off idle stumble

i have to ask alls out there have been working on my bucket and cant get rid of a off idle stumble rebuilt carb,new plugs points,wires etc but there is a pronounceable off ilde stumble,heasitation when one stomps on the gas, works fie if you slowly accelerate but if you put you foot in it you get and hevy heasitation and then she catches and goes good
its getting fuel as the accel pump gives it a good squirt timing is at 10 and the vecume advance is working.
cant figure it out it gets worse if the advance is unhooked.:confused: :confused:
any ideas out there

With the motor and switch off, hang your face over the top of the carb and give the linkage a good pull. Look to see if you're getting a pump shot. The symptom you're describing sounds like a lack of accelerator pump shot.

I had this same problem, (assuming the accel pump is shoddy..) I went and bought a new edelbrock 600cfm carb. after I started to pull the old one apart and found that corrosion had been eating up some critical components and walls of the carburator.

Try a new accelerator pump in it if you can find one, its the cheapest carb fix I know of besides buying a new kit then buying a new carb. =\
You'll more than likely have to re-set your carb's mixture when you do this, but you know this. <3

And check your plugs for any gunk on them as well. Poor spark has done this to me multiple times.

And make sure you,re running on the right side of your carb port. Its the one that has more suction when you hit it.

fuel is ok nice big shot from the accelerator pump, i have to check what denny said re worn dist bushing and also check dwell again. i cant see dwell causing such a blah off the line but i will check it again.
vacume advance is a funny thing i have a new vacume canister on the car and when i suck on the hose the dist does advance but rpms dont jump much. i am to fiddle with this further .

you know denny i never knew what that plate was when i had to set my motor to TDC and i thought. "I wonder what would happen if I took out this LOOSE PIECE OF METAL OUT OF THIS RAPIDLY SPINNING DISTRIBUTOR?"

ROFL, glad I didn't. This is why I'm a noob. I think terrible stupid things :D

Try running a few degrees more initial advance.

i think i found some problems dist bushings are nice and tight no movement, but i found something. i had put a dist advance kit in before putting the engine in and had used the med springs. i thing they are too light as when i took the rotor off the weights were stuck out at about 1/2 way.
i put the heavier springs in it and now just timed by ear tonight, seems to sound better but will have to take on road to see if helped. also oiled up the weiths bushings a little as appeared dry.

riddle me this does the gm vacume advance and mechanical advance all add up together to determine total advance.
or does the vacume advance pull advance out at low vacume full throttle aplications.
i was going to get a advance tape to put on my harmonic balancer but i dont have one or at least on a caddy motor anything that looks like one.

My model A had a 327 in it. I put a new carb on it, an edelbrock thunder series 650 CFM. It would do the same thing, bog when you slammed the pedal down. I cured mine . The accelerator pump has a linkage on it with 3 holes. I moved the linkage so it was in the hole that gave it the biggest stroke. It cured about 90 % of the bogging problem and was free. Also have you doublechecked the carb gasket ? When bought my carb the parts guy gave me the wrong gasket , and it would not let the secondaries open. Check to make sure your secondaries open fully when in wide open throttle.

my mustang was having a similar problem too.... i cured it by changing from a 600cfm carb to a 750cfm carb.... now she launches.... got her out the other night and she was pulling a 6.88 on the 8th mile at 115

Quote:

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Originally Posted by canadianal

riddle me this does the gm vacume advance and mechanical advance all add up together to determine total advance.
or does the vacume advance pull advance out at low vacume full throttle aplications.
i was going to get a advance tape to put on my harmonic balancer but i dont have one or at least on a caddy motor anything that looks like one.

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Total advance is initial plus mechanical. Vaccum advance drops under acceleration unless you use port vaccum then it raises up to WOT then drops.

IF your running a Holley it sounds like accelerator pump is to small,Holley's and Automatic transmissions will at times Stumble just off idle.most peaple think right away that carb is to big so they try a new one.don't no how hot your engine is but even a mild street motor can handle up tp a 750 with vacume secondary'.when you punch it the car falls flat on it's face "waaahhh",thats called a hole in the fuel curve,tranny's putten major load on engine at(WOT)and the engine is running out of FUEL,to fix this problem if you run a holley, install a Holley(REO-KIT)this is a 50cc accelerator pump kit you can also try larger pump shooters and fine tune pump cams. also check to make sure the factory Bronze Holley Fuel filters arn't plugged this will also cause a fuel shortage.filters are located behind the fuel inlet where it screws in to fuel bowle.I yoused to take mine out and run a in line fuel filter,you will have (2) filters in carb bowles front and back if you run a dubble pumper carb,Also check Float level.the hole Holley kit is like $40.00 the 50cc Diaphramis only$12.00 if you start with just the Diaphram first.A new Holley 750 double coast around $400.00, so any way try that if all is well with the timing and problem still persist I hope you get it squared away.

well my car is back together and i worked yesterday pm to get rid of the bog,i seemed to have helped it a bit by drilling a closer accerator pump likage hole which now gives the car a real good shot of fuel. the car is better but there still is a real lean condition right off idle, this engine should hit like a ton of bricks off idle . it even wants to pop back sometimes through through the carb.
if i slowly hit the gas no problems accelerated jsut fine if i hammer it bog pop bang and even stall.
its weird. it seems to have gotten worse when i modified my intake but i brought to back to how it was before by advancing timing timing and now the accel pump change had improved it a little more but its still there
my dist bushings are tight and it is advancing on both mechanical and vacume.
weird

hey den took the top off the carb see what you mean i seem to be getting a really good shot of fuel , i also tighted the secondary door opening up a little bit so it opens slower and it seemed to help but it still does not cure the problem.
i will keep looking it just,seems lean off idle.

it all seems good and i have cured it a bit by modifying the pump lever and i get a real heavy shot of fuel.
i am wondering if ign is the problem?

yeah the bushings are nice and tight no movement, the car even will pop and literaly stall if you punch it from idle. if you load it up againt the convertor and flutter the throttle before floring it it gets up and goes not great but at least it goes.

There seems to be more than a touch of confusion about manifold vacuum and ported vacuum.
I'll make a separate post pertaining to some tests I ran comparing manifold (or full time vacuum) to ported vacuum (also known as timed vacuum.)

Title will be, "About vacuum sources and timing."

further info on off idle stumble.
i think i finally solved it. i timed and played and changed dist springs etc but still had the funny bog off idle that woulf not occur at speed.
went back to the carb and started at sq 1 again. i found that the set screw on the secondary air valve flaps on the carb was kind of screwy it would tighten up but would not perfectly hold the set screw. so the secondary butterflys would open off the throttle and i have them set very agressively when i put the carb together and the air valves seemed ok but would open too fast for the engine and lean bog before catching up. stole another set scew from a old carb set the air valve again and all seems good. nice no bog fry the tires off the line from idle launch.
now to the next goal some kind of traction maybe a mini spool.

if ya mess with something long enough eventually you will figure it out