Issues with slightly modified 318

[TWS67'Fury]

I've been having a very hard time getting my teen to idle good, I've got a weiend intake (with the GOOD fiber gaskets and way too much 1min rtv), medium cam est. .440" 260°, 4 brrl edelbroken. And some stock good heads with the bigger valves. I am getting **** vacuum at idle 3-5inhg when at around 2krpm I get 15-18inhg, I've done intake gaskets multiple times, tried different carbs, reset timing, reset idle adjustments, firing order ect. Now this is the part that will catch someone's attention, when doing compression test got all cylinders around 100-120 (I'm at altitude so psi is a bit lower than it would at sea level) however cyl 6 20psi, took the rocker shaft off and it came back up to 100-120, is the cam bad, lifters, what am I missing here??? Thank you so much in advance.



"Man is born, man lives, man dies and it's all vanity" Cake

 

[CBODY67]

Might need to remove the head to see why #6 appears to not fully close when the rocker shaft is installed. Is the non-closing valve "intake" or "exhaust"?

Were the "larger valves" all of the same length?

There is a recent thread about compression pressures "at altitude", with correction factors.

The cam in THAT small of a motor can be a little bit of the decrease in manifold vacuum. Just as the stock converter can too! Check the vacuum with the car in gear, with the brake on, and the a/c running. It is probably about 10.5"Hg. Then see what the power valve rating is for the power valve in your carburetor/metering rod springs in a Carter/Edelbrock carburetor. Key thing is to ensure that the valve/springs do not allow additional idle enrichment by the power valve circuit. Aim for a hot base idle speed of about 650-700rpm in "P", using the lean best idle misture adjustment method (with a dwell tach).

From what I have figured out, it is highly unlikely that an intake manifold gasket will "blow out" or "leak" if installed half-way correctly. UNLESS the heads have been excessively milled or angle milled.

I smear the sealer on the gasket (all sides and edges) prior to installation, letting it cure overnight before installation. That way, the gasket has a better total seal action AND the surfaces can be cleaned much easier if replacement is needed.

Please keep us posted on your progress,
CBODY67

 

[TWS67'Fury]

Had a thought last night while thinking this thing over, could a bad hydraulic lifter being stuck "pumped up" cause and intake valve to be open on base circle of the cam, I pulled that lifter to inspect and the intake side had rust on the inner edge of the lifter, I'm doubting it's anything head or block related but the can and lifters are the main target, I will keep yall updated.

 

[CBODY67]

What does the lifter bore in the block look like? Smoother or is there some scoring? Friction side of the lifter body, too? Might need a bright light and possibly a borescope?

Do you have another lifter to test your theory with?

CBODY67

 

[TWS67'Fury]

What does the lifter bore in the block look like? Smoother or is there some scoring? Friction side of the lifter body, too? Might need a bright light and possibly a borescope?

Do you have another lifter to test your theory with?

CBODY67

Yes brand new, checked bores before I put the intake on, pulled lifters and they show good signs of spinning while the engine is running, about to start it again and run a "break in" for the new lifters.

 

[MONC440]

Bottom line is you have valves that are not seated in cylinder 6. You can pressurize that cylinder with the rockers installed and see if it is intake or exhaust or both. If the valve seats height were not checked one valve could be seated deeper resulting in the steam being to high. With the rocker arms off put a straight edge across all of the valve stems on that side and see if #6 is higher than the rest, if so you found the problem, if not check the lifters. I would swap from a cylinder with good pressure (number them so you can put them back correctly) and check pressure. If the problem moves to the other cylinder then you know it’s the lifters. If not check the cam lift on that cylinder. If everything checks out then pull the head on that side.

 

[Kuz]

When did this start? After a fresh rebuild? Or did it just start out of the blue?

Can you post a couple pics of your valvetrain? It would help if we knew what hardware you were running.

My first thought was rocker arm interference on the top of the valve spring, keeping the valve open. Similar to the pic below.

You can put #6 at TDC on the compression stroke, then bolt down the rocker shaft while watching #6 valves. If one of the valves is being forced open due to the valvetrain I would think you ought to be able to see it as it happens.