Motor adjustments for running at altitude

Haymaker21

New Member
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10:30 AM
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May 14, 2012
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Location
Greeley, CO
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.
 
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.
 
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.

MikeJ
 
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.
 
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.

MikeJ
 
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.

MikeJ

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.
 
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!
 
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.

MikeJ
 
Mike,
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.
 
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. . . . . .

Thanks!
MikeJ
 
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