Choppy throttle when cruising

[Freightrain]

Seems it has manifested to a point of my insanity.  Likely been doing it awhile??

If I just hold the throttle steady going down the road or highway, the truck "shimmies" enough to shake/wiggle the throttle linkage and it makes it "chop".  I can hear it out the window in the exhaust and in the engine compartment.  If I set the cruise, it stays smooth(cable driven).

I replaced a few heim ends in the linkage to try to take some of the slack out, but it still does it.  I'm ready to convert to cable throttle if need be.  It's just annoying as hell.  As long as I am into the throttle, it is smooth.  Hold a steady speed, chop, chop, chop, chop.....  I don't think it is the govenor or it would do it with the cruise(which it doesn't).

 

Watched a video from Macungie, and watched a B model leaving and I heard that stutter as he just slowly cruised along.  I guess I'm not the only one?

[mrsmackpaul]

Does the injector pump have to levers on the shaft that the throttle linkage hooks to 

 

Mine does, the hand throttle linkage goes in one and the foot throttle in the other

The foot throttle arm has a little spring doohickie in it, I wonder if yours is the same and spring has lost tension

 

Paul

[Freightrain]

Mine only has one arm.   The hand throttle (not there anymore) just went through a hole to pull the same arm that the foot is attached to.

I basically put the cruise control on the same set up.  It is a cable, so any shake doesn't transmit to the throttle.

My only thought is the linkage on the firewall is causing the chop.  Either the engine or cab is shaking and wiggling the throttle.

[h67st]

I've heard other Macks with that chop. My H model will do that and I have to let up/push down to smooth it out, but that doesn't always get rid of it. I wish I had an answer for you.

[mechohaulic]

I'd agree with possible  weak spring tension. either a slightly heavier spring at pump arm OR install secondary spring on fire wall connected to pedal/ firewall linkage . this is a floor mounted fuel pedal ??? how smooth moving is the pedal ? metal pin through alum add a touch of dirt making erratic operation?

[Freightrain]

It will drive smooth, on a smooth road.  Get a washboard surface, and it starts.  It's almost rhythmic.  Taa, taa, taa, taa, taa...

The pedal is mounted to the floor, rod down through the floor to the arm that runs across the firewall.  From there a rod goes to the pump arm.  I need to buy a new pedal as the mounting bracket is broken and really corroded.  It works okay but need to improve it.

The firewall rod is only held by rolled pieces of flat stock.  It's sloppy, and no real way to fix that better.  The left side bracket is literally up against the back of the block.  It is tight.  I removed the passenger side and slipped a piece of 1/16" wall nylon tube to snug up the slop(wear) between rod and bracket.

If the cab or engine wiggles around, it is going to wiggle the throttle.  Thus if I convert to a cable, it won't be able to effect anything.

[seyser]

Larry - I think you are on the right track. A cable setup might be your best option. It sounds like you have already shimmed the current linkage and still not seeing positive results. The cab and engine will always move independent of each other and I wonder if that gets amplified as motor and cab mounts get worn out?

 

[Mark T]

Is the puff limiter disabled by any chance ?

[Freightrain]

2 hours ago, Mark T said:

Is the puff limiter disabled by any chance ?

Fire truck engine doesn't have one.

[mattb73lt]

I'm with mechohaulic with a higher tension return spring. It sounds like a driver induced osculation. If you're hard on it it's smooth, but with a lighter touch or partial throttle your body movement is causing it. I've had it happen in cars, especially my little BMW when it's cold. Tried taking off a few times and it coughs, throwing me forward and then back harshly. So I'm pumping the throttle in 1st gear laughing my ass off for being in a rush. 

I've also had some pilots I was training do that. Give them the controls and have them do a maneuver, like a slope landing or trying to hover over an external load and the aircraft starts hopping around, getting progressively worse. I take the controls back and it smooths right out. Give them back again and it starts to build again. I'd take the controls again, then tell them to reach down and tighten up their seat belt because they're hopping around in the seat. Actually gave me a few laughs messing with them that maybe the aircraft was about to come apart catastrophically and we're all going to die.

[mrsmackpaul]

On the R models out here they have propper bearings on the accelerator shaft on the fire will

Dunno about B models, might have been a RHD conversion thing

 

I'm thinking these all help to reduce movement 

Even propper rod ends

I guess, get the basics spot on first

 

Paul

[Mark T]

1 hour ago, Freightrain said:

Fire truck engine doesn't have one.

IDK, I noticed with it not working like it's supposed to, any bouncy situation telegraphs from your foot to the rack easier it seems.  Almost like the puff limiter dampens it a little.  Also a ridgid seat ( air seat with no air in it ) every little bump is amplified it seems. 

[Freightrain]

Ok fellas, I think I may be onto sumtin.

I was out readjusting the rod ratios to get WOT.  I was a bit off after installing new ends.  The rod was a bit too long and I could tell she was down a few pounds of boost last weekend.

While blocking the pedal at WOT, I was adjusting the hemi on the pump end.  I noticed that there was flex in the rod as I pulled it to attach the pump arm.  Hmmm.  Ya, after watching things I see that the plate on the firewall that holds the rod, was flexing back and forth.  Ya, just like the throttle is doing going down the road.  Ya!  I had replaced the return spring 20? yrs ago and got something "similar".  Well, it is wayyyyy to tight.  Causing the rod mounts to flex, making it waver the throttle.  Dammit, WTH.  So I will be hitting the local hardware which has a great selection of springs to try to come up with something a touch lighter.

I noted the spring, being mounted so close to the pivot point on the pump arm, only stretches about 1" at WOT.  The spring I had was stretched 3" just to get it installed.  Way too stout.

Hope to get back to update everyone soon.  Won't be driving it until the 27th for my next race.

[Mark T]

Maybe at this point someone makes some kinda cable assembly that would be relatively easy to retro. There's all sorts of things like that made nowadays .

[Freightrain]

I know I can make a Lokar cable work.  But I am going to work on spring tension first.

[mowerman]

I had the same problem with my F model glad you got it figured  I had no suggestions other that loose or worn springs which other people already mentioned  … engine manual… actually mentions it..… bob

[Freightrain]

Well I picked up a different spring.  Longer, lighter tension.  Didn't work.  My pump shaft has a snug spot right at the idle position and the new spring would not pull it down all the way.  Just that last touch.  Typically just enough to keep the Jake switch from activating also.  I moved the mounting point way out but not enough.  I reused the original spring, made a new bracket to split the difference between holes on the side of the pump and it seems to be okay.  Tight enough to get it to full idle and repeatedly made the Jake switch work.  Now, next weekend I will see if it does the wobble deal driving me down the road.  I don't think it should as the tension is quite a bit lower.  The rod mounts on the firewall don't seem to be flexing to get the arm moving.

[mowerman]

Excellent good luck as Tom mentioned sorry I didn’t have much time to talk to you so I would’ve liked to spend a little bit more time pretty quick it was all over. I was trying to see as much as possible. I will be there next year for sure. I am going to.spend more time there… bob