Tune-Up Predator 79cc 4-Stroke



LRSimons

Member
Joined
Jul 26, 2016
Messages
37
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.


Logan
 

LRSimons

Member
Joined
Jul 26, 2016
Messages
37
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

Member
Joined
Oct 12, 2015
Messages
93
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!
 

LRSimons

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Joined
Jul 26, 2016
Messages
37
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

Member
Joined
Oct 12, 2015
Messages
93
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)

http://www.sparkplug-crossreference.com/

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

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ColdCruiser

Member
Joined
Oct 12, 2015
Messages
93
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.




IMG_2714.JPG
 

Frankenstein

Well-Known Member
Joined
Jun 24, 2016
Messages
5,078
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.
 
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