My stock 600 cfm Holley finally pooped out on me, it has been running like a top, but the battery failed and it sat all summer, when I fired it up it was puking black smoke at idle. I bought a Street avenger 670 cfm from my friend who had it on his 455 pontiac. I've been in the car the week before I bought it and it ran great. So I thought this would be easy...

What I can tell you about the engine:
Weiend Stealth dual plane intake
stock exhaust manifolds
mild rv cam
Flat top pistons
Auto c6
3.73 gears
Everything emissions has been removed (evap, egr, thermostatic vacuum valves)

Problem is it's hesitating on even mild acceleration (cold and warm), but it idles perfect, WOT is good etc. It feels like it's going to die, but pulls through eventually. Quick neutral WOT snap will hesitate and some times backfire. I fixed the choke wasn't opening properly, now I've verified it's open and same problem. It came with three Vacuum secondary springs, the heaviest was installed, I installed the middle one. Low speed mixture seems good, 21"Hg vacuum.

It had a size 40 accelerator pump nozzle installed, tried a 28, same thing. bought a 31, same thing. I'm using the orange pump cam for all three. swapped the 31 nozzle to the pink cam, no change, and to the blue cam, no change.

Timing @ idle is 10 BTDC and increases about 30-40 when reved. Distributor advance is connected to ported vacuum out of the metering block.

So does anyone have any input on this engine carb combo and where I can go from here or what I can check? I'm temped to buy more accelerator pump nozzles and keep bumping up that, but I'm curious what the norm is...

 

When does the bog happen...at the hit of the throttle? Or does your rig take off, bog, then take off again? Been a while since I studied up, but to my recollection, a bog directly off-idle is an indicator that your squirter is too small...but since you started off with a 40, I doubt that's the problem. Couldn't hurt to try one a tad bigger, but I'm not sure if that's gonna solve anything.

If the vehicle starts to roll out OK, then bogs, that's generally a sign that the accelerator pump isn't shooting enough fuel for a long enough duration, and that means an upgrade to a 50cc pump. Had that problem in the '90s right after I rebuilt the 460 in an LTD II I drag raced. The 50cc pump fixed the bog, and netted me almost a tenth. You might have to play with the pump cams again if you go to the bigger pump.

FWIW, I think the 670 CFM is a good choice for your combo.

Pat

 

I might have to drive it one more time, but I don't think it's right at the crack of the throttle, seems to me it will start to go, then bog. Because every time I adjusted it I thought it was better until it started bogging again.

I was reading that around the 40 size nozzle you are supposed to go to the 50cc pump... And I am shy about buying more nozzles because of the cost, already like $17 for the 31, and even if I order a few more it could add up quick...

Thank you for the reply, I'll try to post updates.

 

Stepping on the throttle in neutral won't tell you a thing.
Tuning has to be done in gear, under load.

What is "the heaviest spring"?
it sounds to me like the secondary's are flopping open.
And that would make sense to me given that the truck likely has less power to weight and a less aggressive cam that the Pontiac.

Spring kits are cheap, and the quick change cover pays for itself in the time you save if you are swapping them.
You likely need a brown or black spring with that combo and vacuum.

http://static.summitracing.com/globa...199r8219-2.pdf

 

Thank you guys for the help, I got the spring kit, which had instructions. Turns out I did have the black spring which is the heaviest. So I'm hoping thats nor it and I can tune that later. For now I'm going to put in the 50 cc accelerator pump kit. It's been rainy and I don't get Home till dark so progress is slow.

 

My brother brought me over the Holley super tuning book. It has really nice charts and numbers for accel pump cams. So I used that too step up too almost the most aggressive, and wow it's much better, almost 90% better. Still a stumbles right at the end of the throttle opening. I'm thinking I might try the smaller squirter again because I'meeing it needs more fuel longer for sure. Didn't pull the trigger on 50cc kit because of cost, and I have much more time than money, LOL.
Also notice the stumble is happening around 8"hg when accelerating. I wish I knew what power valve was in it, but I'm wondering if its too small for the truck. Since my vacuum is about 16-18 in gear@idle I'm thinking the 8.5 or 9.5 pv would be good.

 

I played with a 670 TA a lot on a 460 and it ran nice after I figured it out. The avenger series carbs are finicky, but a very nice unit once dialed in. It could be a rich power valve, mine would go rich and just stumble terribly with a 65 power valve, I am down to a 25 or a 35 I think. Leaner fixed that. Another thing that needs to be addressed is on the TA carbs, there is a checkweight that will allow the idle circuit to run in any position and this caused my engine to need a little more air at idle.

I opened the idle screw up slightly and it idled beautiful but had a flat spot off idle. I finally figured out that it was idling in the transition circuit and when I open the throttle, it would lean out. This is easy to check, close the idle mixture screws (turn them clockwise) with the engine idling, it should kill the engine before the screws bottom, turn them slow. If the engine doesn't die, or doesn't die pretty soon, then you are getting fuel from somewhere other than the idle circuit and that would be the transition circuit.

To correct this if it is the problem I had to open the secondaries slightly to allow more air and close the primaries until they are out of the transition circuit.

 

One of the other tricks if you are running on the transition circuit at idle is to drill a small (1/16") hole in the primary throttle plates to allow the throttle to close more at idle. If you have externally adjustable floats, double check your float levels.

If you think the secondaries are coming in too soon even with the black spring, you can disconnect the secondary diaphram and tie them shut to test. One of the issues I used to see, not with Holleys generally, but TQs and Qjets was too much throttle opening at idle causing fuel pull-over from the main circuit.

Power valves on a Holley are a good tuning tool, they even have two stage ones to allow a leaner main jet, but the ability to add a little fuel at 11-13 in vacuum.

 

Well, after being sick for two weeks I got to mess around with it more, Ordered the 35 squirter, and it was too rich finally. so I tuned down the cam and wow, its running perfect!! I just can't believe how sensitive the accelerator pump shot really is.

So I went ahead and dropped the sping in the secondaries a bit, now they are opening good. Was using the truck to run to the grocery store last night and it all of the sudden started stalling on me at the stop light, got it running, holding high idle, and got off the road, poped the hood to find fuel puking out everywhere. I had no tools, but luckily the clear float sights were in, I could see the main bowl was over filling. so my wife brought me some tools, fiddled with the main needle and seat and got it to close again. so this weekend I will swap that out but otherwise she's running better than ever!!

And for christmas I got all new steering centerlink/ tierod ends. New door panels! I'm all set to do some work to the old truck!! Going to finally rebuild my steering column and install the sport steering wheel, after like 2 years, LOL

But thanks again for all the help!!

 

Sounds good. Be sure you have a good filter in the fuel line. Ford used to have a lovely built in one for a long time, I had one on the pump I put on my '77 when I installed the 390 Camper Special engine. Holley used to have one that was a clone of the one Ford put on the 406 and 427 engines, takes the same filter element as the fuel pump one.

 

I had a similar problem as this guy. Not the same but close. I cant figure out how to do posts so im gonna pop the question on a related thread. I have a 1978 Ford F250 with a 351M and a 2bbl Motorcraft 2150 carb. It runs great at idle but has a flat spot,then when i put it in gear the truck bogs and barely runs in gear. When i hit the throttle the engine nearly dies (it probably will if i hold it down long enough). I'm stumped and i have to have this truck running by August 10th to do something extremely important!!! HELP!!!

 

WHen my holley was doing exactly what your saying, it was the needle that was getting stuck open. I think some debris got in there, or my fuel filter was coming apart. The bowl was flooding the engine through the vent.

 

The needle valve?

 

Float valve on top of the bowl.

 

I think its just getting too lean somehow when i shift into gear.