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Secondary Throttle Idle Adjustment

JedIEG

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I am having a hard time getting my 318 to run right. Its a stock 74 engine 82k miles with a mild street performance cam (I assume similar to a 340 cam; my dad installed it in the 70's) , performer intake and a new 4160 QFT Slayer 600.

It is running rich. My AFR is reading around 12.0 at idle and plugs are coated in carbon. Vacuum is at 13-15" @ 650rpm. I took the carb off to inspect the transition slot and too much was exposed. I need more throttle to get my idle speed up, so I opened the secondaries up to the point that the start of the transition slot is just barely visible. The primaries at .025 slot exposed (1 turn off seated). Does that seem reasonable?
How much does the secondary usually need opened for idle quality? Holley nearly,mentions doing this on their site and the carb came "read to run" with them completely closed.
 
Completely closed , I had this situation on my Holley.... smelled raw gas all the time. I forget what I set mine at but it made a whole lotta difference! Sorry I cant give you better details , but I did this 7/8 years ago
 
Increase your initial advance to gain idle speed.
But first, close those secondaries again.
When it is running that rich you can open the secondaries a lot more to give it more air but the idle rpm will be 1500...guess that is not the plan.

Remove carb, set your transition slot "square" and reinstall the carb, mixture screws at 1 or 1-1/2 turn open and put some more initial timing on the distributor.
Then with the engine at operating temp, and the choke cam off (if you have a choke on it) you can play with initial advance and idle screws to see if you can set your idle to whatever you want it to run at.
DON'T touch the idle screw to reach your target though.
Because that would be cheating. :D

Oh, since it is a small block the distributor turn clockwise, so:
To increase idle rpm, Advance initial timing==>Adjust distributor CCW
To reduce idle rpm, Retard initial timing==>Adjust distributor CW

Once you got the rpm right, then see what AFR meter is saying.
If it is still rich you can close the mixture screws bit by bit.
If closing mixture screws changes the rpm...use the initial advance again to keep the rpm where you want it.

I also have an AFR gauge, do not take the reading for granted at idle.
You say you have a mild cam, but when a cam has some valve overlap you are likely to see a wrong reading.
Keep an eye on the numbers it shows you at idle and cruising and write them down or remember them.
Later you can check your spark plugs and see if that matches whatever the AFR meter has been telling you.
 
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OK I will reset the transition slots back to .025-.030 primary and close the secondary, but I know it will not start with that setting. In wants more throttle than that with my current setup. What initial timing should I be running? I have 14° advance @650 and around 40° @2500 w/ no load .
 
Just to confirm, you check and adjust the transition slot while manually keeping the choke cam off right?
That red cam you can manually keep it pushed down, then the throttle is on Curb idle and you can set the transition slot. Ignore how it looks while it is on the choke cam (High Idle) for now.

The idea is to just get it started and warm up to normal temperature.
If it takes to help and keep you foot at the throttle for now to keep it running, later that will become better.
These things never start by just turning the key, unless it's EFI.
Once the engine is at operating temp and the choke cam is off you can make changes.
Remember to only adjust the timing and mixture screws to get your idle right.
Once your carb is set on a square transition slot exposure that is very where near it needs to be, if you play too much with it again you need to remove the carb again later on to check/set the trans. slot again.....
Yes you can do some fine tuning with the idle screw at the end but at most a 1/4 turn either direction.
Try and get it close with just timing and idle screws.
Remember, advance timing increases idle speed, leaning out (closing) the mixture screws increases speed.

Do you have a PCV valve installed and connected to a vacuum port on the carb?

I have 14° advance @650 and around 40° @2500 w/ no load
The exact advance does not really matter much, i think for SB engines 12-18* of advance is normal so stay within that range.
Likely it will run beautiful at 20* advance but that is too much.
Your 40* that is incl. vacuum advance??
Maybe that is too much advance and too early, i have a 440 engine and that is too much for BB's but for SB's i don't know if that is a normal range.
Maybe someone else here knows that..
 
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Yep, I set transition slots with the choke held open and off the high idle cam, and the throttle stop held against the curb idle screw.
Total timing is 40-42° @2600rpm with vacuum advance.
Vacuum advance is using ported vacuum.
I have removed the pcv and plugged the port for the time being to eliminate that variable.
Idle mixture screws have never made a difference to speed or vacuum other than they eventually kill the engine when turned in far enough. I was assuming the transfer slot was dominating idle fueling making the idle mixture screws insensitive. This is why I was opening the secondaries.
From cold this engine will fire up immediately two shots from the accelerator pump every time but as soon as that fuel is gone it dies unless I open the throttle enough. It doesn't idle any better with the throttle closed once warm.
 
Your PVC valve is providing the engine with an additional air source.
You need to include this during setup, because even if you do manage to make it run good now, you will have to go back adjusting when you hook up the PCV because the A/F mixture changed.
You opened the secondaries to provide more air, that air the engine does need, but can come from the PCV.

The fact the engine does not react on the mixture screws is indeed caused by too much transition slot.
The fueling from there is excessive and the little adjustment on the mix. screws is not going to make a difference.

Try with the following and i believe it should be able to stay running:

-Square transition slot
-mixture screws at 1-1/2 turn from closed
-initial advance 16*
-PCV hooked up
-maybe also better to disconnect and plug the v-can for now.

Also just make sure the basic settings are good, like fuel bowl levels, etc.

Just check the PCV that the valve is free, maybe a clean with some carb cleaner. (if doubtfull, get a new one)
The crankcase breather should also be clean and not cause a restriction.
The PCV should provide the engine with a steady air source as your idle vacuum is enough to suck the check valve against it’s upper seat, that reduces the airflow through it for idle mode.
If the PCV is faulty it could be in WOT mode and allow full air flow through the valve, providing way too much air.

Write down your settings, make 1 adjustment at a time and write down what/how much you changed.
This will help you staying in control, i have spend many hours chasing my own tail on this because i got lost many times because i did not remember what i did 5 minutes ago and is now causing a problem.
 
OK, I tired what you said with the timing and reset the secondaries to 1/4 turn off of fully seated. I bumped static timing to 18° and that helped gain RPM. I ran the vacuum advance with my hand pump to increase idle rpm while running the timing light to see how it liked more advance. It still wants more air though. I tried to keep the idle screw where I set it for the transition slot but when I let it off the high idle cam, it lugged to 450RPM and couldn't sustain that. I had to turn the idle speed screw in about 1/2 turn to get it to idle at all under 20° advance @ 650RPM. I think that would put me at about .040"-.050 transition slot open. The engine was popping thought the exhaust occasionally, so I added about 1/4 turn on the idle mixture screws. Vacuum @650 is a little under 14". There is still not much sensitivity to the idle adjustment screws.
I didn't get a chance to really check O2 as my datalogger is not working well and keeps freezing. I'll see if I can pull it out and run it tomorrow.
 
Ok, well you are going in the right direction as now it can manage to idle although being low.
The 20* advance is already too far, i know its worth in trial though but i think 18* would be the limit.

reset the secondaries to 1/4 turn off of fully seated
Did you visually check the transition slot when setting the primary? That it is about square?

You didn't mention it so i just ask, did you hook up the PCV valve?
 
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Pcv valve was connected.
The transition slot was around .030 exposed when I reset it. Here's what it looked like.
20200515_121814.jpg
 
OK, yeah that transition slot looks ok.
Well, if the PCV valve also was connected and it still wants more air you could try this:
-Disconnect the hose from the pcv, install a fitting with a small ball valve on the hose end and make sure it is leak free.
-Run the engine and gradually open the valve and observe engine rpm until you get your wanted idle speed.
-Leave the valve at that position!

Now you can start fingering a bit on the rest again, try to reduce your initial ign. advance to 16* or so.
RPM will likely drop, try first with allowing a little more air in by the ball valve. If that increases the rpm it means the fueling is still sufficient to support that amount of air, if not open the mix. screws 1/8-1/4 more and see if you get a nice steady idle.

This way you can play a bit with it to reach your target rpm.
If you did not adjust the idle screw and the transition slot is still square your carb is set good, mixture screws should be around 3/4 - 1-1/2 turn open.

Now the only thing remaining is your by-pass air feed.
After setting that valve do not change it, make sure it stays in the same position.
Remove the valve/fitting assembly and hook the hose back up to the PCV valve, start the engine again and see what it does.
Quite sure it will not run as it did with the ball valve setup,
In this case it means your engine is, as you said already, not getting enough air.
To solve that there are multiple solutions:

-Open the secondaries. Now you tried this already and it did not work out. I never tried this to solve my idle issues, but a great help of someone with more knowledge than me told me not to even try it.
The reason is that the air that passes the secondaries is leaning out the rear cylinders A/F ratio, so your forward cylinder will run rich (because you compensate with the mix. screws) and the rears are lean.

-Drill holes in the primary throttle plates. I did try this and it does introduce extra air. The benefit of this over opening the secondaries is that this by-pass air is mixed directly with the normal air/fuel mixture and stabilizes the A/F to equal over all cylinders.
You hook up the PCV as normal, drill 1 hole in each throttle plate, start small (1/32 or 3/64 drill bit) and increase the hole size until you got that same good idle quality and rpm.
Drill the hole near the middle of the plate, on the same side of the shaft as the transition slot (forward) and make a very small chamfer on the hole.
Like below example.
upload_2020-5-16_7-31-3.png

You should have 1-9/16" throttle plates, Holley/QF sells sets of new throttle plates for around $15 so if you end up with too big holes you have an easy solution of restoring it to normal.
When replacing throttle plates strictly follow the instructions, its not hard but needs to be done right.


-Get an M/E Wagner PCV valve. You might have heard of them, this is an adjustable PCV valve which allows you to tailor the air feed at idle to get that right. And also allows you to adjust the point where it will open another valve to increase the air flow at cruising and WOT to make sure your crank is getting properly ventilated. This is the most expensive option though.
 
Well I tried starting it again today and it really did not want to start or stay running. Part of the problem was the distributor didnt lock down so I was getting 28°initial advance, but even with that fixed back to 18° it wants more air still. With quarantine, I'm not going to be able to source a bypass valve easily so I substituted playing with the secondaries by shimming it with a feeler gauge. It wants about .015" more throttle screw cold, but will live with .010" (based on measuring my other 4160, that equates to 1.75:1 movement on edge of the throttle blade).
A new issue I am wondering about is the timing has some erratic behavior where it seems to be firing late some times. Here's a video of that:

Any ideas what would do that? Every part of the ignition system is new...
 
I have come across this locking collar before for BB distributor shafts, but i am not sure it will solve your problem...or even fit the SB distributors.
http://www.hughesengines.com/Index/...el1=SWduaXRpb24gJiBFbGVjdHJpY2Fs&partid=25200

The only thing i can think of is there is a "slack" in the drive to the distributor that causes this.
It could be the total play from drive chain, camshaft/distributor shaft gear and distributor slot that comes all together and shows as an uneven firing.
You should see several degrees difference in initial ign. advance when observing the damper timing marks if this is the case.
If the engine has not had a rebuild and still has the original chain, and distributor shaft i could be likely there is wear.

You can continue playing with introducing more air to get it run better, your eyes and ears can tell what helps the engine, just keep writing down what you changed and if it improved to keep track of it.
Looking at the various adjustments on paper it will become more clear what is the right way to go.
What i ended up doing was cutting a 1/2" long piece of threaded bar/bolt that had a snug fit in the PCV hose, drilled a .100" hole in it and secure it inside the hose with a hose clamp and leave the hose lying there and not connected to the PCV.
After you can try the engine and increase the hole size as required, this acts like a fixed orifice and controls the air flow.
You can write down the hole size, Idle rpm, advance degrees, mix. screw turns and AFR to get a list of variables to compare.
Did you manage to get the AFR reader back up and running?
Leave your idle a little rich like 13.5 or so, don't chase the magic 14.7 for idle settings.
 
I tightened the distributor down more and that seems to have helped the timing, but I will have to get one of those colars. I think I have some more tuning to do, but for the first time ever I got the car to idle nicely and see the adjustment on the idle mixture screws actually made a difference to the AFR and vacuum. I drove it around the neighborhood and it didn't threaten to die in me. If I still need more air I may take your orifice idea to the next level and try to add a bypass valve under the air cleaner stud like on the more expensive Holleys and Demon carbs. There is space for something like that on this QFT body.... Wietse, thanks for all the help.
 
Good job you got it to run now! :thumbsup:

How did you set it up now to get it to run right?


add a bypass valve under the air cleaner stud

Oh yeah, forgot about that.
That is an ideal solution to add by-pass air, my carb doesn't have it so it didn't cross my mind.
That by-pass is specially made for these situations.
 
Your right as Holley says to close the secondary and then go so much on the secondary idle screw after it just touches with the throttle blades closed all the way. 1/4 turn sounds right but I don't remember without looking if that's just right for your carb but it may be. I know once I have my timing set where the eng runs best and if I have to much transfer slot showing I drill the 1/8 holes in the throttle blades and it works good for me as I have done it many times. Holley even says to do it in their book. I like how some newer carbs have an air adjustment screw under the carb stud. Good luck as I hope you get it right. Ron
 
How did you set it up now to get it to run right?

I set the secondaries back to factory settings with a 1/4 turn off seated and the primaries with .025 transfer slot (square). Then I shimmied under the throttle stop to open the secondaries up with a .020 blade just to get it running and warmed up and not effect my primary screw, which made high idle way too fast. Once it was warm and off the high idle cam I could change to a different gauge and see how it behaved. .012" more opening at the screw was about what it wanted to idle right, .010 it was struggling to stay running and didn't want to restart.
Then used a dial indicator on the edge of the throttle blade on another 4160 to get the reading from the feeler gauge and then turn the throttle stop screw to get the same reading (about 3/8 turn). I then turned the throttle stop screw on my QFT the same amount and reinstalled it.
I think the big thing with using the secondaries to adjust idle is having a secondary idle circuit. The QFT has the idle feed hole drilled where the old 4160 I have does not.
 
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On your timing,if you have cross fire from plug wires
running parallel to each other it will cause timing
Light to pickup impulse from other wire.
 
It is worth a try indeed to separate the cyl #1 spark wire and try again.
But modern timing lights have good shielding and should not be affected that fast anymore from whatever is outside the clamp.
Same for the modern spark plug wiring has better shielding.

That bushing on Hughes Engines website says it is for big blocks, does a small block has same diameter distributor shaft?
 
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