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b200ft, heavily skimmed 531, VX3: lumpy and low vacuum idle

Msi

Member
Joined
Nov 21, 2018
Location
TLV, IL
Hello TB!!

Like the title says -

Took a smoothly running b200ft (had a 530 with whatever cam there was, stock).

Put a 531 head w/ a VX3 cam on it.

Before that, sent the head to a shop for checking. Shop said it's heavily skimmed. Dono how much exactly thought.
The shop gave the head a refreshing - making sure that all surface are level (deck, valves, seats).

Result is a lumpy idle, pulling a steady 14"-15" vacuum (was pulling 18" on 530 head).
There are no vacuum leaks.
Pulling 21" on deceleration.
No signs of wear and tear on the cam.
Valve timing seems to be set correctly.

Can this be caused by the elevated static compression? Due to a heavily skimmed head?

OR

Should I start suspecting that the head shop did a poor job of verifying that all surfaces are level? May be caused by valves that do not seal/sit perfectly?

If so, any way to test for it w/o taking the head off?

*all vacuum figures above are in - hg/in.

Thanks in advance!!
Matthew.
 
You probably have it installed with a tooth off. Also, take the VX3 out and chuck it in the trash.
 
I mean, depending on what the compression ratio is now you'll lose a little bit of vacuum with higher compression.

I have no experience with the vx cam but the idle is lumpy like it's hunting or just that it bounces around a couple hundred rpm?
 
I mean, depending on what the compression ratio is now you'll lose a little bit of vacuum with higher compression.

I have no experience with the vx cam but the idle is lumpy like it's hunting or just that it bounces around a couple hundred rpm?

Not hunting, just bouncing around a bit. Not smooth like it used to be.

I'm more concerned with the missing idle vacuum. I figured that elevated static compression could cause that. However, to what extent? It's about 4"-5" lower then it should be. Is that in a normal range? Thought I guess it's hard to answer, not knowing just how much the head
is skimmed.

Thanks!!
 
Definitely worth checking the cam timing the accurate way rather than relying on the marks, if the head has been cut quite a bit. The VX should be pretty damn smooth at idle, and it should pull decent vacuum as well. On my 9:1 build, with the VX at 4* advanced it would idle around 18" in neutral, 16" in gear.

I would also check the valve clearances, see what they are actually set at. Also would be worth putting on a vacuum gauge that isn't damped. A lot of the in-dash ones are damped to some degree so you wouldn't see flutter, which can easily be caused by valves not sealing, which would also cause a funky idle and low vacuum. I went thru exactly that on mine years back, when the valves weren't installed correctly, due to a severely incompetent machine shop.
 
Also would be worth putting on a vacuum gauge that isn't damped. A lot of the in-dash ones are damped to some degree so you wouldn't see flutter, which can easily be caused by valves not sealing, which would also cause a funky idle and low vacuum.

Did that. Saw the needle flutter in the 13.5"-14" range. Is that considered a flatter?

If valve timing correction via the adj., gear won't help - I'll have to check the valve clearance. I hope it dies not come to it though.

Thanks!!
 
It's worth noting that cam timing is retarded one degree for every .010" the cam comes closer to the crank, whether that's by head milling or use of a thinner head gasket.
 
Yes, in theory that is true. It'd be nice to know how much was shaved off the head!
 
A half inch of flutter could simply be valves that are a bit on the tight side or not really settled in, combined with the cam, cam timing, head shaved, etc.

I'd check valve clearance, shoot for around .016". See if you can measure the head height from the valve cover surface to the head surface. I don't have the spec from memory but someone here will. That will tell you how much it's been shaved. From there, advance the cam 1* for every .010" it's been trimmed.
 
I had a similar problem and I THINK it was due to excessive skimming such that the cam timing was too far off.
 
Advancing the cam a silly amount, especially if your adjustable gear is the contemporary type adjusted without any removal, is just a dead simple diagnostic tactic regarding the vacuum at idle, right?
 
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