Mitsubishi FG40K:
Engine surge under heavy load/idle drop Chevy 4.6 vortec

Hello, all!
Recently I had to replace the starter and radiator in my shop's Mitsubishi forklift. After the repair was complete I drove it around the shop to see how it did and it seemed to be a little more touchy with the throttle than it was before. I paid it no mind. After complaints from fellow coworkers about the throttle randomly surging and having them nearly run into things I looked at it further. What seems to happen is when the hydraulic system gets put under high load such as the wheels being at full lock or any of the fork controls meeting it's max (tilt being all the way back, side shift being at it's full max) the idle will drop, vacuum will then spike up and make the throttle surge up. I have checked for vacuum leaks, checked the coolant temp sensor, cleaned the throttle body with carb cleaner, adjusted idle settings and still no change in behavior. I'm not very familiar with this specific propane injection system or it's parts so any help would be great.

Make: Mitsubishi
Model: FG40K
Serial: AF 876-0317
Engine: Chevrolet vortec 4.3lt v6
  • Posted 17 Jan 2020 02:47
  • Modified 17 Jan 2020 02:49 by poster
  • Discussion started by Andrew_tk
  • Virginia, United States
Showing items 1 - 7 of 7 results.
Update! Local construction vehicle mechanic came in and said it was the governor. So we ordered on and replaced it today. Problem seemed to improve slightly but when I tried adjust the idle and throttle via the governor the engine went all over the place in reference to the rpm. Now for some reason no matter what I do it high idling really bad.
  • Posted 4 Feb 2020 06:07
  • Reply by Andrew_tk
  • Virginia, United States
So the counter with will have to come back off, awesome hahaha. Ok I'll try and source those parts and try that and see what happens. Thank you so much for your help.
  • Posted 18 Jan 2020 02:57
  • Reply by Andrew_tk
  • Virginia, United States
It's the part that pushes the carb open just like the cable would if you step on the throttle petal. The part mounted directly under the carb is a governor. I suppose the distributor could have a problem with the vacuum advance causing it to idle erratically. You would have to check the ignition timing with a timing light.
  • Posted 18 Jan 2020 02:45
  • Reply by mrfixit
  • New York, United States
mrfixitforklift@aol.com
So is the dash pot the adjustment screw thing you mentioned before which attaches below the actual carb? And I did in fact mess with that other screw.
  • Posted 18 Jan 2020 02:21
  • Reply by Andrew_tk
  • Virginia, United States
Those "dash pots" can start to act erratic and should be replaced if that happens. I would replace it and lube the throttle linkage and carb shaft. Then try to adjust it to your liking. Also there is another throttle stop right on the carb that you can adjust up to help prevent stalling if you turn down the dash pot.
  • Posted 18 Jan 2020 02:04
  • Reply by mrfixit
  • New York, United States
mrfixitforklift@aol.com
That is correct however I have messed with that control so much and the main results I get is either the problem goes away but I have not enough power to move the forklift or the problem persists and I have higher rpm range.
  • Posted 17 Jan 2020 04:26
  • Reply by Andrew_tk
  • Virginia, United States
Doesn't that have a carburetor with a cable the operates it? If so it may have a vacuum operated device that kicks the throttle up when it senses a hydraulic load right by where the cable is. You can adjust that and some operates say it is so tricky to do fine movements they want it adjusted not to work at all.
  • Posted 17 Jan 2020 04:19
  • Reply by mrfixit
  • New York, United States
mrfixitforklift@aol.com

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