Re: Low rpm bucking problem
Trying to improve off idle low rpm bucking , 475ci bbc 260.050 cam manual trans car pulling away slowly it bucks and you need to get into throttle more to clear it up but there are times on street you just want to go slowly .................... yes big cam is the problem but looking best tuning ideas to reduce issue ....... making the idle circuit richer definitely helps starting to wonder if too much timing is part of the problem ?? I run locked timing 37* with 12* vacuum advance hooked to ported vacuum so no VA at idle but does come in at cruise .......... I can lug car down to 1800 with out bucking which I can live with , its the low rpm take off that I would like better
22lb flywheel dual disc clutch that does not alow much slippage 4.30 rear gear Holley HP 1000
I also have a 3/4 race cam (lol) in my BBC and have fought lots of tuning issues for years.
I run 22 deg of initial timing with 16 deg. of vacuum advance at idle for 38 deg. of total timing at idle. The thing that helped me the most on lean stumbles, bucking, etc... is the "idle-ease". I have a big hole in the middle of my base plate and I turn the idle ease full counter clockwise. This lets a lot of air in and allows me to close my rear and front throttle blades. In addition, you can keep your idle screws 1 full turn out because all that air coming in from the hole in the middle of the throttle plate is leaning out the mixture at idle. So, at hot idle you are sitting there at 14.7 to 1 with the throttle blades barely open and the idle screws set nicely at 1 turn out, then, as soon as you touch the throttle, you immediately go richer because the transition slot fuel pours in with the new air and the idle ease hole becomes inconsequential.
The idle ease is the only way I know of to go from a lean idle to an immediately richer take off.. Without the idle ease, I would go from a lean idle to a lean stumble to a nice richer take off.
Of course, all of this can be cured without the idle ease by just setting every phase of the carb at approx. 12.5 to 1, ie. setting it rich. Holley carbs work great with that rich setting in pretty much all instances with a hot cam. My problem is that I cannot stand a rich idle and I have spoiled myself and my girlfriend to expect no stink and no running eyes with my Chevelle.
1970 Chevelle SS396 (408), Fathom Blue,
Original Block & Forged Crank, KB Pistons, Edelbrock 6045 Heads,Comp Cam 292H, March Serpentine, RPM AirGap,750HP,100hp Nitrous
A/F Wideband,Tremec TKO 600, 12 bolt,
30 spline Moser Axles, Richmond 3:73's, Hotchkis control arms, swaybars, springs, Wilwood 12" rotors D/S, 4 piston calipers