454 BBC isle issues, need help!

[Bmorrow]

Hello,
I recently acquired an 86 c10 with a big block. Big time project but I love the truck. I have been around cars my whole life. I still have alot to learn about them. I am having an issue with it idling properly.
Problems: the truck idles rough. Idle fluctuates from 750-900 at any given time. It's very hard to even set the idle where I want. The timing is set to 16 degree BTDC. I also have some smoke coming from one tail pipe (true duals). It is the passenger side head. Hooked a vacuum gauge to the manifold port and the needles is constantly moving sporadically between 5-10hg at idle. blip the throttle and it goes to 0, then to 20 and back down. at 3000 RMP the needle reads 17HG. The smoke comes when the engine is warm. The color of the smoke is hard to tell for me. It's either white or has a tent of blue. Super hard for me to tell. (doesn't smell sweet and doesn't have any water coming from exhaust) The oil is clean, the coolant doesn't bubble so I'm not leaning towards head gasket. Will run a compression test tomorrow and check plugs.
Specs:454, .540 lift cam. HEI distributor, new plugs and plug wires, long tubes, brand new edelbrock 750 carb.
Any help or advice is appreciated.

[Captainfab]

The idle issue could be due to the duration of the cam and it's LSA. It could also be caused by a vacuum leak. Have you checked for vacuum leaks? Have you pulled the spark plugs from the passenger side head and checked for fouling or correct color after you replaced them?

[cadillac_al]

It sure sounds like a vacuum leak to me.

[Bmorrow]

Originally Posted by cadillac_al

It sure sounds like a vacuum leak to me.

Well, just an update... ran a compression test. All cylinders have 120-135, except cylinder 7. It has 0 compression. Tested it numberous times. Stuck my finger in spark plug hole and turned the engine over and it never blew my finger back. Pulled the valve covers and all rockers are moving same distance. Where to start? Lol. Guess I’ll be pulling the motor
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[weq92f]

The piston could have a hole in it or one of the valves could be broken. I guess the head could have a massive crack too eh, but seems like you feel some compression in that case though.

It's prolly gotta come apart thought

-klb

[Bmorrow]

Here’s an update: took the heads off because I couldn’t find anything that looked out of the ordinary except one thing... I noticed the intake valve on the cylinder with no compressor looked taller, even though it was still in the retainer. This was also the same valve that I heard air coming through the intake. I pulled the head and flipped it over and it looked like the valve seat had been beat up. Took it to the machine shop and he said it had good vacuum on the intake and exhaust ports on the head. He then noticed that valve and said that’s your problem. The previous owner had a shop build this motor, someone at that shop must have put the wrong valve in. My assumption is since the valve was longer, when they tightened up the rockers, the length of that valve forced the valve to stay open at tdc, resulting in no compression. Moral of the story: If you aren’t equipped with the correct parts, don’t do it at all. Do things the right way. I’m a believer in karma. It will catch up to you sooner or later. I learned that lesson a long time ago.
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[RustyBucket]

Lots of info in the search section about cyl. head choices for BBC's.... both factory castings and aftermarket. Since you've got it apart and have to spend $$ on the heads anyway, might be a good time for an upgrade. As always, Mo $$ - Mo FUN.