HF 79cc Carb Main Jet Resizing for High Altitude

Discussion in '4-Stroke Engines' started by MikeJ, Apr 24, 2011.

  1. MikeJ

    MikeJ Member

    Hi All -

    I have run my 79cc engine for just under 10 hours. It was simply underpowered for this altitude in Albuquerque (5500 feet to 7000 feet above sea level). The engine is running much too rich. The spark plug and exhaust pipe have a very heavy layer of plain black soot on both. The carb is starving for air and drowning in fuel.

    I have since swapped out the old exhaust system (too restrictive and too long) to some shorter length and bigger diameter. The air filter is less restrictive now as well. Next is the carburetor; I am looking for ways to make it better. (Governor is gone, magneto pickup has been re-gapped, spark plug has been re-gapped; starts better than before.)

    Attached should be photos of the carb when partially disassembled. The bottom screw holds the fuel sediment bowl to the center post. The screw out the side of the sediment bowl can be opened to drain water and old fuel.

    Just below the plastic float on the side of the center post is what appears to be the main jet of brass (someone correct me if I am wrong). It restricts fuel flow going up into the venturi throat. I measured the main jet opening to be .025 inch (equals .635 mm) using a wire of equal size and a micrometer. This is from the factory; probably optimal for near sea-level air density.

    If this engine is a Honda clone, would Honda have smaller-opening main jets for their 50cc engine that would fit into this engine? I looked at multiple Honda parts web pages and have not seen anything smaller than twice this jet opening.

    I have read of soldering closed these main jets and then drilling out with wire drills that some of us have a hard time seeing. (Under-.025 inch drill is pretty small!) But if this is my only option, I will keep what I have and live with the overly-rich condition (at least for a while).

    I realize this is a very specialized question, but if any experts have anything to say, I'd like to hear from you!


    Attached Files:

    Last edited: Apr 24, 2011

  2. Silvaire

    Silvaire Member

    The jet that you think it the main jet is indeed the main jet.

    The numbers used on jets are arbitrary and are not based on a specific size measurement - only to the relative size of other jets for the same carburetor.

    I do not know if the Honda jets are interchangeable with the one in your clone, but I'd guess it's pretty likely that they are.

    I posted an "old racers trick" in another thread that you might want to try in the meantime:
    Last edited: Apr 27, 2011
  3. Old Bob

    Old Bob Member

    Too bad its not a Honda they have available carbs already set up for operation above 5000 ft.
  4. ocscully

    ocscully Member

  5. MikeJ

    MikeJ Member

    Hi Guys -

    Many thanks for your replies.

    Silvaire - I just happened to read your one posting about the "old racer trick" wire strand in the main jet just two days after posting my original writing. It makes sense... slightly choke fuel flow through the main jet to the venturi tube, and the engine runs leaner (which is what I need). I made note to ensure the wire does not get sucked into the engine.

    I developed an Excel chart of main jet hole diameters as they correlated to wire drill numbers. I came to the conclusion that manufacturer jet numbers have little to no relationship to wire drill numbers. Example: a hole of .025 inch (.635 mm) diameter is a wire drill # 72, but is a main jet size # 63 for Honda. A hole the same size may be a different jet number from another manufacturer.

    I am going to try the wire-strand-in-the-main-jet idea. My engine's stock main jet diameter is .025 inch and the wire strand is .010 inch. My calculations show the jet will be restricted down by about 8 percent. This equates to almost two wire drill sizes, from a #72 to a #74. I don't have anything that states how this relates to jet sizes. The HF engine main jet is NOT numbered.

    I converted main jet opening size to cross-section circular area when computing how additional wire strands affected main jet throughput. Then I could compute how much an additional wire strand affected throughput. Of course, the real test is on the open road.

    I saw a crusty Honda small engine wrencher the other day. He showed me a Honda main jet; it is NOT compatible with a HF 79cc main jet. The size and thread pitch were all different. The wrencher knows of the wire strand trick. He preferred solder filling and drilling using wire drills.

    I purchased an NGK BPR4HS short-reach plug from him. (I want to see what color the plug is after 10 hours of running.) I asked if the plug was too hot; he said not at all. He puts them in Honda air cooled engines for building contractors. (It is not too late to buy a cooler plug if advisable.) He also advised me to place the piston at top dead center to measure how much clearance the plug tip has to the piston. We both agreed: It is a bad idea to bang a piston against the spark plug.

    So you all know my plans. If you see any red flags after reading this, please respond! It is cheaper to fix my plans now than it is to replace the engine.


    P.S. A place for 79cc engine performance parts (that's almost an oxymoron):
    Last edited: Apr 27, 2011
  6. Scoop

    Scoop Guest

    Great info and very nice post. Having same problem in western Pennsylvania. Tried the wire trick found that led resistors worked and made a positive performance but is still too rich even with high flow air intake and it is almost plugged completely very little air goes through when blowing through it. I may need the smallest jet available or smaller lol. Has no low and or hillclimbing power but OK mid range and good top speed (of course it's too rich). Went down to a five NGK one hotter than stock didn't want to go to a four but will try. Tried different spark plug gap's and all. Got it running at least 50% better than stock with air filter pipe and governor removed. Any help would be greatly appreciated -Ryan