BLI will not be able to do squat with your loco on your track, the best they can do is get it to run properly on their track. That said...
First we clear up some nomenclature. A 4-8-8-2 only has one truck with two axles, and in a cab forward configuration, that truck will be the trailing truck. Since you say the "front 2 wheels" aren't resting on the track, I have to assume that:
1. The trailing 2 axle truck does not rest flat on the track and...
2. The forward axle of the two axle trailing truck, in cab forward configuration, is riding high.
If true, then I have to assume that the attachment point for that truck to the chassis is either high, low or torquing the truck at rest.
If the truck is a true trailing truck, the attachment point to the chassis is forward of both axles, in cab forward configuration. If so, the attachment point is too high, and needs to be lowered to allow the front axle wheels to rest on the railhead. The fix could be as simple as backing the screw out a turn, as already suggested.
If the truck is attached to the chassis between its two axles, then the spring or screw is torquing the truck so as to lift the front axle, or the truck itself is twisted or curved from its best configuration. Here, the spring, or the combination of spring pressure and screw tightness/looseness might be the culprit.
Never seen this case, but if the attachment point is behind both axles, (in cab forward configuration) then the attachment point is too low, lifting the front axle off the railheads.
As the truck goes into a curve, it tends to want to go straight, until side pressure from the rails against the flanges forces it to turn. Since the flanges are not vertical, this lateral pressure also induces an upwards velocity component to those wheels. If those wheels are already riding high, off the railhead, then less upward movement is necessary to free the flanges from the rails, allowing it to try to straighten out, derailing that axle and its associated wheels.
If you halt the locomotive as soon as the derailment occurs, I would expect to see the offending (front, high) axle displaced outwards from its desired position atop the rails. If this is what you see, then you need to look at the attachment point's heigth, or the truck itself.
If this is not what you see, we need to double check nomenclature (regarding terms like "leading", "trailing", "front" and "rear"), and if all is well there, start looking for less obvious causes and effects.
leejax01 wrote:I have a BLI cab forward AC-4 loco and it is my best engine, but it tends to derail at the worst times. I know that is how it goes some of the times, but I checked the steamer like I do my diesels and I found that the rear 4-wheel drive trucks doesn't sit absolutley flat. In fact, the front 2 wheels or the rear drive truck sit up about the thickness of a piece of paper. It tends to derail at the rear drive truck as it seems that it jumps in turnouts. Any one with a BLI CF have/had the same problems? I also notice that the front drive truck seems to sit further down than the rear and may be a cause of the problem.
Under warranty have BLI do it.
You can check the 4 wheel tuck for springyness, loosening screws, the drive wheels may be a different story, bent frame for one. Unless your good at springing drivers the drivers would have to be shifted down.
If the sidrods are all solid they cannot be sprung and the lifted drivers is an operationa; problem on the siderods. You don't want that especially if its a bent frame. Send it back to BLI.
They will want to know about manufacture problems, and they will surely want happy customers.
Leejax--
I read your post and was confused for a moment until I realized you're talking about the LEAD truck on the cab-forward (even though it's under the firebox). I've got an AC-5 and I took a look at it. I think you might solve the problem by loosening the screw that holds the truck to the swivel about one or more turns. Also, check the gauge on the front set of lead truck wheels. If they are the slightest bit out of gauge, the smallest bit of uneven trackwork will cause them to ride up the rails and derail. If they are, a slight twist on the insulated wheel should put it back in gauge. But loosening the screw about one or two turns might even the truck up. Try it and see if it works. I've not had any trouble with mine, but your lead truck might have been installed too tight at the factory.
Hope this helps.
Tom
Tom View my layout photos! http://s299.photobucket.com/albums/mm310/TWhite-014/Rio%20Grande%20Yuba%20River%20Sub One can NEVER have too many Articulateds!
You may have a bent main frame, bent truck frame, displaced journals, screws retaining the truck driven too tightly, uneven track, tight curves, improperly finished rail ends, improper joins, tender too tight to the cab (in the wrong hole...although that tends to derail only the tender), and it may even be the other truck that is problematic, not the one you think is.
If it is still under warranty, and the repair tech at BLI has told you what you already know/knew, maybe you should just send it in and make it their problem. It is there problem.