Honda gxh50 / under-mount fuel tank?

Discussion in '4-Stroke Engines' started by mdsman, Jun 19, 2013.

  1. mdsman

    mdsman New Member

    So just starting out down this trail & I'm curious if there is an EXTREME MAKEOVER needed to Under-mount a Fuel Tank if I use this gravity fuel delivery engine?... I've at least seen this application used on a Site where they sell Motorized Bikes but when I called my local Honda Small Engine Expert he said that it may be more trouble then it's worth??? adding that a Fuel Pump is needed which is no BIG thing but then you would need to tap into the Crank with a PULSE LINE...:-/

    This is where he lost me... Can anyone expound with your vast wisdom & experience?

  2. BigBlue

    BigBlue Active Member

    ThatsDax has a rotary valve diaphragm carburetor that doesn't need a pulse line from the crankcase. It uses a prime bulb, like many weedeater engines. The carburetor is $12.99 and the necessary adapter is $19.99:

    Here's Dwayne's YouTube video on how to install the carburetor:

    Might want to watch Terry Blow's YouTube video on how to remove the Honda GXH50 carburetor. Just skip the part about installing the HS carburetor and watch Dwayne's above video to install the rotary valve diaphragm carburetor:

    Good Luck,

    AKA: BigBlue
  3. mdsman

    mdsman New Member

    WOW... that was fast Chris... Really appreciate it!

    > Still open to anymore ideas and/or Real World Experience with this type of Conversion & how it worked for yall

  4. GreenMantis

    GreenMantis Member

    I also mounted up a 15mm rotary carb to mine, and it loves it. However, it isn't actually a requirement to use an undermounted fuel tank. Honda makes a fuel pump for them, it bolts to that 2" round hole near the bottom of the engine block, under the carberator. It's vaccume driven, it has 3 hose attachements, in and out, and one to the pcv crankcase vent, to drive it. In addition, cheap, Chinese mopeds also usually have similar vaccume/diaphram fuel pumps, that mount universally on the frame, someplace. These can be connected the same way, I presume.
    I did my homework on the rotary mod, and found that the 12mm rotary is about the same bore as an overbored (upgrade) original carb, so the 15mm rotary I used is overkill overbore. But it loves it, I have no problems with it. It can be adjusted too lean to run, but it can also be adjusted too rich where you can smell the exhaust, and it blackens up the plug. So there is a comfortable zone in the mixture screw setting that will run great. I mention that since someone advised me against this, because his original cab is very sensitive to mixture settings. I don't have any such issues. I used the air cleaner assembly that came with the rotary carb, and drilled a hole in it, and stuck the hose for the pcv vent in, so that the end of the hose is past the air filter, so dirt cant get in the hose, and into the engine. There are filters you can get that just go on the end of the hose. I bet you a fuel filter would suffice.
    If you have an original Honda GXH50, those have a centrifugal governor that the carb is locked into. You need to replace that with a Huasheng carb just to use the motor any other way, the Honda carb is too lean to use in an on demand application. Take that into consideration, if you have an original, you have to change the carb, anyway, but if you have a Huasheng, all you need is a Honda fuel pump.

    Attached Files:

    Last edited: Jun 28, 2013
  5. 5-7HEAVEN

    5-7HEAVEN Active Member

    The Honda expert meant to say "Tap into the crankcase, not crank".

    Pulse line means vacuum line.
  6. GreenMantis

    GreenMantis Member

    This is provided for, there is a vent hole in the crankcase just underneath the intake port, with a rubber hose stuck in it. My Huasheng simply connected that hose to the airbox, but the original Honda mounted a diaphram fuel pump just below that, and in that case the hose from the crankcase vent hole connects to the fuel pump to provide the pulse for the pump. I think in this case "pulse" means both vaccume, as well as pressure, caused by the (bottom of) piston changing the volume in the crankcase as it moves up and down. In any case, that is provided for. Just take the hose from the crankcase off the air box, and connect it to the fuel pump. This fuel pump: or this one and attach the pulse hose to the third spout on the fuel pump. And cap the plug on the airbox that the pulse hose is connected to now, so you don't suck dirt down your carb. I suggest this route, since it's the cheapest, and least invasive way to mount the fuel tank anywhere you want to. I went straight to the rotary carb, because I already had one, and even reused the intake manifold by elongating 2 of the bolt holes, and making a new gasket, so I could do this immediately, without buying anything. Again, it loves it. I could not get it to work for awhile, I had to keep priming it to keep it going, in the end what it wanted was an adjustment to the metering valve, and that cured it, it LOVES the rotary carb, just for the record.
    Last edited: Jun 28, 2013
  7. Mantis, how did you elongate your bolt holes to mount your diaphragm carb? Did you have a diaphragm carb that was the same as the Hondas carb mounting holes? 42mm C to C I think and I think the closest I have found are 36mm mounting hold. Dax finally got more of the adapters in but they are for the 12mm carb with a corresponding bore. I want to run my 15mm walbro with high and low speed screws. I have a 12mm I could throw on there with the adapter but the only adjustment screw is for idle :snobby: so no go on that.

    I remember you talking about your 15mm either idling off or bogging a little at the low end compared to your 12mm. Is it still an issue?

    I tried to find your thread where you where talking about these things but couldn't come up with it.

    mdsman, plus 1 on the fuel pump for most simple way to fix fuel delivery problem. I am running the Walbro fuel pump and it is more than I need. Once the fuel propagates the lines, this thing rocks! I got mine from
  8. GreenMantis

    GreenMantis Member

    OK, first, all the issues I had initially when I put the 15mm rotary on, were instantly cured by an adjustment to the metering diaphram and needle valve, it loved that. As for the plastic manifold that came with the 15mm rotary, it has 2 seperate sets of holes, one has nuts pressed into the plastic, those holes bolt the carb to the manifold. The other set of holes (there might be 4 holes, but only 2 are used) bolts the manifold to the head (these go on first). As it turned out, these were only a few mm, like 4 or 5 too close together to bolt to the huasheng head. However, there is enough plastic around those that by drilling and some dremmeling, all the way out until the outside corners are as thin as you can get them, the bolts go in and go on. The sealing surface is on the inside, between the bolts, so it doesn't matter what you do to the outside, as far as those holes go. Also note that the carb doesn't sit straight up and down like this, the manifold will be twisted counter clockwise a couple of degrees.
    Look at the manifold for the 15mm, look at it from the front where the carb goes on, the top left, and the bottom right through holes just need to be stretched out to the edge of the plastic, the corner of it. In fact, take the carb and manifold off the 4 stroke, and hold the two together, and you can see that those holes dont have very far to go.
    And like I said, a tweak to the metering diaphram, and my 15mm started to sing like the fat lady, it runs awesome, now.

    There was a tiny issue, and that is that at a couple of corresponding spots, there was just barely enough overlap of the different surfaces to make a seal the whole way around. I got sick of constantly buying new carb base gaskets, and then this, so I found a little roll of gasket meterial, and made my own gaskets, old shcool, like my daddy and grandpappy used to do back in the day. Being able to make a new gasket that took nnto consideration the difference in the shapes of the parts, and made sure that those got sealed good.
    If you want to, me your snail mail, and I'll mail you some gasket(s) to expediate your project for you.

    You know, come to think of it, I have a couple of those manifolds for the 15mm's floating around here, it only took me 5 or 10 minutes to do the holes out, I could do that for you, too, if you want, or dont have one to spare.
    Last edited: Jul 7, 2013
  9. Ok, got it on drilling out the holes. Does the exit port on your manifold look like this one or the black one?

    intake-manifold1.jpg intake manifold.jpg

    The tan one came with my 15mm carb but the exit port is much different than the square intake port on the engine head

    Thanks so much for the gasket offer too
  10. Here is the front side of tan manifold

  11. GreenMantis

    GreenMantis Member

    49cc_motor.jpg DSCN2445.jpg DSCN2446.jpg

    OK, you are starting with something that had a real Walbro carb, I was starting with the ever cheap, Chinese engine called a 44-5, and it's clone of the Walbro carb. Here are some pix of what I was working with, the manifold in this case is unaltered. The side with the wide port goes towards the engine, the round port matches up to the carb. This won't go on straight, as the holes on the manifold are at the corners, so you end up with the rectangular hole on the manifold trying to match up to the square shaped port on the head, and it looks wierd, and at one point on either side, it barely overlapped enough to make a seal, but it did it ok, and worked. There is a "pulse" port on the manifold, which is supposed to pump the internal fuel pump in the carb, but it is not needed, and doesnt have to be connected or anything. All I did is widen those holes, the ones you can see through, to the edge of the thick plastic they are through. in fact, you can reuse the gasket from the original carb on the 4 stroke engine, and that will work, too.

    These Chinese Walbro clones are under $20, and the plastic manifods can be had for about $10, but I did what you are doing: I used what I had available at the time.
    Last edited: Jul 7, 2013
  12. GreenMantis

    GreenMantis Member

    I would also like to pat myself on the back, when I did this the first couple of times (it took me a little while to figure out that it was starvng for fuel because it needed the needle valve adjusted a bit), I used 12 pack boxes to make my gaskets up.
    breaksalltherules likes this.
  13. GreenMantis

    GreenMantis Member

    And just in case I didn't imply it strongly enough, your Honda small engine "expert" doesn't know anything about Honda small engines. They come with the tap in the case on all of these engines, even the clones have it, and they have fuel pumps ready made to run on that pulse, "more trouble than it's worth" is just a matter of buying the $20 fuel pump Honda makes and sells for these engines as an option for mounting the tank elsewhere. That doesn't sond like any trouble at all, and would be more than worth it, if we didn't already have everything we need to convert it over to a diaphram carb sitting in front of us.
  14. GreenMantis

    GreenMantis Member

    That metal manifold you have there looks like it should work exactly the same as my plastic one did. In fact, it looks like it will seal better.
    Here is some benefiet from my experience: the engine should not run, in fact, it should not even so much as sputter, while the choke is on. Turning the choke on while the engine is running should kill the engine in very short order. When mine is running perfectly, it will never start on the first pull. The first pull is done with the choke on, and should not so much as sputter. The second pull is done with the choke off, and it starts every time. If you don't pull it once with the choke on, however, it wont start no matter how many times you try. If the engine so much as sputters while that choke is on, you have an air leak, you have to take the manifold back off, and seal the gasket better.
  15. I guess I needed to learn how to adjust the needle valve anyway.

    Mine might be a little too long, I checked a month or so ago but I'm going to start back up on this and pull my other carb back off.

    I might track down a 3d printer that I could send the thatsdax adapter with the 12mm bore to so they can fabricate the same thing but with a 15mm bore.

    Thanks for the info
  16. GreenMantis

    GreenMantis Member

    I know that the bolt spacing on the 12mm and 15mm carbs is the same, you could just drill out the 12mm adapter.
    Or like I said, you can keep your original carb, Honda makes a fuel pump and the pcv vent hose provides the "pulse" to drive it. Whoever told you that it was too much trouble had their head stuck someplace stinky, it's a $20 part, and 10 minutes worth of installation time. If you don't have everything you need to make the diaphram conversion possible right there in front of you, like I did, it is probably better to go with the Honda fuel pump, instead. I converted mine because I was done in an hour, I didn't have to buy anything to do it.

    Adjusting the metering valve was just a matter of bending the lever on the needle valve that contacts with the diaphram. The setting was just fine when that carb was on the 2 stroke motor, but that motor provided a pulse for the internal fuel pump in the diaphram carb. This engine just needs a different setting that allows as much fuel to flow as possible to run right.
  17. GreenMantis

    GreenMantis Member

    According to AGK, the stock carb bore on these is .425". Their overbored carb is bored out to .478". Now according to my math, 12mm - .472", and 15mm = .59". This is probably why Dax doesn't bother with 15mm conversion kits, it's kinda overkill. It works great for me, though.
  18. I'm actually running the Walbro fuel pump right now. It's really strong but I want my Walbro carb because it's fully adjustable high and low speed and my Super Titan carb only has a single adjustment screw for the fuel (not the idle adjustment connected to the swing arm)

    I was going to get a 3d printer to fab one of the adapters with a 15mm bore instead of 12mm so I could keep the 12mm in case I wanted to go back but if the 15mm carb is working fine for you, I will just drill it out myself and not go through the hassle.

    Do you think the the oversize carb negatively impacts fuel efficiency? If there is an increase power trade off, I'd be fine with that.
  19. GreenMantis

    GreenMantis Member

    I can't really say about the fuel economy, I had to convert mine to ride it, I had no place else to put the fuel tank, so I can't compare it to before. But it gets great economy, I can't complain, and it runs great. I did have to tweak the metering lever just a bit, but that was all it wanted. My Walbro clone only has one screw to adjust, but it will do the range from too rich to too lean, and it's very happy in the middle of the two. I routed the hose from the crankcase back to the airbox, I just drilled a hole and put a fitting on it.
  20. Right on, thanks for the knowledge.

    How did you impregnate your gaskets with silicone? Did you use something like this gasket silicone?

    I bored out the adapter, no problemeo. Going to roughen the intake manifold for better fuel atomization while I have the carb off as well. Eventually I'll end up with a Tourquer v stack with a high flow filter and a rain cover but I'm going to stick with the stock box for right now.