Car likes idling at 1000rpms

[frank1966]

Hey, my engine likes to idle at 1000rpms, when I turn it down to 750 and adjust the mixing screws I cant find a smooth spot. I'm using a compound meter vacuum gauge. It shows in the green - normal engine operation.

The needle is most stable at 1000rpms not sure why. I'm using all new wires, MSD distributor all in one.

I'm not good at using a timing light, how can I best set the idle at 750?
Rotate distributor to max vacuum, then idle screw, then mixing screws? Which is the best order of adjustment?

The engine was rebuilt, don't know whats inside, it does not sound like I have a big cam.

the engine is perfect at 1000, no vibrations or stumbling at all, smooth and balanced sounding.

[frank1966]

sorry duplicate post, I forgot I posted this last night

[Kern Dog]

Frank likes making multiple posts on the same subject.   

[MSRacing89]

Hey, my engine likes to idle at 1000rpms, when I turn it down to 750 and adjust the mixing screws I cant find a smooth spot. I'm using a compound meter vacuum gauge. It shows in the green - normal engine operation.

The needle is most stable at 1000rpms not sure why. I'm using all new wires, MSD distributor all in one.

I'm not good at using a timing light, how can I best set the idle at 750?
Rotate distributor to max vacuum, then idle screw, then mixing screws? Which is the best order of adjustment?

The engine was rebuilt, don't know whats inside, it does not sound like I have a big cam.

the engine is perfect at 1000, no vibrations or stumbling at all, smooth and balanced sounding.

You need to work the idle down and try to figure it out.  Generally if you are having to turn idle up to smooth out your system, you are off getting off the idle circuit to compensate for something.  Most likely a vacuum leak (make sure all your vacuum ports are plugged), gummed up idle circuits, etc.  Make sure your rear throttle plates are closed and you are starting the front plates and the proper location.  If you flip the carb over there is a small idle slot just under the front blades that should be slightly exposed.  Turn the adjuster screws out 1½ to 2 turns.........always a good place to start .

[frank1966]

Hey, my engine likes to idle at 1000rpms, when I turn it down to 750 and adjust the mixing screws I cant find a smooth spot. I'm using a compound meter vacuum gauge. It shows in the green - normal engine operation.

The needle is most stable at 1000rpms not sure why. I'm using all new wires, MSD distributor all in one.

I'm not good at using a timing light, how can I best set the idle at 750?
Rotate distributor to max vacuum, then idle screw, then mixing screws? Which is the best order of adjustment?

The engine was rebuilt, don't know whats inside, it does not sound like I have a big cam.

the engine is perfect at 1000, no vibrations or stumbling at all, smooth and balanced sounding.

You need to work the idle down and try to figure it out.  Generally if you are having to turn idle up to smooth out your system, you are off getting off the idle circuit to compensate for something.  Most likely a vacuum leak (make sure all your vacuum ports are plugged), gummed up idle circuits, etc.  Make sure your rear throttle plates are closed and you are starting the front plates and the proper location.  If you flip the carb over there is a small idle slot just under the front blades that should be slightly exposed.  Turn the adjuster screws out 1½ to 2 turns.........always a good place to start .

Thanks, i dont about this adjuster screw, what does it adjust?

[MSRacing89]

Hey, my engine likes to idle at 1000rpms, when I turn it down to 750 and adjust the mixing screws I cant find a smooth spot. I'm using a compound meter vacuum gauge. It shows in the green - normal engine operation.

The needle is most stable at 1000rpms not sure why. I'm using all new wires, MSD distributor all in one.

I'm not good at using a timing light, how can I best set the idle at 750?
Rotate distributor to max vacuum, then idle screw, then mixing screws? Which is the best order of adjustment?

The engine was rebuilt, don't know whats inside, it does not sound like I have a big cam.

the engine is perfect at 1000, no vibrations or stumbling at all, smooth and balanced sounding.

You need to work the idle down and try to figure it out.  Generally if you are having to turn idle up to smooth out your system, you are off getting off the idle circuit to compensate for something.  Most likely a vacuum leak (make sure all your vacuum ports are plugged), gummed up idle circuits, etc.  Make sure your rear throttle plates are closed and you are starting the front plates and the proper location.  If you flip the carb over there is a small idle slot just under the front blades that should be slightly exposed.  Turn the adjuster screws out 1½ to 2 turns.........always a good place to start .

Thanks, i dont about this adjuster screw, what does it adjust?

Sorry.....Mixing screws! 

[frank1966]

Yes the mixing screws i turn them in until until i get an rpm drop then turn them out one turn

[Kern Dog]

There are plenty of bits of info around on carburetor tuning. One popular method is to set the idle mixture screws  to where you get the MOST vacuum at idle. This obviously requires the use of a vacuum guage.

[XH29N0G]

Yes the mixing screws i turn them in until until i get an rpm drop then turn them out one turn

There are plenty of bits of info around on carburetor tuning. One popular method is to set the idle mixture screws  to where you get the MOST vacuum at idle. This obviously requires the use of a vacuum guage.

I use Kern Dog's method to set the idle mixture screws.  I have read that some people then open them up a small amount (1/8 turn or so to make it a little richer - but look that up). MSRacing 99 is explaining how to set the front blades so the transfer slot is just shaped like a square.  As I understand it, this is the position where you will want to set the idle circuit with the idle mixture screws.  The amount is something like:



I had not thought of vacuum leaks as a possible culprit.  I would check that too.

Generally when all is set up OK at idle, the idle mixture screws are supposed to be about 1 to 1.5 turns from when they just touch/bottom out.  I am also told you want to be gentle when closing them because they have a conical tip.

[c00nhunterjoe]

Why does it "need" to idle at 750 rpm? What do you mean when you said the needle was most steady at 1000 rpm. What did it do at other rpm? What is your actual reading.

[MSRacing89]

Why does it "need" to idle at 750 rpm? What do you mean when you said the needle was most steady at 1000 rpm. What did it do at other rpm? What is your actual reading.

Sensing a little hostility !  No "need", normally just try to keep mild built motors on the idle circuit if we can.  Also, depending on the curve, distributors can start advancing around 1000.  It was more of a general statement if the fact that if you are idling at 1000 with a mild motor.....something is a miss.  I will sign out on this one for now and let you take over.

[frank1966]

Please dont sign off this topic its helpful info for all. From what i know most cars idle between 500 750 rpms, this my 3rd muscle car. When i turn down the idle screw to 750rpms my vacuum gauge needle portex to manifold on carb starts to jump around, not alot and the engine doesnt sound smooth and balanced.  Im not sure how to read my gauge the outer vacuum number says 50 and the inner numbers say 20. According to tbe gauge im in a normal range.

Coumd it be my MSD all in one is the culprite?

[XH29N0G]

Typically vacuum gauges that I know either read in inches of mercury (vacuum is about 30), in PSI vacuum would be about 14.7, or KPA, vacuum would be 101.  I have not seen cm of mercury (That is what we used to measure with real mercury 76 cm).  I know things in inches of mercury (so 10 inches would be low, 15-20 inches would be a mild cam.   Can you post a picture of the vaccum gauge?

I do not know why they would have 50 and 20 as the upper limits.  Maybe if you give a value where one equals the other, we can figure out what 

[frank1966]

Here it is. Compound meter

[XH29N0G]

The one to 70 is cm of mercury - I don't think most of us work with those units unless we are gas chemists from the 1950's to 1990's.  (I was trained on these)

The inner scale is inches of mercury - what I always saw in my dad's car and on my American car.  I think those are the numbers most here will relate to.  

The top numbers are pressure, the 0-10 is in psi and the 0 to 0.7 is in atmospheres or bars.  Those are pressures in excess of atmospheric pressure.

In case you are curious, the gauge needle is probably welded onto something that looks like one of those new years eve spiral things that extends when you breath out and coils up when you breath in. If you take the gauge apart  you can verify this.  

[c00nhunterjoe]

Why does it "need" to idle at 750 rpm? What do you mean when you said the needle was most steady at 1000 rpm. What did it do at other rpm? What is your actual reading.

Sensing a little hostility !  No "need", normally just try to keep mild built motors on the idle circuit if we can.  Also, depending on the curve, distributors can start advancing around 1000.  It was more of a general statement if the fact that if you are idling at 1000 with a mild motor.....something is a miss.  I will sign out on this one for now and let you take over.

No clue what you are talkin about??

[c00nhunterjoe]

The gauge shown, you read the 0-25 scale.  If there are small variations when the engine stumbles, that is ok, otherwise it should remain steady. What tach are you using to verify rpm? Forget the 750 number. If it has a bigger cam in it then stock, it most likely wont want to idle that low, especially if it is a low compression engine.
  Start with the idle transfer slot as pictured in an above post. If the engine idles too low, do not open the screw as that will contribute to an off idle bog and other tuning problems. If your carb has adjustable idle air bleeds, that is a plus, if not, you will be drilling the throttle plates to increase airflow.

[frank1966]

I realized i never checked the gauge. I just installed a dakota digital dash!

[sccachallenger]

if this is a Holley, you can adjust the secondary throttle stop a little to open the blades a little more.
This will allow you to close the primary side a little and may make the idle screws function properly at your desired rpm.
A quarter turn is a good first try, don't go crazy!

[frank1966]

The gauge shown, you read the 0-25 scale.  If there are small variations when the engine stumbles, that is ok, otherwise it should remain steady. What tach are you using to verify rpm? Forget the 750 number. If it has a bigger cam in it then stock, it most likely wont want to idle that low, especially if it is a low compression engine.
  Start with the idle transfer slot as pictured in an above post. If the engine idles too low, do not open the screw as that will contribute to an off idle bog and other tuning problems. If your carb has adjustable idle air bleeds, that is a plus, if not, you will be drilling the throttle plates to increase airflow.

Its not my gauge, i turned down the idle mix screws until i get an rmp drop then turn it out a half turn. I still cannot get the idle below 1000, car runs perfect at 1000. Most cars idle between 500 and 750, that why i use that reference. Id hate to idle higher than necessary.

[HPP]

A carb or throttle body or air valve are simply a controlled vacuum leak used to modulate engine speed. If you can't get the idle to adjust properly by tuning the carb, then you have another leak somewhere else. Power brake boosters can be a common culprit.

Start by spraying carb cleaner any place there is a seam, joint, nipple, or hose and  if you hit a spot that changes the idle, you have found your  leak point.

[frank1966]

Thanks, when i turn it down to 750, its not stmbling or about to stall, its only an increase in engine vibration, shaking not a whole lot. If i have a vacuum leak by increasing the idle to 1000 offsets a vacuum leak?

[c00nhunterjoe]

If it has anything over a stock cam, it will not purr at idle. Can you take a high quality video and upload it to youtube. Having it idle at 1000 and 750, shot from the rear, and looking under the hood.

Shot like these of mine.

https://youtu.be/9gpncCxfZ44

[frank1966]

I will do that. I have pretty quiet mufflers on, it does not lope like that. Maybe its muffled and i cant hear it, it does not sound like there us a big cam in there.

[HPP]

Thanks, when i turn it down to 750, its not stmbling or about to stall, its only an increase in engine vibration, shaking not a whole lot. If i have a vacuum leak by increasing the idle to 1000 offsets a vacuum leak?

The leak is introducing additional air into the engine without the fuel to stabilize the idle. Turning the idle speed up is allowing enough fuel to draw from the carb to offset the extra air coming form the leak.

But as others have said, there are a lot of variables involved. Some cams flat out wont idle at lower speeds. The 300*/.590 in my 360 idled at 1200.