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OEM Carter AVS tuning issue
Posted By: SomeCarGuy
OEM Carter AVS tuning issue - 07/15/09 02:41 AM
I found a new Eddy AVS for sale but decided to try a 71 440 AVS on my 383 first since it was on the shelf. It replaced the hated Eddy 750 AFB. I had the AFB running OK but I think the AVS will be better.
I rebuilt the AVS I have this afternoon. It runs OK but is rich(more on that in a minute) and the real problem is that after it gets reved to about 3K, it idles about 1100 RPM. Curb idle holds about 750 where I want it, returns after blipping the throttle lightly.
It only has one port for the vac advance so I ASSUME that it is ported. The AFB idled fine after a rev fest. Timing is 10/34.
The engine is a stock 70 383 HP with a HSD intake and headers. Good chain.
Thr rich issue at idle maybe caused by the idle screws not screwing in the same since I had to tap one that was stripped. I noticed they aren't the same but they are all I have at the moment. That shouldn't matter on the idle speed after reving.
What might do something weird is the rods. I have a 630 AVS that I had to one from since this carb had a BENT rod.
The 630 even had a mismatched set so I ended up with one 575 rod and one 572 rod. Could that be a factor in the hanging idle?
Posted By: RapidRobert
Re: OEM Carter AVS tuning issue - 07/15/09 03:05 AM
Quote:
It only has one port for the vac advance so I ASSUME that it is ported.
The 630 even had a mismatched set Could that be a factor in the hanging idle?
(yes ported) (2) no. check that it's not catching on the fast idle cam steps. that the throttle blades ain't hanging up in the bores and that you have a strong enough return spring.
Posted By: SomeCarGuy
Re: OEM Carter AVS tuning issue - 07/15/09 03:12 AM
I looked at the fast idle and it isn't hitting. I have the choke wired open.
Plenty of return spring. Two actually.
THe throttle bores sticking could be a winner.
Posted By: SomeCarGuy
Re: OEM Carter AVS tuning issue - 07/15/09 03:15 AM
I'll have to get a vaccum gauge to get the idle mix correct. It has kind of a rough idle compared to the AFB.
Any good ideas on where to get some matched rods? I guess I could go to the two step rods but I bet the 3 step will be worth the effort and will be fine if both are matched.
Posted By: SomeCarGuy
Re: OEM Carter AVS tuning issue - 07/15/09 03:21 AM
I found this interesting on a search-
"Check to see if you have the "hot idle compensator valve" on your carb., its basically a bi-metal strip that when open, allows a passage to the manifold vacuum. Most hot-rodders (in my day), fashioned a block off plate to eliminate this really crude emissions device. I remember that being a common problem."
Could this be giving me trouble? Either for the high idle on return or the general rough idle? I know I have issues that will cause the idle to be rough currently but is this something to invesigate? I know a compensator isn't in there.
Posted By: RapidRobert
Re: OEM Carter AVS tuning issue - 07/15/09 03:32 AM
AVS strip kits are extremely rare nowadays & I think I just saw a parts AVS for cheap in one of the parts for sale sections which'd get you some rods. they might not be 100% correct but they would be matched but with that being said the AVS was an emission carb of that era of gov't regs that were starting to get severe(& the carb tech had not caught up and the AVS with its 3 step rods w the intermediate step are inferior to the earlier AFB(w 2 step rods)for performance so I would get an eddy strip kit & as you know the flat covers. I would take out the idle screws & straighten that out and anything else that's off especially w the rods getting bent by a prior owner
Posted By: SomeCarGuy
Re: OEM Carter AVS tuning issue - 07/15/09 03:42 AM
I thought that the 3 step rods were better for getting a precise mix since it has another stage?
So I would be better serving to run the AFB parts then and not fool with getting AVS rods?
Posted By: Jim_Lusk
Re: OEM Carter AVS tuning issue - 07/15/09 03:46 AM
Before you do anything else pull the throttle cable off the carb and try it with just the return spring on.
Posted By: RapidRobert
Re: OEM Carter AVS tuning issue - 07/15/09 03:46 AM
Quote:
I know a compensator isn't in there.
It has the compensator recess but no bimetallic strip to open the air bleed so it is open all the time? If so I'd epoxy it(the extra air bleed hole) shut.
Posted By: SomeCarGuy
Re: OEM Carter AVS tuning issue - 07/15/09 03:50 AM
I'll go out now and pull the cable. It was in the same spot as the old carb needed it so??? But worth eliminating.
I honestly didn't pay any attention to the compensator area other than it didn't have one. If it were bleeding fully all the time I would think I wouldn't have such a rich idle.
Or would that cause the high idle right there?
Posted By: RapidRobert
Re: OEM Carter AVS tuning issue - 07/15/09 04:03 AM
Not sure but it is a controlled vac leak(which can raise the idle speed)(though a small one) but I think other issues going on especially with you being able to blip the throttle & it straightens out and did you say it was part time which(in this case) point to a mechanical hangup.
Posted By: SomeCarGuy
Re: OEM Carter AVS tuning issue - 07/15/09 04:12 AM
Quote:
Not sure but it is a controlled vac leak(which can raise the idle speed)(though a small one) but I think other issues going on especially with you being able to blip the throttle & it straightens out and did you say it was part time which(in this case) point to a mechanical hangup.
It ran fine in the garage, so i took it down the road. Noticed the hangin idle and came home. Tried to pinpoint and figured out it was around 3K that it caused it not to idle right. A few blips and it clears.
Posted By: SomeCarGuy
Re: OEM Carter AVS tuning issue - 07/15/09 04:18 AM
I went out to take the cable off and test and thought better about waking up the world.
So I looked around for my AFB stip kit to see what rods and jets I still had. Getting discouraged, I made a discovery that is hard to believe.
Hidden in a box on the top shelf was ANOTHER 71 440 AVS! Praise the lord! Not only is it the same 4966 carb, it appear to have never been touched- ever. It still has the blue carb tag on it, still had the factory air cleaner stud. Dirt and grime appear consistant all around.
I sprayed the idle screws down and carefully worked them back and forth- move pretty well. They are dead even sticking through the bore.
I pulled a rod and it is a 572 like the other carb. SO I guess tommorrow I will put the new accel pump in it and see what we get.
One other note- the hot idle compensator is still in this one. I'll give it a whirl as is before I go and lead over anything.
Posted By: JohnRR
Re: OEM Carter AVS tuning issue - 07/15/09 12:56 PM
Quote:
I went out to take the cable off and test and thought better about waking up the world.
So I looked around for my AFB stip kit to see what rods and jets I still had. Getting discouraged, I made a discovery that is hard to believe.
Hidden in a box on the top shelf was ANOTHER 71 440 AVS! Praise the lord! Not only is it the same 4966 carb, it appear to have never been touched- ever. It still has the blue carb tag on it, still had the factory air cleaner stud. Dirt and grime appear consistant all around.
I sprayed the idle screws down and carefully worked them back and forth- move pretty well. They are dead even sticking through the bore.
I pulled a rod and it is a 572 like the other carb. SO I guess tommorrow I will put the new accel pump in it and see what we get.
One other note- the hot idle compensator is still in this one. I'll give it a whirl as is before I go and lead over anything.
FYI the 4966 is a 650 carb , not a 750 , it's application is 71 non HP 440 and it will be RICH on your 383 since it is calibrated on the rich side for the 440 it came off of. The factory calibration was a compromise to save on warranty issues.
Posted By: SomeCarGuy
Re: OEM Carter AVS tuning issue - 07/15/09 02:45 PM
That's right, I do remember looking that up awhile back.
Posted By: JohnRR
Re: OEM Carter AVS tuning issue - 07/15/09 03:09 PM
Quote:
That's right, I do remember looking that up awhile back.
I installed that same carb on a 330HP 383 and it did run very well for the few times I did drive the car .
Posted By: SomeCarGuy
Re: OEM Carter AVS tuning issue - 07/15/09 03:11 PM
I bet it will do what I want- smooth performance all around. THe 750 Eddy just doesn't do that.
I noticed the throttle response seemed better with the cobbled up avs last night.
Posted By: SomeCarGuy
Re: OEM Carter AVS tuning issue - 07/16/09 01:36 AM
OK, after much fiddling today, I got the untouched carb on there and working. It had a few issues but i worked them out.
Right now I have two problems with it. One is the idle seems erratic. I smoothed that out with adjusting the mixture but it still seems rough compared to the Eddy 750 AFB.
Also, I feel a slight lean surge at part throttle.
I saw a comment online while searching today that a drop base air cleaner will cause the carb to idle strange as it outs the lid too close to the idle bleeds. Might this be my issue? I could drive a little with it off and check I suppose.
So far it has been less smooth than the Eddy was and that was why I wanted to change it out
Posted By: RapidRobert
Re: OEM Carter AVS tuning issue - 07/16/09 02:59 AM
Quote:
One is the idle seems erratic.
Also, I feel a slight lean surge at part throttle. that a drop base air cleaner will cause the carb to idle strange as it outs the lid too close to the idle bleeds. Might this be my issue? I could drive a little with it off and check I suppose.
(1) you might take out the idle mixture screws(count the turns) & blast a 1 second shot of starting fluid into each hole then reinstall the screws. (2) I was going to suggest a rod w a thinner "cruise" step then I remembered you have no other rods/jets to play with (3) with the lid off you'd find out in several seconds. Good luck on it.
Posted By: SomeCarGuy
Re: OEM Carter AVS tuning issue - 07/16/09 03:10 AM
I went for a longer cruise to see if maybe it would clean up a bit. I noticed that there seems to be a lot of popping in the exhaust now. Not loud but the engine definetly isn't happy. Lots of popping sounds on decel. Would seem to be lots of unburned fuel igniting in there.
That seems out of wack as it is surging. I was thinking a K&N might lean it out but then that wouldn't help a surge. Maybe dirt is in something???
The air valve isn't spot on as it doesn't seem to be opening up quick enough. Oddly, I punched it from a 25 MPH roll in second and it bogged like they flew open too quick. Maybe a pump shot issue? I had to take it back apart and I cleaned the accelerator pump circuit and nothing came out. I decided to use the shooter from the other 4966 and it worked. Before i wasn't getting any shot unless i went half throttle or more. I checked the setting and it is spot on per the rebuild kit papers. I could change the rod hole and get a bigger shot by going to the top hole from the middle hole.
Posted By: RapidRobert
Re: OEM Carter AVS tuning issue - 07/16/09 03:25 AM
how is it doing so far
Posted By: SomeCarGuy
Re: OEM Carter AVS tuning issue - 07/16/09 03:55 AM
I took the idle screws out and sprayed in the starting fluid. I couldn't really tell if it made any difference. I held my hand over the choke area nd if I got a good seal it raised the RPM so I went from 2 turns out to 2 1/2. Better but same test= same results. I ended up at 3 1/2 turns.
That would seem to indicate a vaccum leak. The idle now smells rich but it holds steady and returns well. I took a spin and it flet much better.
I forgot to tune the secondaries
, that might make a differnce at speed but the idle mixture made it much better overall.
I was able to turn the idle speed screw quite a bit after richening the mix, now holding pretty solid at 850.
I still seem to have the off idle stumble some. I think there is something in that circuit as it isn't a constant problem. Will almost bog enough to die in first sometimes.
Posted By: RapidRobert
Re: OEM Carter AVS tuning issue - 07/16/09 04:16 AM
Quote:
I still seem to have the off idle stumble some. I think there is something in that circuit as it isn't a constant problem. Will almost bog enough to die in first sometimes.
Progress. I'm assuming the floats are correct and can you go in a bit on the mixture screws and when you touch the throttle is there an immediate shot of gas and check that the secondary door isn't too loose(I dont know what the baseline setting for them is) but iirc you turn it until it is closed then a # of turns further.
Posted By: SomeCarGuy
Re: OEM Carter AVS tuning issue - 07/16/09 04:29 AM
Yes, I checked the level and drop both today.
I have gas but the stumble is intermittent so...
Earlier today I had no gas so what I did when I broke it back down was an improvement. Has the shooter from the other carb in it now.
The air door setting is 2 turns after slack is taken up. That is for all AVS carbs. Looks like that won't work here and really I think that is the baseline and every engine wuld need some tuning from there.
I think 3 turns would be worth trying on the idle mix. 3.5 seems like a lot for a mostly stock engine.
Posted By: RapidRobert
Re: OEM Carter AVS tuning issue - 07/16/09 04:51 AM
You might tighten up the air door several more turns just to take the secondaries completely out of the picture as this(appears) to be a pri system problem or even removing the secondary link so the secondary throttles dont even open(kind of brainstorming on that) but anything to eliminate/narrow down the problem but keepin it simple I would lean the idle mix screws a bit & mainly concentrate on the AP system especially after what you did so far made a dramatic improvement. I'd check for immediate tip in, that the plunger lever is in the hole furthest from the fulcrum then readjust the plunger to max height & possible enlarging the shooter nozzles
Posted By: SomeCarGuy
Re: OEM Carter AVS tuning issue - 07/16/09 04:58 AM
Here's the rub-
It would havemade much more sense to just swap rods, fix idle screw adjustments and then deal with the hanging idle on the other 4966. I can't believe I messed that opportunity up.
That carb shot gas out w/o issue at all. So enlarging anything on this one seems out of wack. In fact the shooter from the other carb is in it so that leaves the circuit in the body messed up somehow.
Posted By: RapidRobert
Re: OEM Carter AVS tuning issue - 07/16/09 05:07 AM
yeah but didn't you say that the idle mix threads on the other one were cross threaded or something (it sounded bad
)
Posted By: SomeCarGuy
Re: OEM Carter AVS tuning issue - 07/16/09 05:24 AM
It was stripped and I retapped it. The screw went in OK. I bet you could baseline it with the carb off and eyeball or in some way measure the tip of the screw sticking out.
The hot idle compensator is missing from that one, could be the hanging idle problem.
I also have a 69 383 stick trans AVS also but that one has the tip of a brass idle mix screw broken off in it.
Hopefully I can get the pump shot right on this carb. I think the air door thing will be no problem at all.
Posted By: JohnRR
Re: OEM Carter AVS tuning issue - 07/16/09 12:24 PM
The idle hang on the other carb could be worn primary shaft in the carb body causing the misalignment , this is why 40 year old carbs belong on show poodles that get pushed on and off trailers .
[swingin/72] Just buy a Holley 750 DP and be done with it [/swingin/72]
Posted By: SomeCarGuy
Re: OEM Carter AVS tuning issue - 07/16/09 02:43 PM
I'll take donations so you all don't have to endure this anymore
Posted By: SomeCarGuy
Re: OEM Carter AVS tuning issue - 07/16/09 06:06 PM
I tuned the air door and that seems to have come around. Slight bog then takes off well.
I tend to think the bog might be from that missing pump shot. I'm going to try the lever in the top hole and see if that generates enough shot to work. i noticed that the arm was in the top hole when i took the carb apart yesterday. The middle hole should be all that is needed but since this isn't working...
I turned the idle mix in 1/2 a turn and reset the speed. Seems OK. An interesting thing is that the cam now sounds much more choppy. The Eddy idled so smooth you would almost not hear any of te overlap.
Engine still isn't as smooth as the Eddy carb had it at cruise but is not as bad as it was.
Posted By: SomeCarGuy
Re: OEM Carter AVS tuning issue - 07/16/09 10:10 PM
I think I have the secondaries about as close as they will get to perfect.
The top hole adjustment helped the stumble but I still feel it. When i flushed it nothing came out of the tunnel and I sprayed from both the accel pump side and the shooter side.
Not sure what to do about that now. I could drill one of the three shooter i have but I don't think it is going to help since you still need fuel there to come out.
Maybe try another accel pump???
Posted By: hooziewhatsit
Re: OEM Carter AVS tuning issue - 07/16/09 10:55 PM
In the troubleshooting section on edelbrocks site, they mention that an off-idle stumble is usually caused by the idle mixture screws being too rich, or too lean.
Posted By: SomeCarGuy
Re: OEM Carter AVS tuning issue - 07/17/09 01:13 AM
I really think it is that I'm not getting any shot immediately for some reason. I've had the stumble with the idle so rich it smelled rich and with the idle so lean it barely ran.
Posted By: RapidRobert
Re: OEM Carter AVS tuning issue - 07/17/09 01:44 AM
I'd continue to search for why there's no squirt on tip in, maybe check the brass weight under the shooter and the gasket for starters. Since you have several shooters to play with you might drill one a few thou larger since that is the restriction AFTER tip in is reestablished
Posted By: SomeCarGuy
Re: OEM Carter AVS tuning issue - 07/17/09 02:34 AM
I don't think there was a brass weight under the shooter. New gasket.
Posted By: RapidRobert
Re: OEM Carter AVS tuning issue - 07/17/09 02:39 AM
Quote:
I don't think there was a brass weight under the shooter. New gasket.
We've got it 
EDIT grab the weight out of the other one
Posted By: SomeCarGuy
Re: OEM Carter AVS tuning issue - 07/17/09 04:54 AM
I went out and looked. Sure enough there is a needle that was in the other carb. When I took it out, it looked like I remember the one on the car looking.
I looked all over the bench and in the pan that I cleaned and didn't find a check ball or a needle. The exploded view said you can have either one.
Since the pump shot was in the top hole when i dug into it, I bet that was a mask for the missing part from a prior rebuild. It is better with the bigger shot.
If this makes the one on the car as responsive as the mucked up one, we will have a pretty good piece. Thanks Robert.
Posted By: SomeCarGuy
Re: OEM Carter AVS tuning issue - 07/17/09 05:24 PM
Installed the needle in the accel pump circuit-
Noticed it still had a stumble off idle at first. Seemed to get better after running awhile but it was nearly fully warmed when i took off from the house so...?
Car is running smoothly down the road.
WOT doesn't have the bog the 750 Eddy AFB had BUT it doesn't pull near as hard either. I checked the WOT adjustment and with some weight on the pedal it is about 1/16th short of full open. I would like to check with a helper tonight and see what I think, the weights are hard to wedge and I don't have the old 2x4 I used to use as a dead man anymore.
Drone seems less with this carb. Maybe harmonics changed with the smaller butterfly's.
Idle sounds wicked now. I don't understand why the Carter does this and the Eddy was smooth. Anybody have any ideas why that would be?
I will be testing going back to the top hole for a bigger pump shot. Also will try a K&N filter in the current breather(open) and then with a K&N lid. Might be an improvement if the carb doesn't like the drop base putting a restriction close to it with the solid lid. Will also try different idle mixes, hopefully I will find my vac gauge.
This 4966 doesn't feel as crisp as the other one I tried that had the boogered up stuff. It had the one off size rod, makes me think this one could use some tuning but it is hard to go off a cobble carb as a guide. It seemed to really jump on throttle movement more than this 4966 does.
Thanks for all the help!
Posted By: SomeCarGuy
Re: OEM Carter AVS tuning issue - 07/18/09 01:17 AM
I checked the WOT when a helper arrived and found that I can't get it any closer. It is about 1/16th off. Not the end of the world.
I got out the K&N lid and found out that I had to run a K&N main filter because they made it so it won't fit down in a stocker. I wanted to run ONLY the lid to see if that made any difference.
The engine then idled roughly and was at only about 650 RPM, not the 850 or so it had held steady before. I had to turn the speed screw up, but wrote that off on something to do with messing around with the throttle cable. Doesn't make any sense, but that is all i came up with on that.
The drive showed a worse running engine, so i returned home to reinstall the old lid and paper filter. No dice, the car now runs crappy. I dorve around with the "good" setup awhile and concluded that it runs like it did before the accel pump fixes today.
So, the air is cooler tonight. Like last night. When it ran good today, the water temp was 170-180. Tonight it ran 160-170. Maybe the more dense air makes it too lean???
Posted By: RapidRobert
Re: OEM Carter AVS tuning issue - 07/18/09 02:04 AM
Quote:
WOT doesn't have the bog the 750 Eddy AFB had BUT it doesn't pull near as hard either.
It doesn't have near the CFM that the eddy does, is 40 years behind it techwise and was never designed for good WOT performance. I am keeping up on your posts & if I think of anything substantial that will might be of benefit I will holler. Stay on it!
Posted By: SomeCarGuy
Re: OEM Carter AVS tuning issue - 07/18/09 03:35 AM
The tech really isn't all that different, the CFM is for sure. After I had it all dialed in right today, it was pulling pretty good. And with no bog it would likely be a close race. I think having a carb that runs w/o a bog is better overall since you won't have that letdown in sensation.
I went out and found an old FSM and it says subnormal engine heat will cause a lean condition. So that would support the theory that the cooler air has something to do with the big drop in performance.
So that leaves two things I can do-
1. Richen up the carb.
I could swap to AFB Eddy jets/rods if I can find my strip kit.
I could drill out one of the two other sets of jets. That would require buying a small drill set, maybe a pin vise.
2. Change to a hotter stat. I looked and i have a 180 Robertshaw and a 190 that looks to be a Robertshaw that isn't stamped by them. Definitly a high flow unit. I have had these on the shelf for years. I also have a gasket.
Problem with option 2 is that I really like that my engine runs as cool as it does. I know that I might be seeing some increased wear, but the comfort in knowing that gauge has never, ever went past 180 makes for some serious piece of mind when driving. I bet that it would hold whatever temp the stat I would put in but still.
I'm headed out of town for a few days so I will have time to weight it out. I really need now to drive it on another hot day to get it up to 180 again and see if that will cure it. i sure don't want to change stuff on a goose chase. I just can't see any reason why it would be anything else.
One long shot could be the plugs got fouled when i was checking the WOT. I drove around quite a bit and beat on it so I think that would rule that out.????
Posted By: SomeCarGuy
Re: OEM Carter AVS tuning issue - 07/18/09 03:45 AM
A few other notes-
Could the Eddy have had the bog from being too cold as well?
The AVS doesn't start as fast as the Eddy does. The Eddy usually was running so fast that you couldn't hear the starter. Really. The AVS starts right off but i would say it takes a full second longer. That isn't a problem but I found that interesting.
I've been running a 3/8 "rebuild kit" spacer under both of them sandwiched between two gaskets. The Eddy seemed to like it and I can't run anything that is thicker. Never had a hot start problem with either carb. Eddy was fine hot starting w/o the spacer. Never tried the AVS w/o it.
This thread is the third thing to pop up on a Google search on AVS tuning.
Posted By: SomeCarGuy
Re: OEM Carter AVS tuning issue - 08/04/09 03:50 AM
Update-
The 190 stat helped but still not got it right.
The accel pump went bad, based on popping on tip in occasionally.
Put new pump in and checked the pump shot adjustment and figured out that I hadn't backed the idle screw off the last time. The adjustment was off.
I took an arm off a parts carb and bent it to work. I have the other to use as a baseline if needed for some reason.
Still not running right. Got back home and looked at the shooter yet again. Noticed this time that every so many pumps results in the fuel coming out from under the gasket. Guess I will put another new one in.
Hope this gets it. Getting sick of taking the carb off and can't take the hurt back to lean over the fender. Probably bend it if I did anyway.
Posted By: SomeCarGuy
Re: OEM Carter AVS tuning issue - 08/04/09 11:59 PM
I pulled the carb again and changed the shooter gasket.
Still have fuel seeming to spill out from under the shooter.
I doubled up the old gasket with the new and still had the same problem. So I took a light and closely watched as I ran the pump. The fuel I'm seeing is actually a "drip edge" like effect where it is curling back up the bottom of the shooter nozzle and then spilling out on the bore of the carb.
So is this normal? I'm about at my wits end on this one. If I solve the off idle stumble I should have a pretty decent carb all around.
Maybe I need to drill out one of the parts shooters? Maybe fill one and drill a smaller hole to cause it to shoot out with more PSI, but that maybe in turn cause a volume issue.
Posted By: Rug_Trucker
Re: OEM Carter AVS tuning issue - 08/05/09 01:05 PM
850 seems like a really high idle for a stock 383 Magnum.
I like AFB's and AVS's. I am planning on rebuilding a 340 Buick AFB for a slant.
Posted By: SomeCarGuy
Re: OEM Carter AVS tuning issue - 08/06/09 07:19 PM
I went with a shooter out of the 69 383 AVS. Really no change. This is a hit or miss problem. If you give it enough gas taking off you are fine, still even with the new pump it pops on hard throttle sometimes.
The idle is more like 750 right now. No change. At 800 it is still on the idle circuit. One chart i had called for it to idle that high anyway with a stick trans. Not sure it really matters.
I can still go with the top hole "big shot" and see if that cures it. Could try a pump with the neoprene(sp) seal instead of the leather.
This has been both a fun and frustrating deal. The carb works 90% of what I would want.