440 w/ AVS stumbling at cruising speed.

[RJRENTON]

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.

That particular carb has an "adjustable" off idle transfer feature, calibrated during the carb's manufacturing, that provides a lean off idle calibration point to reduce emissions, especially during deceleration. There is no real advantage to changing the origional settings. Your problem MAY BE attributable to a stuck in the down position, one or both of the primary metering rod STEP UP PISTONS, which control the position of the metering rods. If the piston(s) are stuck in the down position, the mixture will be lean as the throttle is opened, as the metering rods cannot move up, allowing greater fuel flow thru the metering jets. Remove the covers to check....in the engine off position, the step up piston should be at the top of their bore and move freely up/down. Additionally, there may be dirt or crud in the internal fuel passages restricting fuel flow causing the symptoms noted. Just my opinion of course.....
BOB RENTON

 

[lewtot184]

wonder if there might be a fuel delivery problem?

 

[Geoff 2]

No, it s not a fuel delivery issue. No stumble when throttle opened, secondaries opened.

OP got the car last fall, says he has no history on it. That is why it is back to basics, as per post #19.

There are soooo many things that can cause this that can be hard to find. Example, here is another one: Exh heat x over blocked in intake, either from carbon build up or purposely blocked with a plate. Fuel does not vapourise as well through lack of heat, causing a stumble on light throttle application.
Another example: slightly larger cam fitted. Idle is still smooth, so no external indication. But the slight reduction in cruise vacuum for a carb that was calibrated very lean, is enough to cause a tip in stumble.

 

[lewtot184]

No, it s not a fuel delivery issue. No stumble when throttle opened, secondaries opened.

OP got the car last fall, says he has no history on it. That is why it is back to basics, as per post #19.

There are soooo many things that can cause this that can be hard to find. Example, here is another one: Exh heat x over blocked in intake, either from carbon build up or purposely blocked with a plate. Fuel does not vapourise as well through lack of heat, causing a stumble on light throttle application.
Another example: slightly larger cam fitted. Idle is still smooth, so no external indication. But the slight reduction in cruise vacuum for a carb that was calibrated very lean, is enough to cause a tip in stumble.

the reason why i ask about fuel delivery is because i see so many people put a cellulose fuel filter on the suction side of a mechanical pump. restriction on the suction side in combination with winter blend alcohol gas isn't a happy combo.

 

[superbird77]

I can confirm it was rebuilt with a slightly bigger cam from factory. Trying to find info on it for reference. Thanks guys.

 

[Geoff 2]

Then most likely cause then will be lean mixture at cruise.
This is a simple non invasive test to test for leanness. Engine warmed up & idling, slowly close choke blade until engine note changes. Back off slightly & wire the blade in this position. This richens the mixture.

Test drive to see if surging is improved/gone.

If fixed.....

Many factors will determine whether at 55 mph cruise you are still on the idle cct, main cct....or in between. I would lean [ pardon the pun ] towards the idle cct which controls off idle fuel & transition to the main cct. Particularly so with this carb because of the very lean idle cct.
Richening the idle cct would require enlarging the idle tubes 0.002-3" [ thin tubes hanging down from the base of the primary boosters ] . However, if this does not fix the problem, it is a bit hard to put the metal back......


So........I would try richening the primary jetting first. Either slightly larger jets or met rods that are 2-3 thou smaller on the cruise step. If you do not have these on hand, you can do this: remove the piston springs & make up spacers to fit under the pistons so that the piston top is 1/4" down from the piston cover. This will ensure the met rod is on the intermediate or power step & richen the mixture for a test at cruise speed. Use anything you like for the spacer, nail, screw etc as long as it larger than the vac hole under the piston.

 

[SlinktRR]

It's probably just needing some slight tuning for cruise. With an Edelbrock carb take the air filter off and take the metering rods out. There is a four digit stamp on it, first two numbers are for cruise and last two are for WOT. Check the metering chart for your carb and see what it is set to, if you haven't touched it the setting will be in the center of the chart. If your cruise is acting lean you can decrease the first two numbers to make the fuel curve at cruise richer without affecting WOT (i.e. change 7352 rods to 6852 rods). The chart will show you rod/jet combinations to go richer/leaner. Use the tuning kit with metering rods and a screwdriver and take the car out for a ride and change rods until it feels right. The best thing about AVS carbs is fine tuning cruise is so easy and quick compared to a Holley, don't have to drain bowls to make changes. Also make sure you have the right step up springs in there, should be about half the idle vac and they should not bounce around at idle with the caps off. Good luck.

 

[Geoff 2]

This engine has a factory [ Carter ] AVS. They have different jets & rods to the Edel AVS. Carter AVS has taller pri jets & three step met rods, compared to Edel two step rods.

 

[prostockstan]

Thank you as well. I appreciate it.

Check your vacuum at idle in gear if automatic. Change the color coded step-up springs to the value 1/2 of what the vacuum reading is. The needles are dancing at cruising speed and causing erratic surging. You can flip open the needle covers and tighten them half open and see what the needles are doing. At idle in gear the needles should be all the way down. Use the strongest spring possible that will still allow the needles to stay all the way down at idle. The stronger springs will allow the needles to rise sooner. If cruising 55 and the needles go down to far the fuel will go lean causing detonation like surging. You must play around with the needles, jets, and step-up springs. You also have multiple holes for the accelerator pump. The hole closest to pump is richest the farthest hole from pump shaft is lean. I hope this info helps. I have lots of time with these carbs. When you finally get them tuned correctly they are great. It does take time and patients.

 

[Geoff 2]

There is no way of knowing if the surging is [a] mixture related or timing related from the info provided so far. Nor is it known if the surging is due to leanness is caused by the idle cct or the main cct, from info provided so far.
That is why post #19 should be the first step.

 

[Geoff 2]

I cannot believe some of this BS. The hole position in the acc pump arm has NOTHING to do with surging at cruise. The OP reports going deeper into the throttle from cruise there is no stumble, which means the acc pump is doing it's job. If leanness related, stronger met piston springs are not needed because there is no miss/stumble when opening the throttle & the met rods transition to the power step.

 

[ka0mek]

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!

or a fuel pump push rod. i had the same thing till i changed the pump push rod check yurs with a known good one or a new one

 

[4vnut]

I can confirm it was rebuilt with a slightly bigger cam from factory. Trying to find info on it for reference. Thanks guys.

Any progress?

 

[dvw]

I researched this a bit for a friends 70 Charger R/T. Since cruise is on the idle circuit My assumption was that the Idle cicuit was lean. Screws didn't have a large effect on idle quality. Since todays premium has 10% alcohol that reinforced my lean diagnosis. We ended up opening up the idle feed restrictions in the bottom of the primary clusters. That made it better. I also agree to much timing will cause the surge. We all add timing to our stuff. The vacumm advance may now give it to much overall advance at cruise. Depending on the distributor the advance can be tailored with a allen wrench thru the vacuum port of the advance canister.
Doug

 

[Dave6T4]

Doug beat me to it, but I was going to say timing, too. Try blocking off vacuum advance and see how it runs. With advance hooked up, you could be running as much as 50 degrees advance at cruise with good vacuum. Today's fuels have changed a lot since Chrysler mapped out ignition advance on our 50+ year old cars.

 

[RJRENTON]

I researched this a bit for a friends 70 Charger R/T. Since cruise is on the idle circuit My assumption was that the Idle cicuit was lean. Screws didn't have a large effect on idle quality. Since todays premium has 10% alcohol that reinforced my lean diagnosis. We ended up opening up the idle feed restrictions in the bottom of the primary clusters. That made it better. I also agree to much timing will cause the surge. We all add timing to our stuff. The vacumm advance may now give it to much overall advance at cruise. Depending on the distributor the advance can be tailored with a allen wrench thru the vacuum port of the advance canister.
Doug

"Depending on the distributor the advance can be tailored with a allen wrench thru the vacuum port of the advance canister...".
This is true IF/AND ONLY IF, the distributors vacuum advance assembly has this provision...not all do...
BOB RENTON

 

[lewtot184]

I researched this a bit for a friends 70 Charger R/T. Since cruise is on the idle circuit My assumption was that the Idle cicuit was lean. Screws didn't have a large effect on idle quality. Since todays premium has 10% alcohol that reinforced my lean diagnosis. We ended up opening up the idle feed restrictions in the bottom of the primary clusters. That made it better. I also agree to much timing will cause the surge. We all add timing to our stuff. The vacumm advance may now give it to much overall advance at cruise. Depending on the distributor the advance can be tailored with a allen wrench thru the vacuum port of the advance canister.
Doug

i went from the factory .031" idle jet to .037" and it made a big difference in the idle and low speed running.

 

[RJRENTON]

i went from the factory .031" idle jet to .037" and it made a big difference in the idle and low speed running.

The REAL issue is the fuel.....gasoline has a heating value or energy of roughly half that of gasoline ...therefore, the higher percentage of ethanol in the fuel mix, the richer the fuel mix must be increased.

i went from the factory .031" idle jet to .037" and it made a big difference in the idle and low speed running.

Thar would be very difficult as removing (making larger) by 0.006" diameter material from an orifice that is only 0.035" in diameter initially. The idle jet is part of the primary booster venturii assembly, as a swaged reduction of the end of of the brass tube and IMO, it would almost be impossible to accurately increase the size of the orifice. You would be slightly better off to REDUCE THE SIZE OF THE AIR BLEED ORIFICE FEEDING THE IDLE CIRCUIT ... not by adding more fuel but to reduce the air volume....to accomplish the same result. Not all fuel mixture issues deal with liquid but are in concurrence with an air change. Not saying what you did was wrong....just very difficult to accurately accomplish.....
BOB RENTON

 

[lewtot184]

The REAL issue is the fuel.....gasoline has a heating value or energy of roughly half that of gasoline ...therefore, the higher percentage of ethanol in the fuel mix, the richer the fuel mix must be increased.


Thar would be very difficult as removing (making larger) by 0.006" diameter material from an orifice that is only 0.035" in diameter initially. The idle jet is part of the primary booster venturii assembly, as a swaged reduction of the end of of the brass tube and IMO, it would almost be impossible to accurately increase the size of the orifice. You would be slightly better off to REDUCE THE SIZE OF THE AIR BLEED ORIFICE FEEDING THE IDLE CIRCUIT ... not by adding more fuel but to reduce the air volume....to accomplish the same result. Not all fuel mixture issues deal with liquid but are in concurrence with an air change. Not saying what you did was wrong....just very difficult to accurately accomplish.....
BOB RENTON

have you ever worked this before? .035" is pre-smog or smog manual trans carb. automatics are .031". working air bleeds, and especially "all" the bleeds on a carter will never give enough volume. reducing bleeds can give a stronger signal to the circuit but in the end it's a volume problem.

 

[RJRENTON]

have you ever worked this before? .035" is pre-smog or smog manual trans carb. automatics are .031". working air bleeds, and especially "all" the bleeds on a carter will never give enough volume. reducing bleeds can give a stronger signal to the circuit but in the end it's a volume problem.

Short answer is: yes. But to INCREASE the diameter of the idle fuel metering orifice from 0.031" diameter to 0.035" (or some other diameter +/- 0.001" determined quantitatively) can be done but how? Through the use of number drills (#61 - #80) or reaming or ??. Not "air bleeds" just the one that controls the VOLUME of air introduce to the idle fuel circuit of the booster venturii. By making the air bleed smaller, yes, the differential pressure signal will increase, allowing greater fuel flow.....thereby increasing the Air/Fuel ratio generated....Boyle's Law applies.....in proportion to cube of the flow. If you've solved your issue, congrats.....because the orifices involved are small, the utmost care and accuracy is necessary to accomplish the task.....
BOB RENTON