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Why does my carb have two curb idle speeds (not talking about high idle)

thegreatjustino

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Red Head Grease Monkey
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Jan 23, 2002
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15,624
Loc.
Stockton, CA
I have spent several days now trying to get the carb on my Bronco adjusted correctly. '68 with a stock 289.

Using a timing light with tach and a vacuum gauge, I have set the timing correctly, adjusted the idle mixture screws until I have the highest steady vacuum, and used the regular idle screw to set what is supposed to be 700 or so RPM. All of this has been done with the engine at normal operating temperature.

At highway speeds, the Bronco pops wheelies. Accelerating onto the freeway has plenty of power. Idling on my driveway the vacuum is not perfectly steady but has a small needle sweep between 15 and 16 pounds.

If I have the idle set around 700 RPM (according to the tach there is a 40-50 RPM fluctuation as the engine is idling) and then rev the engine or go for a drive, when the engine returns to idle speed, the idle is now at 550. If I adjust the idle speed screw so the RPMs are back up to 700 then again rev the engine for a bit or go for a drive, the RPMs are then at 950-1,000 when the engine returns to idle speed. Then I have to set the idle speed screw back down and this whole thing starts all over.

Driving around town, half the time the Bronco idles decently, other times it wants to die. If I bump the RPMs with the screw and keep driving, sometimes the idle is OK, other times it's way too fast. This is just driving around town, not shutting off the engine, just stopping for lights and stop signs.

What on earth would cause the idle to fluctuate after driving here and there and/or revving the engine for a bit on the driveway? I can't seem to get it to idle smoothly but freeway driving is great.

Am I fighting a tiny vacuum leak?
 
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Slowleak

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Sep 12, 2013
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3,732
Loc.
Georgia
Could be a vacuum leak, bad vacuum module, or it could be a gummed up distributor. A sticking weight would cause timing variations which would affect idle speed.
 

Rustytruck

Bronco Guru
Joined
Feb 24, 2002
Messages
10,875
Try wiggeling the throttle shaft while ideling. May be loose shaft. Also move the shaft in and out and see if the butteryflies hang up on the bore.

While the engine is running watch the vacuum gauge spray around any vacuum sources, edge of intake where bolted to the head. Base of carb and intake. Replace every vacuum line or port plug you can find.

Check the distributer vacuum canister. When was the last time you lubed the distributer? Pop the cap, twist the rotor by hand and quickly let go. Does it snap back on its own? If sluggish pull the rotor and down the hole in the shaft there is suppose to be a felt wick. Put a couple of drops of oil on there. It lubes the advance shaft. Good time to check the points and adjust them on the wide side of tolerance.

A 1 inch vacuum vacuum swing is a good sign the engine has no major problems but you can go nuts finding the vacuum leak, poor fuel adjustment or other mal adjustment causing the issue.
 

pcf_mark

Bronco Guru
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Jun 11, 2010
Messages
3,574
I had this when the advance weights in the distributor had old sticky grease on them.
 
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thegreatjustino

thegreatjustino

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Red Head Grease Monkey
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Thanks for the suggestions. This Bronco has been driven infrequently for years. I had a massive vacuum leak last summer at the back of the intake and a new gasket fixed that.

I'm going on vacation for a few days, but will try the suggestions when I get back. Would like to get this figured out before I take the top off and enjoy the summer driving season.
 

SHX669

Bronco Guru
Joined
Jan 9, 2009
Messages
1,997
Addressing your "two curb idle speed screws " . You didn't say what carb you have but some have " 4 corner air fuel mixture screws " my Barry Grant did. Those types aren't actually an idle speed screw but a fuel/air mixture screw and you adjust those the same as you do a carb with only 2 screws - just twice as many -LOL.
If you're asking why is there an air/fuel adjustment screw on each side of carb { as in two total } - then that's because there are "jets on each side " of the carb.
If that's was just a Rhetorical question then they do that just to confuse us.
 

jckkys

Bronco Guru
Joined
Mar 15, 2012
Messages
5,196
"Stock '68 289" implies to me, a 2100. The symptoms indicate a variable vacuum leak. A sticky PCV valve could explain it.
 

73azbronco

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Bronco Guru
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Nov 11, 2007
Messages
7,796
Or as mentioned a wonky dizzy throwing out variable timing cues.
 

73azbronco

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I think there are opportunities for vacuum leaks once engine comes to operating temp on thermactor systems, right? Hence the term thermactor.
 

jckkys

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Mar 15, 2012
Messages
5,196
The Thermactor system gets full manifold vacuum to the divertor or anti backfire valve at all times, if the engine is hot or cold. The name may refer to the fact that pumping air into the exhaust makes it hotter.
 

DirtDonk

Contributor
Bronco Guru
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Nov 3, 2003
Messages
47,346
What about an anti-dieseling solenoid in this case? If it's present, check the plunger's position each time you have this issue. Perhaps it's cutting out once in awhile and letting the idle fall down.
You did say you were doing this only with the idle screw though, so I'm guessing your '68 does not have the solenoid. But figured I'd bring it up anyway.

When it's idling high, does the main idle screw work in a linear fashion? Reason I ask is that sometimes, even with the choke plate fully open, the high-idle screw plate will stick and the screw will just contact the first step up on the cam. When this happens your idle is falsely high. But it might seem as if you can still adjust it via the curb idle screw.

A long shot there, since you do get some inconsistent adjustments from the curb idle screw when the high-idle system is sticking.
But just on the chance it is doing something, it's a quick-n-easy fix (usually) to spray the choke side down with carb cleaner then give it a good lubing with some WD-40 or similar.
It's not a permanent fix, but it can get you going long enough to determine if that was your issue.

Paul
 
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thegreatjustino

thegreatjustino

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Red Head Grease Monkey
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Manual choke. Cable is pushed all the way in. Choke plate open and seems to be working correctly. When I have the low idle issue driving around town, I'll drive with the choke on to get the rpms up.
 
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