I’m thinking of using 140 or 150 mm forks on my trek fuel 9 build.
Should I then look for a flat bar to keep my weight lower over the front?
Seen some ‘on one’ bargains.

nope, you should look for a replacement frame as it will be more likely to fail. You will also need to bear in mind that Trek will not warranty it as its not designed to take those forks.

The frame is second hand so no warranty prob, why would it fail?

I put some 140mm uturns on my fuel ex8 and its great, works well climbing when dropped and feels mint descending, I have put a low rise bar on tho.

Adil.

How much travel does the frame have? The new ones are 130mm and would probably be fine with up to 150mm forks.

Since you’re raising the front end, either move the stem down if there are any spacers under it, or swap to a lower rise bar to keep your position the same.

Left field answer- add an Angleset headset and let the change in head angle soak up most of the longer fork without raising the bars and bb or slackening the seat tube angle. That is if a slacker head angle sounds like a good thing to you.

I’ve seen 15mm difference axle to crowning the same travel fork so that is a more useful measure. Simple answer is to try it with what you have then look at different bars depending on how it feels. There us no formula or rules

Fork length itself isn’t what breaks frames, someone riding with a longer fork is more likely to be getting more extreme.

The trek fuel is a great bike and is very capable, more so than its 120mm travel would suggest. I have to ask why you would want to mess with something that’s already one of the best bikes in its class. It isn’t designed for rough DH tracks but is fantastic for flowing single track, I also found it jumps really well and would be happy using it occasionally for that sort of thing as long as the jumps had smooth transitions.
I assume you have the 120mm version, in which case adding longer forks will raise the BB and slacken the seat tube, which will compromise the bikes performance in the areas that it currently excels.
Personally I would fit a pair of 120mm forks with a 15mm axle. You could fit an angleset but I don’t think it needs it.

Thanks all.
The frame can take up to 130mm forks as designed, but they are not that easy to find.
Wheels need 15mm maxle.
There is more choice at 140mm, and I did fancy a little more travel.

I’d say go for it. Put Fox Talas on my old EX8…had 100-120-140 travel and found it excellent given the ‘bottomless’ travel feel of the rear: 100 for long climbs, 120 general trail riding, 140 for steeper downs