1971 360 question
I get "sense" from your posts, it just seems to me that you have a narrow understanding of a complicated topic and think that repeating the same very specific advice to everyone is good for them regardless of the situation.
You say you don't recommend any idle timing numbers, but yet you'll brow-beat anyone that uses more than you- very helpful. I didn't say 20º advanced was "big", I said- "not every build should have 20° advance at idle, but not every build is gonna be right at 10° either". I said that because the OP said his timing was "around 20 degs". The OP's words (post #45), not anything out of my "own daymn head" as you say. Maybe read the thread before you regurgitate your same advice and come after me?
Tire spin as a reason to not add idle timing? Give me a break! That's a lousy reference. I could blow off the tires on my Challenger with a boring 318, a 625 carter OOTB, 2.76's and a 904 with a stock converter. No where near an idle tune. If you couldn't spin the wheels up running the all season tires you run, in an A-body with a 360 and a 4 speed I'd tell you to do a compression check not change the idle timing. And that's before we get to a lack of traction not implying a perfect tune.
Not every engine needs to be tuned the same. Not every engine will perform best when tuned the same, because not every engine build is the same and not every application is the same. You want to tune for absolute gas mileage? Neat. Someone else might want to tune for maximum power, and that will be a totally different set up.
Regardless of what you or the banned member formally known as Yellow Rose say, there are A LOT of different ways to tune a carburetor. Some are definitely better than others, but there's a lot in the middle too. And what works best in one particular car will depend on the engine build, car weight, gearing, vehicle use and even the drivers style and preferences.
Saying I'm "lazy" because my car doesn't run best at 12º advance at idle and won't tolerate a 12.5 PV just shows that you only understand a very small window of a very complicated topic. I'm no expert, but it's pretty obvious you aren't either.
I haven’t seen an engine that can actually use 1/2 of idle vacuum and have it be correct.
No where have I read Holley claim it’s a starting point, and if they have they never tell how to tune from there.
Even Holley doesn’t practice what they preach. How so you ask? I can find several examples of OE Holley carbs that came with 12.5 inch opening power valves. That would mean the idle vacuum would be over 24 inches.
Of course you’ll say that they started at 1/2 vacuum at idle and then tuned back to a 12.5 power valve. I’ve never seen how they got there though.
Interesting note for those who care. If I get this wrong maybe
@Mattax can come in here and clean it up.
Not long after Colt Industries bought out Holley they did what all these buy out/merger/acquisition companies do and that’s cut costs. And of course, most of the tooling was old and used up and Colt wasn’t going to spend a bunch of cash retooling.
This is when the faulty power valves started showing up. And this is about the same time all these articles and books were being written and when the “half idle vacuum” started.
Once that was put down on paper it was the de facto method of power valve tuning. And of course it’s wrong. How else do you explain he above reference of the OE Holley carbs using a 12.5 power valve.
Let’s also remember (I forgot to mention it was an OEM GM carb) that the vast majority of testing for the carb was done by GM.
At any rate, the idea that idle vacuum or anything related to idle vacuum is even a good starting point is at best wrong.
I see it all the time. Carbs with the MAB so big it would swallow a golf ball and a power valve opening at 6.5, which has now become the de facto starting point for guys who don’t even bother to use a vacuum gauge.
That big MAB starts the booster way too soon, and if you couple that with an “early opening” power valve you get a nasty, slobbering, pig rich condition. The actual fix is correcting the MAB first and then tuning power valve opening timing.
That never happens. Or at least it happens so little that it’s like it never happens. Does delaying power valve opening help with the massive MAB? Certainly. Is that the right or correct way to tune these carbs, taking into account that while there are exceptions to the rule (which you freely admit) the answer is an unequivocal NO.
I can’t tell you how many engines out there that I know of that have an idle vacuum so 7-8 inches. By the wrong method they should be STARTING with a 3.5 power valve. Of course, at a cruse most of these engines have vacuum of 14-16 inches.
It doesn’t take a rocket surgeon to see the HUGE delay in opening the power valve. And that SHOULD cause a lean stumble. And it will IF the MAB is sized correctly.
You can be butthurt all you want. I couldn’t care less. The fact is it’s wrong to keep perpetuating the myth of half idle vacuum to start power valve opening.
To start tuning power valve opening the MAB sizing needs to be checked and remedied before anything else. I’m not going into how to size the MAB but if the MAB is bigger than an .028 on most 4150 carb’s you need to find out why that is. Then the car needs to be driven with a vacuum gauge hooked up and look at cruise vacuum under various loads and conditions. Then you can pick a starting power valve. Which should be 2-3 inches LESS than cruise vacuum to start.
Annular booster tune differently than down leg boosters and down leg boosters tune differently than straight boosters.
Which leads to this last statement. I know guys love a good hack. A corner cutting time saving hack. This is a bad place to use a hack.
I’d bet a lot that most guys won’t bother to get enough hose to run from the engine compartment to the cabin to read the gauge while driving. They can use the half idle method which is quicker and requires much less effort.
That’s why the dreaded “Holleyitus” is so prevalent. It’s one reason for sure. I would think at some point we’d be wanting to correct this way if tuning. Of course, tuning correctly takes way more time and thought and effort and maybe that’s why it isn’t being done.
TL;DR using half idle vacuum to start power valve opening is a fools errand.