So what’s it running for ignition timing at idle then?

 

Couple things I thought of:

That cam you have a XR-270HR, it might not be as mild as you think it is.
It is a nice street performance cam.
It will have a mild "lope" to the idle.
A lope like a L46 350-350 HP, or an L82, or a Comp 268H. It has 53 * overlap.
You will hear it. I would call it a little rumble, or an oscillation.
The overlap is almost double the stock L48 cam you had in there before (35*)
Maybe you are just not used to a performance cam, being overseas?

I can not hear any stumble in your video.
Perhaps try for idle sound back by the tailpipes.
We could tell you if it sounds normal.

As I recall, you bought this as a former race engine.
With a big nasty cam.
A big cam like that needs stronger valve springs to run 7000rpm.
Did you change them out?

A recommended spring for that kind of cam could easily run 130# seat & 350# open.
And some "drag race" guys put some ridiculously strong springs on their engines, well, just because...
Those type of springs could easily overwhelm your comp cam HR lifters. And prevent proper opening.
IE: they bleed down.
So if you did not put comps recommended springs in there, a quick check is warranted.
The recommended 986 comp springs are recommended for near 7000rpm cams.
They do not need to be any stronger.
Before I would buy new springs, I would pull one and take it to an engine shop, and have them test it with a spring checker.
Take the Comp Cam specs with you.

A quick vacuum estimate shows you should have near 18" at idle.
Is that near where you are at?
Yes it might twitch a little it, but not too much.
A video of the vacuum gauge movement might help us help you.

You could also be having random misfires for a couple of reasons.
Air fuel mix, ignition, etc.
An easy old school trick to isolate one problem cylinder may help.
I have found this very helpful at times.
Pull each spark plug wire, one at a time, and measure on an underhood tach how much the rpm "drops" for each cylinder.
Write all eight "drops" down.
They should all be the same.
If one cylinder does not drop nearly as much, or not at all, you have found your weak or misfiring cylinder.
You already have one burned plug wire, there may be an underlying lean condition.

You could be lean.
Set your idle mixture using the 50 rpm drop method, and back out a 1/8-1/4 turn, or shoot for highest vacuum.
Both methods should be similar.
You could be lean. Or it could be in just one cylinder due to a mechanical or lifter or misfire problem.

Hope this helps.

 

OTOH
And, If springs are too weak, the valvetrain will become out of control.
OP indicates he will replace valve springs "and see what happens" --- agreed.

Moving forward: I trust OP will also measure for Coil Bind and address that; if any exists.
I also trust OP will closely inspect each rocker arm for any damage or undue looseness-wear.

 

OP
** after more thought:
strongly suggest you closely inspect each lobe's Entire ground surface.
Seems your cam P/N 12-422-8 is ground on a cast iron core (not any billet steel core). Seems your valvetrain is noisy; which has yet to be determined why. Seems little is known about valve springs. However, if your valvetrain had been, or is now going out of control, your lifters' hardened steel roller wheels may Not have remained in Constant contact with those comparatively soft cast iron lobes. If and when any such lack of wheel-to-lobe contact occurs, the roller wheels can and do beat on (hammer) those lobes and damage them. All of that is a "what if" but (given the noisy valvetrain) IS a distinct possibility. It costs little more than time and aggravation to determine condition of camshaft surfaces.
Again, strongly suggest you closely inspect each lobe's Entire ground surface.

As you remove lifter-pairs for lobe inspection; strongly suggest close inspection of each-all roller wheel, axle, needle bearing, link bar and link bar rivet.

The fact it's now a great season for enjoying our cars (and related events) is not lost on me. Notwithstanding that, I strongly encourage you to resolve any valvetrain issues; sooner, rather than later.

 

duplicate

 

Thanks ! OP certainly will. . I do hope the tools needed will come in tomorrow and not get caught in customs... as is often the case.

I plan to carefully measure installed height, seat pressure, etc. and order the exact valves, retainers, etc. that I need .

 

That bottom pic with rocker arm OFF--- clearly shows valve TIP.
There's a cross X or star-like pattern on TIP: that pattern CAN, repeat CAN be result of valve float.

On the rocker arm's Roller-tip to Left of Star --- there's an ugly wear pattern developing on that roller.

 

I was hoping all the tools necessary would have come in this Friday but of course FedEx and customs nixed that. The parcel made it to Belgium and then, according to tracking scans, went back to Paris only to return a couple of days later. FedEx gave me a song and a dance to explain why what I was seeing was not actually what was happening. Then they said that customs could not reach me. Pushing further they confirmed that customs had yet to begin the clearance process.

Hot potato!!! And a weekend of corvette progress squandered. Hm. There is not point paying for express service Waste of money.

 

DELETED reapeat

 

Last week I was hoping my motorcycle parts would be done at the machine shop, and I'd be able to spend my days off building.....not so. I'll have to wait for this week.
So I kind of know how you feel. Hopefully your stuff will arrive soon so you can get at it!

 

A lot of the questions above I cannot answer... for now.

The belts are off (to confirm that the water pump's bearing was out).
Spark plugs are out and a couple of rockers are loose.

Now, I could put it all back together... but at this point, it makes sense to simply replace the valve springs of unknown specs and quality and the water pump. This being Belgium, I am not entirely convinced I could quickly find a shop to examine my springs. And, since I am going to replace them anyway, it will not be worth the cost.

Tonight, I will inspect the seat pressure.

Timing. Vacuum needle behavior, etc., will need to be revisited in a few weeks once I have the new springs on there.

I thank you all for the brilliant feedback and... Stay tuned

 

I am a science guy.
I always like to verify that I found the problem.

Any engine rebuild shop should have a spring checker tool that looks something like this:

A spring check should take basically 30 seconds.
There is also a less expensive one that can be used with a press.


If you really want to know, and can't find anyone, mail me one spring set, and I'll check it for you.

Cars, heck most symptoms for most things, seem to have a funny way of hiding the root cause.

 

So, I finally came home a bit early…


#90 at the seat


Rocker wear seems minimal


I don’t see anything terribly concerning here either.


Dual springs.


And in other news.

 

I cannot tell from photo-lighting

is One of the springs (inside) made of a flat steel ?
OR, are Both springs made of a Round wire ?

100 psi relative to what ? Closed, partially open, fully-open ? at what valve height ?

 

Outside spring is round. Inside is flat.

90 lbs at the seat. Maybe even a tick less.

This weekend I will measure the installed height

For *****’n’giggles I eyeballed full lift and it seemed to be less than 200 but more than 150

 

Well those pressures are the opposite of what I was expecting, but still a problem.
I am so glad you purchased a spring checker.

Those are basically stock springs. 90/180#
Here are Chevy's specs:


And since you have heavier non-factory roller lifters, and a cam with a much quicker ramp speed, I would go with Comp's suggestions, which are significantly stronger.
Like 30-40%. Nearer to 134/280#
Too light of a valve spring can cause valve bounce on closing, and it is possible that is the source of you idle concerns.
Light springs can also cause valve float at higher rpms, maybe not even so high in this case.
That means check the valve tips and the rollers on the rockers very carefully.

How many miles did you put on it like this?

 

Yes - those must be stock springs. What I don't understand is why the previous owner had such a hairy cam in it with essentially stock springs.





I will pull the intake manifold. I was never happy with the silver paint - too much bling. It needs to get Chevy Orange, just like the new water pump. Then I can get a look at the lobes and cam.

Perhaps this weekend ?

 

Hm 986 springs are discontinued.

Need to find alternative with correct specs.

 

Duplicate

 

They also recommended a beehive spring. Much more modern and better valve control.
986 are basically decades old Z28 off road springs from the 70s.
Try #26986 beehives instead.

Or even better just use PAC springs