Idles fine - runs great when I mash on it - stutters/stumbles with a little throttle

[Brent Mills]

Idles great, drives great except when just cruising and then give it a little more gas it stumbles/stutters quite a bit. Give it a lot of pedal and it goes and feels pretty good.

What should I look for? Timing is 38 degrees total. Windows are about half on both ends.

[fsiewert]

Hey Brent;
It sounds like you have a lean run condition when the throttle first opens. Check how far the throttle opens before the accelerator pump starts to squirt. There are some adjustments you can make if the squirters don't squirt until the throttle opens substantially. Lengthen the spring loaded bolt that contacts the accelerator pump arm so the accelerator pump squirters squirt sooner. You can also change the setting on the accelerator pump cam to change when and how they squirt. By making them squirt sooner, your lean run condition is eliminated.

Also check to make sure that your timing is changing as the RPM changes. It should retard a bit when the throttle is first opened then return to full advance as cruising RPM is achieved. I don't know what kind of distributor you have but there should be some method that advances and retards the actual spark timing as the RPM changes.

Hope this helps,
Fred

[Brent Mills]

Thanks Fred...I'll check the accelerator pump tomorrow. I double checked the timing today and it is fine.

One thing I noticed and incorrectly stated above is that the front bowl is at the halfway mark on the sight while the back bowl is down at the bottom. I read on the holley site that they should both be at the bottom, yes?

[Ralphy]

Brent found some useful videos.

http://www.youtube.com/watch?v=w3F3ssOb8lg

http://www.youtube.com/watch?v=eiRIfcrc2yA

[Hotfingrs]

Not knowing what carb you have, I'd say bigger squirters. Go up a couple sizes and if the helps, go up a couple more until problem is gone. Check the contact on accelerator pump first, then play with the squirts.

[55312]

Holley says both float bowl levels are at the bottom of the site window so it just barely seeps out. Adjust the rear float level before you dive in any further.

Tim

[ZOERA-SC7XX]

Despite what most everyone here thinks, I think you should connect your vacuum advance. I chased the same problem for a year before I connected it and instantly the car behaved well in the low rpm range. My initial timing is at 15 deg advance, add to that the 10 or so with the vacuum advance, (but not constantly). The mechanical advance will add to that up to 21 deg, so you may have to install the black button which will limit the total. Either way, there's lots of combinations to play with. The vacuum advance allowed my car to become quite driveable in city/traffic conditions. Detroit engineers were pretty good, don't you agree. It may not be the answer for the guys who track their car, but may be a good idea for general cruising.
I hope this helps. Good luck with that beautiful GT.

[Bill Bess]

I had a big stumble when I gave it a lot of throttle quickly. I finally installed larger 50mm Holley acerator kit and cam on the throttle shaft and cured my problem. It now gets more fuel an prevents that stumble. It's been running great for over two years in extremely hot weather.
Bill

[cdnus]

You stated it idles great & drives great so that would indicate the pump,squirter's & float level are OK.
It sounds like you may have to work on the transitions.
1st. pull the carb & adjust the primary's for .020 exposure of the slot, you need at least that to accomplish a seamless tip in. install the carb, warm up car--short drive, if idle is too fast, reduce speed with secondary adjustment.

If that doesn't solve the problem you may have to start messing with the Idle air bleeds or restricters.

JMO

Craig

[jhv48]

Before you start replacing things, do the following:

(Make sure your timing and advance is spot on first, before any changes.)

1. Adjust both float levels to the base of the sight plug (assuming you have a Holley). Test drive.

2. Make sure accellerator pump is squirting immediately when the throttle is moved. Test again.

3. Try opening the idle air screws an additional 1/4 turn (richening) from their current position. Test drive.

[xlr8or]

Brent what power valve do you have in the carb? You need to measure vacuum level in the manifold to determine where you should be on power valve.
It may be as simple as changing it to an appropriate rating.

[Rick Parker]

Ralphy's first video reccomendation pretty much covers any real issue you might have, pump cam tuning is further down the road.

[CWizard]

Brent -
Your problem sounds a lot like the one I just squared away. My car ran great at any throttle postion except when just maintaining cruising speed on a level road, and there it would cut out, or hesitate noticeably. After some terribly enlightening troubleshooting I found two main problems. One - the timing advance was squirrelly. It would stick at a total advance of 34 degrees and then ramp on up (I had 17 initial + 21 mech advance, 38 total at that time). Worse, the timing would bounce up and down by as much as 4 degrees at lower rpms, and specifically in the rpm range where I would be trying to maintain cruising speed. Never found a reason for this strange timing advance behvior, but I cured it with a new distributor. Note that to find this kind of problem you have to map the rpm vs timing advance curve at about 200 rpm intervals to really know what's happening, ie, don't just rely on a point at idle and a second point when the advance is supposed to be full in.
My second problem was related to idle transition in the carb. I had adjusted the idle mixture screws (4 corners) to get max vacuum at idle and then leaned it about 1/8 turn from there. The engine did not care for that set-up, and continued to hesitate a bit while trying to maintain a constant, low load speed. So I started backing out the idle mixture screws and finally, after about a half turn out (richer) on all 4, it's pretty much ok. I also found one of the idle mixture screws was a bit rounded at the end, which certainly complicates the adjustment process, so it will be replaced soon.
Hope this is of some use to you. My carb is a Holley 4150 HP street, mech secondaries, no choke or vac advance.

[elmariachi]

Originally Posted by jhv48

Before you start replacing things, do the following:
(Make sure your timing and advance is spot on first, before any changes.)
1. Adjust both float levels to the base of the sight plug (assuming you have a Holley). Test drive.
2. Make sure accellerator pump is squirting immediately when the throttle is moved. Test again.
3. Try opening the idle air screws an additional 1/4 turn (richening) from their current position. Test drive.

X2 on all counts.

Plus check idle vacuum and install correct power valve accordingly. A tip-in stumble is almost always a temporary lean condition unless you get black smoke, then its likely rich because of too much PV.

Jim

[PANAVIA]

I am a big believer in vacuum advance as well, and it should be functioning properly or you are just driving a thirsty tractor.

For the fuel level in the bowls, holley states and I concur that the level should be at the base of the window, now to the transition.

Are you running any sort of base plate on your carb like a spacer ?

If so is the spacer the same design as the intake ?

Now to the transition , of it is stumbling quite a bit, it may be too much fuel as well .

Take a peek and let us know.

Steve

[zrayr]

Holleys sometimes work best with fuel even lower than at the base of the inspection hole/window. But I've never heard of a Holley working better with the fuel level appreciably higher.

Z.

[Brent Mills]

Guys, I totally messed up. I thought it was a holley, but it's a quick fuel carb. Looks like after reviewing again with the pump on it's at the center of the sight windows on both and there's a line in the casting to indicate that's where it should be.

Second thing is that there was a little bit of a vaccum leak as one port was missing a cover ....Runs a lot better now (go figure), but not perfect. Haven't had any time to do much more with it, but will take a look tomorrow more as the weather should be nice.

Thanks for all of the advice everyone!

[Rick Parker]

Just a Holley with earings.

[Michael C Henry]

Do they mention float levels for carbs not sitting level? Most manifolds are designed for engines sitting at an angle, Front higher than rear. In my Cobra replica there is very little if any angle if any. I have a Dove Tunnel wedge intake manifold, now my carbs are angled donward in front. 2 side hung single inlet 4160s ===1850

[Gonein60]

Novice here but just got back from the dyno with my 750 and this same problem was cured but switching out the power valve. Mine had apparently been damaged by gulp of air and a little too much fuel. Everything else everyone has mentioned was already in check on mine so this was the last ditch effort...