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Help on Propane carbureted scissor lift. Seems to be going lean on fuel.

Discussion in 'Forklifts/Telehandlers' started by fastline, Jul 7, 2016.

  1. fastline

    fastline Senior Member

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    Bought a big scissor a while back. Guy thought it was something to do with the hydraulics but I could tell the engine was not pulling the load right. When under harder loads, the engine stumbled and bogs down but otherwise idles perfect and if I tickle the high idle right, it will run at top RPM fine. I think when it wants more fuel from a hard load, it is going lean. I have never been in a Propane system and to make this worse, someone has bastardized the system to run on the small 20lb bottles and all the plumbing is not OEM. I need to see how we figure out if there is something going on with the regulator and valving or if the carb just needs cleaned and tuned?


    Anyone sharp on propane stuff?
     
  2. lantraxco

    lantraxco Senior Member

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    Not sharp, but I do remember working on them.

    Is this an air cooled engine? Does it have a heated converter (liquid to vapor) or just a regulator for vapor only? 20lb bottle will freeze down and fail to provide vapor if you're pulling much out of them.

    Sometimes it's nothing more than wear in the linkage. The old forklifts and such I worked on the carbs were bone simple butterfly for the air, metering valve for the fuel, and there was an adjustable link between the levers for them. Loosen locknuts rotate center barrel to shorten as I recall makes it richer (just move the fuel lever in the same direction the throttle linkage moves to open the carb up.) couple turns at a time until it pulls good, one or two more to keep it running cool. You can smell it in the exhaust if you get carried away. But then maybe you have a newer style carb.

    Spark plug gap can be an issue too, these engines run so clean the plug don't need changed for fouling, but eventually the spark erodes the electrodes until the gap is so wide they'll misfire under hard load.
     
  3. fastline

    fastline Senior Member

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    I ran some numbers and want to see if I am on track. I estimated that at full load on this 24HP engine, I could need as much as 250K btu/hr from the cylinder. It does not seem like a 20# cylinder can produce enough vapor to supply the engine at full load. Can anyone verify this? The OEM tank was a 60# or 15gal. The scissor will not need full engine power too long but when lifting, it struggles and the outriggers pull it down to a stall.


    However, when you consider some forklifts, the math does not compute. I know I had a big fork running a big V6 rated at probably 150HP or more, running off a 15gal cylinder.
     
    Last edited: Jul 7, 2016
  4. lantraxco

    lantraxco Senior Member

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    That's what I was saying, most forklifts pull liquid from the tank and a heated converter makes the vapor to deliver to the carb. Only small engines, usually air cooled, use vapor direct from the tank, and I have seen air cooled engines with heated converters as well but it's not as common.

    If you have a liquid system and the tanks are giving you vapor that won't work well either.

    The tanks will be freezing if you're pulling more gas than they can vaporize with ambient air temp.
     
  5. fastline

    fastline Senior Member

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    Well, this all seems to be lining up as I can pretty much verify this is a vapor only system. Vapor coming off the top of the BBQ style tank. Not even sure if you can even get an emulsion tube in one.....

    So, it is dual fuel but of course the gasoline fuel system is missing so I will have to setup a gravity system for now to get going.

    However, I do like the idea of propane as they work well in a building. Assuming that I want to convert this BACK to the liquid system that was likely on it to start with, What all do I need to do that? How do these work at start up if there is no exhaust heat?

    I did talk with one company that indicated I would need to place the converter next to the exhaust, which is no biggy, but I am stuck trying to figure out how those systems work now. Why doesn't the convert just freeze up? The heat from the exhaust is enough to keep things going? Do you then go through a low pressure fuel reg like the Garretson I have or does the converter feed vapor to the engine directly?
     
  6. lantraxco

    lantraxco Senior Member

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    Most converters have the regulator function built in, the liquid cooled engines run coolant through to warm the liquid propane. When starting there is no fuel flow so no heat is absorbed until the engine begins pulling fuel, and by that time coolant is circulating.

    24hp air cooled, you should be able to run on a vapor system, but you may need a bigger tank, not sure.

    Have any pictures of the current setup? There should be a filter in the line somewhere, or maybe that's only on liquid systems....
     
  7. fastline

    fastline Senior Member

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    I think to do this right, it should be on liquid. However, I just don't have the time so I have devised a plan. I am going to run 2 20# tanks in parallel which should double my BTU capacity and run them mostly full all the time. The vapor capacity is a function of how many gallons I have on board so it if ran half ass on an 1/4full tank, it might be enough to get us by on 2 full tanks. This assumes the engine would not really be at full capacity. They actually built this scissor with a 20HP engine so I am hoping the numbers will wiggle out and I might be down on HP just a bit.
     
  8. lantraxco

    lantraxco Senior Member

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    Actually vapor delivery is a function of ambient temperature and surface area of the tanks. The pressure in the tanks keeps the propane in liquid form, when demand pulls vapor off the top the pressure drops and heat is absorbed through the tank walls to replace what's lost in the conversion from liquid to vapor. Adding a tank should essentially double vapor delivery capability. You'll find out soon enough if that solves the problem.
     
  9. walkerv

    walkerv Senior Member

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    newer style bbq tanks also have a flow limiter built in so adding 2 tanks for a 24hp engine should help in that department also , found this out triing to run a portable genset on one tank along time ago
     
  10. fastline

    fastline Senior Member

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    Well, we are MUCH closer! Machine will now at least operate without stalling and will take high idle without stuttering. It won't take a high idle boost while operating a function but we can live with it for now. I ended up running two BBQ grill reg/hose units to the lockoff valve, then to the fuel valve. Works great!

    However, I now have a couple other issues showing up.

    1. Wheel brakes are NOT working and that became a quick point of safety just testing it on a little grade! It is designed to be towed so I am hoping there is something simple here like a brake bypass lever? Sort of need brakes! I can hit reverse and stop it but it gets interesting.

    2. Wheel power seems to be nearly nothing. It has good flat ground travel speed but give it any incline at all and even at high idle, it won't do it and it almost sounds like I am hitting a bypass or pressure relief valve. I know it should be able to handle some grade. I am going to see if my high idle is up to spec but I suspect something more is going on because I should be able to crawl into position, not power my way into it. I hope the drive motors are running ok.
     
  11. lantraxco

    lantraxco Senior Member

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    Scissor lifts typically are not designed for much if any grade, basket lifts on the other hand usually will climb some.

    Make and model?
     
  12. lantraxco

    lantraxco Senior Member

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    I had forgotten that, I'm old school, no fancy gadgets and if you want liquid turn the tank upside down, set it on the collar.
     
  13. fastline

    fastline Senior Member

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    Lift is a Marklift RT40G.

    Being a Rough terrain lift, I would sure think it could handle some grade. Were doing some testing and when the wheels are under load in a climb it goes right to a pressure bypass and the engine really does not bog much. I think it has more in it and suspect the valve has an issue. I cannot find ANY service manuals for this thing so I am purely guessing with it.

    I have a parts manual and it mentions a brake bypass valve and even an old sticker on the machine at the back but there are no hydro lines to any brakes and certainly no valves. Just two lines to the drive motors, that is it. I am a little confused right now.
     
  14. lantraxco

    lantraxco Senior Member

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    Ah, my bad you are correct, brain cramp.

    http://manuals.gogenielift.com/operators/English/TerexAerials/Marklift/17205.pdf

    Early versions apparently just used the motors for braking, there is what looks like a needle valve mentioned as a "Freewheeling Valve" which you open for short distance towing at low speeds, this may well be your issue with lack of drive power and braking ability. If there's a valve with two lines on it, make sure the knob is tightened to the right to close off the bypass.

    There are also some cushion cylinders that have quad ring seals on the pistons, if those are toast they may also cause similar symptoms, or of course a bad motor.

    Later units apparently were fitted first with disk brakes and even later spring brakes.
     
  15. fastline

    fastline Senior Member

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    Well here is what I know. The drive motors are run with only two lines, no case drains and I believe them to be the gerotor type. I know what the manuals state but i can tell you about half of this machine has been completely screwed up. The cushion cylinders are GONE, there is no brake valve, there are no lines to any brakes, etc.

    I know the machine was built beyond 1986 because the drawings I have show the valves that are what I have. There is basically only one spool valve that operates the drive motors. Push/pull and when one side goes high, the other side opens up to the return to the tank.

    However, i got to thinking about this and VERY confused as to why when I stop all motion, why does it freewheel like a bicycle? There is pretty much no resistance!

    Now, one might reason that freewheeling with no prob coupled with no drive power is likely two toasted drive motors and massive internal leakage BUT I am confused now as to why once the motors get in a bind trying to work, the system hits a relieve valve and you can hear it bypassing like crazy. I would not think a motor that was that worn out as to freewheel would plug things enough to hit a relief valve. I am still wondering if that relief has problems though since it seems to come in rather early before the engine really bogs much.

    I guess a simple test might be to jack it up, spin the wheels, then blockoff the hydro lines and see what it does.

    I would be just fine with the motors doing the braking but I also got to thinking about how they would handle the shock of a stop? When you slam the spool closed, it really should block off all flow and they should lock up. However, I determined the brakes are in there, a wet multi disc type, and they have GOT to be complete toast because there is no line at all to them and with bellevilles in there, they should be locked down. So the brakes are not taking any pressure off a stopping condition, no cushions for the motors, it seems like things are setup to break the way it is.

    When I was testing, when I got off the travel button, it ran right down a hill, but I could use reverse to slow it down. I did not know if it would hurt something but I sort of needed to stop. It certainly did not immediately change direction, but it worked.

    From the looks of things, I think as things broke, some super intelligent person took those broke parts and file 41'd them as "not needed". Always pisses me off to see how people maintain stuff.
     
    Last edited: Jul 8, 2016
  16. lantraxco

    lantraxco Senior Member

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    If it's the later style with the internal spring brakes, there would have been a brake valve which diverted pressure to release the brakes when you start to move the machine, you say it's not there? Typically given the spring brake, the control valve would have a "motor" spool which means both lines are open to tank in neutral, and that makes sense given the instructions for using the later model freewheeling valve, you close the valve and then bump the drive, this releases the spring brakes and locks the fluid until you reopen the freewheeling valve. The motors have to be open to tank to freewheel.

    Cushion cylinders were only used on the earlier machines with no brakes I think, so those machines would have had a normal cylinder type spool which blocks both work ports in neutral, this give the braking effect and why they needed the cushion cylinders?

    Best thing of course would be to repair the brakes and install a brake valve, the only other alternative would be a counterbalance valve to lock the motors hydraulically, but it's a compromise at best, hard on the motors as you say and not a secure parking brake.
     
  17. fastline

    fastline Senior Member

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    I honestly still cannot even see a port where the hydros would hook up the brake but I see something in the diagrams. I am so used to CAT books. This stuff is a joke. Get all wound up to learn what that fitting is, the books just says "fitting". So helpful.

    I am still amazed the brakes are not hooked up. Nothing.... There is a port on the valve block that says "brake" but plug looks factory. No way this machine is safe without brakes but will use is on soil, not concrete for now.
     
  18. lantraxco

    lantraxco Senior Member

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    Yeah, confusing much.

    Okay, I think in the PDF I posted the link for the right hydraulic drawing is: SCHE Page 8
    Section 2 figures 3 and 5 show tube and hose connections for the brake valve
    And the brake valve itself is Section 3 Figure 42.

    After some more perusing, it looks like the control valve is indeed a motor spool, all ports open to tank in neutral.
    The brake valve contains a shuttle to provide spring brake release when the control valve sends pressure to the motors. It also contains a counterbalance valve to provide cushion and hydraulic braking before the spring brakes lock up and some orifices to control motor speed.

    There are two needle valves for freewheeling on this variant, one with a check valve to hold pressure in the spring brake lines when closed so they stay released, a second basic needle valve that is opened to allow the oil to flow in a loop between motor ports.

    Sounds like you're missing all the valving.
     
  19. fastline

    fastline Senior Member

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    Thank you for the help!

    So if I simply connect a brake to the brake port, the port should be hot all the time UNLESS I am traveling, in which it will remove pressure? I don't need any return loop?

    What I am considering is trying to just connect one of the brakes to that port after port testing and see if I can just get one brake to work. I would be fine with that for now and certain it will be enough for now.

    Also, could you help me verify which relief valve is for the travel motors? I am looking for a way to splice in to test the pressure as I am pretty sure that machine should have more power than it does. When I am lifting the platform, the engine bogs, when I drop the outriggers, it bogs, but when I try to travel up any incline at all, it just hits the relief but does not seem to pull hard on the engine. I really think being an RT lift, it should be able to handle a "little" grade. I am fighting to get it up even the smallest incline.


    I posted a couple pics of the hub/drive motor area. You can see there is next to no room there. In the second pic you can see a small threaded area that I can only suspect is the right port. There is only a small window to access the area and there is no port visible on the other side so someone could have had that hub off and did not rotate it right.


    EDIT...... Scratch that. Head in the clouds. with no pressure on the brake and still holding nothing, it is obvious the brakes are toast. That won't work.
     

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    Last edited: Jul 8, 2016
  20. lantraxco

    lantraxco Senior Member

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    In the hydraulic schematic, in the "Drive control" section there's a dump valve with a 900 psi relief to tank. It needs to be activated to close off the line to the relief valve, and then the pressure can rise to the 1,800 psi main relief setting.

    Don't know why I missed it, but finally found the right electrical schematic, it's page 56 in the pdf, SCHE Page 10. There's a "Hi Torq" solenoid valve controlled by a 12 foot cut-off limit switch. Apparently once you've lifted to 12 feet, the limit switch cuts out high speed drive and also lowers the drive torque dramatically.

    So, either you have a bad switch, broken wire, or bad solenoid coil?