Hi

First post here. I have an 06 F4i with 18k miles. New to me 1k miles ago. This is a European spec bike.

I'm chasing poor fuel economy (30-38mpg), a flat spot at 7-8.5k and a slightly lumpy idle when cold and warmed up. Bike occasionally revs up to 4k on restart when warm, but settles back down to 1300 rpm after a minute or so. Otherwise the bike runs well, plenty of power at the top end. Bike is stock except for black widow downpipes (still has the stock can).

I've replaced plugs and air filter to try to remedy. No change. Also done oil, filter and coolant as general maintenance.

I bought a replacement FPR, which I believe is the new redesigned version 16740-MBW-J32. This will fit onto the fuel rail and vacuum hose but the return pipe is of a larger diameter than the original, so much so that it's very difficult to get the return hose to fit over it. I tore the hose in the process of trying to fit it to the new FPR and the hose clamp would not expand enough to fit over the sealing buldge of the new FPR, so I trimmed the hose back and left the original FPR in place, presumably this is 16740-MBW-J31. I haven't seen anyone mention this issue in FPR replacement writeups. Has anyone come across this? I can fit a new return hose with a larger ID but then I may have issues fitting a larger ID hose onto the tank pump housing? Given that it's fuel under pressure I'd rather get this one right! The fuel return hose looks original and it's probably time for a new one anyway.

I've attached photos to highlight the difference between the two units:

FPR that's on the bike - straight return pipe

Replacement FPR with cranked return pipe, larger OD than that fitted

 

I lifted the tank and took some measurements:

Stock FPR fuel return pipe OD: 6 mm
Tank fuel return pipe OD: 6 mm
Replacement FPR fuel return pipe OD: 8 mm

Any suggestions on how I should fit the fuel line to the new FPR and the tank return pipe? Can I use 6 mm ID hose over both connections or should I get a 8 mm - 6 mm reducer?

The stock FPR isn't dripping fuel from the vacuum outlet when the tank is primed but the vacuum hose does smell of petrol, so it does look like it needs replacing.

 

Yes! I ordered a replacement FPR through Amazon a few weeks ago. Pic, description, and part # all matched the OEM but what I got was similar to yours, i.e. vacuum hard line at wrong angle. I saw that and did not even try to hook up the main hose. I returned it, bit the bullet, and ordered the OEM part...

As for mpg, mine sits right around 34-38 right now. When I first bought the bike a couple years ago, I was getting 38-40. Our models aren't known for getting excellent mpg, so you might not be able to do much about that.

Good luck and would like to hear what, if anything, you find out!

 

Hey Doc

Thanks for your reply. It's never straight forward is it! It's going to cost too much to return the new FPR so I might as well have a go using it.
I've ordered 6 mm and 8 mm fuel line and a 8 mm - 6 mm reducer so I can plumb in the new FPR. Hopefully this eliminates the flat spot and improves the MPG. I'm hoping to get 40 mpg when riding steady. It certainly drinks it at the top end! I'm planning to fit a wideband O2 sensor with a power commander to fine tune the fuelling but I'd like to get it somewhere near first rather than just plug it all in and wreck the sensor with it running way too rich. The stock map for the power commander is mostly a negative map to lean things up, so it looks as though these bikes do run rich as standard.
The old plugs were a little fouled so likely there's savings to be made. I do a fair few miles and gas is so expensive that it's worth trying!
While everything is in bits I removed the PAIR system and blanked off the ports on the valve cover.
Will report back with results.

Bet you miss the Forester!

 

You are quite welcome! I try to respond with help whenever possible since it's so quiet here.

No, no it is not! I've had mine in pieces for a little over a month now, trying to do the valve adjustment myself and few other odds and ends. Going a bit mental at this point!

Yes, yes I do! Had her for 10 years, my first experience with extensive modding, but ended up selling it after my mechanic told me the rear diff was starting to whine. Sold it to a fellow who contacted me about a year later and he told me it was running just fine. I was glad for him but, yeah, I do miss her...

 

Hey Doc

Update on the FPR: I got the replacement unit pumbed in. The cranked return pipe brings it very close to the #2 injector electrical connector but it will fit. I used 8mm fuel hose then a 6mm step down connector then 6mm hose to the return pipe on the tank. Will post a photo next time I have the airbox removed. I also replaced the fuel filter. It had never been changed - 18k miles on the one I took out. Cut it open to see condition. It had filtered 😄 Can't believe the fuel filter isn't listed as a service item.

I've only run one tank through the bike, but is much improved. Throttle response is miles better, smoother at the bottom end and the flat spot at 7-8k is much reduced but not completely gone. 38 mpg from this tank with mixed riding and a good but of hoonage so it's been worth the time and moving in the right direction 👍 Not sure if it's the new fuel filter which has made all the difference.

Next up will be compression test and check valves. Replace vacuum lines and balance the throttle bodies. Then put on the autotune to get rid of the flat spot.
What's the trouble you're having with checking the valves? I try to get stuff back together asap. The longer it's in bits the more chance of it never going back together!

I had a wrx wagon for 3 years, loved it but could never get around the feeling that a big bill was coming 😄 They have a different creak and groan, rattle and shudder each week it seems but just keep ploughing on! It least with bikes you can fix most stuff without the need for a lift.

 

Nice to hear! So good when things come together (literally) like that!

Ugh. So, overall, I'm happy to have a clean garage to work in but it is so cold right now. Even with a heater going, it sucks the heat out of me... Got the valves checked just fine but they were all either too tight or at the bottom of spec towards too tight. Replaced them all and, following the shop manual, left the tensioner out when I went to rotate the crank before checking the new clearances. Chain jumped a couple teeth on the intake cam sprocket so that's been fun to figure out. I'm really anxious about making a mistake and having to tear into everything again, just want to make sure I'm getting it right. Plus, I'm just cranky and old!

Yeah, I was so tuned to how the Foz sounded that every new noise (and it seemed there was about one a month) had me anticipating the worst! Some of the work I did was fun but, yeah, I definitely prefer working on the bike!

 

I'd definitely be interested in your picture as I'm looking to change my FPR soon! Looking around, it looks as though all of the J32 parts have the bend at the end? I found this one on eBay that looks the same but is a genuine Honda part. Did you purchase any specific hose with specific pressure ratings or anything like that?