignition coil). Condenser on the distributor is old, maybe I should replace this? Only other thing I can think of is a vacuum leak? I plugged all the hoses from the carb though. Kind of at a loss for what is wrong. Low fuel pressure maybe? Anyone know why the choke light comes on suddenly now?