Another timing curve question
Vizard led me down the rabbit hole. Fortunately I found some other crazies that eventually helped me find my way out.
(That rabbit hole included a $400 bill from a well known company to rework my carb, purchase of an LM1, auxbox and I then a step by step effort 'til I finally could see my way clear.
I do have a wideband - shows about 13.5-13.8 at idle currently. Not sure how reliable it is at idle with lots of overlap, since it's only measuring the oxygen in the exhaust.
I think with a hot cam like that, anything from 11 to high 13s could be what it will end up liking.
I agree that 4-corner idle would be a big help, but by the time I finish modifying (or replacing) the metering blocks and the main body, I might as well buy the carb I should have in the first place! :D But this one cost me something like $60, not $600, so I'm willing to experiment and learn a bit.
It just needs a rear block and you have to have guts to drill the idle port. I didn't have the guts. For 'poor mans four corner' its possible to cheat and not have to make the tricky angle. But for an engine with low vacuum at idle, true four corner would be better.
Does the increased advance at idle reduce the torque (at idle) significantly? If it's cleaning it up and probably creating more vacuum, isn't that an indication that it's running more efficiently?
It's cleaning up because there is almost zero load so the engine can be run leaner than when there's a load. However the additional vacuum is also pulling harder on the idle and transfer ports. The typical 4180 was probably set up assuming vacuum at idle in the 11 to 14" range. When your engine is making 8"Hg its still not drawing as much fuel as Holley intended to put in the ballpark.
So yes your engine is probably running more efficiently - but only for a no load situation. If it was running richer it probably would have a bit more power. And being richer it should burn quicker, so less timing needed. Power at idle speeds is very relative. What might your engine be making to overcome friction at 1000 rpm? 6 Hp? An increase of just 2 Hp would be alot. So when I wrote efficient I meant its ability to produce torque and power. I'm sure you've driven vehicles where you can just slip the clutch and put it into first at 600 rpm with hardly any blip of the throttle. In this case, we not that's not going to happen.
Back to the carb.
If I was in your situation, my course of action would be to do a few experiments with richening idle with the distributor setup as it is. That way you know the idle timing is prety much whatever you set it at, baseline being 22.
I would not use toothpicks - except maybe if I wanted to test the extreme case of no airbleed. Even then I'd use something else.
One check would definately be to reduce the primary idle air bleeds. Get some wire between .016 and .030. Stick it in one airbleed and run it up over the choke tower if its still there and down under the air cleaner gasket. Then do the other one. Another way is to make both out of one wire. Harder to make but also harder to lose.
If that seems promising, then see what effect closing the secondary and opening primary has. As well as just doing one or the other.
Two goals here. To observe the effects of more fuel for a given situation and whether changing any one or combination it will run strong enough that you can reduce the idle rpm a little. If your at 1100-1200 now, getting it to a stable 1100 or 1050 would be big deal.
Without going 4 corner idle, idle can be made richer by reducing the primary idle air bleeds permanently. But if reducing them alot (say they are .074 now and your reduce them to the equivalent .66 - that's alot) then increase the primary idle feed restrictions instead.
