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I'm trying to nail down an issue in the morning with the truck. Starts up nicely every morning with 1 or 2 pumps of the pedal, but has a pretty rough idle and feels close to dying. After a couple minutes or so it runs perfect. Or when I have light pressure on the pedal the idle is fine.
Again this is only in the morning and not throughout the day, or when the engine is warm. Side note, let's say I drove to work and let it sit there for 4 hrs the engine is cold but it's sunny out then the idle's fine.
'85 7.5 460 w/a Holley 4180
My initial thought is that there's an issue with the primary stage in the carb, or the idle system?
It's nothing serious as the truck is perfectly operational, but over the next month or so I am getting ready to do a partial fuel system replacement (both tanks, sending units, fuel lines, return lines, selector valve, relay) and I would like to address this as well.
What is the RPM when the choke is on?
I ask cause you said it will die if you take your foot off pedal .
Dave - - - -
It won't necessarily die when if I'm off the pedal, just runs pretty rough and feels like it wants to die. Good question about the RPM in the am it fluctuates a little bit between 5-8
What is the idle speed when fully warmed up?
You should look at the idle speed setting on the emissions label of the radiator support.
Maybe the fast idle cam behind the choke element is sticking and you're not seeing 1,100 rpm when the truck is warming up?
Bill Vose has mentioned this is correct and his advice is sound for me. (he did own and operate a carburetor and tuneup shop for many years)
What is the idle speed when fully warmed up?
You should look at the idle speed setting on the emissions label of the radiator support.
Maybe the fast idle cam behind the choke element is sticking and you're not seeing 1,100 rpm when the truck is warming up?
Bill Vose has mentioned this is correct and his advice is sound for me. (he did on and operate a carburetor and tuneup shop for many years)
Idle speed when fully warm is a very steady 5
In the am when cold it's bounces around 5'ish
OK I will check that setting on the radiator support.
With the motor overnight cold, before you start it, remove the top of the air cleaner and depress the throttle. The choke flap on the top front "air horn" should close about 2/3 of the way. If it just stays wide open, your choke is definitely not closing.
Over on the passenger side of the carb there is a fast idle cam that should rotate into position when the choke is closed. When the fast idle cam is in position it holds the throttle open resulting in a faster idle when the motor is cold.
It could be that everything is dirty, sticky and dry. You might try spraying a little lube on the choke flap pivot points on top of the air horn and on the wad of springs / fast idle cam on the throttle shaft passenger side of the carb to see if that frees anything up.
Also, you might post some pictures of the passenger side of the carb. Hoping to see the yellow fast idle cam in particular. There is an arm that sometimes gets broken off and if that's the case you will need a new fast idle cam to get it to work right.
Bone cold mine will idle at about 1,200 RPM in park, 900 in gear until it warms up. Then it's about 850 in park, 650 in gear.
What is the idle speed when fully warmed up?
You should look at the idle speed setting on the emissions label of the radiator support.
Maybe the fast idle cam behind the choke element is sticking and you're not seeing 1,100 rpm when the truck is warming up?
Bill Vose has mentioned this is correct and his advice is sound for me. (he did on and operate a carburetor and tuneup shop for many years)
Originally Posted by Brnfree
Sounds like a choke / fast idle cam issue.
With the motor overnight cold, before you start it, remove the top of the air cleaner and depress the throttle. The choke flap on the top front "air horn" should close about 2/3 of the way. If it just stays wide open, your choke is definitely not closing.
Over on the passenger side of the carb there is a fast idle cam that should rotate into position when the choke is closed. When the fast idle cam is in position it holds the throttle open resulting in a faster idle when the motor is cold.
It could be that everything is dirty, sticky and dry. You might try spraying a little lube on the choke flap pivot points on top of the air horn and on the wad of springs / fast idle cam on the throttle shaft passenger side of the carb to see if that frees anything up.
Also, you might post some pictures of the passenger side of the carb. Hoping to see the yellow fast idle cam in particular. There is an arm that sometimes gets broken off and if that's the case you will need a new fast idle cam to get it to work right.
Bone cold mine will idle at about 1,200 RPM in park, 900 in gear until it warms up. Then it's about 850 in park, 650 in gear.
Ok thank you for the input, I will get in there first thing tomorrow am and take a look at your suggestions. I will also take a few pictures and post and update tomorrow.
In the am when cold it's bounces around 5'ish
Isn't 500 rpm kinda low when fully warm? That's definitely too low when the fast idle is supposed to be active. If the warm idle speed needs to be increased, I wonder if that will also help with the cold fast idle, too.
1. First pic choke fully open, the choke plate is basically vertical (almost)
2. Second, throttle engaged choke plate is fully closed position. It does not seem to close 2/3 in my opinion, it really only looks like the choke plate rotates 25% but I do not have another truck to compare it to so hopefully you can tell from the pictures
3. An overview of the carb
4. Close up of the fast idle cam assembly. The yellow cam assembly bottoms out at the screw behind the assembly, which prevents the choke plate from closing more than in the #2 picture.
5. Final picture is of the connection on the choke cap. I need to replace this as the rubber insulation is crumbing off. What is it?
Follow up to kr98664: Idle speed: When fully warm idle is 500 in gear 750ish in park. Does idle need to be adjusted?
Nothing is frozen or gunked up, choke plate and assembly move freely. So am I wrong in assuming that the screw behind the fast idle cam assembly (I believe it's the "fast idle speed screw") needs to be backed off to allow the choke plate to close further?? Or is this whole issue related to the idle speed only?
Choke plate fully open
Choke plate closed as far as possible
Overview of carb
Close up of fast idle cam assembly, stop screw behind assembly not pictured
First off, you have to open the throttle just a little bit to free up the linkages that allow the choke to close. By design.
When I first open the throttle on mine after its good and cold I'm careful to notice two clicks as I slowly push open the throttle to pump it... *click, click* I actually can feel the choke snapping shut and the fast idle setting through the gas pedal.
DO NOT Ajust any of the little fast idle screws. That's how the plastic arms get broken off of the fast idle cams. Don't go there.
The base idle adjustment screw is located over on the driver's side. If you have a 460 with a C6 transmission hot idle in gear should be about 650 RPM.
BUT before you go adjusting your idle speed you really need to check your timing. Should be about 10* BTDC with the vacuum advance Disconnected.
No point in setting your idle speed to compensate for incorrect timing. Speaking of which... try this.
With the motor off disconnect the hose from the vacuum advance diaphragm on the side of the distributor. Stick about a one and a half foot piece of vacuum line on the distributor diaphragm and suck on it. If you can draw air through it your vacuum advance diaphragm is shot and needs to be replaced. A bad vacuum advance could very well cause a low idle. It does with my setup. Had one go out on me and it idled at about 525 in gear as a result.
My spare 4180 is in the 40* F garage and as you can see the choke wants to be all the way closed (after goosing the throttle to release the linkages).
Left side. Choke is closed. Fast idle is on.
Right side is looking up at the fast idle stop screw from underneath. Top, the fast idle is all the way on (Note that the fast idle cam has graduated steps)
Bottom, fast idle is off. Note the stop screw is not resting on the cam. Throttle should be resting on the idle speed screw over on the driver side when the fast idle is not engaged.
Follow up to kr98664: Idle speed: When fully warm idle is 500 in gear 750ish in park. Does idle need to be adjusted?
I'll defer to brnfree on this one. The low idle speed might be a symptom (caused by ignition timing, for example) vs. being the root cause. Definitely follow brnfree's advice.
Brnfree: First off, you have to open the throttle just a little bit to free up the linkages that allow the choke to close. By design.
When I first open the throttle on mine after its good and cold I'm careful to notice two clicks as I slowly push open the throttle to pump it... *click, click* I actually can feel the choke snapping shut and the fast idle setting through the gas pedal.
I guess I was too anxious to check it out last night, may not have been totally cold yet because when I opened the throttle it only closed partially (as pictured above). However, this am when it was overnight, I opened the throttle and the choke plate snapped shut very quickly. I forgot to look at the fast idle setting.
DO NOT Ajust any of the little fast idle screws. That's how the plastic arms get broken off of the fast idle cams. Don't go there.
The base idle adjustment screw is located over on the driver's side. If you have a 460 with a C6 transmission hot idle in gear should be about 650 RPM.
Yes I do have the 460 w/C6. Hot idle in gear is 500 maybe a hair higher.
BUT before you go adjusting your idle speed you really need to check your timing. Should be about 10* BTDC with the vacuum advance Disconnected.
This morning I checked the timing, factory recommendation is 8* BTDC, markings have it between 8-10. Checked with an inductive timing light and the timing is right on point.
No point in setting your idle speed to compensate for incorrect timing. Speaking of which... try this.
With the motor off disconnect the hose from the vacuum advance diaphragm on the side of the distributor. Stick about a one and a half foot piece of vacuum line on the distributor diaphragm and suck on it. If you can draw air through it your vacuum advance diaphragm is shot and needs to be replaced. A bad vacuum advance could very well cause a low idle. It does with my setup. Had one go out on me and it idled at about 525 in gear as a result.
Followed your suggestion. Unplugged vacuum advance diaphragm hooked up hose, could not suck air through it, I was able to get strong suction when I tried to draw air.
Below is an overview of the throttle linkage on the drivers side. Not sure if it helps or not. Also below a picture of the engine information sticker.
This morning when I manually opened the throttle and saw the choke flap close, I turned on the truck and it didn't idle rough. Although it was still low. Even dead cold it high idled in park at around 800-850, it was smooth, but man that seems low.
Okay, the fact that your choke closes all the way when cold is a good sign.
Now, with the choke closed look on the driver side at the idle adjustment screw. The stop on the throttle shaft should not be against the idle adjustment screw because the throttle should be held open by the fast idle cam over on the passenger side. Look over there and see if the fast idle stop screw is up on one of the fast idle steps.
Once the motor is warm and the choke is off the fast idle should be disengaged and the throttle stop should be against the curb idle screw on the driver side.
Now when you checked the timing did you disconnect the vacuum advance? Vac advance must be disconnected to correctly set base timing.
Next, remove the disctibutor cap and repeat the vacuum diaphragm test only this time you want to see the magnetic pick up plate inside the distributor move in response to vacuum being applied to the diaphragm. If it doesn't move it's stuck and needs to be freed / lubed.
Beyond all that, if everything is working like it should, cold idle in park should be 1,100 ish and 900 ish in gear. Hot idle in park should be about 850 and 650 in gear.
Because (in theory) the throttle is stopped against the fast idle cam when cold the curb idle adjustment on the driver's side should have NO effect on cold idle speed. Cold idle is purely a function of the choke and fast idle cam.
With the choke and fast idle disengaged the curb idle screw will work to adjust your hot idle speeds.
Oh yeah, not that it effects your idle, but I noticed the metal vent tube has slipped out of your rear carb float bowl. Should push that back in.
Thought some more about your low idle and I'm bothered why both the cold fast idle and hot idle are each about 175 RPM too low. Since they are each set independently from one and other it's hard to believe both of them are out of adjustment in equal amounts?!
And that brings us back to retarded timing. You checking it with a light is still no guarantee the timing is correct because the balancers have been known to slip on these models. If the rubber inside the balancer looks bad it may be suspect.
Does it run reasonably well but seems sluggish and lacking in power? If so, that plus the slow idle suggests retarded timing.
Assuming your vacuum advance is correct, I wonder what would happen if you advanced your base timing until your hot idle hit 650 with the vacuum advance connected? How would it run? IF you advance the timing and start pinging under a load, and/ or the starter is struggling to turn the motor over (and you've got a good strong battery) then the timing is definitely too advanced. But if it runs and drives great, with a noticeable boost in power, then you have to question the timing marks.
If you do decide to experiment, be sure to mark your current timing so you can always come back to it. Probably also best to make any adjustments in small increments.
I know mine likes extra base advance and I've probably settled on about 12* BTDC, + 14* manifold vacuum advance at idle = 26* of advance at idle! Seems to work though, Sharp off the line and better power and mileage than I got at 8*.
11-17-2016, 02:42 PM
