Hoping to pick a brain or 2. 79 F250. 6.6L. Mods done: bored .060 over. Racetec pistons at roughly 10:1. Howard’s rattler hyd roller cam installed at 0. Edelbrock intake and AVS2 carb at 650CFM on trickflow power port heads. I’m set at one step leaner per the tuning chart with yellow step up springs in the carb. I’m having trouble getting a good initial timing. I’m currently at 18degrees but just can’t seem to find the sweet spot. Also have a little dieseling occasionally at shutdown. Idle set at 700 rpm. Idle screws out 1 1/4 as it seems happy there. I know several of you guys have similar setups. What’s you find to be a happy initial timing? TIA!

 

18 degrees initial timing is gonna put you at 38 total. With those cylinder heads, and 10:1 compression, that's probably too much advance. That also probably explains the dieseling. Do you have an Air/Fuel Ratio gauge? With that kind of compression, pre-detonation is dangerous, and you should monitor your AFR.

Are you running a vacuum advance distributor? If so, where do you have the vacuum line run to? Did you disconnect the vacuum line and plug the port during timing? Your PCV routing can effect idle quality too. How is it set up?

 

thank you for the reply. I don’t have an AFR gauge. I’ve been contemplating that as well as a vacuum gauge. I’m using ported vacuum in the carb. PVC is going through a pvc valve on just the right side rocker cover with the other side plugged off. The static CR is a little of 10:1. Maybe 9.5. I’d have to dig the paperwork back up to remember for sure. I haven’t heard a single ping even under hard acceleration. I’m running an HEI style distributor with vacuum advance

 

Way to much advance for 62 CC chamber heads on a 414 CID motor. With that set up you should be in the 10-12° at most initial. Also the idle mix should be set with a vacuum gauge, adjust for max intake vacuum number of turns in/out is irrelevant.
I think if you dial the initial timing back and properly adjust the idle mix you find it will run much better.

 

these are the 72cc chambers to try to avoid over compression

 

Better, but still too much initial advance.. still stick to the 10-12°

 

You'll thank yourself for installing AFR and vacuum gauges. I have both, side by side. Using both gauges makes tuning a carburetor a breeze. Proper air/fuel ratio is important in any engine, and downright critical for 10:1 engine.

 

You need to filter incoming air for the PCV to work properly. Right now all you're doing is creating a higher vacuum inside the crankcase (not a bad thing in and of itself, but not right in this situation) and potentially sucking dirt inside through whatever orifice the vacuum finds to relieve itself.
Either run a hose up to the air cleaner from the opposite valve cover, or add a simple filtered vent.

With the HEI maybe the total advance is not that high. Or you're running richer than you'd like to be. Those could be reasons you don't hear pinging yet. But I'm with the others that you should maybe keep it a little lower initially until the whole system gets balanced.
Part of the running rich thing could be the lack of a fully functioning PCV? Not sure how much effect that would make, but it's possible.

You might want to run through the rpm while watching the timing marks so you know what your distributor's timing curve is.
And double check the vacuum (or lack thereof) at the timed port. If for example the cam dictates a slightly wider carb opening to get that 700 rpm idle, you might actually have vacuum there. Does not sound like your issue, but it's still worth verifying if you have not already.
Besides, lowering the timing a little may get rid of your dieseling and that's one distinct goal right now. You didn't say what exactly it is you're using as your criteria for getting the timing right. Just said you can't get it right?
What's it actually doing that you're not seeing? Maybe you're trying to get it to ping, so you can find it's limit then back off? Not sure what is causing that lack yet, but I would not use initial timing as your entire criteria for finding the pinging point. Other things at work, and the dieseling is telling you something else is going on.
Your idle does not seem high, but this is exactly why they used to use the "anti-dieseling" solenoid on the carburetor linkage. Use it to set the curb idle, then when the key is turned off the throttle blades shut down even further and reduce the instance of run-on.
Don't think I've seen one worked out on an AVS carb, but it's possible I'm sure.

Paul

 

Timing pretty much needs to be wherever the engine says it needs to be. there is no exact number you can use.

As a general rule, engines like yours would be 10-15 initial, 30-38 mechanical all in, 45-55 mechanical all in and vacuum advance hooked up. if you're in that range you should be able to dial it in by ear peaking it out. if your much out of that range you probably have some work to do on your curve to even play ball.

 

thanks Paul. I’m running the pcv through a typical Pintle valve in the right rocker cover and the other side is simply blocked off as to not allow a leak or contamination.

im trying to run as much advance as I can without getting any pinging. I’m not actively searching for a ping point. Over all, she runs pretty good. I advanced the timing slightly and the idle rpm came up so I was able to back the butterfly’s down and I think I cured the dieseling issue. I do think there’s an ARF setup in its future to help me get her dialed in better. Still richer than I’d like but I prefer that over too lean for sure

This is the wife’s truck and I just want a fun reliable driver for her. The work I’ve done already is a vast improvement over the most stock 400 that it had before it dropped the #7 exhaust valve.

 

Take it up until you hear pinging then back it off a little at a time until you can't get it to ping. then check it with a light to see where your advance points are.

 

And fix the PCV system like it was originally set up. Not sure if by "typical pintle valve" you're just referring to a starndard PCV valve using it's proper name, or if you have substituted some other type of device that is supposed to work similar. But if the latter, is it sized correctly for what it's intended purpose? And if a store-bought PCV valve, is it the correct one for your engine? They are different, and do vary not only by shape and size, but by design and weight of the "pill" as well.
But either way, you MUST open up that blocked off end. Sure, you are trying to keep contamination at bay, but not at the expense of flow. You MUST use a filtered media/medium to allow for proper flow through the crankcase and out of the valve into the carburetor.

And speaking of that... Where is the valve connected to the engine? You can't run a PCV system to any single individual intake runner. Upsets the flow too much. It must run to a common area, either the intake plenum of the manifold itself if there is a port, or more typically to a port on either a carb spacer with fitting, or the carburetor itself.
Full vacuum, but to the common plenum area and not just one or two runners. It's ok to run a brake booster, or trans modulator, or some other devices from individual intake runners, but not the PCV.

Paul