Bucking '67 L79
My '67 L79 has direct vacuum and correct carb but previous owner had installed a comp cam "nostalgia" cam, similar to but also different from the 151 Chevy cam. Problem is: idles fine and pulls strong at full throttle especially toward upper rpm range, but has little low rpm torque and sometimes "bucks" -rpm goes up and down abruptly when trying to hold slightly opened throttle even in 1st or 2nd gear- not like lugging. Is this due to vacuum advance going in and out? 14in engine vac at idle. Vacuum advance is adding advance at this idle but I haven't done full mapping.
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Re: Bucking '67 L79
William,
My thoughts are that the engine vacuum is not pulling hard enough on the idle circuit causing the surge. If there is no timing issue the way to correct this is to reduce the size of the idle air bleed.
The Holley 3810 has a .070 diameter idle air bleed, I would experiment with a .063 +- diameter bleed and see if that helps in the right direction. The bleed can be drilled and tapped with 8-32 threads and a brass set screw 8-32 x 3/16" long installed with the new size bleed hole. You may also be able to use some wire to stuff the hole and reduce the bleed hole then see if you are going in the right direction.
As a test you can also pull the vacuum advance and plug the hose to see if retarded timing helps, also look for a obvious vacuum leak like a blown vacuum control. -
Re: Bucking '67 L79
That cam is nothing even resembling the L79 cam. I suggest something not “racetrack” orientated. A 270 grind (224@.050) or smaller is necessary to signal the carburetor correctly.Comment
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Re: Bucking '67 L79
I know the compam is different and wish I had stock, but I didn't put it in and
I know the comp cam is different but it's there and I'm going to have to deal with it. So; would appreciate thoughts on the "bucking".
Duke, what say you? FYI, it does have a 3367 carb with stock jetting as used on '66 L79's rather than 3310 used in '67.
Thanks in advanceComment
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Re: Bucking '67 L79
While I certainly don’t have the technical knowledge of many on this board I did have a similar experience with our L79 a few years back. We checked pionts, timing, vacuum, etc as many have suggested here. Unfortunately in our case none of this helped with the bucking. I decided to change plugs since it was relatively cheap and easy to do and wouldn’t you know it I haven’t had a problem since. I’m not saying this is your issue but it might be worth checking out.
Good luck!Comment
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Re: Bucking '67 L79
The 3367 is the same basic carburetor as the 3810 as far as calibration, you problem likely is in the idle transition circuit. Make sure the secondary blades are closed just enough to not stick and try to idle only on the primary side.
I believe what happens is the vacuum pulls on the idle/transition circuit but also pulls a lot of exhaust dilution back into the cylinder so these cams need a richer mixture under the high vacuum conditions like small throttle openings.Comment
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Re: Bucking '67 L79
As I've said so many, many times before, reporting "idle vacuum" without actually specifying the actual engine speed that it was measured at is absolutely MEANINGLESS!
Typical L-79 idle behavior with an OE equivalent cam is 14-15" @ 750 RPM with a slight lope. Raise the idle speed and manifold vacuum increases and decreases idle speed is lowered.
You also need to report the VAC ID and test it with a vacuum pump to determine the vacuum required to start it to pull and the vacuum required to pull it to the limit.
DukeComment
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Re: Bucking '67 L79
Thanks, Duke. Will do tests you recommended , as recommended.Comment
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