Carb/ accelerator linkage mystery

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D

DFXR

SILVER Star
10 Year 15 Year
Joined
Nov 18, 2010
Threads
45
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1,096
Location
Oakland, CA
Short story is my accelerator linkage is binding. Seems like the warmer the truck gets, the higher RPM it sticks at. It feels mechanical since I can bring it back to idle by pulling up on the gas pedal, or pulling back the linkage at the carb by hand, or goosing the throttle.

When the motor is cool, or warming up, it returns almost to idle but not quite - it sticks around 750 instead of 650. As I drive it more, it starts hanging up anywhere from 1500-2000RPM but will return to 750 if I goose it.

Long version:

This happened to me once before, about 10K miles ago, and I "fixed" it by cleaning the linkage with carb cleaner. No hint of this problem since then, til now.

Carb is in good shape, rebuilt @ Mudrak about 5K miles ago.

Here's what I've done this time around:

Pulled, cleaned and inspected linkage. All there, in good order, intact and working. I've tried running it with the linkage both dry and lubed (white lithium grease). Same issue either way.

Pulled the return springs at both the gas pedal and the carb base - both were in good shape and installed correctly.

Checked the function of the carb itself - it works fine when the linkage is disconnected and I work it by hand.

The only tinkering with the carb that happened any time recently was me putting in a used OEM choke cable a few months back. It worked fine and did not bind or hang up, but I went ahead and disconnected it at the carb while I deal with the linkage, just to rule it out.

I've read through old threads - I know this is not unheard of - but most people deal with it either by cleaning/lubing or replacing springs. Or they just keep dealing with it but never completely solve it.

** Big clue and hint that this is an old inherited problem rearing its head - the PO or PPO added a third return spring in the engine bay, where the gas pedal comes thru the firewall and meets the vertical rod of the linkage. I've kept this spring in place since I've owned the truck, and for the most part it's been running great. This go-around with the linkage, I even replaced with a new, tighter spring. This actually "solved" the problem for about 20 or so miles...but now it's back. But it does tell me that I'm not the first to deal with this. However, when this spring is not attached, the difference is not particularly noticeable.

Maybe when the PPO pulled and re-installed the motor after a rebuild it was a tiny bit off spec, and the third spring was his solution?

Any ideas? Anyone out there replaced their whole linkage including the pedal assembly anytime recently? Wondering how much of that is NLA.

Thanks in advance for any help-
 
Mine did this and I replaced the return spring at the pedal. Solved my problem.
 
Might as well start replacing things at this point. Hard to tell with these little springs how far worn they are from new.
 
I was hoping Jim C or Mark A would chime in.
 
Last edited:
I did a couple of turns to the vertical rod the create some very minor free play. That and new springs everywhere and my problem went away. I might add the new springs seemed to have the same exact tension as the old so I think the linkage adjustment was the key.

My problem was temperature sensitive, fine when cold but worse when hot. I'm certainly no expert but that tells me that expansion and contraction with different temps had something to do with it.
 
Bear Lake Cruiser said:
I lengthened rod 78104 by a smidgen.
Click to expand...

Thanks Bear Lake for the info. But did you lengthen or shorten vertical rod 78104?

Spike, thanks for that - the HZ rod is definitely installed in the right orientation.

We all hope for JimC to reply to carb threads I think... with a two-word answer that solves all problems. In the meantime I did find an old "sticky linkage" thread where he observed that the return springs in the assembly don't really wear out, in his experience. Reflects what Bear Lake says above. Hard to tell with these little springs.
 
Once the truck is hot enough to induce the problem, disconnect the last piece of linkage AT THE CARBURETOR, rev the motor up a few times [obviously by hand on the carburetor] and see if the idle stays high or comes down. Should help narrow things down.

I have seen this happen a few times. In the majority of cases I have seen, it was the linkage on the carb [IOW, I did the SAME test I just described, and the idle remained high, so I focused on the carb]

HTH

Mark
 
65swb45 said:
Once the truck is hot enough to induce the problem, disconnect the last piece of linkage AT THE CARBURETOR, rev the motor up a few times [obviously by hand on the carburetor] and see if the idle stays high or comes down. Should help narrow things down.

I have seen this happen a few times. In the majority of cases I have seen, it was the linkage on the carb [IOW, I did the SAME test I just described, and the idle remained high, so I focused on the carb]

HTH

Mark
Click to expand...


Mark, you were right (no surprise). I got it hotter than I have been, disconnected the linkage at the carb, and the sticking/binding continued.

So now my focus is the carb. The linkage all the way to the accelerator pump arm appears to be in order and I don't see any obstructions. It's clean, too. Is it possible for the accelerator pump to hang up inside the carb body (under the black rubber boot)?
 
Just a hunch; check the pivot bolt on the pump arm.
 
In case anyone was wondering...

I am back to normal, have put 50 or 60 miles on without issue. Followed Mark A's advice and isolated the sticking to the carb. I backed the accelerator pump arm pivot bolt off just a hair - it was very tight, now it's just tight.

However - by the time I got to this I had already shortened the vertical torque arm in the accelerator linkage as per Bear Lake's experience. I shortened it by about 1/8" or 3 full turns (this will make sense if you ever pull your linkage to clean it up).

So, this is a less-than-scientific resolution for my sticky throttle. If I had to do it over again I would start with the carb (easy), then clean the linkage (easy and probably due), and only then shorten the torque rod (still easy but not sure what the long-term effect is).
 
DFXR, similar problem. Only it takes a minute or two for idle to reach 1000-1100. Floats around there even after driving for awhile. Vac test says leak. Tried propane and smoke test with nothing showing up. Thoughts?
 
Well, it's back. So if you're experiencing something similar take everything I said above with a grain of salt.

I am "sticking" again, failing to return to normal idle once the engine is fully warm/hot. Sticks in the 1000-1500 range, more sticky the hotter it gets. A pump of the pedal usually brings it back to normal, but now there is something very strange - if I throw it in reverse, back up a short distance and stop quickly (like we often do in a 3 point turnaround), idle will drop dramatically to 500 or less, stumble, and occasionally stall. Again, a few pumps of the gas while this is happening and I'm back to normal.

Acceleration and power are smooth otherwise when actually driving.

Vacuum at normal idle (~650) is 20.

Just to complicate things, my EGR valve sticks occasionally - idle gets a tiny bit unsteady, vacuum drops to 15... I then tap the top of the EGR valve and it smooths out and returns to 20hg. Of course, this is not uncommon with EGRs and could easily be a parallel problem that is unrelated. However, I don't have and never have had the classic EGR stumble on acceleration.

I've checked for intake leaks with carb cleaner,
checked the carb mount bolts,
done a lean drop adjustment as per FSM,
backed off the accelerator pump arm pivot bolt,
checked carb visually for obstructions,
blasted the sh*t out of the carb with carb cleaner, etc.

I've also (based on Mark A's advice above) replicated the sticking issue with the accelerator linkage detached, which tells us that it is not and probably never was a linkage issue.

I have read plenty about the classic EGR stumbling issue and experienced it in another 60. But this is different since it happens at idle. Can a failing EGR cause problems at idle like I'm experiencing?

What about vac advance on the distributor? Along the way I found that the vac line from the advancer port on the carb (under idle mix screw) was disconnected. This is the line that goes through the hard line, passes through a black-blue VTV, and attaches to the vacuum advance port closest to the distributor. I re-attached, with no great result other than a little less power, which was what I expected.

So anyway... if anyone has any great ideas I'm all ears. If you've read this far you deserve thanks regardless.
 
DFXR said:
Can a failing EGR cause problems at idle..
Click to expand...

Definitely.

The EGR valve can leak in 2 places. The piston sealing the exhaust (the sealing valve) and it can also leak at the thin connecting rod bushing that leads up to the diaphragm.

Over the years, I have purchased several old used EGR valves to have as spares. They all behave a little differently. One leaks a lot more through the top pin bore than the others. Another one leaks a bit at the valve no matter how well I tried to clean it.

If the piston part of the valve is not sealing perfectly and consistently, it could stick open a tad and create a small EGR vacuum leak. But then when you pump the pedal, the valve could move a bit and reseat properly.

To rule out the EGR valve, just pull the vac hose from the top of it and plug the hose.

Swap the vacuum hoses on the distributor so the outer goes to inner and vice versa (if you're below 4000 ft.)

Then try it out.

If the engine is still is acting up, it's not the EGR.

>>>

Have you tried sticking your toe UNDER the gas pedal and lifting it back up when the idle gets too high? Does that return it to normal?

>>>

I can't imagine how putting the transmission into reverse would cause such a weird problem. Can you repeat what happened again? It may have nothing to do with reverse. Maybe coincidence.
 
Thanks.

I'll take the EGR out of the equation today and see what happens. That would make sense that there is some occasional sticking/vacuum leakage since vacuum is good (20) when idle is normal. When you've cleaned your various EGRs have you been able to get at the piston?

>>>
I can pull the gas pedal up to bring the idle back down - that's why I initially took this to be a linkage problem. But the problem persists even with the linkage completely detached at the base of the carb. Which led me to the accelerator pump arm.

What's also new is that pulling the gas pedal up can not only bring the idle back to normal - it can actually drop the idle down and starve the engine as described above. Like it's pulling the accelerator pump back below idle. Same issue as when reversing as described above.

I don't think it's actually reverse that is causing the idle drop - as you said there's no real link. I think it's more about stopping short and there being some kind of vibration or movement. Maybe engine mounts?
 
If you can pull up the gas pedal and reduce the idle, that's a mechanical carb issue. But since you've disconnected the linkage and it still won't always return to good idle, that's pointing to a mechanical sticking issue inside the carb.

When was the last time the carb was rebuilt? (By a pro)

If the answer is never. Now's a good time.
 
I get what you're saying. The pedal action has always made it seem mechanical. EGR could just be its own thing. But the carb was rebuilt at Mudrak around 5K miles ago.

The accelerator pump arm pivot bolt was a promising lead for a carb-specific mechanical issue... but it obviously did not pan out.
 
Disabled the EGR and the problem continues.

I did feel the mythical, magical tiny boost in power that comes with a disabled EGR though, so at least there's that.
 
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