Couple of observations on the HF79

Discussion in '4-Stroke Engines' started by professor, Jan 23, 2011.

  1. professor

    professor Active Member

    Talk here about 2 stroke timing made me think of checking the HF. Took a bit of doing but this one has about 27 degrees of advance. It didn't change with rpm and I left it alone.
    Looking at the engine with a thought of more power, the carb seems to be real small. Probably great for the rated hp and rpm though.
    I am not kicking about the power, it seems like an old Honda 90 untill about 35 mph where it runs out of steam, Maybe the small carb is why.

    With the disconnected gov, more open muffler and different air cleaner mesh, I think I am around 3.5 hp.
    The exhaust port looks like it could be opened and probably the intake too.
    Anybody do mods to one yet? Am thinking a bigger carb and some porting would add another hp.
    I have no idea what rpm this thing is doing, am imagining around 5000 it is flat out.
     

  2. samuari86

    samuari86 Member

    id like to know to im working on one rigth now so, go to Frame-Mounted Drive-Trains and look for samuari86, all so need some help and you will see my hf motor and bike,lol. but good look on yours!
     
  3. 4950cycle

    4950cycle Member

    Speaking of frame mount, I'm going for a rack mount . I don't see a fuel pump on mine , at a glance it looks like gravity feed for fuel. This is going to take some doing for a rack mount . It will be hard to get a fuel tank higher than the carb on an aluminum Mongoose full sus. mountain bike. I bet an HS honda clone 50cc:idea: carb is too small ? I think they make one of those with a diaphram fuel pump. Any ideas ?
     
    Last edited: Feb 2, 2011
  4. professor

    professor Active Member

    If you absolutely have to mount the tank lower than the carb you can buy a crankcase pulse operated fuel pump to adapt to your engine. This means putting a fitting into the crankcase somewhere but this whole deal would only work if the engine has a check valve on the breather, like the old Kohlers or Techumsehs.

    Or change the carb, but you probably would need to fiddle with jetting.
    Of course, you need a carb that actually fits the engine. The HF I have was a tad lean, so I opened up the main jet a couple of thousandths. No longer need to leave the choke on for five minutes.

    Why not leave the tank on the engine?
     
  5. 4950cycle

    4950cycle Member

    Kinda ugly Professor, The blue tank way up in the stratta. lol Heck, A rack mount is ugly by nature anyway. My 1985 30 hp Tahatsu outboard has a crankcase pulse fuel pump. I like them. I will bugger rigg it with fuel jacked way up in the air somehow to test the MB once I getter built. Then tweak it with a propper diaphram carb of some sort. I wonder if the intake throat is close to the same size as the Heushang 50cc or the genuine Honda ? If it is I am well versed at drilling my own main jet as you do Professor. Nice to know someone else out there doe's that. It was the only way I could get my 750 Kawi to run right 13 years ago I remember. Tried stopping any little possible air leak I could see first.
     
  6. professor

    professor Active Member

    I did build a rack mount bike and put the tank on the handlebars. You could do a tank up there and hide it in a leather pouch or handlebar bag.
     
  7. 4950cycle

    4950cycle Member

    That might be the only way to test the build Professor. Tank clear up on the bars as you say. Since most of that clear fuel line hardens after a time, I wonder how long I could run with that idea ? Remember , I have to steer and the line must be flexable if the tank is on the bars.
     
    Last edited: Feb 4, 2011
  8. professor

    professor Active Member

    The whole line need not be flexible- just the places where they need to be.
     
  9. 4950cycle

    4950cycle Member

    This is true, And that is what I'll do. Run a handlebar fuel tank camoflaged or not camoflaged in a bag. Maybe run hard line where fuel line run is motionless. And run a short length of flexable (clear) line at the steering head. And it will also put a pinch of weight on the frontend where I am weight poor with a rack mount setup anyway. Thanks for walking me through this and helping me to get to a point where I won't have to do any carb. work Professor. BTW , I got lucky I guess. My motor only needs the choke to start it. I can turn it off as soon as shes running. Probably running rich enough. But it's so new I havn't checked the color of the plug yet either.
     
    Last edited: Feb 4, 2011
  10. professor

    professor Active Member

    Happy to help, do an oil change after you run it for an hour or so also.
    I use a suction devise with a tube on it into the fill hole rather than make a mess.
    Lean the bike over on that side as you do it- works great.
     
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