I do the vacuum idle tuning because that’s what I’ve heard everywhere basically. Everyone says turn the screws for peak vacuum and then back them off a bit for cushion. But to summarize (it’s actually similar to what my dad told me he did back in the day.) and correct me if I’m wrong.
You start with 1.5 turns out or so, just equal on all 4. (Is .028 IFR and .070 MAB too much/little for my setup in your opinion?). Then start with one screw and turn it in slowly until idle speed drops, back out until it goes back to what it was and let the idle stabilize. Then turn them out slowly until idle drops and then go back until it’s normal and stabilize. Do the same on all 4, and adjust as necessary for any tip in.
For WOT tuning, there is a quarter mile strip not overly far from me where I can go to tune in the WOT stuff when they do test/tune days, I’ll have to do some digging about them.
In terms of my plugs, I’m running NGK FR5s gapped at .040”. They’re a step colder than stock. I read that colder would help with preventing pinging so I did it just to experiment. And as a reminder I’m currently running 16 degrees initial.
- 700–1000 rpm: 16°
- 1500 rpm: 28°
- 2000 rpm: 32°
- 2500–3000 rpm: 36°
The bowls are set currently at just about halfway on the sight glass (not plugs). Pressure is just under 6psi.
This may be an odd question but I’m curious if there’s any insight about it. Let’s say I tune my wot for a quarter mile pull and it’s all good in that environment.
For this fictional scenario let’s say I’m hauling somewhat heavy and for some reason I’m on the highway on ramp and I floor it going up hill. Given the load and the incline, the amount of time is the same as the quarter mile, but the distance traveled is not. Is this going to be the same thing to the carb, or is the whole situation different because of the load? (Or is the simple answer don’t floor a truck up hill heavy lol)
In my world, just because everyone says it doesn’t mean it’s right. It also doesn’t mean it’s wrong. In my world the majority is wrong far more than they are correct. How to set Holley power valve timing is an example.
For decades everyone was using 1/2 idle rpm to set the opening and at best it’s half wrong most of the time. And then every tuning decision made after that is based on an error. How do you correct that.
Start where you are. Then set it like I explained and see if it idles better or worse. I could be dead wrong. That’s how you learn.
Im not a big fan of cranking an assload of initial timing in an engine to make it idle. More times than not you can get the same idle or better if you tune the carb but in my experience if you need todo that the build has too little compression or too much cam for the compression. That’s how I see tuning for the highest manifold vacuum. To what end? If it’s lean missing, even if you can’t hear it then it’s still wrong. But you have 2 more inches of idle vacuum. What’s the gain?
One thing I forgot to mention is when you get idle nice and clean and the mixture screws set I take one screw and turn it in and count the turns just so I know. That will give you an idea of what the engine wants for an air/fuel mixture at idle. Turning the screws in and out only changes the AMOUNT of air and fuel getting to the engine. Changing the IAB or IFR or both changes the air/fuel ratio.
To that end, let’s say you get the screws set and you screw them in and you are three turns out. That tells me that the air/fuel RATIO you are giving the engine is on the lean side.
If the screws are only 5/8 of a turn out you know the air/fuel RATIO is rich. I make this point because that’s the two extremes. You only have about 3/8 of a turn in to lean the carb down. If you are 3 turns out you have about the same amount of adjustment if you need to fatten the idle up a bit.
Certainly you can run it and probably never get to the point you’ll be out of the adjustment one way or the other. If you don’t drive the car to huge elevation changes from your home it’s not a big deal. Im about 3 hours from the ocean by car which is about 120 miles as the crow flies. I live in a high desert valley. The change in elevation and weather conditions causes a change in IAB and sometimes a jet change and probably a timing change. That’s why I try and get the screws centered as best I can. It’s in hope of not needing to change an air bleed or IFR. Turning screws is quicker, easier and cleaner.
If you are say 3 turns out and you are lean, you can go down .004-.006 smaller on MAB to get the screws back in. If you go much more than that you made need to make other changes to go with that. If it needs more than .006 smaller I’d drop the IFR down .002-.004 and see how it goes. If it’s too fat then go back to up on MAB .002-.003 to clean it up.
The thinking is the same if you are only 5/8 out, you a drop the MAB down. If it needs more than say .006 you need to drop the IFR .002-.003 and see where you’re at. If it’s a bit lean, you can open the MAB up a couple of thou.
I hope that makes sense.
As for your timing 36 is a TON of timing at that rpm UNDER LOAD. We have to consider load (manifold vacuum) when looking at timing. At a cruise you engine may want as high as 50 degrees of timing. If you crank all that timing it (50 degrees of mechanical advance) when you stand on the throttle to pull a grade, or you have a trailer on it will rattle its brains out until it spits a rod out.
What you are doing is trying to get the engine happy with timing you cant achieve so you compromise with (using you example) 36 at a cruise. You may be as much as 14 degrees retarded on timing. That causes elevated exhaust temperatures, increased fuel consumption and a massive hit in torque at and around peak. How do you fix it? You use vacuum advance.
But you can’t have a curve like you have and add vacuum advance or it will rattle. I’ll give you an example. Im it saying this is what you need but it’s an example.
Let’s say that peak torque is 3500. And you have vacuum can that will give you 14 degrees more timing at say 18 inches of vacuum.
What if you rolled the timing back to 30 degrees at a 2700 rpm cruise and you added 14 degrees more timing with the VA? You now have 44 degrees of timing at a cruise. Fuel consumption goes down as well as exhaust temperatures. When the vacuum goes away (under load) you go back to just the mechanical advance. And it’s happy without all that timing at that load.
You get the best of both worlds. Is it quick and easy. Nope. But the benefits of doing it that way are so valuable that IMO it’s worth it.
That’s how I think about timing. I know this. Too much timing at peak torque will kill 20 foot pounds so fast your head will spin. And the fact that you have to retard the timing to keep it from detonating means that timing is retarded at higher rpm.
I hope this makes sense.
