Motor adjustments for running at altitude

[Quenton Guenther]

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,

 

[Deleted member 12676]

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.

 

[Quenton Guenther]

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,

 

[bigoilbob]

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...........

 

[Skyliner70cc]

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 ;-)

 

[MikeJ]

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.

MikeJ

 

[Old Bob]

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,

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.

 

[MikeJ]

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.

MikeJ

 

[MikeJ]

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?

MikeJ