Tune-Up Predator 79cc 4-Stroke

Discussion in '4-Stroke Engines' started by ColdCruiser, Oct 13, 2016.

  1. ColdCruiser

    ColdCruiser Member

  2. ColdCruiser

    ColdCruiser Member

    Hmm, start with new gas!
  3. LRSimons

    LRSimons Member

    To get a quick look at your AFR, you can take a non-contact infrared thermometer and compare the head temp to exhaust port temp. Higher head temp is richer, higher exhaust is leaner. This can be helpful because the plug can stay one color if it's not new.

    Someone on here told me that, don't remember who. Thanks to them.

    Hope the gas and some fiddling can help you out. Most small engines are tuned pig rich from the factory to keep them cooler and intact or whatever. The air filter/exhaust/ whatever else you've done has probably thrown towards the lean side if you haven't rejetted.

    Heck, it's a four stroke, you could run an O2 sensor if you wanted to get fancy.

    ColdCruiser likes this.
  4. LRSimons

    LRSimons Member

    Whoops, saw you did rejet. Take temp readings, plug chops, and go from there. Don't know how the carbs are designed on the HF engines, but the idle is usually a screw for speed (opens/closes the throttle a tiny bit I think) and sometimes a mixture screw. Otherwise check the needle/jet(s) for the mixture.
    ColdCruiser likes this.
  5. ColdCruiser

    ColdCruiser Member

    Thanks LRSimmons for your response.

    I will try to keep your suggestions in mind. I found someone who has worked with these to check it out. We took the carb apart and cleaned everything real good. He explained this carb was not really adjustable because of smog laws. Mentioned there are adjustable carbs out there. The guy also thought there might be a rev-limiter in addition to the governor because the engine starts cutting at a certain rpm range.

    I have rebuilt the clutch with a heavier spring so I could turn the idle up without the bike taking off. I still have to feather the throttle to keep from stalling at stop. All-in-all it's not too bad, at least I can ride it and it's a lot of fun!
  6. Tony01

    Tony01 Member

    Post a pic of the spark plug!
    ColdCruiser likes this.
  7. LRSimons

    LRSimons Member

    Sounds like a bigger/more adjustable carb would be a good idea. The stock one being designed for smog and reliablility is probably really restrictive. I suppose it doesn't really need adjustability considering most of these hf engines go on generators or snowblower or whatever- boring crap that only needs to run at one speed.

    I've taken a ton of inspiration from your build. Super cool setup without much compromise. Glad to hear it's going along nicely.
    ColdCruiser likes this.
  8. ColdCruiser

    ColdCruiser Member

    Tony, pic of the plug?! Ok.

    I actually have had a problem with the plug. The engine fits in there so tight that I can't get the plug out without dropping the engine! I tried to get more room but the off-the-shelf motor bracket uses a little too much space. SO, I've been wanting to find a shorter plug and perhaps a different plug wire that's a little more low-profile.

    Here's my list of plugs that should fit but I don't know which are good or shorter.

    - - - - - - - - -

    $6.50 - NGK BPR8HIX - Spark Plug - Iridium
    $5.00 - NGK BPR5HS - Spark Plug
    - NGK BR-4HS - Spark Plug

    Torch (E4RC)
    NGK (3322 / BR4HS)
    Bosch (WR10AC)
    Brisk (NR19)
    Daihatsu (99906850P9002)
    Denso (IWF16 / W14FR-L / W14FR-U)
    Honda (98076-54757)
    Motorcraft (AER6)
    Autolite (425)


    Autolite RacingĀ® Hi-Performance Spark Plugs (425)
    AR3910X Racing Spark Plug??

    - - - - - - - - -

  9. ColdCruiser

    ColdCruiser Member

    LRSimmons, thank you for the kind words.

    The bike gets a lot of attention. I took my dog to a car show last weekend and many people wanted to know about the bike and where they could get one, lol. Great conversation starter!

    I originally thought using a readily available utility motor was a good idea. Now I see it needs to be heavily modified to work better in a bicycle. I thought about buying another HF79 so I can work on it at my leisure on the workbench then swap it out when I'm finished. I'm not in a hurry to do this though.

    Another thought is to use a different motor. Lately I've been wondering if I can get a Honda CH80 and trans from an old Elite scooter from the 80's. Maybe that would be the same thing as just adding a torque converter though(?). I like the fact that you can jump on one of those old scooters and take off, easy-peasy. My ride takes a bunch of tinkering and is somewhat dangerous. I probably can't let anyone else ride it because they will break the bike or get hurt.

    I need to take some better pics but here's one I just snapped of the other side. It barely shows the new air filter and breather port. I also finally got the second rack mounted to the front.

    Cozmik Mezzenger likes this.
  10. Frankenstein

    Frankenstein Well-Known Member

    What cruiser is this? Neat looking and been pondering a 4 stroke fat tire, most frames are just too small for an acceptable engine. This however seems like oven the larger 212 could fit if wanted, I think I could dig up a 98cc diggy buggy motor if I looked around. That would probably be a perfect fit too.
    ColdCruiser likes this.
  11. ColdCruiser

    ColdCruiser Member

    This is a "Sikk Bike UFO" (black on black). Steel frame. With disc brakes and a hub-shifter they are ~$400.

    I can not recommend the hub-shifter because I just spun mine out and now need to rebuild/replace the rear-end.

    This bike also has forward-facing drop-outs and that has been an issue with the torque (see other post).

    Another thing, there's not as much space as it seems. I added a wide crankset so I can reach the pedals. Any wider and I couldn't turn. Still, my legs are very close to the exhaust and gears. I think a larger motor would be too wide and also unnecessary. I can pull the trailer w/ 75 lb. dog at 35 mph with no trouble and the governor attached. The rest is the gearing. I could get it up to 55, 65, whatever but I don't need that.

    As you can see above, I don't even have room to get the plug out. The mounting plate I bought uses a little more space than it would need if I made it custom. Then it could sit lower and that would be better. If I were to make a custom mounting plate, I could also tap-out the other oil fill plug and then tilt the whole thing to make better use of the space. As it is, I have it sitting flat.
  12. Street Ryderz

    Street Ryderz Well-Known Member

    most mx guys when they change to high flow filter have to go up (richer) sometimes as much as 10 jet sizes!when I rebored my cns and put hp filter stock jet was a 68 and now runs a 82 so yes very restrictive stock form.
    ColdCruiser likes this.
  13. Frankenstein

    Frankenstein Well-Known Member

    Width isn't too much an issue, I like it big too, just feels nice.

    I wonder how hard it would be to get the frame for this bike, I have a kinda crazy idea for a larger motor, maybe give a double frame a shot and weld them up nice. Crazy maybe but not too crazy.

    I think it would be a fun build, I already have a bike that needs a motor but that's not really important lol, can't fit a 4 stroke in it...
    ColdCruiser likes this.
  14. Frankenstein

    Frankenstein Well-Known Member

    How big is this anyways? The 79cc motor I mean without the tank on there? I'm trying to size up a frame but have nothing to compare...
    ColdCruiser likes this.
  15. ColdCruiser

    ColdCruiser Member

  16. ColdCruiser

    ColdCruiser Member

  17. Cozmik Mezzenger

    Cozmik Mezzenger Active Member

    When mine started doing that it was the stupid float. But yours has not yet started to overflow. But it is worth a look. I am going to eventually get a better carb than stock.

    Attached Files:

  18. ColdCruiser

    ColdCruiser Member

    Cozmik, I cleaned everything real good and put new gas in it and now the motor runs fine again.

    However, I spun-out the freewheel on the 3-speed hub. I'm not sure how I'm going to resolve that issue. I was thinking about doing a left-side direct drive like you have. How's that working out for you? What gear ratios are you using?
  19. Frankenstein

    Frankenstein Well-Known Member

    Internal hub?
  20. ColdCruiser

    ColdCruiser Member

    Yeah, Sturmey Archer SX-RK3

    The manufacturer was less than helpful. "Our hubs don't work with motors." ...which is a ridiculous response. They should have said, "Our hubs can only tolerate the smallest amount of torque." ...or something like that because the fact that it's hooked to a motor seems irrelevant to me.

    Even if I put a fixie hub won't help because that would still need a freewheel connected to the drivechain. That is why I'm thinking a left-side fixed drive might be a better idea.