Engine stumble at idle

1968 skylark with a 1975 buick 350 t/a manifold,edelbrock carb, k&n filter freshly recharged
new plugs,wires,cap,rotor,coil,ignition module,alternator,voltage regulator
timing is perfect 12btdc
18in of vaccum at idle off manifold
when the engine is at idle or in traffic its a little worse there is a slight stumble the rpms drop about 50 rpms
the lights dim the voltage drops from 14 to 12
where should i start looking?
I believe the idle mixture is right if i adjust it the vacuum drops a little and it runs rougher
any ideas appreciated (except change to q-jet, someday just not now)
thanks
chris

 

Could be time to clean up your fuel system (debris in the tank, clogged fuel filter) or rebuild the carburetor.
Sounds like you are approaching this problem well.... starting with the smaller, cheaper maintenance items until you find the culprit.

 

It sounds like you are doing all the right stuff. We are experiencing the same w our 73 centiry. We finally dropped the fuel tank and cleaned it out and removed debris off of the fuel pick up sock. That helped alot. Also do a plug reading. Look at the insulators to check color. Change plugs if fouled or rich. Also. Something we dont really think about alot but e should. The fuel quality these days includes ethanol. Try different suppliers. Try mixing a gallon of race fuel in ur regular tank mix. We r still using the stock 2bbl rochester and installed bigger main jets as these came lean from the factory. Also if ur still on points like us check ur dwell setting. Adjusting points to compensate for the ethanol helps

 

Problem:Edelbrock carb

Fix:Q-jet or Holley


I had a new Eddy on my 440 mill, never could get rid of the off idle stumble and bog when secondaries came in.I bought a calibration kit and tried every combination-what a waste off money!

Problem with Eddy's is that the accelerator pump has a very lazy design, primary plates are half open when the acc. pump starts to squirt fuel down the carb, second thing is that the air valve's are not adjustable??? Some said to grind off the air valve weigh and others said to add weight.

Here's a video when I had Eddy on my Charger: http://www.youtube.com/watch?v=eYap_Vtq9Kk&feature=channel_video_title

 

this may be a dumb question but what cam is in the engine? or did this problem just come out of nowhere?

 

I'm hoping that you mean a stumble when starting back up to speed at a light.

Over-advanced timing will NOT cause a stumble, it'll actually cover up some lean conditions.

I had this problem with my Edelbrock carburetor when I first installed it. All I had to do was go four numbers larger on the primary jets and it cleaned right up.

Dirty fuel tank socks lead to surging/cutting out at high fuel demand, not at idle/off the line.

 

Accelerator pump

 

I am using ported vacuum to the dist. to the best of my knowledge, the hose is coming off the front of the carb on the passenger side i use the other port on the driver side to hook the vac gauge up to set idle mix

 

stock cam to the best of my knowledge the previous owner had this car all messed up (wrong plugs, fuel filter in backwards,etc.) so after finding out what exactly i had for a motor i put in the correct parts tuned it and it runs better but the stumble.

 

when in park at idle it runs ok but about every 5 seconds the rpms dip from 800 to 725,750 range with a little engine shake, setting the idle lower makes it worse

when in traffic or a stop light it does the same a little quicker than while in park like every 2 seconds

If i mash it to the floor it hesitates real quick then takes off, no hesitation going from a stand still to 1/2 throttle regardless of how quick i press the accelerator 0 to 1/2 or creeping up to 1/2

runs good while driving can't notice and stumble or mis

 

That is why a Holley is such a great carb to use. You can change the pump cams easily and if that does not work you can change the shooters on top of the carb. A five minute fix for either one of these. Take the 4 screws off the front of the bowl and change the jets, another five minute fix.

But if you have a stock intake with the large holes in the back of the intake the Q-jet is the best carb to use. The adapter used for a Edelbrock or others with a square bore just do not work on the stock spread bore intake. Been there done that.

You have to have a TA intake to be able to use the other type of carbs that are square bore or spread bore. Then you can use 4 hole or open spacers to taylor your engine if you want but not always needed. When using a adapter you have to have a larger pump shot because the carb is now farther from the floor of the intake.

 

ttt

 

It sounds to me like you have a fouled plug or a bad plug wire... Have you replaced the plugs ad wires?

 

yes all have been replaced and gapped i do have a megaohm meter does anyone know what the resistance should be the wires are mallory highfire ignition 8mm

 

I had a thread a while back about checking the resistance of plug wires... if I remember right 10k to 15k ohms was a good resistance.
http://www.popularmechanics.com/cars/how-to/maintenance/1272351

http://www.msdignition.com/page.aspx?id=3302

 

I had a slight stumble off-idle on my Riviera once I got the timing dialed in. My engine is just about stock, but I think the machine shop who did the balance/blueprint job on the 430 may have put a slightly aggressive cam in. The manual said initial timing at 2.5deg BTDC, but I had it set at around 6 degrees since it seemed to idle smoother. I just backed it down to 4 degrees, and the stumble went away. Has noticeably more zip off the line, too. Amazing what can be done with a snap-on digital light that actually works (I had to repair the cheap ribbon cable connecting the keypad to the main PCB).

Keep the idle below 800 RPM, or you'll start kicking in the mechanical advance too soon...and cars with auto trans can stall out.

 

I will try messing with the timing a little and ohm out my plug wires,
I will report back when i do thanks guys
:beer