Eddy AVS idle tuning question.

[XCELLR8]

Have a Eddy AVS 800cfm carb on my stock smogger 77 400. The bottom end is stock with stealth heads, comp thumper cam, performer intake, headers, and AVS carb. I have been fighting this carb's idle on this set up for the 8 years since. It idles if I use the transition slot, but wants to die in gear and when I come to a stop. If I get off the transition slot like it's designed, it wont idle well, if at all. In researching the issue I have too questions. 1. Has anyone drilled holes in the butterflies like I'm reading about? 2. How many turns out on the idle mixture screws is reasonable? The Eddy book states that factory is 2 1/2 turns, but in reading I'm seeing up to 7 to 8 turns. Some do both drilling and lots of turns. I hate to drill unless its needed. I know the cam is the problem but its installed and I see many with bigger cams on here. Thanks

 

[Snook]

If you don't want to swap the cam, go with a looser converter. My 2 cents.

Chuck (snook)

 

[R/T Boy 67]

What is your initial timing set at? With low compression and a bigger cam you need 15- 20 degrees. Make sure there are no vacuum leaks. There is no magic number of turns on the idle screws that will work for all engines, give it what it wants.

 

[Rishi]

If it idles in neutral, but not in gear, things to measure and test are idle speed, idle mixture, engine timing, and vacuum at idle.

I've never needed to drill holes in the secondary butterflies.

I always thought the whole "counting turns of the idle mixture screws" was about a STARTING position for these screws, when the idle mixture is unset, unknown or changed by a carburetor overhaul or engine swap.

With emphasis on "starting position" we then alter and tune the screws according to procedure. The idle mixture, along with the idle speed, MUST be set and fine tuned on the running engine, unless you're very lucky.

So... any instruction telling you where to place those screws can only be an approximation, good enough to start the engine and begin testing. But maybe not set perfectly.

 

[493 Mike]

I've had to drill primary throttle plates to get enough air flow to use the idle circuit. Sometimes the idle restriction needs to be opened up a little also (richen the idle mixture).
Mike

 

[XCELLR8]

Timing is set at 26* (little higher than I thought it was set at.). The 2 1/2 turns on the idle set screws was what Edelbrock states is the factory (get you started) setting. I was shocked at reading 7/8 turns. I was asking if it was normal, so I will be turning the screws out.

493 MIke - How big of holes did you have to drill? This is something I do not want to do, if I can get the carbs working.

 

[oldkimmer]

Have u ever changed the metering rods or the step up springs? Where is your base timing at? Kim

 

[Bee1971]

Plus

Eight years with this issue you said

Was the carb brand new when you bought it or used

Have you opened up the carb and checked float levels , what jetting is inside the carb now for primary and secondary

Cleaned the idle circuit

Plenty of Edelbrock Carb issues brand new out of the box , whether it’s the primary circuit or secondary circuit

 

[493 Mike]

Timing is set at 26* (little higher than I thought it was set at.). The 2 1/2 turns on the idle set screws was what Edelbrock states is the factory (get you started) setting. I was shocked at reading 7/8 turns. I was asking if it was normal, so I will be turning the screws out.

493 MIke - How big of holes did you have to drill? This is something I do not want to do, if I can get the carbs working.

Start with a 1/16" hole in each primary throttle plate. Place them in the same location.
Mike

 

[66 Sat]

7 to 8 turns and the screws would nearly be out of the carb surely...

I wouldn't be drilling any holes yet either, as you can't put the metal back in there easily.

As others have said, timing, vacuum leaks, other carb issues first.

 

[Orig69]

Some pictures please.

 

[Geoff 2]

- Low compression & Comp Crapper cams are your main problem.
- I understand you do not want to change cams.
- There is no right or wrong number of turns on the mixture screws.
- With the engine idling & in gear, remove the PCV valve. I will bet it is nmot seated &/or dancing around.

The starting point to fix this is idle ign timing.

Warm up engine, put in gear if auto, & engine idling, SLOWLY advance the dist until you get the highest rpm reading; just do it by ear. Then check & see what the timing is. Wouldn't surprise me if it was 40* with your combo. [ see below ]. Do NOT touch the carb for now. Report back & we can take the next step.

 

[XCELLR8]

- Low compression & Comp Crapper cams are your main problem.
- I understand you do not want to change cams.
- There is no right or wrong number of turns on the mixture screws.
- With the engine idling & in gear, remove the PCV valve. I will bet it is nmot seated &/or dancing around.

The starting point to fix this is idle ign timing.

Warm up engine, put in gear if auto, & engine idling, SLOWLY advance the dist until you get the highest rpm reading; just do it by ear. Then check & see what the timing is. Wouldn't surprise me if it was 40* with your combo. [ see below ]. Do NOT touch the carb for now. Report back & we can take the next step.

View attachment 1707828

To answer some of the questions. I bought this carb new, and has been cleaned and rebuilt due to car sitting a few years. The jets are stock and metering rod are 2 steps richer, this was done when I was idling and driving on the transfer slot in the carb. I changed the metering springs per Geoff2's reply in another thread, now the rods stay seated. Timing is at 26* at 850 rpm. I worked on the carb last night and it will idle with the choke on, but as soon as the choke opens up it dies. I turned the idle screws out to 8 turns doing 1/2 turn at a time. No change. I'm trying to get this motor to idle off of the idle circuit, so the jets and rods should not affect anything. Am I correct in thinking this way?

Geoff2 - Thanks for posting the article. I was looking for it, as I had seen it in the past. That is the next step to try.

 

[Geoff 2]

Xcell,
Unfortunately the parts combination is veeeeeery poor. The 'performance' can be improved but it will take time & effort & maybe some $$$$.
The main culprit is the cam. The 383/400 Ch engine has the highest rod/stroke ratio of all the popular BB engines. In laymans terms, they do not tolerate cams with a lot of overlap; they lose low end tq & idle rough. This is the reason [ below ] for the warning from Erson cams. The R/S ratio of the 400 is even higher than the 440, making the problem worse.
Also, the 800 cfm is a bad choice. A 650 AVS would have been better & more responsive in the low/mid range.
If it was my engine, I would replace the cam with an Isky 264 Mega. The low comp ratio is the big killer here & limiting choices. Fine tuning of the ign & carb will still be needed, but a lot easier.

 

[Geoff 2]

If you decide to keep the existing cam, then the starting procedure is as outlined in post #12. Engine will need a LOT of timing at idle & you do that with manifold connected vac adv. See below

 

[XCELLR8]

Xcell,
Unfortunately the parts combination is veeeeeery poor. The 'performance' can be improved but it will take time & effort & maybe some $$$$.
The main culprit is the cam. The 383/400 Ch engine has the highest rod/stroke ratio of all the popular BB engines. In laymans terms, they do not tolerate cams with a lot of overlap; they lose low end tq & idle rough. This is the reason [ below ] for the warning from Erson cams. The R/S ratio of the 400 is even higher than the 440, making the problem worse.
Also, the 800 cfm is a bad choice. A 650 AVS would have been better & more responsive in the low/mid range.
If it was my engine, I would replace the cam with an Isky 264 Mega. The low comp ratio is the big killer here & limiting choices. Fine tuning of the ign & carb will still be needed, but a lot easier.

View attachment 1708289

Again, thank you for the articles. The cam choice was because I was looking at the Hughes Whiplash cam which was suppose to help a low C/R engine. At the time, Hughes cam was out of stock and the Thumper cam had almost the same specs ( more lift on the whiplash). So that is why I went with it. The stealth heads were bought because they are 80cc closed chamber heads (that look stock) compared to the ~90 CC 452s. Also at the time 915 heads were going from 750 to 1,000 a set, then need rebuilt. So why not go with the lighter head. The 800 cfm Edelbrock was bought because I had seen where the Thermoquad was 830 CFM, I have put a 600 Holly on it and there was no change. The Mopar distributor and performer intake was off of a 400 I bought and had machined for a 451 stroker motor. I am going to work on the timing to see what it does. Future plans are, I am going to finish the stroker motor or buy KB240 pistons to up the C/R in the current 400. I'm just trying to get this car drivable for the remainder of the summer/fall. Please note that all this was done in 2012 and a lot has changed since then.

 

[Geoff 2]

The Crapper cams, the Whiptrash cams, the Bootcrapper cams.....they all work on the same 'principle': lots of overlap, created by excessive exh duration & tight LSA. The exh valve opens early & you get as very noisy cackling exh note to attract the girls. The rough idle, poor low speed tq.....well you just have to live with it...

 

[XCELLR8]

I agree with the cam not turning out like I wanted. A cam change will be done regardless what motor I do, as I am going to a roller cam.
I did work on the car yesterday and messed with the timing until my timing light quit working. I put the idle mixture back to the Eddy starting point of 2 1/2 turns out and adjusted the idle speed screw to get 800 rpm at 26* with vacuum advance unhooked. I advanced the timing and the idle increased to 1050 rpm taking little steps at a time. I ended up at 42* at idle and the motor never sounded this smooth. When I hooked the vacuum advance up to manifold vacuum it would go to 58*. At this setting the car still would flat out die when putting it in gear. When trying to start the car it was having a hard time turning over and my battery went dead. So battery is on the charger and I'm looking for a good new timing light.

 

[Gary Gaz]

If it dies when putting into gear you will need to have a higher idle speed. so end up up with about 850-950 once in gear, everyone else is right get the timing sorted before you start on the Carb

 

[Geoff 2]

So it sounds like 42* is the magic number for idle timing for this engine [ see below ].
Sooooo, with this newfound knowledge, I would do this:
- set initial static to 20* [ not 26 ].
- use an adj vac adv unit & connect it to a manifold vac port. [ Buy a new dist if needed with an adj VA unit ]
- adjust the Allen Key [ AK ] in the VA unit fully CW.
- start engine & check/set static timing & then connect VA; between 40 & 44 is ok [ static + VA ]. If more than 44*, you need to make a stop or limiter for the VA actuator arm to limit travel to 22-24*.
- make sure timing is steady with VA connected.
- come back after this is done for the next step.