Part-Throttle Surge "Bucking"

[455 Formula 4 Speed]

Cliff,

We have discussed this via e-mail, but I thought this might be of some benefit to this forum, so I am posting this here.

I have a hybrid Q-Jet built from a 1978 Olds main body (17058553) and 1975 Pontiac (7045568) throttle body and air horn. My idle/main upper/lower air bleeds have all been sized to .070" and I am currently running a .072" (size verified) main jet with a 40K primary metering rod that actually measures .0385", tapering down to .035" with the usual .026" power tip. APT is 2.5 turns out with correct tip-in of -100 rpm @ 2500 rpm no load.

My idle tubes are .037" with a .055" down channel. Idle bypass is .125" (up from .109" stock) and idle is a very stable 650-675 rpm @ 13-14 in/Hg with 231°/235° - 110° camshaft and Rhoads Lifters in 413 CID Pontiac.

This carb works perfectly everywhere (371 RWHP/393 RWTQ on first dyno pull without any tuning) except at very light, part-throttle, such as 'creeping' through a parking lot in first gear. This is where there is a definite lean surge that clears up as soon as you open the throttles past the transfer slots.

As I recall, idle transfer circuit is 100% fueled by the idle tube/down channel restriction, so would an increase in tube/restriction be prudent in this case?

Thanks..

[jstew]

455,

Saw your post and I couldn't help but offer my 02cents on this problem.  I have a 69 350/350 Corvette that was having the same issue, very light and off throttle (when coasting) bucking.  I too was focusing my attention on the Quadrajet transfer circuit, but in the end, solved it by switching from manifold vaccuum advance to ported or "timed" off the carburetor.  Basically, I was getting way to much spark advance in the throttle range you were describing. It was like night and day when I made switch. 

The only explanation I could come up with was from retired GM engineer - today's fuel don't allow for as much part throttle vaccuum advance as they once did. I hope this helps someone someday, I know I spent A LOT of time trying to work this problem.


Good luck,

John

[73 Z28]

Very light /part throttle surging usually indicates too lean. Raise the apt piston 3 to 3.5 turns up from seated and try that. If it doesn't help, try this, disconnect the lever arm to the accelerator pump and flick the throttle with the engine idling. If the performance improves, you are running too rich, if it gets worse, too lean, if no change,check your accelerator pump circuit for a problem usually check ball not seating.

Good Luck!!
Paul

[77cruiser]

If it's like you say on the transfer slots, I'd go bigger on the down channel a few thou.