Thread: Tunnel Ram Redux

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Dual Holley vacuum secondary 390's with stock jetting on a dual-quad tunnel ram 289 intake, on a 5.0L H.O. roller cam. Cam specs are .471/.441", 219/209 @.050, 115 deg sep.

Just fired her up today. (Finally!) Runs KILLER, amazing throttle response (roller cams are COOL), lays rubber like a bad dog.

Except the only way it'll idle is at ~1500, with the timing WAY advanced -- 20-30 degrees.

I had this exact induction on a '78 306 flat tappet block / cam and had a 700-800 rpm idle. That was with a cam that wasn't as "plickety." I've double and triple-checked for vacuum leaks.

Am I stuck with this idle? Would rejetting help, and if so, would I lose some of that killer throttle response?

"I've double and triple-checked for vacuum leaks."

I'll go out on a limb here and suggest that you're not checking at the right place, because you can't with the motor buttoned up.

When I've experienced in the past the same malfunction you're experiencing, I found it to be a vacuum leak on the bottom side of the intake. In other words, the intake is sealing at the top where you're checking for leaks, but it is wedge-shaped in reference to the intake/head interface and gapped open on the crankcase side, allowing atmosphere from the crankcase to be sucked into the intake runners and leaning out the mixture.

That's a pretty mild cam and you certainly should be able to idle the motor down to 700-800 rpm's, particularly with the lobe centers ground at 115 degrees.

A lean mixture will require more ignition lead because it burns slower, so that may explain why you have to put so much lead into the motor to get it to run.

As I said, I've experienced this enough times because of using parts from different manufacturers that were supposed to line up but didn't, that I developed a fix for it that I now use on every motor I build. I wrote a paper on it for another forum and offer it to you here.....
http://streetmachinesoftablerock.com...opic.php?t=350

Richard, I never thanked you for this. This turned out to not be the problem, but it's a well-written article and it makes a lot of sense to check the underside of the manifold for gaps; I'll do it from now on every time I swap manifolds. Cheers!