[MadHippo]

Here is the setup in my 67 GTO convertible.

428 stock rods and crank. .30 over.
#62 heads
1.65 rollers
comp cams xe268h
edelbrock 750 and performer manifold
1 5/8" headers.
PYPES 3" exhaust w/ X pipe
MSD billet distributor set 12 degrees btdc
muncie 4 speed

The car pulls like crazy from around 1500 to about 4000 rpms. The problem is that it has a stumble from the line. If i keep the rpms up before letting off the clutch, its fine. If i take off really really slow, its fine. If I take off normally, it stumbles/bogs, almost to the point of stalling. The guys at the dyno think that it may be a carb/cam combo problem. Also, the HP seems to fall off fairly early if you look at the dyno sheet.

Please have a look and throw me some suggestions if you have some.

thanks,

matt

[se428bandit]

Try adjusting the accelerator pump,there should be 3 adjustment,i had the same problem [stumble] with my elderbrock.Have you check your fuel pump,it might be starving for gas in the upper RPMs.

[440GP69]

Wow whats your compression ratio? 10.5:1? with a small Xtreme cam= detonation, turn up advance to 16-20 initial and 36 or so total, small cam+small intake+not so good carb yeah it shouldn't pull past 4800 at the most, E-carb and intake need to go put the factory 4 barrel back on its one of the best intakes there is and will pull to 6000rpm second use your Q-jet get it rebuilt, still with the small cam it will not want high rpm, IMO if you have over 9.5:1 and went with Q-jet/iron intake 1" 4 hole spacer and at least the comp 274 atleast!, Fuel pump is probably not doing well put a guage on it, e-carbs are easy but suck to put it plain, not trying to be rude but you could have used factory parts and had way better performance remember a pontiac 428 is not a chevy 350 and if you build it like a 428 should you will have a monster JM2

[Cliff R]

We tested that cam in a 400 engine with ported #16 heads. DONE by 4800, if not 4500rpm's. The engine idled great, strong off idle and mid-range power, just not enough camshaft for big high rpm power.

We tested two Edelbrock 750cfm AFB carbs here, they did the SAME thing, stumbled/hesitated/bogged anytime we went quickly to full throttle. Despite our best efforts, we could never get them to transition smoothly onto the secondaries. The lower the engine rpm's and vehicle speed, the worse the stumble. They ran fine everyplace else, after I tuned them with the Edelbrock tuning kit......Cliff

[PONTIAC DUDE]

If you gasket match the intake you will see a world of difference. You need to see if it's on the primary or secondary side when you mash it and it stumbles. I ran a #068 milder cam with a 750 and performer intake on a 455 and ran great. But did have to play with the carb. Most stumbles, especially with Pontiacs is the pump shot. Even Holley on a Pontiac usually require more pump shot.

Might try more intital. I have seen that make a big difference. I ran 18 with 36 total.

[Cliff R]

Ken, when we tested the 750 Edelbrock AFB clones, the accl pump shot was increased to maximum capacity. This really helped, but there was still a lean spot at the onset of the secondaries. Keep in mind that my engine is making over 500hp and accellerates very fast. They may fair better on engines making less power, as lots of folks use them with decent success? In any case, even with positioning the accl pump to the highest possible point in the well, for full delivery volume, it wasn't enough to get the secondaries thru transition without troubles......Cliff

[RAIV55]

Originally Posted by Cliff R

Ken, when we tested the 750 Edelbrock AFB clones, the accl pump shot was increased to maximum capacity. This really helped, but there was still a lean spot at the onset of the secondaries. Keep in mind that my engine is making over 500hp and accellerates very fast. They may fair better on engines making less power, as lots of folks use them with decent success? In any case, even with positioning the accl pump to the highest possible point in the well, for full delivery volume, it wasn't enough to get the secondaries thru transition without troubles......Cliff

What is the trick? On those carb when tipping into the secondaries at midrange RPM, they lean stumble. Fatten it up, the stumble goes away but pig rich. Had several that's done the same thing.

[Cliff R]

Those carburetors simply lack any adjustment for the opening rate of the secondary airflap. I'm told by an AFB expert that the flap is NOT designed for Pontiac engine power curves. My testing indicates that he is correct. No matter where we set the accl pump, or how much secondary jet was used, we could not get rid of the lean spot at the onset of the secondaries. Some mods helped, but the engine still hesitated/stumbled/bogged when we went to full throttle.

The slower the engine/vehicle speed, the worse the stumble. It was barely noticable at high speeds or at high rpms in any gear, and really bad from standing starts off idle. There is NO WAY that I know of to tune this out. Our answer was to ditch those carbs and go back to the Q-jet.....Cliff

[MadHippo]

So bigger cam and/or Qjet carb of the same size? If the wife lets me spend some money, ideally I get a bigger cam and new carb? or can i get away with just a more adjustable carb?

[Jeff Walker]

Well, a properly set up Q-Jet should get rid if the stumble. Also the stock iron pre-EGR era intake has been show to often outperform the performer intake.