Carburetor Idle Return Troubles

[S4Cruiser]

I'm running a newly rebuilt JimC oem carb that functions flawlessly except I'm having an issue with it not returning to idle after driving. Start truck and idles perfect at 650 rpms. Drive truck and come to a stop light and idle is high (no tach yet to say exactly). Use foot to attempt to lift up pedal and no change. Tap pedal with some force, truck revs and then returns to appropriate 650 rpm idle.

I'm positive it has ZERO to do with JimC's work but more so how I have it connected to the throttle, etc.

The carb is on a 1987ish 2F in my freshly rebuilt '70 FJ40. I'm running a 60 series air cleaner and throttle cable (vs linkage) which required me to also source and run the bell crank mechanism that was used in early '73.

Based on my searching and reading, it seems I am missing a spring that connects the end of the throttle arm (see pic below) to a part of the bell crank mechanism.

Is the purpose of this 'spring' to keep the arm from returning to fully horizontal? Based on just looking at how a spring being attached to this part, it would attempt to prevent it from returning to horizontal. I'm trying to understand it's function as the bell crank already has a spring on it too.

Moving the linkage by hand, it returns the circled part below to horizontal. The movement is fairly smooth but with some noticeable notchiness (very technical description) at the end of the return rotation.

SOR Image:

MUD Image with referenced 'spring':

Pic of my carb with bell crank:

 

[LAMBCRUSHER]

I am running an fj60 carb with a 73 bellcrank and fj62 cable...I am having the exact symptoms you describe. it's driving me to drinking...put a heftier spring in and it was good for about a week. doesn't always stay high- it is very intermittent...I suspect a worn shaft or bushing, and the throttle plate catching the side of the carb bore... acts worse following a rain.

 

[S4Cruiser]

I am running an fj60 carb with a 73 bellcrank and fj62 cable...I am having the exact symptoms you describe. it's driving me to drinking...put a heftier spring in and it was good for about a week. doesn't always stay high- it is very intermittent...I suspect a worn shaft or bushing, and the throttle plate catching the side of the carb bore... acts worse following a rain.

At least we can be frustrated and driven crazy together!!

Hopefully someone can provide some helpful direction and solve both our issues.

 

[LAMBCRUSHER]

I'm sure it doesn't help that we've mixed and matched several decades of technologies together to solve our bastardized problems... I am planning on rebuilding another carb, and paying close attention to the shafts, bushings and clearance of the plates in their bores, as well as looking very hard at the linkage ball studs. might go so far as to fab new link arms and ball studs via McMaster Carr. of course, If I was serious about fixing this, I'd just drop that 3FE in and be done... Keep me posted, as I will you.

 

[thebigredrocker]

When you come to a stop would your rig idle high indefinately if you didn’t pop the throttle? If so, what about using calipers to measure distance between a point on throttle arm and spring bracket. Then pop throttle and measure again. Sounds like you are pretty sure throttle is stuck open a bit, a measurement might help confirm suspicions.

 

[S4Cruiser]

When you come to a stop would your rig idle high indefinately if you didn’t pop the throttle? If so, what about using calipers to measure distance between a point on throttle arm and spring bracket. Then pop throttle and measure again. Sounds like you are pretty sure throttle is stuck open a bit, a measurement might help confirm suspicions.

When you come to a stop would your rig idle high indefinitely if you didn’t pop the throttle?
- yes indefinitely, unless I popped the throttle

My next step this weekend will be taking the measurements you proposed above.

 

[LAMBCRUSHER]

same same on mine

 

[S4Cruiser]

Bump for any other ideas.

 

[White Stripe]

I guess first I would disconnect the cable from the carb and start the truck. Blip the throttle on the carb by hand and see if it returns to idle or not. If it returns to idle fine maybe your cable is gummed up. If it doesn't return to idle then inspect what's sticking as far the the throttle shaft or linkage. Looking at the pics that bellcrank seems to scream to me as a red flag. The design looks prone to binding. If that's the case I would ditch it and get a long lokar cable setup and just have it curve and come into the side without the bellcrank.

 

[Casey E]

Cable sounds like it needs more slack.

 

[Dizzy]

A general rule of carburetor adjustment is that the idle speed screw should be backed-off (turned out) to the highest degree without starving the motor of air. (This can be a challenge at higher elevations where the air density is lower.) If the throttle plate is cracked open too much (by turning in the idle speed screw in excessively), the carb technically is at partial throttle and the engine is idling on a combination of the fuel from the progression hole immediately above the idle mix screw, as well as fuel thru the idle mix screw opening. This can cause the inconsistent idle you report. Back off on the idle speed screw and open up (turn out) the idle mix screw to insure that the fuel is only delivered from the idle mix screw passage at idle.

If you really want to get out of the dark, my recommendation is to get an O2 bung and broad-spectrum O2 sensor installed in the exhaust and connect it to a air fuel/ratio gauge for purposes of tuning, either temporarily, or as a full time in-dash installation. I suggest this because running to lean at any driving condition toasts engine components, and creates peak cylinder pressure too advanced relative to the crank angle, and, running rich is no good either.

Now, if you have a vacuum leak (anywhere between your valves and the carb base), or low engine vacuum, you could have a possible reason for the excessive opening of the throttle plate )via turning in too far the idle speed screw). This is because the the carb is not getting enough engine vacuum for proper Venturi function. Vacuum leaks that affect idle can be at the carb base, the PCV system, a corroded EGR system, the brake booster, really anything below the carb throttle plate, or the intake manifold itself (a crack above the exhaust manifold).

 

[S4Cruiser]

Cable sounds like it needs more slack.

This is my thought too and next attempt at fix. Weather sux today but should be able to go for a ride tomorrow.

 

[LAMBCRUSHER]

I guess first I would disconnect the cable from the carb and start the truck. Blip the throttle on the carb by hand and see if it returns to idle or not. If it returns to idle fine maybe your cable is gummed up. If it doesn't return to idle then inspect what's sticking as far the the throttle shaft or linkage. Looking at the pics that bellcrank seems to scream to me as a red flag. The design looks prone to binding. If that's the case I would ditch it and get a long lokar cable setup and just have it curve and come into the side without the bellcrank.

Have done. not the cable. I run a brand new fj62 cable on this set-up... the hang up is on the carb side in my case. I suspect shaft and flap to bore interference...

Cable sounds like it needs more slack.

again, not in my case...

not my first rodeo...

 

[LAMBCRUSHER]

A general rule of carburetor adjustment is that the idle speed screw should be backed-off (turned out) to the highest degree without starving the motor of air. (This can be a challenge at higher elevations where the air density is lower.) If the throttle plate is cracked open too much (by turning in the idle speed screw in excessively), the carb technically is at partial throttle and the engine is idling on a combination of the fuel from the progression hole immediately above the idle mix screw, as well as fuel thru the idle mix screw opening. This can cause the inconsistent idle you report. Back off on the idle speed screw and open up (turn out) the idle mix screw to insure that the fuel is only delivered from the idle mix screw passage at idle.

If you really want to get out of the dark, my recommendation is to get an O2 bung and broad-spectrum O2 sensor installed in the exhaust and connect it to a air fuel/ratio gauge for purposes of tuning, either temporarily, or as a full time in-dash installation. I suggest this because running to lean at any driving condition toasts engine components, and creates peak cylinder pressure too advanced relative to the crank angle, and, running rich is no good either.

Now, if you have a vacuum leak (anywhere between your valves and the carb base), or low engine vacuum, you could have a possible reason for the excessive opening of the throttle plate )via turning in too far the idle speed screw). This is because the the carb is not getting enough engine vacuum for proper Venturi function. Vacuum leaks that affect idle can be at the carb base, the PCV system, a corroded EGR system, the brake booster, really anything below the carb throttle plate, or the intake manifold itself (a crack above the exhaust manifold).

have backed the speed out to a low rumble and even slower... issue I seem to have is I cannot add more idle mix screw without being too rich, but I cant back the speed off too much or there isnt enough air...no vacuum or intake leaks. I have considered this throttle plate to transition slot relationship; not convinced this is it, but it could be, and I am thinking about drilling a hole in the throttle plate like the old hot rod guys would do when they had this issue... if you have enough fuel but not enough air, add air... I run a Delta KC289 cam. sea level vacuum of 16.5" at idle.

 

[JohnnyC]

When it’s doing this have you gone and looked to see if the throttle plate is actually open a tad? And not returning

To rule out that maybe advance isn’t sticking ... then unsticking when you snap the throttle

If the throttle is sticking open then the issue is binding ... something is binding ... all those pivot points look dry

Clean them and lube them

 

[LAMBCRUSHER]

my linkage has been doused in dry graphite lube. your offering of the advance not unloading all the way is golden. I do know that my dizzy may need the bushing///some love...

 

[JohnnyC]

my linkage has been doused in dry graphite lube..

That was more so for the OP because of the pictures he posted

 

[S4Cruiser]

my linkage has been doused in dry graphite lube. your offering of the advance not unloading all the way is golden. I do know that my dizzy may need the bushing///some love...

Interesting as I saw zero change in sticking when switching from a non serviced to freshly JimC serviced dizzy.

 

[S4Cruiser]

That was more so for the OP because of the pictures he posted

Will lube her up later today...definitely dry!

 

[S4Cruiser]

When it’s doing this have you gone and looked to see if the throttle plate is actually open a tad? And not returning

To rule out that maybe advance isn’t sticking ... then unsticking when you snap the throttle

If the throttle is sticking open then the issue is binding ... something is binding ... all those pivot points look dry

Clean them and lube them

I have not visually checked the throttle plate.

I have manipulated the throttle by hand and can feel / see the linkage (rod that travels through the carb) not 100% returning. I need to try this with the cable not attached and see if there is a difference.