A question concerning Robin 35 with walbro carb.

Discussion in '4-Stroke Engines' started by motorbikemike45, Oct 21, 2009.

  1. On my new bike (GEBE drive system with a Robin EHO35) I first mounted the 1/2 gallon remote fuel tank about 12 inches below the engine. It started, idled, and ran under light load fine, even at high RPM, but it seemed to starve for gas in the midrange under hard acceleration with WOT. I remounted the tank about 6 inches higher, but still no joy. Now I have again moved the tank higher, to about the same level as the carb and now everthing seems to be working as it should.

    I thought the stock Walbro diaphram carb would pump the gas up from a foot or so, but I was obviously wrong. Is this typical of this type/size diaphram carb, or is it a problem with this particular carb I happened to get on this engine? I'll rebuild the carb if I thought new diaphrams would help. It looked a lot better when it was lower, but function overules looks every time. Ideas?
     

  2. augidog

    augidog Banned

    in another thread, the one about setting up an aux. tank, it was noted that the 4-stroke won't have much vertical drawing power...lower revs & half the "pulses"...if that's correct, that would explain it.

    my PF40 will draw 14 vertical inches.

    make sure all grommets and fittings are tight tight tight, same with however you "plugged" the original vent, and be sure your new vent is venting...take advantage of capillary-action by using dinky 1/8" line. we use a thick-walled line for good grommet sealing and so far it hasn't collapsed or starved the engine out.

    i'm just rambling & trying to help...if quay's setup was here i'd be able to tell you better how we did that one.

    there's remote tank info scattered around here we could gather & use as reference if we need to, how did you set yours up?
     
    Last edited: Oct 21, 2009
  3. I buy Robin engines without the standard fuel tank. On my first 2 bikes I used 3/4 gallon base mount tanks from Staton. This time I bought the side mount 1/2 gallon tank from Staton. I use the tanks as supplied with whatever vent and gas lines come standard. This is the first time I've attempted to mount a tank below engine level. It will also be the last time. Everything is working beautifully now and the GEBE system is really smooth and quiet. Almost have the engine broken in and I've tried a few short WOT runs. With the 13 tooth gear acceleration and hill climbing are adequate and I'm getting 29 mph (plus a smidge) on the flat with little breeze. I'm fully satisfied with the set up and any faster on a bicycle would be pushing it beyond what this old timer's slowing reflexes can handle.

    I just posted this to see if I might have a bad or substandard carb on this engine. I know some readers here must have tried mounting a fuel tank on the lowdown with a Robin and wondered if anyone managed to make it work for them.
     
  4. augidog

    augidog Banned

    ah, not an aux tank...well, then, that explains that about the 4-stroke's weaker draw, i'm glad to know it's all good now.

    btw-me too no original tank, but i removed mine on purpose. i can carry a 1.25 gal container with me, and my only tank is the "scooter" style one that staton offers, 'bout a 50 mile range with a full load.

    http://www.motoredbikes.com/attachment.php?attachmentid=21273&d=1253840810
     
  5. Silvaire

    Silvaire Member

    It is well known that 4-Strokes do not do as well as 2-Strokes when it comes to providing the negative pulse that powers the fuel pump on a diaphragm carburetor. It's just how the basic design of the engine is.

    However, with the stock tank, the Subaru EH035 engine draws as much as 8" vertical from the bottom of a nearly empty tank up to the carb.

    One issue that I have encountered with Walbro WYL carburetors stems from one of the check valves of the pump portion of the carburetor being a flap of rubber diaphragm material. With the garbage fuel that we have to use, over time this rubber will get brittle and distorted. When that happens the fuel pump may still work - but not as well as it will when the rubber flap valve is soft and flat. If I had problems with a remote tank not feeding well, one of the areas that I would take a good look would be the condition of this part in the carb.

    It's just my opinion, but I think that anyone running a engine with a diaphragm carb should have either a carb gasket kit or complete carb kit on hand (and some correct fuel line too) - just to have around for when one has to deal with a fuel related issue. A carb gasket kit is usually about a $5 item when bought from a competitive source.

    One issue that I have encountered with the Walbro WYL carbs stems from one of the check valves for the pump portion of the carburetor being a flap of rubber diaphragm material. With the garbage fuel that we have to use, this rubber will get brittle, curled and disorted. When that happens the fuel pump may still work - but not as well as it will when the rubber flap valve is soft and flat. If I had problems with a remote tank not feeding well, one of the areas that I would take a good look would be the condition of this part in the carb.

    It's just my opinion, but I think that anyone running a engine with a diapharagm carb should have either a gasket kit or full carburetor kit on hand - just to have it around for when you have to deal with a fuel related issue. A carb gasket kit is usually about a $5 part when bought from the right source.
     
  6. I bought the engine from a member. It may have been in storage a while so the diaphragms may have dried out some. I'll replace the diaphragms over the winter. Thanks for the info. Much appreciated.
     
  7. loquin

    loquin Active Member

    There's a very good description of diaphram fuel pump performance differences when used with 2-stroke and 4-stroke engines in Walbro's Pulse Fuel Pump Brochure.
     
    Last edited: Oct 30, 2009
  8. Silvaire

    Silvaire Member

    That's some very good info Lou!

    I have known for some time that it is very difficult to adapt a diapharagm carb from a 2-stroke engine onto a 4-stroke engine, and now I know the specifics as to why this is.

    The bottom line is that the crankcase differential pulse pressure of a 2-stroke is as much as 8 PSI, while the intake manifold pulse pressure of a 4-stroke is only about 2 PSI.



    Below is the info from that link which is specific to this subject:

    "A pulse fuel pump uses the pressure differential produced by the engine to move a diaphragm inside the pump body. This pressure differential is generally transferred via a pulse tube to one side of a flexible diaphragm in the fuel pump. On the opposite side of the diaphragm, check valves are positioned in the fuel channels to only allow the fuel to be drawn from the fuel tank and delivered to the carburetor.

    In a two-stroke engine the pulse tube of the fuel pump is connected to the engine crankcase. As the piston ascends and descends the pressure in the engine crankcase transitions between positive and negative. The pressure differential can be greater than 8 PSI. As this pressure differential is transferred directly to the diaphragm, fuel pressures are nearly the same as the pressure differential of the crankcase. Fuel flow is also directly related to this pressure differential.

    In a four-stroke engine, the engine crankcase contains lubricating oil. Therefore, the pulse tube of the fuel pump is connected to the intake manifold instead. As the piston ascends and descends, the pressure in the intake manifold transitions between approximately atmospheric pressure and negative. This pressure differential is usually less than 2 PSI. Because of this low pressure differential, a spring is added to move the diaphragm back when the negative pressure returns to approximately atmospheric pressure.

    A combination of the pressure differential and the spring force is transferred through the diaphragm and fuel pressures are nearly the same as the pressure differential of the intake manifold. Fuel flow is also directly related to this pressure differential.

    So, for a given pulse fuel pump, a two-stroke engine will provide a greater pressure differential and correspondingly greater fuel flow and pressure than a four-stroke engine."

    __
     
    Last edited: Oct 30, 2009
  9. Old Bob

    Old Bob Member



    This pressure differential is usually less than 2 PSI.


    More internet gossip/mythes.:ack2:

    Tests with Hondas the differential is much higher than 2 psi, in fact the differential is no less than 5.5 psi.
    While its true two stroke have a stronger pulse, the ability for a four stroke to pump adaquate fuel is not a problem with a reasonable head distance.
    The spring added to pumps has nothing to do with the engine being a four stroke per se, fuel tank position and carb position determines whether or not its equipped with a spring.
    Get you facts straight.
     
  10. loquin

    loquin Active Member

    Old Bob: Tell it to Walbro - They make the aftermarket pumps, and they're where the quoted information came from - straight out of their brochure...

    A few cases where the pressure is greater than 2 psi wouldn't make the statement invalid, either.

    "This pressure differential is usually less than 2 PSI"
     
    Last edited: Oct 30, 2009
  11. Old Bob

    Old Bob Member

    Not talking about after market pumps, talking about the built in pump in the carb. and manifold vacuum/pressure.


    What is the document you refer to and its part no.?
     
  12. loquin

    loquin Active Member

    The link in post #7, above. Walbro also makes carbs with the diaphram pumps built-in. The after market pumps I was talking about are separate, diaphram pumps that use the manifold vacuum or (in a 2-stroke) the crankcase pressure/vacuum.
     
    Last edited: Oct 31, 2009
  13. Old Bob

    Old Bob Member

    Its still off topic and Walbro should do real world testing.Those pumps are for larger engines than the EHO35 I never measured a lawn mower engine so it may be that low, but not in the mini four strokes.They don't take into account spit back which is caused by the intake valve shutting and the mixture flow stopping abruptly, causing a spike in the intake port presssure. These small engines are plagued by spit back. They also have a fairly high intake port vacuum because of the small port and short valve timing.
    The question was about the internal pump in the carb used on the EHO35, all the WYL series carbs are internal pump carbs.
    Honda used 6 different carbs on the GX31 alone, half of them had the return spring the other half didn't.The spring does not make it a four stroke carb.
    98% of all pumping issues with the WYL are related diaphragm problems or clogged inlet screen or clunk.
     
  14. loquin

    loquin Active Member

    I agree that a 2 PSI (if it is or isn't valid for these motors) shouldn't a problem. After all, 2 PSI is equivalent to the pressure of over 4 feet of water column and over 5 feet of gasoline. If you assume that half of this head would allow the fuel pump to push/pull enough gas to keep the engine running, you're still looking at two and a half feet...

    As far as some carbs having a spring, and some not, if the diaphram is 'springier' it could be act as it's own 'spring.' Or, if the carb was intended for use on a motor with an attached fuel tank, the diaphram could be springy enough to work just fine...

    However, the physics of the design of a separate diaphram-style fuel pump are exactly the same as an attached fuel pump. The basic pump design is the same as well. (Specifics, of course, can be different.) So, in that regard, the information provided by Walbro describing how separate diaphram fuel pumps actually work is applicable.
     
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