440 engine stumbles off idle

[1970GTX]

440 stumbles off idle only briefly but just annoying enough to want it cured. Fuel octane rating makes no difference. Engine is fairly fresh, about 5000 miles since rebuild. Retains stock bore and stroke.

1970 440 stock manifold with edelbrock performer 1407 carb

stock exhaust manifolds

Comp Cam -4 hydraulic flat tappet
http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=717&sb=2

timing at 5 BTDC
idle @850
We've adjusted the idle mixture screws to no avail.

I have a rebuilt Carter 4738s carb. Do you guys think it's worth bolting that on and seeing if things improve? Any other suggestions appreciated.

 

[RustyRatRod]

Usually off idle stumbles equate to a lean pump shot. I would inspect that and see if you can improve it.

 

[1970GTX]

layman's terms?

 

[70rr-brian]

Accelerator pump. Inside the carb.

 

[RustyRatRod]

layman's terms?

With the car not running, remove the air cleaner and look directly into the primary (front) barrels. Take the throttle lever and open the carburetor all the way slowly, as if you were acellerating up to speed slowly. Look for a good strong stream of fuel in each primary barrel. I bet it's not very strong, or it starts after you move the throttle lever.

 

[Mopars & Missiles]

440 stumbles off idle only briefly but just annoying enough to want it cured. Fuel octane rating makes no difference. Engine is fairly fresh, about 5000 miles since rebuild. Retains stock bore and stroke.

1970 440 stock manifold with edelbrock performer 1407 carb

stock exhaust manifolds

Comp Cam -4 hydraulic flat tappet
http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=717&sb=2

timing at 5 BTDC
idle @850
We've adjusted the idle mixture screws to no avail.

I have a rebuilt Carter 4738s carb. Do you guys think it's worth bolting that on and seeing if things improve? Any other suggestions appreciated.

You'll go a long way to fixing this issue if you increase the timing to around 12-15 degrees initial (vacuum disconnected) and reset the idle.

5BTC is not enough for most HP engines today. That was the factory setting (very conservative) back then and you have made changes to the engine since and it needs more initial.

Be sure to check total (initial + mechanical) timing, you want around 35-36 degrees with the mechanical advance all in at around 2000-2500 RPM. Once that is all correct, you can hook up the vacuum and you should see around 52-55 degrees total (initial+mechanical+vacuum) at highway cruise speeds (low engine load). And, yes you do need vacuum advance if it is a street driven car. Race only can disconnect and ignore the vacuum.

After the timing curve is set correctly, rotor phasing would be another step to improvement.

Rebuilding the carb accel pump may also be part of the issue as others have pointed out and should be addressed as well. But I will bet you will see an improvement by getting your timing straightened out too.

Good luck.

 

[1970GTX]

ok thanks for all the help. anyone have thoughts on the Carter AVS 4738??

 

[67 coronet]

you are prob better with the edelbrock.its a bit more performance oriented then the old carter.try the timming change as recomended first before you mess with the carb any more.other then checking the accelerator pump squirt.

 

[Montclaire]

You need to find true TDC and mark your dampener accordingly. MA just did a full write up on just that. If you had more time with the engine and knew how it reacted you could really set it by ear, but in your case I would recommend a timing light and accurate readings.

As for the carb you can start out with changing the position of the pump shot arm, but I'm guessing you will need bigger jets as well. Eddy sells a squirter pack to help get you over the off-idle lull.

 

[Meep-Meep]

I tried a factory AVS on my 440 for a few passes and it went 12.90's, so unless yours has major problems I doubt it will be an issue. Low vacuum will mess with it as well as any other carb so do check your idle vacuum. And if you have an automatic, check the vacuum in gear. As mentioned above, check the pump discharge and set the timing to 15 BTDC. Personally, I think that cam will give you problems when tuning the factory carb.

 

[benno440]

yeh agreed thats a low/mildish cam for a stroker motor. get a new carbie mate IMO

 

[67 B-body]

I'm betting the timing advanced to 15 - 16 BTC base timing will correct the stumble!

One more thing,,,, be prepared to lower the idle screw to bring the RPM back down after you advance that timing.... Then you'll be using the IDLE circuit to control the fuel mixture, and you'll also be increasing the manifold vacuum with the throttle blades being closer to the closed position...... This is why it gets rid of stumble conditions....

 

[Montclaire]

It may, but it won't if he doesn't know where TDC is

 

[1970GTX]

ok, thanks guys. Once she catches she pulls strong through the entire power band. With 3.91s she can give ya a kick in the *** on the 1 -2 shift. The stumble is just a little annoying driving around town. I'll get after it tomorrow,

- - - Updated - - -

It may, but it won't if he doesn't know where TDC is

not a problem.

update, checked the squirt as I was advised and there was an immediate and steady squirt in both barrells. so set initial timing at 10 no vacuum advance, then checked again with vacuum advance and was seeing about 20 to 25. reset the idle, Then to my amazement the sun came out and the roads were dry so, yup took the BigX for a road test. Just a very slight hesitation off idle but a big improvement so, were going at it again tomorrow and we'll start at 12 initial and see if it keeps improving