high idle after fuel mixture adjustment

[Wew]

Hi all,

So my engine has been rebuilt. (by me) Got about 300 miles on it to date. Everything is going fine, however there is still some tweaking that needs to be done with all the systems surround the motor.
Vacuum leak highly unlikey since all seals vacuum lines etc. are brand new, including injector housing sleeves and rings etc.
Couple of things I have noticed are the car was bucking at low rpm’s while driving even when warm.
I would also get a lot of exhaust backfiring when decelerating.
I had the car running and took off the cover to the air box and I heard a lot of air noise.
This was the first time I removed the airbox cover with the car running so I do not have anything to compare too. However, I tried testing the mixture by pushing up and pulling down on the metal arm that hold the the air intake plate. I pulled slightly down and the car would buck violently. I push up and it would purr. I could push it up quite a bit farther before it would not like how much distance I was pushing.
So I turned the fuel mixture screw in a clockwise direction until I got it right about in the middle of where I would push the metal arm up or pull it down about the same distance before the car started reacting negatively.
The result of this was, way less exhaust backfiring, and a car that seems to be running much better and smoother.
The down side is the idle now is high even after turning the idle screw all the way clockwise. I think I have bottomed it out. It is idling at about 1200.
Prior to richening the fuel mixture I could get the idle down to 850 – 900 no problem.
Of course the car wasn’t running as nice either.
Please share your thoughts and experience regarding this, as all input is much appreciated.

Thanks,

Gary

[Bob Kontak]

Originally Posted by Wew

Vacuum leak highly unlikey since all seals vacuum lines etc. are brand new, including injector housing sleeves and rings etc.

Nah - Vacuum leak is very, very likely.

If you can screw your air bypass screw all the way in and it does not die or sound like near death (i.e., chugging at 200 rpm) you have a vacuum leak.

Check silly stuff like the oil breather on the back of the air bellows and the hoses to the brake booster. At 1,200 RPM you have a big leak. Cap on the oil tank?

Edit: Scratch the oil breather system - It will cause your RPM to drop if sucking air.

[Bob Kontak]

Originally Posted by Wew

Couple of things I have noticed are the car was bucking at low rpm’s while driving even when warm.

Regarding specific experience, I did have bucking at low RPM when cold and occasionally at warm. Air was sucked in at the base of the #4 intake runner after I refreshed the gaskets and such (had CIS off). I found it by spraying carb cleaner at the base of the runners at idle.

Also had a putt putt chuffing noise out the exhaust before I tightened it. Like a backfire but without a pop if that makes sense.

Had snugged it but had forgotten to tighten. Best I never go into aircraft maintenance.

[Wew]

You know what Bob, you are the pelican bogwahn. I do recall stepping on my brake pedal I will check again for vacuum leaks. man all new hoses gaskets everything and still a potential vacuum leak. crazy.. just crazy... I will fire it up tonight and remove the oil cap to see what happens..
Also when I start it up it idles low, then about 5 seconds after initial start up the idle jumps up to about 2k. before going back down to it's current resting level of 1200.
I believe the increase in idle after the initial start is the WUR kicking in.. Is this correct?

[Bob Kontak]

Originally Posted by Wew

I believe the increase in idle after the initial start is the WUR kicking in.. Is this correct?

WUR starts off rich when in the full cold position. As it warms (electrically) it gets leaner. At first blush, I would say the lag is not attributed to the WUR if you are sucking air somewhere. Extra air likes extra gas.

That said, your car has four systems for bleeding air into your intake past the throttle plate. The air bypass screw, the auxilary air regulator, the decel valve and the auxilary air device. Your car is pushing 40 years old. Are they all working just like the day they left the factory? I would not worry about the short term lag before it comes to life. Work on that after the real false air issue is dealt with.

One trick (regarding the start lag only) is to plug the vacuum retard hose at the distributor and that gives you a wee bit more advance when you start the car. That can help. I do not know if the 78 has both advance and retard hoses in the module on the side of the distributor. The way to tell which is which is if the hose is sucking air at idle. If it has vacuum it retards the timing. The advance hose kicks the vacuum in after the RPM's increase. Mind you, I am on thin ice as I do not know what your car has. Most older (pre-SC) cars have vacuum retard and the advance is all mechanical but I am not sure of the early SC's.

[Hil911]

Could the 5 second surge be the extra fuel from the cold start valve? Doesn't it go for 7 seconds or something? The WUR response is much more gradual.

[Baby]

Wew, after my first-time rebuild I tested fuel pressures and vacuum leaks before I started messing with fuel and air. I found a few little leaks around injectors and hoses where things weren't seated quite right. Easy fixes for issues that surely would have complicated the pursuit of a proper A/F mix, which to my taste was complicated enough!

[Wew]

O.k. So I fired it up tonight and during warm up rpm was about 1300. I removed the oil cap and idle dropped. After rpm settled down, it sat at 950-1000. Go figure. I don't get it.
However, once it settled in at the 950 -1000 I again removed the oil cap and no change in idle this time? Perplexed. I used card cleaner and sprayed around the intake manifold housings checked vacuum lines and everything was connected. I have a concern it may be my box itself. The motor doesn't leak a drop since the rebuild except ���� in the triangle of death. Go figure. I changed the thermo sensor. Looked at it with a mirror and no leaking. It's gotta be the thermostat, which I put a new rubber ring gasket. The oil breather hose got some stuff going on and may have to be replaced. P.s. I am shocked with how many bolts have rattled loose since the rebuild. And I was meticulous getting everything torqued to spec. Wondering if those self locking bell shaped nuts are no good. Particularly on the exhaust. It just melts away whatever is coated on them.

But I digress. Point being I can't find a vacuum leak unless it's underneath the airbox where the brake vacuum connects.
Stay tuned and any insights of course are welcome

Gary

[Moozak]

Bench test the decel valve. I had a very similar situation when I refreshed my CIS. My decel valve was out of adjustment and was allowing air through during idle. When I adjusted to spec, my idle dropped back down to normal. You can try plugging the decel valve and see if the idle comes down. If so, test it with a mityvac.

[Wew]

Thx for the advice. Will check decel.

[dos531]

Did you replace the bypass valve seals while it was out? I had a high idle issue and it ended up being the bypass valve allowing air past the throttle body and raising idle speed. Was a pain to find since there was no external vacuum leak.

[T77911S]

what year is it. the CIS was different for different years.

the decel vavle (DV) was one of my thoughts. the early ones were adjustable.

check timing. make sure vacuum advance is working and not pulling in at idle.

has anyone messed with the throttle plate stop screw?

[Wew]

1978 is the year. Stock. Note sure how to check or repair a Decel Valve. Have done a lot of searches but no luck on a step by step so far.
I will check the timing.. I just had Dizzy rebuilt.
How do you check to make sure vacuum advance is working and not pulling at idle?
I am not sure what the throttle plate stop screw is on a 78 sc. Have researched and seen some pictures that show the general assembly but no pointer or specific reference to this exact piece or screw. Please share or clarify so I can check that as well. Thanks,
Gary

[Bob Kontak]

Originally Posted by Wew

I will check the timing.. I just had Dizzy rebuilt.

How do you check to make sure vacuum advance is working and not pulling at idle?

Get your car into that 950 RPM range by hook or by crook.

Set you timing light up. What is the timing?

Pull the vacuum hose at the pot. What is the timing?

Should be the same if vacuum advance is working. No change should be present at 950 RPM.

Take it up to 2,000 RPM. Pull the hose. Timing should move closer to TDC a few degrees.

Vacuum advance assists the mechanical advance but it functions ONLY after off idle. That is why you need to not be idling at 1,200 rpm.

I am pretty certain (after some searches) that the 78/79 distributors have vacuum advance but no vacuum retard. Retard works only at idle. If timing at idle moves from TDC to 5 BTDC when you pull the hose, it's retard. But I don't think that is the case.

[Wew]

Awesome. Thx bob. Will do that tomorrow. Was messing with my ride height and after adjusting rears and front and bouncing the car up and down numerous times I think I got it pretty close to good.
Fronts 25.5" n rears 24 7/8". Something to keep me busy while chasing down vacuum leak.
Will follow your steps tomorrow with an ice cold one. Suppossed to be in the low 70 ' s n sunny tomorrow. Might even take that targa top off.
Bottoms up. Thx again bob,

Gary

[T77911S]

you need to roll it.

you may have a vacuum leak if you can not bring the idle down.
is the vac line to the brake booster good
are all the intake bolts tight. they can losen

the DV is located behind the throttle body to the left. it has 2 larger vac hoses and one smaller one. the smaller one should have a T on it and one side goes to the throttle body and the other to the TV then to the WUR. the DV also has and adjustment screw. the way i checked mine was with a hand vacuum pump. if you dont have one, you can get creative and find a port that has vacuum on it with the engine running and connect it to it. the idle should go up.
you need tol keep vacuum going to the WUR or the mixture will change giveing you a false problem.

[Wew]

Beautiful.
Thanks for the detailed explanation.
Gary

[Bob Kontak]

Originally Posted by Wew

Beautiful.
Thanks for the detailed explanation.
Gary

T77 is very intelligent. I would stick with beer, though, rather than rolling one as he suggests.

You can always "force" feed 5-10 psi of air pressure into your air box and squirt the engine with a soapy water solution. Kids bubble blowing stuff works great.

I just duct tape the big air cleaner rectangular hole over and slit a hole for the air nozzle. Cock open the throttle plate a bit with a wedge of some sort.

Better ways to do it. boyt911sc (Tony) submerges the air box (car not attached) for some applications. That is serious but it sure puts the issue to bed.

Alternatively, find a below throttle plate vacuum port and feed air into that port or the hose feeding it. e.g., your decel valve hose.

If you hooked up air to the brake booster hose in the front trunk, you could listen for leaks in the engine compartment as the hissing noise from the nozzle would be a good ways away. Never did that one. Just talking. There is a one way check valve you may have to bypass.

You still need to close off the air cleaner side of the air box and even the muffler outlet to keep the pressure in so it bleeds out at the vacuum leaks.

[boyt911sc]

Gary,

Until you could prove that you don't have unmetered air going into the system, you are guessing!!!!!! Stop guessing and it would be easy to find and fix your problem. The oil cap removal test is performed with WCP (warm control pressure) at max. Or engine fully warmed up. You could test the decel valve for leak by using a hand held vacuum pump.

You are in good hands. These guys from Ohio and SC are very capable. I promised my wife to stay away from the Internet during our vacation unless necessary. Was able to resist it the first week. Presently, sitting here in a Bangkok shopping mall waiting for my wife and 3 nieces to finish their shopping. Why do women don't get tired shopping? This is their 4th day of shopping since we arrived 4 days ago. Got to sign out now before I get caught.

Tony

[kodioneill]

The EGR valve can stick open causing a induction leak.