secondary hesitation

[edwin]

i’m having a problem with my ’79 trans am, 454 chevrolet (big cam) engine, 700 cfm holley 4 corner idle).
it performs very well on the primaries, but when i go into the secondaries it hesitates, then the engine comes back but the performance is flat compared to the primary side.

i have tried 30cc and 50cc accelerator pumps and various cams, and the condition is best with almost no gas from the accelerator pump,
adjusted jetting with and without power valve, settling with no PV and size 84 jets (73 jets on the primary side).

hanging out at the drag strip, everyone tells me it’s too rich, but when it hesitates and after no black cloud from the exhaust.

any suggestions woult be helpful,
thanks,
charlieJ

Topic

[Bluesnut]

I assume this still has the Quadrajet carb on it?
It’s been many years since I’ve messed with a QJ but you might inspect the secondary and choke linkage carefully.
I vaguely remember a tiny pin in the linkage (passenger side) that can become dislodged and when this happens the secondary butterflys will not open even if the primarys are wide open.

[Zac Aleksovski]

Place a 455 Pontiac under the carb. Just kidding (sort of). Vacuum or mechanical secondaries? If vacuum, Holley sells different springs to change the rate the secondaries open. Had a 650 spread bore on my ’75 TA. Seems like ran more consistent with a Q-jet. Been over 25yrs. since I’ve messed with carbs other than on small engine stuff.

[edwin]

mechanical secondaries, 4150 series

[Zac Aleksovski]

So your cruising part throttle then get into the secondaries & have a hesitation? Does it recover & rev out ok after that? How does it launch from a dead stop?Holley also has different sized orifice accel pump nozzles as well as different cam profiles & capacity pumps as you mentioned. Maybe get on the Holley website?

[edwin]

thanks for the input guys.
so almost stalls when i get into the secondaries, recovers to a point, but much less performance than on the primaries. performance on the primaries is very good.

i have tried both 30cc and 50cc accelerator pumps, with and without power valve, and gone through the procedure for jetting and tried several cams.

right now it’s at 30cc accelerator pump, no power valve, #84 jets, and orange cam #2 position, which gives almost no gas from the pump. this is the best performance that i have gotten.

i have gotten many opinions that suggested and agreed that it’s too rich, but i had someone follow me on a test drive and no black smoke when i get into the secondaries or after, where there is smoke on the primaries. makes me think it’s too lean.

[Zac Aleksovski]

Did it just recently start running like this? What do the spark plugs look like? How is fuel pressure & ignition system? Running electric pump & regulator with larger lines? Float levels on primary & secondary bowls ok? Went through all that stuff on my brother’s Chevelle but that was back in the stone ages so I don’t remember too much. He made so many changes can’t remember them all. I think ended up 850 double pump, the red (I think) Holley fuel pump, 1/2″ lines with a fuel cell, 4500RPM stall converter (I think), 4.56 gears, don’t remember the dia. of the slicks. MSD ignition stuff don’t recall the advance curve in the distributor, & 3-stage nitrous NoS probably? 9.73 don’t remember MPH. Someone broke into my dad’s garage where he had it stored & helped themselves to a bunch of parts off it. That ended that.

[edwin]

so this is my first BBC project, rebuilt and installed the engine this spring, so ever since the rebuild i’ve had this problem.
this car runs 11.3 seconds in the quarter, 112 mph even with the problem. 3.70 gears

and yes, i did write holley an email 6 weeks ago, still no response from them.

[Zac Aleksovski]

Had the engine running in the car before rebuild? If so was the same carb, intake ignition on the engine & didn’t have the stumble? Plugs look like too rich or lean after a run? Did you get the carb new? Was it worse with the jetting the carb came with? Did you go radical with the cam from what was in there before? Other than making sure fuel pressure & volume is good, float levels & ignition system, I don’t know. I don’t think we ran without a PV just got one that opened @ a vacuum lower than idle value. We borrowed a Holley tuning book from a friend that was helpful. I would think you should be able to get you hands on one somewhere.

[edwin]

the engine did run before the rebuild, but it was basically stock and therefore a gas guzzling dog. i went through all the standard tuning practicing procedures for the carb, and no love. i was thinking about setting the idle, which was at first difficult and i could only get it to ‘idle’ at about 1600 rpm. i realize that this is technically running, but i did eventually see some old school videos that got the idle @ 600rpm, lopey, but steady. combined with the fact that i had someone follow me when i tested and they said no smoke at all when i got into the secondaries, but plenty when i was in the primaries. so i put a 50cc accelerator pump, power valve and 84 jets (73 jets in the primaries) in it. Basically throwing a bunch of gas at, knowing it was way outside the parameters i should be tuning. during the test, fuel pressure dropped to 4psi, indicating the real problem, the stock 5/16 fuel line was inadequate for the engine and carb. i’ve got to say, thanks for all the suggestions and help. Stay dirty!

[James]

wanted to say thanks for all the suggestions and help i’ve got here. i’m new, just registered, but i’ve already got lots of helping by browsing through your posts guys! been wondering if i may ask you questions? thanks!

[Author]

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