Engine Timing

Ok, I do apologize in advance if we are getting too esoteric, but I am in the mood to go on, so here goes. Ignore if you like, as always!

The idle speed is adjusted by using the secondaries for air flow, with only final adjustments to air flow being done with the primaries. The reason for this is that with an engine with a large cam, you probably have to open the primaries so far with the idle speed screw that you are actually running well into the main jets and operating circuits, not the idle jets and circuits. Pretty much like sitting at a stop sign with your foot on the gas. The problem with that is that it will run rich and no amount of adjusting the idle mixture screws will help. Like trying to regulate a fire hose!

If you open the secondaries for air flow, that allows you to close down the primaries and that puts it back into running on the idle circuits rather than the main jets. If your primary idle screw is more than about 1 full turn or maybe 1-1/4 in from where it starts to move the throttle linkage, you are too far in for proper idle adjustment.

This is another thing you can live with, the car will run ok, but forget getting the idle mixture right because it won't happen.

It sounds like you have the timing pretty close(other than my blathering about vac advance!), so one MAJOR success there!

One other thing, the port that Phat is referring to is not on the manifold. It should be on the side of the carb on the primary metering block on the opposite side from the linkage. This does matter because you will get manifold vacuum from the manifold and ported vacuum from the carb. Manifold vacuum is pulling at idle and also at a steady cruise. That is why it will up your timing at idle as well as at cruise. Ported vacuum only works when there is significant air flowing through the carb. At a proper idle it should have zero vacuum, which is why Phat made the comment about if your idle is too high on the primaries you will see vacuum at the ported vacuum source, even at "idle".

Lots to absorb, I know! Sleep on it and maybe read up a bit more online. Once you get the basic principles it will all start to come together and the understanding will work wonders on your ability to tune your engine.

Thanks and keep stretching the comfort level, you will be glad you did!

Chris   

Latest update. I am currently running 18/36 (initial / All in) timing with no vacuum advance connected. I have the big black bushing and a light silver and a blue advance spring installed. This seems to be working fine - but  .... my idle quality is reasonable given the cam, around 900 rpm with each primary jet at two full turns out.
My problem is that it still stumbles significantly when I get into it while driving and takes a bit to recover.. Seems to me to be fuel system related at this point??? I ohmed the plug wires and they are all good. Tomorrow I am going to double check my bowel levels again but aside from that where to go? The carb itself is fairly new, power valves shouldn't cause this stumble under acceleration? I do wonder if I am getting enough fuel but I do run a 110 gpm electric pump - with no return? The tank is not vented and does build up a fair amount of pressure when its driven some??? Any thoughts?

The size of the squirters for the accelerator pump will determine the duration of the fuel shot and the cam determines the amount of fuel overall. There is also some fine tuning that goes with profile of the cam.

I would suggest that you up the size of the accelerator pump nozzles in small steps until you get a decent launch. If it starts out ok but then falls down after a second or two, you need to start looking at the pump cam(s) to make sure they are delivering enough fuel overall. Finally, you can tune the cam profile for best around town drivability.

Pump nozzles are usually .025, but you may find that an .031 or .035 does the trick.

Thanks!

Chris

PS: Most of what Spiritdude says is spot on, particularly the float and accelerator pump arm adjustments. One thing, I would not worry too much about the fuel pressure because the carbs operate on the float bowls. Too little fuel pressure will generally not show up upon initial acceleration but rather somewhere around 3rd or 4th gear at WOT,when the float bowls run dry.

Edited by ccowx - Jul/24/2017 at 10:08pm

6 to 6.5 psi is good. More importantly you don't want it below 6 psi when you are WOT in high gear after going through all the gears at WOT.

Dave Z wrote: Would total timing stay the same between cast iron and aluminum heads? Dave Z

Everything being equal, you can go with a bit more if you are using aluminum heads. Start out with 1-2 degrees more.