Motor adjustments for running at altitude

Discussion in '4-Stroke Engines' started by Haymaker21, Jun 15, 2012.

  1. Haymaker21

    Haymaker21 New Member

    Hey guys,

    So i live in Colorado almost 6,000 feet above sea level. My owners manual says that i should have it "professionally adjusted" to run above 5,000 ft. It's a 49cc. I know that i'll get worse preformance than someone at sea level, but my average speed on flat ground is 25.5 mph. Will adjusting my motor to run at altitude increase my preformance? I'm just curious if this is something i should seriously consider or what the deal is. Any thoughts are appreciated.

    Thanks all.

  2. bigoilbob

    bigoilbob Member

    Yes, if it has not been adjusted for altitude, you are running rich. Power is probably down a little, and economy is certainly off. If it has not yet been adjusted for altitude, get it adjusted, but don't forget to get it readjusted if you go lower. If you don't, you will be running lean, and might fry your engine. Google around, you might be able to do it yourself with your adjustment screw and some indication of when you are lean enough.
  3. MikeJ

    MikeJ Member

    Haymaker21 -

    I am also in COS. My GX-160 has a #72 main jet. I have been told by Honda engine pros that a #69 jet would be better; maybe a #68. (Jet numbers refer to the diameter of the hole in the jet itself. #72 = .72 mm diameter, #68 = .68 mm diameter, etc). I have not found a supplier of smaller jets (yet), but I have not looked real hard either; my engine runs fine at a fraction of full throttle. There are alternate ways if Honda-provided jets are not available, but that is another discussion. I will look in the Yellow Pages (paper phone book) for Honda engine pros; you do the same and let's post findings for other riders.

  4. Skyliner70cc

    Skyliner70cc Active Member

    A lot of times you can get away without having to mess with the carburetor jetting by adding a tuned pipe such as one from sick bike parts.

    To see if your engine needs to be leaned out do the following. Warm engine up by riding around for at least 10 minutes. Find a safe spot to ride full throttle for several minutes. Go to full throttle and simultaneously shut off fuel flow and observe what happens when engine runs dry. If engine speeds up a lot then you can consider leaning out jet, if engine just dies without speeding up then you are at mixture or slightly lean. I run this test up a hill and measure by change in acceleration and speed up a hill when I shut fuel off.
  5. MikeJ

    MikeJ Member

    Haymaker21 -

    Disregard my post. For whatever reason, I thought you had a Honda 50 cc 4-stoke engine. My comments do not apply to you. Skyliner70cc was right on the money; follow his advice and keep in touch with him. In the past, I did run a 66 cc for over 1300 miles until it became hard to start (probably needs a good carb cleaning). In retrospect, it could have used a smaller jet; it 4-stroked a lot at higher speeds, indicating it was running too rich and below max possible power.

  6. Skyliner70cc

    Skyliner70cc Active Member

    My bad, I thought it was a 2 stroke. I didn't realize the original post was in a 4 stroke forum. Disregard my advice on a tuned pipe if engine is a 4 stroke. Fuel shut off test also applies to 4 stroke engines and if you determine it is running rich, going down a jet size should fix your issue. Also, many small engines have severely restricted intakes due to crappy foam air filters. If you put a less restrictive intake, that could help with your rich mixture.
  7. Haymaker21

    Haymaker21 New Member

    So i should 1) Do the fuel cut off test 2) get a better intake than my stock crappy foam one. If i do have that sudden burst of speed, how can i correct running rich? Also, sorry about the confusion on the 4-stroke. I should have mentioned that in the post.

    Thanks guys!
  8. MikeJ

    MikeJ Member

    Hi Haymaker -

    Here is an answer I received from a COS lawn equipment dealer and repairer: They do not consider 6,000 high enough for 4-cycle engines to justify installing a smaller jet in the carb. They listen to the engine and watch the exhaust. If the engine sputters or exhaust is more than the tiniest trace of black smoke, they say a new jet expense can be justified, for mixture is too rich. They draw the line at about 9,000 feet. I have ridden my GX-160 up to about 7,000 ft heading east out of COS to 5,000 way out east. It never skipped a beat. (Not running at wide open throttle probably has some influence.) When I see my plug covered with excessive black soot, then I will move to a smaller jet size.

    See how your engine runs for a while. Then if you still have the itch to try a smaller jet, try one that is two or three thousandths of an inch smaller in diameter. Finding the jet is the hard part; installing is fairly easy. Just note the orientation of the parts you remove.

  9. Skyliner70cc

    Skyliner70cc Active Member

    A new jet isn't a big expense to get the best performance out of our engines at altitude. I rejetted my 3000 watt generator so it could run my travel trailer airconditioner which it couldn't do before at stock/sea-level jetting.

    An engine loses approximately 3% power output for each 1,000 feet gain in altitude and that is with correct jetting. At 6000 feet, you are down 18% in power and running stock jetting actually worsens that figure significantly.

    Your lawn equipment dealer can't justify to its customers the labor fee to rejet a carb but the cost is cheap if you it yourself.
  10. MikeJ

    MikeJ Member

    Hi Skyliner70cc -

    Right you are! Rejetting a Honda engine is quite easy. Another similar thread started by rowens mentions getting a few jets (easily available for the Honda engine for $7 to $8 each from Affordable Go Karts (AGK)), soldering them shut, then drilling them out with a drill set that sounds to be readily available. And if I don't like the hole size, solder it shut and drill it out again, as often as I want. Buy a few jets and have a complete set, like Fabian has. I can see now there is no excuse to not have an optimal jet in any engine carburetor. . . . . .

  11. Quenton Guenther

    Quenton Guenther Motored Bikes Sponsor

    After racing motorcycles for 15 years, jetting was important and here is the way Pro racers jet a 4-stroke carburetor.

    Warm up motor, then run at full throttle under load for approx. 2 min. Hit kill button. Take out spark plug and look to see if too rich or too lean. A rich carburetor will make the outer edge of the spark plug black, and a lean motor will make the plug look same a new [white or very light gray].

    This test only checks the main jet and the object is to have a spark plug a medium brown.

    Never do this test without a warm motor.

    Best way to save money is to solder a jet close and use a number drill set to arrive at the correct opening.

    If you are using the Honda GXH50 carburetor, the main jet is smaller than the HS 142A motor by 2 sizes.

    A restriced intake breather will RICHEN the mixture not lean it as it acts like a choke. Less air more fuel.

    A tuned exhaust on a 4 stroke WILL have a major influence on jetting, and normally leans out the mixture.

    The HS142A motor has a much higher top speed than the Honda motor and all is because of the main jet in the carburetor [both use a 15 MM carburetor].

    One of our test bikes has the Honda motor and uses a stock HS 142A carburetor [original main jet] with a tuned pipe and the jetting is 100% on the money at sea level.

    Have fun,
  12. jaguar

    jaguar Well-Known Member

    There is less air pressure at high altitudes so you need to shave the head or ditch the head gasket to bring the combustion pressure back up to normal. Around .5mm is what works. If you dont do that then there will be less compression and less power, regardless of jetting.
  13. Quenton Guenther

    Quenton Guenther Motored Bikes Sponsor

    Can't mill the heads on the Honda GXH50 or the HS 142A as they are part of the crankcase and aren't removable.

    Have fun,
  14. bigoilbob

    bigoilbob Member

    Shave heads? You're spectacular, man. These are tiny engines. The goal should be to run less inefficiently, and no more. You're going to lose power at altitude (unless you want to turbo/super charge that GX35 or GX50), so don't deny it. For high enough altitudes, rejetting might be practical. That's about it...........
  15. Skyliner70cc

    Skyliner70cc Active Member

    Rejetting is more than practical, it is essential at altitude. Spoken be me, I've been living at <6000 feet ASL for quite a while now

    I'll be rejetting two 2 stroke generators this weekend. The only engine I haven't rejetted yet is my chain saw. I've owned it for 9 years and it gets maybe 12 minutes of use each summer. I can't justify pulling the jet on this one..I'm too lazy ;-)
    Last edited: Jun 27, 2012
  16. MikeJ

    MikeJ Member

    I will give rejetting a try. I ordered three spare Honda engine jets from AGK and an inexpensive set of wire drills from Amazon. I can check diameters with a micrometer. All this may arrive before this weekend. If so, I will press on and report back later.

  17. Old Bob

    Old Bob Member

    You'll never get a valid plug reading with unleaded fuel.A/F meter is the best way to determine mixture.

    If you contact the right Honda mechanic they can supply high altitude jets.
  18. MikeJ

    MikeJ Member

    Just an update. . . I received the three main jets from AKG as ordered; good service. The Chinese micro drills from Amazon are a bit different. A set of 20 micro drills did arrive. I prefer knowing true diameters in inches rather than drill numbers, so I took a micrometer to each of them. I noted in the set I received three drills of .030 inch diameters rather than unique sizes. The next step down were two drills of .025 inch diameters. This is a visibly big jump in sizes. But I can live with this.

    The from-factory main jet in my 160 cc Honda 4-stroke is .025 inch diameter. I have a drill at .0235 inch diameter. This is about three jet sizes smaller than the stock jet. I will try the .0235 inch drill. In reality, I expect the final hole to be about .024 inch in diameter. The original jet will not be altered just in case I want to go back to using it. It will be a few days till my next update.

    To get an idea of what .024 inch is like, that is six new one-dollar bills in thickness.

    Last edited: Jul 6, 2012
  19. MikeJ

    MikeJ Member

    A smaller jet in my 4-stroke engine is no-go. With the jet installed as planned, my engine barely runs above idle and can't pull me at all above idle.

    I computed the hole made by the .0235 inch drill has 88% the cross-section area of the stock jet. The smaller hole is just enough to render my engine virtually useless when trying to carry me on the road. So I removed the modified jet and returned the factory main jet back into the carburetor. A test drive showed all is good once again.

    2-stroke engines are much more altitude sensitive than 4-strokes. Re-jetting them makes sense.

    Have not had a flat tire for the past few hundred miles; the engine runs well; lights and helmet and jacket keep 4-wheel drivers away; I join others for rides once in a while; and all law enforcement officers go after dumb and bad drivers, not me. What could be better with this hobby?

    Last edited: Jul 7, 2012