I purchased an 04' Honda Rubicon (TRX500) last year in Phoenix, AZ - elevation 1400 ft. I told the dealer I would be riding it mostly in Colorado at altitudes of about 8000 ft. They told me to have it re-jetted for higher altitudes by a local dealer since they would know best what jets to use, etc. I got it to Colorado, and the dealer here said they don't rejet 04' Honda Rubicons and that they just adjust the air screw 7/8 of a turn clockwise from stock. I had them adjust the airscrew and rode it, but it seemed to eat a ton of gas, not perform too well, and backfired quite a bit coming down hills.

Being confused about this becuse purchasing dealer and honda manual both say to rejet if riding at higher altitudes, I took it to another dealer (not honda) where they did the re-jetting for me. The ATV doesn't die on me at high altitudes like it used to before occaisionally and performance may be very slightly improved. However the engine still backfires out the tail pipe occaisionally when I'm going down hill, let off the accelerator and allow the engine compression to slow me down.

Is this normal to backfire at high altitudes? What's your opinion on re-jetting vs. air screw adjustment? And finally, is there an adjustment I could further do to either the idle or airscrew to eliminate the backfire?

Thanks from an avid ATVer, but someone who is mechanically challenged.

Russ1767

 

The backfire comes from a lean idle circuit. Go ccw on the mixture screw (richer), and that will disappear. This adjustment only effects the idle circuit, it wont change the mixture in the midrange or main circuits.
Jetting isnt all that difficult, and if you can change oil and such, you should be ok doing the rejetting.
Heres a good article on rejetting, and it has a helpful guide for symptoms of rich and lean, to help you figure which way to go.

jetting

Going up in altitude, makes the bike richer, due to the lower air pressure.
Some people cut holes in the air box lid, to let more air in, instead of decreasing the jet size, for high altitude riding. Removeing the air lid can work sometimes as well, but it may make it too lean. Using holes in the lid also allows you to ride at lower altitudes, if you cover those holes with duct tape...you dont want to ride at lower altitudes with high alt. jetting...itll be too lean.

 

Thanks, I will give that a try. I saved the old jet in case I ever go back down to lower elevations.

Is the CCW adjustment on the air screw just something I'll have to play around with to see what works best (like 1/4 turn increments at a time)??? Fortunately I did obtain a Motion Pro carb adjustment tool to get under that little devil. I think I forsaw this coming for some reason....

You make it sound easy and I know it all boils down to fuel and air and finding the right happy medium for the conditions.

 

I do believe the backfire is from a rich mixture. When you go from a low altitude to a higher altitude, you lost oxygen which made your air/fuel mixture richer. A lean mixture has a small amount of fuel and more air, less fuel, less likely to backfire.

We live at 4200 ft and always ride anywhere from 6500 to 11,000 ft. Our dealer installs a 150 jet in every Rubicon while doing their PDI. With a 150 je our Rubicon runs quite well at high altutudes. There are 4 Rubicons in our riding group, our 01, an 02, 04, and a new 05. They all have 150 jets because we all bought them from the same dealer and they all run like a million bucks.

 

I bought mine in Phoenix, AZ (1400 ft. elevation). The jet that came with the ATV was a 158. My dealer here in Colorado put a 152.5. Does that sound like a jet that would work fine in higher elevations around 8000-10,000 ft.? Hopefully at this point it's a matter of fine tuning the air/fuel mixture just to get rid of that back fire I get once in a while coming down hills, hands off the the throttle.[img]i/expressions/face-icon-small-confused.gif[/img]

 

The other website , i linked to, has a procedure for setting the mixture screw...theres no exact setting, so much depends on altitude and humidity and engine condition. I usually go by the drivability symptoms, and if you have poping on decelleration, the idle circuit is too lean. And seeing how its fairly new, i doubt you have an intake or exhaust leak, so it leaves the mixture screw. You also said the dealer turned it in 7/8 turn. Id turn it out at least 1/4 turn, and if it still pops, keep going 1/4 turn, til it goes away.
An easy confirmation, is to pull on the choke, just before throttle letoff, this will enrichen the idle circuit...and the popping will stop. If it were caused by a rich circuit, it would get worse.
You can go by the drivability symptoms for the other circuits too. Doing a plug chop doesnt work for most people...its not that easy to do on a auto clutch bike, and the gas is formulated with cleaners, and aditives, that mask the coloring of the plug. And if the procedure isnt follwed exactly...the results arent acturate.
Your carb is a cv carb, and is somewhat self adjusting in the midrange, so just focus on full throttle and idle circuits. The mixture screw and pilot jet effect the idle, and the main jet effects the wide open.

This comes from the Mikuni carb website

4: Backfires in Exhaust

Note:

It is normal for many high performance exhaust systems to moderately backfire or pop when the throttle is closed from mid-to-high rpm. In fact, one should expect a well-tuned high performance engine to "pop" and "crackle" when the throttle is closed at high rpm.

The popping is a result of the air/fuel mixture becoming very lean when the throttle is closed and the engine is rotating well above idle speed. It is also necessary that the exhaust system have rather open mufflers.

Why This (normally) Happens:

1) When the throttle valve is in the idle position, fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system.
2) The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, in turn, causes a high air flow rate through the small gap formed by the throttle valve and carburetor throat.
3) Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there.
4) When the odd firing of the lean mixture does occur, it is sent, still burning, into the exhaust system where it sometimes ignites the raw mixture that has collected ---- the exhaust then pops or backfires.
5) Completely stock Harleys do not do this until open-end mufflers, such as the popular Screamin' Eagle slip-ons, are installed. The exhaust must be both free-flowing and have an open exit for the popping to occur.

Other possible causes:

Air Leaks:

Any source of fresh air into the exhaust system can create or worsen the conditions that bring about exhaust backfiring. The most common entry point is the junction of the header pipes and mufflers. Even a small air leak can dramatically increase the intensity or likelihood of exhaust system backfiring.

A high temperature silicone sealant, as can be found in many auto parts stores, may be used to seal the pipe/muffler junction.

Lean Carburetion:

While exhaust system popping may be considered normal, it is certainly made worse by an overly lean idle circuit.

Be sure that your carburetor's pilot jet is the correct size and that the idle air mixture screw is correctly adjusted before looking for other causes of popping. The procedure for adjusting the pilot circuit is covered in the Tuning Manual.

Ignition:

If exhaust system popping is very loud, irregular and accompanied by loss of power, then you should suspect that the ignition system is not performing as it should. If, for some reason, the ignition sometimes fires at the wrong time, then exhaust popping can become very energetic (loud). Look for failing high tension leads (plug wires), failing ignition coil(s) and especially switches or connectors as possible causes.

 

I agree, lean is most likely

 

Thanks a bunch. You all have helped a bit in educating the "non-mechanic".

The article from the Mikuni website that was posted which says a backfire is normal occaisionally when throttle is closed from a mid or wide open position actually makes me feel a lot better. The quad wasn't doing it all the time so it was never a constant problem - just a small concern that I didn't know how to fix. I almost feel like it's okay to leave it the way it is, but nevertheless I'll tinker a bit and find the optimum setting.

 

Right...on a modified motor, its ok for the pipe to pop, if its an open type. Which means free flowing.
If your rubi is totally stock, it shouldnt pop, because the stock pipe isnt free flowing(theres a designed backpressure).
It probably isnt harmful for it to pop now and then, but be aware the idle circuit is lean, and when the weather turns cold, it will get worse, and itll be more difficult to start, and will take longer to come off choke.
Before you start adjusting...turn the mixture screw in , all the way, lightly, until you hit resistance. Count the number of turns it takes to get there. That way, if you want, you can always return it to where it is. But turn it in lightly, because the screw has a taper, and the bottom of the well isnt. Its posible to dent or deform the taper, if turned in too tightly.
Play around with the setting, and youll get more comfortable, and youll notice the difference in drivability.
welcome to the forums.