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Old 06-12-2019, 05:52 PM   #1
Msi
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Default b200ft, heavily skimmed 531, VX3: lumpy and low vacuum idle

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.
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Old 06-12-2019, 05:59 PM   #2
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Also, idle RPM is normal and AFR at idle is bouncing around stoic - 14.X
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Old 06-12-2019, 05:59 PM   #3
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You probably have it installed with a tooth off. Also, take the VX3 out and chuck it in the trash.
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Old 06-12-2019, 06:12 PM   #4
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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?
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Old 06-12-2019, 06:33 PM   #5
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Quote:
Originally Posted by Harlard View Post
You probably have it installed with a tooth off.
I'll look into it. I have it installed with an adjustable gear, so I can easily check for it.

Thanks!!
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Old 06-12-2019, 06:38 PM   #6
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Quote:
Originally Posted by Marvelous3 View Post
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!!
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Old 06-12-2019, 07:11 PM   #7
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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.
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Old 06-12-2019, 07:27 PM   #8
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Quote:
Originally Posted by gsellstr View Post
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!!
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Old 06-17-2019, 12:12 PM   #9
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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.
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Old 06-17-2019, 07:59 PM   #10
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Yes, in theory that is true. It'd be nice to know how much was shaved off the head!
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Old 06-17-2019, 10:11 PM   #11
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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.
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Old 06-18-2019, 05:47 PM   #12
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I had a similar problem and I THINK it was due to excessive skimming such that the cam timing was too far off.
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Old 06-20-2019, 10:37 PM   #13
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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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