Re: SBC 406 & Holley 670 rich idle & poor transition issues?
Red67Camaro, thanks for your reply and suggestions.
Compression ratio unknown, motor built previous to my taking ownership (found some documentation that stated Performance Engine Design of Tacoma WA did the build, but no idea when or if they would have records that detailed). Based on my time with engine thus far, I believe it to be a low compression build, how low I just can't say. Wish there was an easy way to determine CR without tearing it all down but I don't know of a way?
While at a show today spoke to someone that suggested I try ported vacuum instead of manifold vacuum to prevent vacuum advance from retarding the timing when throttle is opened. So I gave it a try when I returned from the show even though I've read much on manifold vacuum being the way to go (perhaps that's true up to a certain size cam, but then changes when the cam is even bigger?). Ported vacuum is really just manifold vacuum that is being throttled up and down relative to the throttle butterfly position of the carburetor (in other words, ported vacuum will be less than manifold vacuum when the throttle is closed or minimally open but will never be greater then manifold vacuum).
This change did accomplish what I thought perhaps a vacuum retard delay valve would do when inline with manifold vacuum as far as not retarding the timing when the throttle is opened and got rid of the huge hesitation and stumble when coming off of idle to release clutch from a dead stop. I'm paying very careful attention to any signs of pinging and can say the performance with ported vacuum advance is much snappier then it was with manifold vacuum.
So now the remaining issue is what I would describe as rough running when cruising just above idle (I'd say holding the accelerator pedal no more than a 1/8" to 3/8" from idle position) between 1,000 rpm and say up to 2,500 rpm (upper rpm's depend on load on engine). This rough running causes what I guess could be described as a gurgling from the exhaust rather than a steady exhaust note and causes the car to almost buck while the free lash in the drive train is taken up and released. The further the accelerator pedal is pushed/held and/or the higher the rpm's of the engine, the less this symptom is present until it fades away.
It feels like she's lean to me, and I believe I'm beyond the idle circuit due to accelerator pedal position, I'm not in the accelerator pump circuit as I'm holding steady pedal pressure, I'm not into the power valve circuit as vacuum would be much higher than power valve value and I'm not into the secondary circuit so I'm thinking perhaps if I am lean in this window, that it might be the primary jets are too small. If I get on her and really let her go, she runs real strong, very steady and smooth, but of course she's well into her vacuum secondary circuit with help from the accelerator circuit and power valve.
With this in mind, the next chance I get I'm gonna pull a few spark plugs, clean them up real good, put them back in and take it for an easy cruise. While cruising I'll try to stay out of the secondary circuit and only run from idle to primary circuit as gently as I can. Then I'll pull the plugs I cleaned up and see if they have a story to tell either confirming my thought that the primary jets may be too small and creating an off idle lean condition when the accelerator pump and power valve are not contributing or perhaps it will reveal something else?
I'll also do as you suggest and double check the idle transfer slots. Thanks again for the reply and your thoughts on this, I will post an update once I have something new to report from what I try.
69 Camaro Z/28 RS, original Azure Turquoise, M21 & 3.73 12-bolt posi. NOM 406ci, AFR 210 heads, Straub hyd. roller cam & Dynatech 1-3/4" > 1-7/8" headers, QuickFuel 750 Annular. Photos at:
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