Fuel Mixture Screw doesn't change the idle.

[48ply1stcar]

I have seen a lot of threads about my issue, but no answer.  Hard to tune-up my car when the idle mixture screw doesn’t change the idle. 

Many said you, “must have a manifold leak.”  Re-installed manifolds with new gaskets.  Rebuilt the carb no change.  A member of my Plymouth club brought his carburetor to my house one night after I re-installed the manifolds.  He installed his carburetor, and I quote, “I hope my engine runs this smooth after I get it back together.”  So, I must be doing something wrong with my carb that I’m not seeing.  I have disassembled and cleaned my carburetor, made new gaskets because the others had center holes that were bigger than the throat of the carb (Summit Racing).  This carburetor is cleaner than most of the coffee cups I used in the Army.  What am I doing wrong or what am I missing.

[garbagestate 44]

You could be pulling in air through the throttle plate shaft hole. They do wear out. Last time I rebuilt one I had to pick through several junkers before I found one that didn't leak.

[P15-D24]

Check for vacuum leaks. Seal off the line to the vacuum advance on the distributor. Also check for leak in the carb body seams indicating a failing gasket. And lastly the mounting bolts and seams to the  manifold. Make sure you have the correct gaskets installed and no air passages blocked in the carb body. 

[knuckleharley]

I doubt you are doing anything wrong. You are probably just overlooking something.

 

What does the screw look like when you take it out? Are there grooves worn in the tip,or does the tip seem to be bent?

 

Have you taken a straight piece of steel,plastic,etc,etc,etc that is close to the OD if the idle screw and stuck it in the hole to see how deep the hole is and then compared it to the length of your idle screw to make sure you don't have the wrong one?

 

Can you try to take a flash photo of the idle screw seat in your carb to make sure it isn't damaged and that the hole is not deformed or partially blocked with trash of some sort?

 

Could it be a throttle plate problem? Have you checked the bores in the throttle plate for burrs? Checked the throttle plate insert to make sure no air is seeping around it due to wear? As far as that goes,have you checked it to make sure the plates are the right size for the bore? These cars are old,and who knows who has switched what over the years?

 

I once bought a car that had a trans that worked perfectly on the test drive,and then the trans started slipping after just a few miles of highway driving. Come to find out the previous owner had not inserted the speedometer cable in the rear of the trans for some reason,and had just wrapped the tailshaft with masking tape. I didn't discover this until I put the car on a lift. I still can't even begin to guess why he didn't just insert the speedo cable and bolt it down. I was right there.

 

If all else fails,buy a old carb with the same parts number and rebuild it using your new parts and see if that solves the problem.

[48ply1stcar]

53 minutes ago, garbagestate 44 said:

You could be pulling in air through the throttle plate shaft hole. They do wear out. Last time I rebuilt one I had to pick through several junkers before I found one that didn't leak.

Swap that out from my box of carbs.

 

I drove the car from 97 until 2012 with this carb.  I switch the block from a 53-218 to a rebuilt 56-230.  I just transferred everything to the new block.  The car still has my 68-69 High School Parking permit.

Edited October 2, 2018 by 48ply1stcar
Additional information

[P15-D24]

Did you transfer the distributor or use the one in the 230?

[48ply1stcar]

I'm sure I'm using the distributor from the 218.

 

[Lloyd]

All the idle mixture screw does is allow air to bypass the throttle plates when they are closed.

I've found when adjusting carburetors and assuming I have no vacuum leaks if the adjustment screw does not change the idle then my throttle plates are usually open to far. Which is usually the idle speed set to high although a remote possibility could be the fast idle cam on the choke as well. If the idle screw is holding the throttle plates to far open then the mixture screw will have little or no effect. Although one time when I had removed the butterfly plates during a rebuild and had not centered them in the bore before tightening them on the throttle shaft. Another thing I found on these B&B's is the rebuild diagram actually tells you which side is up and which side is down when installing the throttle plates on the shaft. As I recall it states looks for the engraving on the plate.

You can check either one by removing the carb and while holding the throttle plates closed shine a flashlight at the bottom and look through the top of the carb. One other thing to check while you have it off is make sure the idle port is just above the throttle plates when the plates are closed.

Edited October 4, 2018 by Lloyd

[Don Coatney]

I agree with Lloyd. One additional thing to check is the carburetor  linkage stop screw that holds the butterfly open too far not allowing the idle circuit to come into play. The small screw seen on the linkage to the left go the idle needle valve in this photo.

 

[kencombs]

2 hours ago, Lloyd said:

All the idle mixture screw does is allow air to bypass the throttle plates when they are closed.

I've found when adjusting carburetors and assuming I have no vacuum leaks if the adjustment screw does not change the idle then my throttle plates are usually open to far. Which is usually the idle speed set to high although a remote possibility could be the fast idle cam on the choke as well. If the idle screw is holding the throttle plates to far open then the mixture screw will have little or no effect. Although one time when I had removed the butterfly plates during a rebuild and had not centered them in the bore before tightening them on the throttle shaft. Another thing I found on these B&B's is the rebuild diagram actually tells you which side is up and which side is down when installing the throttle plates on the shaft. As I recall it states looks for the engraving on the plate.

You can check either one by removing the carb and while holding the throttle plates closed shine a flashlight at the bottom and look through the top of the carb. One other thing to check while you have it off is make sure the idle port is just above the throttle plates when the plates are closed.

OK, that differs from my understanding so let's discuss.

It has always been my understanding that the idle air bleed only serves to atomize the fuel being metered by the idle mixture screw.  The bulk of the air ingested at idle comes from the almost closed butterfly valve.  The control of idle speed is from the butterfly stop screw.  The idle mixture screw meters the correct amount of fuel, not air.   It is of note that all the manual that I have available state that turning the screw counterclockwise (opening) richens the mixture and clockwise leans the mixture indicating fuel metering.

 

How wrong am I?

Anyway, with my understanding,  I think the idle fuel passage is plugged or partially so.  That is assuming the idle speed is within range.  If not, start with lowering it.

 

To the op, did you remove the rivet plug above th,e idle mixture screw and carefully clean the passage, as well as the one from the mixture screw to the carb's bore.

 

Edited October 4, 2018 by kencombs

[Lloyd]

No need for discuss ken. You are right the idle circuit is a metered air and fuel mixture. It’s almost like a little carburetor in a carburetor. I think the suggestion I was offering is that if your throttle plates are open then the idle circuit don’t work so that’s something to look for. 

[knuckleharley]

5 hours ago, Lloyd said:

I think the suggestion I was offering is that if your throttle plates are open then the idle circuit don’t work so that’s something to look for.

 

• That was my understanding of the point you were making.

I have no idea why that heart in blue is there. I have tried to delete it,and can't.

Edited October 4, 2018 by knuckleharley

[Don Coatney]

15 minutes ago, knuckleharley said:

 

• That was my understanding of the point you were making.

I have no idea why that heart in blue is there. I have tried to delete it,and can't.

 

Hit the like button again then hit the "X" that pops up to unlike it and the heart should be gone.

[knuckleharley]

4 hours ago, Don Coatney said:

 

Hit the like button again then hit the "X" that pops up to unlike it and the heart should be gone.

Nope,didn't work.

 

I like all my posts,and don't want all the others getting jealous because I didn't put a heart up with them.

[kencombs]

18 hours ago, Lloyd said:

No need for discuss ken. You are right the idle circuit is a metered air and fuel mixture. It’s almost like a little carburetor in a carburetor. I think the suggestion I was offering is that if your throttle plates are open then the idle circuit don’t work so that’s something to look for. 

 

 

And I agree wholeheartly.  The wording of your original opening sentence confused me, not that it's hard to do that!

 

I think we agree that the Idle speed is the first thing to check.  If it can't be reduced without stalling, time to check the fuel passage under the rivet plug.

[Fernando Mendes]

On 10/1/2018 at 10:52 PM, 48ply1stcar said:

I have seen a lot of threads about my issue, but no answer.  Hard to tune-up my car when the idle mixture screw doesn’t change the idle. 

Many said you, “must have a manifold leak.”  Re-installed manifolds with new gaskets.  Rebuilt the carb no change.  A member of my Plymouth club brought his carburetor to my house one night after I re-installed the manifolds.  He installed his carburetor, and I quote, “I hope my engine runs this smooth after I get it back together.”  So, I must be doing something wrong with my carb that I’m not seeing.  I have disassembled and cleaned my carburetor, made new gaskets because the others had center holes that were bigger than the throat of the carb (Summit Racing).  This carburetor is cleaner than most of the coffee cups I used in the Army.  What am I doing wrong or what am I missing.

Turn your distributor in counterclockwise direction.

[Jeff Balazs]

You need to check your vacuum readings with a good gauge. I am betting that it is too low for the idle mixture screw to work. Try adjusting the timing until you get a higher reading .....18" to 21" is about optimum. Then see what happens as you adjust the mixture screw.

Hth, Jeff

[Dodgeb4ya]

Do you have a slotted base gasket under the carb?

[Dartgame]

When you rebuilt the carb did you check the float ? If the float has holes in it, then it will sink and make the mixture impossibly rich and completely insensitive to idle mixture screw adjustment. 

 

If not the float, then try spraying some carb cleaner around the throttle shaft while running and see if the engine speed changes or smooths out, if it does then the throttle shaft is loose and needs to have bushings installed on the carb base.

 

I’d carefully check the assembly of the carb as well, make sure any check valves (steel balls) and rubber o rings are present. There is an o ring that sits under the step up piston - I missed it and have rebuilt many carbs over the years....my car ran, but idle was less than sensitive to mixture changes at idle. After i found the missing o ring, and installed it, voila smooth idle and sensitive mixture screw.

Edited October 5, 2018 by Dartgame

[48ply1stcar]

Thanks everybody for the information.  Maybe I'll get a chance to work on the car in the next couple of days.  And I will post the solution.

[Dennis Detweiler]

I got the same results when I put a rebuilt Stromberg on my 39. I couldn't get the rebuilt Carter to idle properly, so switched to a professionally rebuilt Stromberg. Like you, I had the idle setting a little too high and the mixture screw had little effect until I slowed the engine down as slow as I could get it before stalling, then the mixture screw (needle) was more sensitive and adjustable.

[48ply1stcar]

14 hours ago, Dodgeb4ya said:

Do you have a slotted base gasket under the carb?

Yes I do, I've noticed that they have different thickness.  Multiple gaskets per set.

[Dodgeb4ya]

The only things left are carb base/bowl to insulator gaskets wrong or leak....... or carb idle circuit plugged up... this as long as you can keep idle under 550-600 IMO..

[linus6948]

If you want it restored to like-new you can send it to  Dimitrios and he will do it, his work is the best I have ever seen and he is a truly nice guy.

 

http://www.oldcarbrebuild.com/

 

2022 Update:

Apparently he has retired and is no longer in business.

Edited April 5, 2022 by linus6948

[48ply1stcar]

While doing my latest carb rebuild I noticed the two things.  New idle/mixture screw is loose, it wiggles a bit.  It's longer than the old one so I probably didn't notice it before and the throttle base was swaped out. ( I'll search for the other and compare) and secondly, Ithink I've been sticking my small ball in the wrong hole.  I have been putting the small ball in the slotted hole under the trottle plunger.  Maybe it belongs in other hole to keep fuel from going back into the bowl. I finally thought of that when the ball fell out of the slotted hole after the retaining spring was installed.  Let me know if I'm right please.