2000 RPM Surging / Chugging / Jerky / Hesitation

[79SCBRAD]

Hi Everyone, hoping for some help.

The car is a 79 SC, 3.0, stock CIS, stock ignition.

It starts and runs very reliably (and fast). I have noticed, though that in any gear, with partial light throttle, there is some surging or hesitation around 2000 RPM. This is most noticeable on a 50mph highway, lightly crusing through speed zones. It goes away with a slight application of throttle, but it is noticeable because it sets up an oscillation between coasting and not.

Timing is on spec, 5 degrees BTDC at idle, looks like a quick and smooth advance to roughly 25 at higher RPM.

Vacuum advance works.

Checked the idle mixture last night (no CO meter, but followed the CIS Primer instructions) and it appeared VERY lean. Got things adjusted and have noticed an immediate off-throttle improvement and smoothness. I *THINK* that this may have improved my issue as well, but I was still able to notice a light surge on a test drive.

So, I'm wondering if this problem could be CDI box or coil related. Is it possible for the boxes to start to fail at a particular frequency (RPM)? Green wire from distributor looks new with no rubbing or abrasion. CDI box shows 12.8V at the plug with ignition on, so I don't think it's a bad connection issue??

I'd love to swap another CDI for a couple minutes to see if there's an improvement, but I need to find one first.

Any thoughts or suggestions welcome.

Thanks.

[LIRS6]

Suggest to identify where you live - maybe someone can assist you with your request.

Jason

[79SCBRAD]

Originally Posted by LIRS6

Suggest to identify where you live - maybe someone can assist you with your request.

Jason

Thanks Jason.

Oshawa (Near Toronto), Ontario...

[stlrj]

Check plugs and replace dis. cap.

[Tippy]

Lean AFR's will definitely surge. You're probably fine, but have masked vacuum leaks..