Clark GPX30:
GPX30 won't stay running

My GPX30 starts and runs for a short time then stalls. It will run for 20-40 seconds at a slow idle before it dies. It'srunning really lean- if I press on the accelerator it dies right away. If I try starting it again right away it will catch for a few seconds or not at all, but if I let it sit for a few minutes it will run longer. The longer I wait the longer it runs, up to about 40 seconds. I'm not really familiar with propane systems, but it seems like it isn't getting enough fuel. It seems like some component slowly fills up like a reservoir and allows the system to run, but then there isn't enough flow to keep it going. It has an Impco 100 with a vacuum regulator. There is coolant going from the vaporizer to the regulator, and there is a strong vacuum at the hose going to the valve cover breather. I pulled the back side of the regulator and the diaphragm looked good. I played with the timing and checked the rotor and cap-they aren't perfect, but it doesn't misfire so I don't think that's a problem.

For the record, I had just fixed another problem (fwd/rev not working, directional fuse blown) and I had the instrument panel off and the directional switch disassembled, and when I put everything back together the stalling problem started. I can't see how the two are related because I don't think the Impco system has any electrical connections, but it was running fine before that (other than overheating from low coolant), so the timing is a little suspicious...
  • Posted 13 Jul 2014 14:27
  • Modified 13 Jul 2014 14:29 by poster
  • Discussion started by junkyard
  • California, United States
Showing items 1 - 15 of 23 results.
The system works entirely on vacuum that's sourced in the throttle bore of the mixer. If there's low vacuum in this circuit it may be low enough to not turn the regulator "on"- so to speak- especially while cranking while engine vacuum is at it's lowest.

I would check the hose between the mixer & the regulator- even a pinhole in this line will make a big difference.

I wouldn't run the lift w/out the PCV system working- the engine is designed to "breathe" & just venting the engine will cause excessive sludge buildup, etc.

In answer to your question- is it an on- off system?- The fuel system is a constant flow type that is controlled by the vacuum pull of the throttle bore of the mixer. This vacuum works against diaphragms which are spring loaded- there are different regulators which flow different flow rates at the same vacuum.
  • Posted 26 Jul 2014 00:34
  • Reply by bbforks
  • Pennsylvania, United States
bbforks (at) Hotmail (dot) com
Customers love technology- until they have to pay to fix it!
It is a JB-2 (from total source). The case says W.C. 0.08 lbs, but the specs I found for the JB-2 are -1.5. The old regulator says W.C. 0.2 lbs, which apparently corresponds to the red spring. Now for the good news. I pulled the line that goes to the PCV valve and plugged it, and it started and ran perfectly- smooth idle, no hesitation on acceleration, no stalls. It may be that plugging the line boosted the vacuum enough to make up for the wrong spring rate, or it may be a bad PCV valve. If the PCV is good I can still plug the line and run a "breather" on the PCV, but I'm wondering if the higher vacuum requirement of the regulator will change the mixture or fuel curve. I know this is handled at the mixer, but does the amount of fuel delivered by the regulator change by the amount of vacuum...or is it an on/off type of delivery?
  • Posted 20 Jul 2014 09:56
  • Reply by junkyard
  • California, United States
I'm not in front of it, but I still had the box in my car- the label says IMPJB-2. From what I can see, that is the blue spring model. Do I need the orange spring? Can I just change out the spring? When I get to the shop I will look at the W.P. rating on the case to make sure.
  • Posted 20 Jul 2014 07:34
  • Reply by junkyard
  • California, United States
What's the part # of the new regulator? The reason I ask is that there is 2 types of Impcos, standard and low vacuum operation. They have different springs in them, blue or orange. The GPX uses the low vacuum orange spring type. If you try to use the blue spring regulator on it will act just like you are describing with needing the hand over the carb to get it started but once running it keeps going.
  • Posted 19 Jul 2014 20:50
  • Reply by mrfixit
  • New York, United States
I put the new lockoff on and it ran well when I first started it, but on subsequent starts reverted to the fuel-starved condition. Through trial and error I've found that I can choke it with my hand while cranking, then pull my hand away and it will start. Sometimes it keeps going, and sometimes it dies. It seems to need a lot of vacuum to gt the regulator going, and once it's started the engine vacuum is enough to keep the propane flowing. It's pretty much the same problem I started with- I think the regulator problem was incidental to the original problem.
  • Posted 19 Jul 2014 14:49
  • Reply by junkyard
  • California, United States
Does anyone know what the timing should be? Pretty sure it's a 4G54, and there is an '89 date code stamped on the upper right side. The local auto parts stores don't list anything by engine anymore, but I think the rotor/cap is the same as an 1989 Montero. I'm not sure what kind of car would have an equivalent coil and resistor, though...
  • Posted 18 Jul 2014 16:58
  • Modified 18 Jul 2014 16:59 by poster
  • Reply by junkyard
  • California, United States
I finally got time to get back to it, and I put a new regulator on it. Unfortunately, the case on the VFF30 cracked when I removed a fitting, but I don't feel too bad, since it was suggested that I replace it anyway-just a pain because I wanted to test it. I plugged the vacuum line and plumbed the propane direct to the regulator to test the system and it fired right up and ran nicely. It didn't do as well once it was warmed up-not sure if it is because I was running it without the lockoff or if there is something else that needs attention, but it looks like the regulator was the main issue. I'll post again once I install a new lockoff. Thanks everyone for your advice-it was spot-on!
  • Posted 18 Jul 2014 16:52
  • Reply by junkyard
  • California, United States
I always do my best to replace the VFF-30, Cobra regulator & the mixer diaphragm at the same time so the entire fuel system is fresh & clean. Nothing like chasing your tail 'cause you tried to save a buck. Chances are that if one part is faulty/dirty in the fuel system then the other parts really aren't that far behind.
  • Posted 15 Jul 2014 22:15
  • Reply by bbforks
  • Pennsylvania, United States
bbforks (at) Hotmail (dot) com
Customers love technology- until they have to pay to fix it!
The Clark part number for the Diaphram in the IMPCO 100 mixer is #998838......if your going to replace it!when you take it out inspect it and the mixer bore for black tar goop???if the tar has gotten into the mixer then the Vaporizor is full of it as well as the VFF30 lockoff....
  • Posted 15 Jul 2014 11:38
  • Reply by kevin_t
  • Pennsylvania, United States
I really appreciate all of the input everyone has provided. I'm not sure when I'll be able to get into it again, but I'll check the pressure. Now that I've taken all the stuff apart I have a little bit of a feel for how the system works. I'm going to check the mixer next, and I'll just replace the diaphragm while I'm at it. It's a model 100, so the parts are cheap and probably easy to find here in So Cal.
  • Posted 15 Jul 2014 11:29
  • Reply by junkyard
  • California, United States
AT LEAST DO ONE THING.....on the back of the vaporizer the one with the two coolant lines,is a 1/8 NPT plug,remove it with the engine OFF and put a pressure guage in it and crank the engine over and note the pressure reading to us..............
  • Posted 15 Jul 2014 11:06
  • Reply by kevin_t
  • Pennsylvania, United States
If that is the case then I still don't know what is bad, and nothing has really emerged as a definitive diagnosis. I really don't want to start replacing things ****-nilly. That's kind of the point of trying to diagnose the problem-

From what you're saying I guess I need to pull a vacuum on the lockoff and the regulator output at the same time to check the regulator.
  • Posted 15 Jul 2014 10:49
  • Reply by junkyard
  • California, United States
and while you are at it, replace the diaphragm in the mixer. the cobra regulator and mixer diaphragm should cost less than 200.00 together, maybe considerably less depending on who you contact to purchase them from.
it requires vacuum on that hose, between the model j regulator and the mixer, for their to be 'normal' gas flow, so no flow with the hose off is expected.
  • Posted 15 Jul 2014 10:21
  • Modified 15 Jul 2014 10:25 by poster
  • Reply by edward_t
  • South Carolina, United States
"it's not rocket surgery"
REPLACE THE MODEL J VAPORIZOR WITH A IMPCO COBRA REGULATOR..........
  • Posted 15 Jul 2014 08:01
  • Reply by kevin_t
  • Pennsylvania, United States
I pulled all of the hoses and fittings between the tank and the lock-off-didn't see any problems. I never saw any filters or screens, by the way- After I emptied the lines I tried running it and cracking the valve as suggested earlier. It didn't run at all with this method. I pulled the input side of the lockoff and observed the actuator arm while cranking the motor, and the arm moved and opened the port, so I guess that eliminates lockoff issues, and of course vacuum issues.

I think it is the regulator; tell me if this sounds right: I pulled the propane hose off the model J (the hose that goes to the mixer) and cranked the motor and there was no propane flow. I can get propane by pushing the button, but if I get it running that way the button pushes out so hard that I can't hold it in.
  • Posted 15 Jul 2014 07:57
  • Reply by junkyard
  • California, United States

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