FZ Losing Power & "Bogging Down" at the Top End

[jb.junior]

Heya.  I can't seem to get this figured out.  Wondering if anyone here has an idea.  This is for my built, MotoAmerica superbike FZ.

Problem:

·         When running on the track, the bike starts to lose power and “bog down” at around 8.5k RPM, up until redline (~10.4k).  It does push through that range, but barely.  And stays bad up until redline. 

·         This started happening at COTA in warm up.  Continued in to race 1, worse.  Top speed of 124mph.  Before the problem, I was at 139 mph.

History:

·         I put in the YZ 450 injectors, and auto tuned them for about 30 runs, back in May (2024).  It seemed to finally smooth out. The bike has had problems since.  (This is supposed to add 1-3 hp...)

·         In June, I had to stop race two at RA early because the bike was bogging down. 

·         Upon inspection, the spark plugs were fouled – bad.  I replaced the plugs, and the problem went away.

·         At Brainerd, the next month (June 2024), the problem seemed to come back but quicky went away.  Not sure if the plugs started to foul again and the residue blew off.  I changed plugs again after.

·         The bike ran good for the next two weekends.

·         At ECR, in August (2024), the bike ran great, until the last race.  Started to bog at high RPMs again. 

·         After ECR, I changed the fuel pump, looked at the spark plugs, put a different map in it and tested it.  It seemed to run fine.  Ran it at a track day, and no problems.  

·         Next month, at Hallett, the bike ran fine for the first half, but then had a problem with bogging down, again.  I put in an old map, that I knew worked, thinking that my O2 sensor was bad and giving the PC5 bad autotune data, and it did not fix the problem.

·         I then had both the OEM and YZ450 injectors flow tested and cleaned. The OEM ones had some minor restrictions (8%) but not horrible.  YZ450 were perfect.  Same with fuel rail and hose.

·         For the next event, I put the OEM (FZ / R7) injectors back in and put the old, original tune (for those injectors) back in.  And put new plugs.  Fresh fuel.  Bike started and idled and runs great (as always), until I get it on the track at high-end RPMS, and then it bogs down again.

·         Looking at the auto tune suggestions (Power Commander), it’s asking for 25% (max allowed) fuel from 70-100% throttle, from 8.5K – 10.5K.  So it seems starved of fuel? 

·         Got a season on this engine. 94 hp.

Diagnostics of this problem / things I’ve tested or replaced recently:

·         Took off fuel tank and inspected.  Clean.  No water in tank. 

·         I’ve tried multiple fuels.

·         Took off Yoshi airbox.  Looks good. 

·         Swapped fuel tanks (and thus trying another fuel pump) with spare bike.  Didn’t fix.

·         Looked at throttle bodies.  Looked clean.  Minor film.  Cleaned with carb cleaner.

·         Multiple maps tried.

·         Swapped O2 sensors with spare bike.  (Don’t think these are bad.)

·         Compression test.  Great compression: 240 psi each cylinder.

·         Replaced coils (May; used)

·         Replaced RR (May; used)

·         New stator (May) [it had a bad ground and was shorting out bike]

·         New Power Commander 5 (May)

·         New Dynojet QS (May)

What else to try?

·    Get a brand new fuel pump.  Is it possible that the current fuel pump(s) work on the superstock bike, because they don't pull as much fuel, but won't work on the superbike because it needs to much more fuel?     

- Swap out harness?

·         Swap out Power Commander 5.  I have on on my SS bike.  And I have a newish PC6 I can try.  I'd love to run without the PC, but I'm pretty sure that would be damaging to the engine.  

·         Test other things on the engine.  (Maybe it’s not electrical / computer?).  Maybe vales, but wouldn't that make it not run right at idle and all RPM ranges? 

·         Swap out MAP sensor.

·         Swap to spare engine (I don't think it's an engine problem)

- Battery test

-  Blockage in exhaust?

[br4nd0n]

Have you tried removing the Power Commander and switching anything back to OEM that would require it? It seems like one of the only things in the list you haven't tried.

[jb.junior]

15 hours ago, br4nd0n said:

Have you tried removing the Power Commander and switching anything back to OEM that would require it? It seems like one of the only things in the list you haven't tried.

Hi.  Thanks for the input. Well, that's a great idea, and I have not done that.  But... because the engine is so built, I don't think it's safe to run it without the PC on the OEM flash / tune.  What I will try, today, is to swap to a different power commander and see if that does it.

I changed around maps and did a lot of test runs yesterday.  The auto tune module kept demanding more fuel in the upper-ish range (100% throttle; 8-9k rpm).  So I would give it to it and go test.  The bike ran worse and worse the most I increased fuel.  So it's something dynamic, not static.  When I went back to a leaner tune, the problem went back to being less pronounced.  So I think it has to be a problem with the Power Commander or the fuel pump.  

Is it possible that the fuel pump works fine under "normal" fueling for this bike (it works on my superstock bike with a stock engine) but on this superbike, that needs a lot more fuel, it can't keep up?  

 

[M. Hausknecht]

Jeff, personally, I wouldn't use a PC if I had a choice but I'm not sure that is your problem. I use an aRacer Super2 and, if nothing else, its data logging allows me to record extensive data from a session and then download and review it later. On my 92rwhp motor the stock injectors are right on the edge duty-cycle wise. I  can see that on the dyno and when viewing an on-track session. My understanding is that they just don't flow enough fuel to get to 94hp with the stock fuel pump at its full pressure. Do you know how much more fuel the YZ injectors flow over the stock 07 injectors? Anything less than 10-15% could be problematic. The PC indicates the motor wants more fuel (so the AFR is lean) but you also say the plugs get fouled. Are they fouled from fuel or from oil? Your compression is good but the valve guides could be leaking oil badly; you have run the motor a full season after all. 

Switching fuels adds variables and introduces inconsistency. Different levels of oxygenation and/or specific gravity differences can make a big difference in AFR. I always use the same race gas, just to keep the fueling consistent. I gather you haven't checked your fuel pump for flow or pressure; I'd do that. My best guess is that you're lean and getting oil into the combustion chamber through the guides.