Do they have a BMAP sensor, just a MAP sensor or do they have both BARO and MAP sensors?

 

If I recall correctly they have MAP/TP, IAT and ECT sensors, and also take a signal from the rear VSS.

 

Thanks.

Only having a MAP and no BARO tells me that the only time the PCM knows the altitude is when the key is in the on position and the engine is not running.

The PCM reads the BARO pressure before vacuum is applied to the sensor after the engine is started.

This means that the computer does not change A/F mixtures or alter ignition timing as the ATV gains or looses altitude but rather only after you have gained altitude, shut off the engine and restarted it. Then the computer will make the necessary changes for that altitude from where you re-started the engine.

So, the mixture and timing don't compensate for altitude change as you ride up and down the mountain.

 

I had never researched this for myself, but had heard that this was the case. Our ascents are steep and rapidly climb from 4,000' in the valley to over 11,000' in the mountains (eastern Sierra), and with kids in the group that means fairly frequent rest stops anyway, so it has never been any kind of paractical issue!

 

Even out here where we change altitude quite a bit we still stop frequently too so I don't think it is much of an issue rather than just a FYI.

7000 ft is a big change and we love the views from the top. We start at 6k and end up at 12k where we live.

I guess that we have been fortunate with our carbureted ATV. Other than the usual loss of power I have really never really had any issues.

 

As long as it knows intake pressure, air temp and engine RPM it should have what it needs to calculate mass of air, and thus know how much fuel to inject.

If ambient pressure is lower, there will be lower manifold pressure at a given RPM and throttle position.

 

This is true.(insert wink)

 

On our carburated machines, we run Thunder TPI's. Open them as you ascend to get more air, and close them again as you descend. Above 9,000' we often pull the airbox lid off completely.

Thunder Products - Performance carburetor and clutching for snowmobiles, motorcycles and ATVs.

This is indeed a rejet, but works by increasing the air intake rather than by decreasing the fuel. Works pretty well, although having these in your lid requres that you be careful on deep water crossings!