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 16 - 23 of 23 results.
I'm not aware of any type of valving controlling the vacuum to the shut off valve- there should be a hose directly attached to the carb which goes directly to the shut off valve.

The components of the fuel system shouldn't get cold- in fact the regulator should get warm- it's plumbed into the cooling system of the engine.
  • Posted 15 Jul 2014 07:28
  • Reply by bbforks
  • Pennsylvania, United States
bbforks (at) Hotmail (dot) com
Customers love technology- until they have to pay to fix it!
I have disconnected the bulkhead fitting, which looks to be a spring-loaded brass valve- I assumed it was a pressure relief. I didn't see anything that looked like a filter in there.

My question about the lockoff vauum hose routing was: Where the hose connects to the engine is there any sort of valving that controls the vacuum?

The Impco compnents get cold after a while, but not cold enough to frost.
  • Posted 15 Jul 2014 04:18
  • Reply by junkyard
  • California, United States
Ignition timing won't cause insufficient vacuum to run the system- as long as the engine's running there's enough vacuum to keep the fuel system "turned on".

As far as the vacuum hose routing- the vacuum shut off ( the first big round thing in line from the propane tank) should have a small vacuum line from the mixer (carb) feeding it.

Is there frost developing on any part of the propane system- this is an indicator of a blockage w/the subsequent pressure drop.
  • Posted 14 Jul 2014 22:59
  • Reply by bbforks
  • Pennsylvania, United States
bbforks (at) Hotmail (dot) com
Customers love technology- until they have to pay to fix it!
There is a filter in between the the tank and the lock off in that dome shaped bulkhead. That would cause the lean symptoms if it's flow isn't enough to keep the motor running but will fill the hose while shut off. Starts ok and runs on the fuel in the hose but then starves when the filter can't keep up.
  • Posted 14 Jul 2014 20:18
  • Reply by mrfixit
  • New York, United States
Thanks for the info- I'll try that. I've bled the lines and run the engine to pull everything out of the system in case something was stuck or had some kind of pressure/vapor lock. It really seems like it is fuel-starved rather than rich. As I said, it seems that something fills up with propane if it sits for a period. Could a backfire rupture the diaphragm in the carb or change the mixture setting? I haven't re-set the timing since I played with it, but it runs ok for the short period that it runs... could bad timing create a low vacuum that keeps the lockoff from operating? I'm not near it right now, but if I recall the lockoff vacuum line goes to a fitting on the back of the water pump. There is another line close to it that runs to the carb, and that line is atached to a fitting that is in a raised area of the valve cover. Is that a PCV type of unit...or maybe something that regulates pressure to the lockoff valve?
  • Posted 14 Jul 2014 08:51
  • Reply by junkyard
  • California, United States
I have a funny feeling the Model J just happened to go bad while you were working on the dash (I've seen it before)

I would do the following:
Bleed the line from the propane bottle to the vaporizer (bleed it away from you- it's very cold & EXTREMELY flammable- make sure no open flames & plenty of ventilation for fumes to dissipate) & hook line back up to bottle

Start engine-if engine fails to start- just quickly crack open & close the propane bottle shut off valve.

Once engine started- let engine begin to run out of gas- as engine begins to stall- crack open (very quickly) & re-close the shut off valve once again. Engine should pick up & run normal once again.

If you can keep the engine running doing this procedure then the Model J regulator is faulty (passing to much fuel & flooding the engine). Replace with the newer Cobra regulator.
  • Posted 14 Jul 2014 04:25
  • Modified 14 Jul 2014 04:25 by poster
  • Reply by bbforks
  • Pennsylvania, United States
bbforks (at) Hotmail (dot) com
Customers love technology- until they have to pay to fix it!
Pretty sure it's a J. I'll check the pressure. Does the oil pressure switch interact with the regulator or carb? I feel the button on the regulator pulsing if I push it while cranking the engine, and when I stop cranking the button pushes out hard- I can't hold it in.
  • Posted 13 Jul 2014 15:37
  • Reply by junkyard
  • California, United States
Ther is a couple possibilities, 1)... When you had dash apart a wire for the oil pressure switch broke or came loose( going to the light in the dash)..or the PSI switch itself maybe bad....2) the regulator,even tho it " looked god" May need rebuilt/replaced..,or possibly the Diaphram in the air mixer needs replaced,just a few things to check.....on the back of the IMPCO. regulator(probably a model J?) there is a 1/8 pipe plug,put a pressure gauge here and see what your pressure is,should be around 2-3 PSI I believe.....IF you plan on replacing the regulator,I recommend installing a IMPCO Cobra model regulator,they are more reliable than the model J,little more $ put worth it in the long run,TRUST ME
  • Posted 13 Jul 2014 15:06
  • Reply by kevin_t
  • Pennsylvania, United States

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