440 w/ AVS stumbling at cruising speed.

[superbird77]

Hey all, hoping to get some help on an issue with my 70 GTX w/ 440. Stock build, Carter AVS, auto trans. The issue I am having is when driving and maintaining a consistent speed of lets say 55 mph. The engine seems to consistently stumble lightly under partial throttle only. It does not do this at idle, accelerating or if you get on it, only when cruising and maintaining a consistent speed. When secondaries are engaged its fine. The engine pulls very strong, carb seems tuned well enough, no ignition issues to report. Could this be a carb issue, maybe secondaries not staying closed when not being engaged creating a lean effect? Just thought I would throw it out and see if anyone had an issue like this before. Thanks everyone!

 

[pnora]

Running a vacuum advance? If so unplug and cap it and see what it does.

 

[steve340]

From your description it sounds like it is a little lean at cruise.

 

[superbird77]

Yes, I am running vacuum advance currently.

 

[superbird77]

From your description it sounds like it is a little lean at cruise.

I'm versed with Holley's, so possibly increasing jet size on the primaries of the AVS? I'll admit I need to learn a bit on the Carter.

 

[pnora]

Yes, I am running vacuum advance currently

It is simple to try what I suggested above.

 

[superbird77]

It is simple to try what I suggested above.

Thank you!

 

[steve340]

Try removing the vacuum advance as suggested by pnora first. Make sure you block the tube from the carburettor.
On the AVS you would change the needles most likely. If it turns out to be the carb the best way is to use an AF meter or take it to a reputable tuner with a suitable rolling road - were they can load the engine and maintain cruise RPM.

 

[superbird77]

Try removing the vacuum advance as suggested by pnora first. Make sure you block the tube from the carburettor.
On the AVS you would change the needles most likely. If it turns out to be the carb the best way is to use an AF meter or take it to a reputable tuner with a suitable rolling road - were they can load the engine and maintain cruise RPM.

Thank you as well. I appreciate it.

 

[steve340]

Just remember that pulling off the vacuum advance may seem to help or eliminate the problem without being the actual cause. If the engine is stock it will be best with the vacuum advance fitted as this promotes a complete burn of a lean mixture - such as what is required at light throttle cruise.
It takes a bit of experience to find the root cause at times.
An AF meter and a dial back timing light are very handy tools.
The AVS is a great carb - spend some time getting the combination of ignition and carb dialled in and I am sure you will be happy with the AVS.

 

[pnora]

Reason I suggested trying it with the advance disconnected is it will narrow it down. Very common problem is over advancing. If it helps it the poster will have to go over his timing curve. That starts with simple timing tests. I have found lots of times these cars are pulling way to much timing at light tip in when your vacuum levels are the highest. No sense messing with anything else until the timing is corrected.

 

[peabodyracin]

Assuming all the air bleed ports on the top of the carburetor are clean, I would suspect the step up rod/piston/spring combination. I've run into this problem in years past but in retrospect think it was usually with an engine that had seen a cam change. That would affect the vacuum pulled at any given RPM. Take the access caps off, pull out the step up rods and see if the pistons and rods look clean yet.

I'm not clear on if this combo had been running right for you before, and just recently developed this problem. In theory at least, the AVS would have been tuned properly when made for the combination it was going to be installed on. You can do some rough tuning by changing the step up rod springs, to see if a stronger spring would richen things up a bit and make it run better at cruise, since I agree it sounds like a lean condition.

The secondary's, like you're used to on Holley's, are vacuum activated, but use a completely different means of opening, and counterweights to close back up. It would be pretty easy to check that they move freely reaching down through the top of the carb with a screwdriver (and the carb pulled to the wide open throttle position, engine shut off of course) That said, I've chased a lot more secondary opening problems with Holley's over the years than AVS or AFB's.

As mentioned, an air/fuel meter would remove a lot of the guess work, but does involve finding a shop with the right tools, experience (and motivation) to tackle the problem.

One other thought, which is sure to get someone here excited, is to enlarge one of the bleeds inside the primary assembly, which can be removed from the top without much trouble. I had been told years ago that the current fuels required a slight enlargement of this port in order to improve drivability. I have to do some digging to get you the specifics and I know I have a drill bit of the right size. I can tell you I made this change to the AVS on my 69 Roadrunner and it helped improve drivability for me. Others will tell you it's a band aid fix for another problem. I don't know, I just know it cured my hesitation problem. I've been running the car for a number of years now without any other issues surfacing from the change.

I'll be quiet now and others much more experienced and smart than me add to the discussion.

 

[GTX JOHN]

What fixed it several times for me was stretching the springs longer (on the needles) thus making it a it richer at cruise
speeds.

 

[superbird77]

Thanks for the insight guys, all is helpful. Unfortunately I had acquired this particular car last fall so I haven't had much driving time or the previous history to draw from. My other B bodies I know the details of what I setup. It runs great in all other situations, its the dang cruising/maintaining same speed where it does this stumble. Once I accelerate slightly it goes away and runs fine.

 

[steve340]

The AVS is calibrated quite lean from the factory so it might "flutter" but it is unlikely to hurt the engine as there is no load.
My AVS has a very slight flutter on cruise but it causes no issue. I can tell it is there but a passenger cannot.
So I just leave it alone.
There is another adjustment but it is sealed from the factory. I did unseal mine and adjusted it. Did not completely eliminate flutter but made it virtually unnoticeable.

 

[lewtot184]

currently i use the factory .101" primary jet with a 16-575 in the 4618s i have in my '69 r/t. the car drives pretty good with this and it is the factory jetting. i do a mopar performance electronic distributor with the 13 degree plate, but i have changed the springs around. the vacuum advance i use is a factory 9 degree unit. something that can be done is use a 3/32" allen wrench inserted in the vacuum nipple and check to see how turns in or out the screw is. you may need to tighten yours up. i'd suggest 4-5 turns counter clockwise from the adjusting screw being seated (this means the adjusting screw will need to be turn clockwise until it stops and then back-off 4-5 turns). if this doesn't help then a jet or metering rod change may be in order.

 

[AR67GTX]

Thanks for the insight guys, all is helpful. Unfortunately I had acquired this particular car last fall so I haven't had much driving time or the previous history to draw from. My other B bodies I know the details of what I setup. It runs great in all other situations, its the dang cruising/maintaining same speed where it does this stumble. Once I accelerate slightly it goes away and runs fine.

That’s another symptom of too much combined advance at highway speed on light throttle - all centrifugal advance is in due to light springs and since vacuum is high at light throttle, full vacuum advance is also in. Give it a little gas and vacuum drops and vacuum advance with it and the constant hickup goes away instantly. With as much as 56 - 60 degrees of advance all in (say 12 initial, 24 centrifugal and 20 -24 vacuum advance) the ignition is firing so far in advance of TDC that max cylinder pressure is leaking to early almost causing a slight kickback. Just plug the advance hose and go for a quick drive - either it will stop it or you can rule it out.

Good luck

 

[RJRENTON]

I'm versed with Holley's, so possibly increasing jet size on the primaries of the AVS? I'll admit I need to learn a bit on the Carter.

If you are considering a metering change, its far easier to change the primary metering rods, on the cruise to power diameter. The origional primary metering rods were three step design (3 different diameters). You could consider a two step metering rod with slightly smaller diameters to richen the mixture slightly. But your problem just may be too much ignition advance from the vacuum advance. As suggested, try disconnecting the vacuum advance hose and plug the hose with a golf tee to see if the symptoms go away.....
BOB RENTON

 

[Geoff 2]

This could be many things & since the car is a recent purchase, needs a back-to-basics approach.

Also, AVS carbs do NOT have counterweights, post #12, so you can stop looking for them.....

This is either going to be a lean problem [ at cruise ] or excessive ign timing.

- remove carb top & check float level. Unlike brand H & clones, you will not need a new gasket if you are careful.
- check that the sec throttle blades are closing, shaft not binding. Warped base, thick gaskets can cause this from overtightening. Any binding here will cause air leakage & lean mixture.
- check that power pistons have the met rods attached & move freely in the bores.
- if the vac adv diaphragm is ruptured it will let air in & cause a lean mixture. Apply suction to the VA unit & make sure the plunger moves.
- gummed up PCV valve. Remove & shake it. Should rattle.
- check intake man bolts are tight.
- make sure initial timing is correctly set, not over advanced. Worn dist weight springs or wrong dist can cause excessive timing.

I would check these things & report back. If the engine is stock, it should NOT require jet/rod changes. There is another problem....

 

[superbird77]

I tried the vacuum advance suggestion but no change, still does the stumble. I will check timing and some of the carb suggestions mentioned next.