Troubleshooting lean idle

Paul_G said:

It is getting un metered air from somewhere. If it is not above the intake where you can find it, look below the intake. Could be a bad seal on the intake ports, on the valley side. Look for signs of oil being sucked in to the intake ports. will have to pull the intake to see. If you have exhausted all other ideas.

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Already tried that. Hooked a smoke machine up to it and pulled the distributor but saw nothing. I replaced the intake gasket for the hell of it and it made no difference. If it is getting unmetered air, wouldnt the AFR be affected above idle as well? I have a wideband O2 sensor and as soon as i give it any amount of throttle, it richens up into the mid 13s.

Have you tried partially closing the choke butterfly? if the idle smooths out try readjusting the idle mixture with the choke still partially closed. If that produces favorable results pull the top off that carb and open up the idle restrictions with the next size number drill. Have you tried any other brand of carb?
Mike

Not sure about the valve overlap you have on that cam, but in my case also i had noticed the AFR at idle needs to be taken with a grain of salt.
As due to the overlap you do get a certain amount of air entering the exhaust side which makes it look like it runs lean.

Are there any issues with idle quality remaining?
Do you still require a high initial ignition advance? What is it set at now and what idle rpm are you having?
How much transition slot is currently exposed below the throttle plate? Make sure when you check it that the throttle is not on the choke cam.

Because as above, you could sort of ignore the reading at idle because it is not showing correct.

Wietse said:

Not sure about the valve overlap you have on that cam, but in my case also i had noticed the AFR at idle needs to be taken with a grain of salt.
As due to the overlap you do get a certain amount of air entering the exhaust side which makes it look like it runs lean.

Are there any issues with idle quality remaining?
Do you still require a high initial ignition advance? What is it set at now and what idle rpm are you having?
How much transition slot is currently exposed below the throttle plate? Make sure when you check it that the throttle is not on the choke cam.

Because as above, you could sort of ignore the reading at idle because it is not showing correct.

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Yes I'm still having issues with the idle. It wont idle unless the screw is turned almost all the way in. Initial timing is 18 but it seems to want way more than that to idle. It barely runs if I try to lower it below 1000 rpm. The mixture screws are backed out as far as they will go, but they dont seem to make any difference. I have had the carb apart to check it out, and everything looks to be normal. I blew out all of the passages with compressed air just to be sure.

493 Mike said:

Have you tried partially closing the choke butterfly? if the idle smooths out try readjusting the idle mixture with the choke still partially closed. If that produces favorable results pull the top off that carb and open up the idle restrictions with the next size number drill. Have you tried any other brand of carb?
Mike

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It does run considerably better with the choke held partially closed. I already drilled out the holes for the mixture screws and it made a little difference but not much. I think my next step with be to drill out the idle restrictions. I just dont understand why it worked fine before the rebuild but is giving me all this trouble now. It pulls a steady 17" of vacuum and runs flawlessly above 1200 rpm. I havent tried any other brand of carb since all I have are Qjets, but I wouldnt think it would make much of a difference.

If you have the idle screw turned in all the way in it is getting a **** load of air, so it will need a **** load of fuel as well to get a combustible mixture.
As the throttle is opened that far there is already a lot of fuel being pulled out of the transition slots, the idle mixture screws will be come unresponsive due to that.
Initial advance of 18* is ok, just make sure you have a total of around 34-36* max.
Increasing the initial advance to 22-24* will indeed give a better idle, although it is too much.
The peak combustion pressure has a sweet spot (believe somewhere 20-25* ATDC) where the engine will run strong, but you want to be in that area while driving and not during idle which is what happens if you give it too much initial advance. When rpms increase due to driving you are already past that sweet spot.

The picture you posted earlier (below) shows the rectangular transition slot, when the throttle is set in the basic ballpark that rectangular will appear almost square when looking from the bottom.
When you check this, make sure the throttle linkage is not on the choke cam as it will keep it more open than normal.


Another thing is to make sure the secondaries are fully closed, which i believe you already confirmed.
To rule out the PCV valve, disconnect the hose from the PCV and install a small ball valve on the hose, this will feed the engine an additional (adjustable) amount of air and rules out any "fluctuating" or excessive air flow from the PCV.
I am not sure if your carb has it but some have an idle air by-pass screw, this is used to give the engine additional air at idle. (the old way is to drill small holes in the throttle blades)
If you do have it that can be used instead of a ball valve on the PCV hose.

It might be easiest to run the engine prior to trying below to have it warm which will help to keep it running idle.

I would take the carb off and set the idle screw in the position where the transition slot shows square when looking from below. (throttle must be off the choke cam!!)
Set the idle mixture screws at 2 turns open from closed.
Install the carb back and start her up.
If you get it up and running it might not stay running idle by itself yet.
Slowly open the ball valve that you installed on the PCV valve hose and see what the engine does, as long as the fuel feed supports the extra air the rpm will keep increasing by opening the valve more until it leans out too much and starts to run like ****. You can hear it going wrong.
The AFR gauge will help to give some guidance on what the mixture is, bear in mind it will like report a leaner mixture then it really is due to valve overlap and some fresh air flow in the exhaust.

You can make small adjustments on the idle mixture screws, the ball valve on the PCV hose and initial advance and see what the effect is, make only single adjustments and write down what you do so you can keep track of the changes and see what improves things and what makes it worse.
Since the transition slot is set, you should not adjust the idle screw at all, but you could try and give it a 1/4 turn MAX. (this is your last option to play with though)

Try and get the rpm down to 800-850 rpm without touching the idle screw.
I worked mine the same way as above to find a happy medium, in the end i got a steel plug with a 0.120" hole in the PCV hose which acted as a fixed orifice and over rules whatever the PCV valve is doing.

Carb is not drawing enough fuel thru the idle circuits=17-19 af ratio. Something clogged them up, backfire when the bearing went? Shaft bent? Some bad gas with particles? Something changed in that carb especially since you can close the choke and it helps.

Ceedawg said:

Carb is not drawing enough fuel thru the idle circuits=17-19 af ratio. Something clogged them up, backfire when the bearing went? Shaft bent? Some bad gas with particles? Something changed in that carb especially since you can close the choke and it helps.

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He used different carbs and got the same problem...

myk r sanchez said:

He used different carbs and got the same problem...

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I once tried 3 rebuilt master cylinders before I got a good one.
Mike

Well the odd thing about it to me is that all three worked fine on the motor prior to the rebuild and now they don't. Assuming he hasn't tampered with the carbs I would assume the problem lies elsewhere...

myk r sanchez said:

He used different carbs and got the same problem...

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But used the same cam with the same characteristics, it’s lean on the idle circuit. Stock carbs are all lean with bigger cams. The original could of had trash in the idle AIR bleed which would make the fuel circuit draw more gas

Wietse said:

If you have the idle screw turned in all the way in it is getting a **** load of air, so it will need a **** load of fuel as well to get a combustible mixture.
As the throttle is opened that far there is already a lot of fuel being pulled out of the transition slots, the idle mixture screws will be come unresponsive due to that.
Initial advance of 18* is ok, just make sure you have a total of around 34-36* max.
Increasing the initial advance to 22-24* will indeed give a better idle, although it is too much.
The peak combustion pressure has a sweet spot (believe somewhere 20-25* ATDC) where the engine will run strong, but you want to be in that area while driving and not during idle which is what happens if you give it too much initial advance. When rpms increase due to driving you are already past that sweet spot.

The picture you posted earlier (below) shows the rectangular transition slot, when the throttle is set in the basic ballpark that rectangular will appear almost square when looking from the bottom.
When you check this, make sure the throttle linkage is not on the choke cam as it will keep it more open than normal.
View attachment 1045127

Another thing is to make sure the secondaries are fully closed, which i believe you already confirmed.
To rule out the PCV valve, disconnect the hose from the PCV and install a small ball valve on the hose, this will feed the engine an additional (adjustable) amount of air and rules out any "fluctuating" or excessive air flow from the PCV.
I am not sure if your carb has it but some have an idle air by-pass screw, this is used to give the engine additional air at idle. (the old way is to drill small holes in the throttle blades)
If you do have it that can be used instead of a ball valve on the PCV hose.

It might be easiest to run the engine prior to trying below to have it warm which will help to keep it running idle.

I would take the carb off and set the idle screw in the position where the transition slot shows square when looking from below. (throttle must be off the choke cam!!)
Set the idle mixture screws at 2 turns open from closed.
Install the carb back and start her up.
If you get it up and running it might not stay running idle by itself yet.
Slowly open the ball valve that you installed on the PCV valve hose and see what the engine does, as long as the fuel feed supports the extra air the rpm will keep increasing by opening the valve more until it leans out too much and starts to run like ****. You can hear it going wrong.
The AFR gauge will help to give some guidance on what the mixture is, bear in mind it will like report a leaner mixture then it really is due to valve overlap and some fresh air flow in the exhaust.

You can make small adjustments on the idle mixture screws, the ball valve on the PCV hose and initial advance and see what the effect is, make only single adjustments and write down what you do so you can keep track of the changes and see what improves things and what makes it worse.
Since the transition slot is set, you should not adjust the idle screw at all, but you could try and give it a 1/4 turn MAX. (this is your last option to play with though)

Try and get the rpm down to 800-850 rpm without touching the idle screw.
I worked mine the same way as above to find a happy medium, in the end i got a steel plug with a 0.120" hole in the PCV hose which acted as a fixed orifice and over rules whatever the PCV valve is doing.

Click to expand...

Thanks for the reply. The ball valve on the pcv line worked and i was able to get it to idle nicely while still being on the idle circuit. Oddly enough, it seemed to still want a little choke as well as the ball valve cracked open to run smoothly. Carb screws are still backed out 4.5 turns, but at least i dont have the idle screw turned all the way in to get it to run. Seems like my issue is definitely carb related although im not sure why it was fine before. Should drilling small holes in the throttle blades be my next step to a permananet fix?

Good you are getting in the right direction.
A lean idle could be caused by lack of fuel or excessive air, the carb will only give the basic mixture.
Adjusting the fuel can be done as easy as opening the idle mixture screws, adding air can be more difficult depending on the carb in use.
As mentioned some "more expensive" or "more advanced" carbs have an idle air screw below the air cleaner stud to bypass additional air if required.
The other options are PCV line and drilling holes in the throttle plates.
Thing with drilling holes in the throttle plates is that it is permanent, and once too big or it did not work out you need to change them.
Playing with an orifice in the PCV hose is then very easy by trying different hole sizes and see what gets you the best results.
A lot of times with bigger cams that produce low vacuum is that the PCV valve turns into "full flow" mode and provide way too much air, your engine is pulling a decent and strong vacuum so that will not be your case.
As you mentioned it still wants some choke means that it is getting a little too much air/or too much air/fuel mixture from the carb. (with the idle mixture screws open that far it seems like that at least)
If the carb transition slots are in the ball park and the idle mixture screws are about 2 turns open try and keep the valve on the PCV closed and see how that runs.
Do not use the choke to do any adjustments, if it stays running like that you can crack the valve open and see if it improves. If the engine needs that extra air the rpm's will start to slightly increase.
If that is the case you can retard the distributor a little and the rpm will come down again.
If the rpm remains to low to run normal you can feed a little more fuel by the mixture screws and feed a little more air by the valve on the PCV.
You have to finger it like that till you find the best mixture.

If you found it you need to make some sort of plug that fits snug inside the PCV valve hose and drill a hole in the plug that is about equal in surface area as the opening in the ball valve.
Start with a small hole and try, increase the size slightly step by step till you found that same happy medium.
That hole size is the only thing you need to change at this point, do not alter anything else without writing things down what you changed so you can always go back to the basic setting you found previously.

Wietse said:

Good you are getting in the right direction.
A lean idle could be caused by lack of fuel or excessive air, the carb will only give the basic mixture.
Adjusting the fuel can be done as easy as opening the idle mixture screws, adding air can be more difficult depending on the carb in use.
As mentioned some "more expensive" or "more advanced" carbs have an idle air screw below the air cleaner stud to bypass additional air if required.
The other options are PCV line and drilling holes in the throttle plates.
Thing with drilling holes in the throttle plates is that it is permanent, and once too big or it did not work out you need to change them.
Playing with an orifice in the PCV hose is then very easy by trying different hole sizes and see what gets you the best results.
A lot of times with bigger cams that produce low vacuum is that the PCV valve turns into "full flow" mode and provide way too much air, your engine is pulling a decent and strong vacuum so that will not be your case.
As you mentioned it still wants some choke means that it is getting a little too much air/or too much air/fuel mixture from the carb. (with the idle mixture screws open that far it seems like that at least)
If the carb transition slots are in the ball park and the idle mixture screws are about 2 turns open try and keep the valve on the PCV closed and see how that runs.
Do not use the choke to do any adjustments, if it stays running like that you can crack the valve open and see if it improves. If the engine needs that extra air the rpm's will start to slightly increase.
If that is the case you can retard the distributor a little and the rpm will come down again.
If the rpm remains to low to run normal you can feed a little more fuel by the mixture screws and feed a little more air by the valve on the PCV.
You have to finger it like that till you find the best mixture.

If you found it you need to make some sort of plug that fits snug inside the PCV valve hose and drill a hole in the plug that is about equal in surface area as the opening in the ball valve.
Start with a small hole and try, increase the size slightly step by step till you found that same happy medium.
That hole size is the only thing you need to change at this point, do not alter anything else without writing things down what you changed so you can always go back to the basic setting you found previously.

Click to expand...

Hey thanks for this; been following this thread

Will be trying to tune an old Holley 750 for 10” of vacuum pretty soon. Pretty sure I’m way into the transition slot also.

Was not understanding that putting orifice in the pvc would have the same effect as drilling the butterflies; makes all kinds of sense though...

Thanks again!

Does that orifice need to be cleaned regularly?
Would imagine it might get gunked up after a while then affect the tune

That_Tall_Dude said:

Was not understanding that putting orifice in the pvc would have the same effect as drilling the butterflies; makes all kinds of sense though...

Click to expand...

Not really a 1 on 1 solution, in my case the engine was pulling only 7" vacuum which puts the standard PCV in "full flow" mode.
So i had the full flow available all the time, in case you are on the border line where the engine vacuum transition point is for the PCV (the vacuum where the valve changes from restricted to full flow mode) you cannot really rely anymore on that constant "full flow".
I am not sure at what vacuum the PCV transition point is, but guessing 10-12"
In this case you might have to install an ordinary hose tail without valve on the valve cover to work with an orifice.
(I.e. the PCV might already be restricting the flow, therefore the orifice hole size makes no difference at some point as the limitation is caused by the PCV itself)

Anyhow, due to a low vacuum at idle a standard PCV valve is going into "full flow" mode where it allows a bigger air flow being pulled out of the crankcase.
This is what was causing me big issues with idle as it is getting more air then the fuel feed could support, then you end up high in the transition slot as you will start to finger the idle screw, which will give issues later on at the end of the transition slot, the main fuel feed is not flowing yet and you have run out of transition slot...ends up with a bogging engine as it is with a lack of fuel for some time until the boosters start feeding fuel.
I did the installation of a fixed orifice after finding the right hole size and all was good.
As an option, which is not cheap but for sure a well made quality product, is the ME Wagner PCV, which is what i bought and installed and works really well.
This valve is fully adjustable in air flow quantity during idle and an adjustable transition point to full flow mode.
Engines with serious cams will require this to set it to work properly.
At the moment i still have it in dual flow mode, but since i installed EFI i will change it to Fixed Orifice mode.
Being at this point with EFI, i could have avoided buying a Wager PCV as making an orifice that works is easy.
Then again, the Wagner is a very good piece of kit and the adjustment will be with an Allan key instead of installing/testing/removing and drilling a bigger size hole.
They can be a pain but by all means.....do not remove the PCV function as this is very important for the engine!

Just FYI for someone who might not know:
A standard PCV valve has 2 flow modes, during high vacuum conditions as in idle and cruise the check valve inside the PCV is sucked up all the way against a seat, this causes a restriction in air flow.
During low/no vacuum conditions, during speeding up and high throttle/WOT conditions the check valve will be in a "float" mode where it allows maximum flow as it is not against the seat.

Thanks everyone for all of the help and advice. I finally got the idle problem resolved. I found out that my "rebuilt" Q-jet had the idle air bypass JB welded shut. The tip from Wietse on using a ball valve on the PCV line pointed me in the right direction of where to look. I ended up removing the JB weld from the idle air bypass holes that pass through the main body of the carb, and drilled out the idle air bypass holes in the base plate by .010". I got it to idle pretty well after that but it was still a little lean even with the mixture screws all the way out. The final fix for that was to drill out the idle restrictions in the main body of the carb by around .005". It now has a nice steady idle in park and in gear with a 13.8 A/F mixture while running on the idle circuit.

Great news.