Bad mojo on dyno printout...

I have a nearly stock VF750F. I had a feeling for a while now, that the bike was running lean at high RPMs. I work near a shop with a dyno machine, and figured it would be a useful diagnostic tool (it has an EGA built in). The first run went well, but dropped out early, and the EGA was still off-line. The last two runs had very erratic power readings, and the air/fuel ratio was just plain screwed up. A dyno printout should be attatched to this thread somewhere, the red circle highlights the problem.

Not to sure if being a California model has anything to do with it, but usually an AFR of 17:1 would be considered... LEAN, right? Im pretty sure this bike should also be making well over 75 horsepower... Isn't stock somewhere around 87?

Being lazy with the effort, but not with the head (or the yak), I don't really want to pull the carbs again until I have some more input. When I got the bike, I noticed it had a dead carb slide. Like I expected, a slide diaphragm was torn. Since it was such a small tear, and I didn't happed to have a spare 25 year old part lying around, I repaired it with VLP (if it will fix a torn seat, it should fix this). After I "fixed" it, the slide operated perfectly, and in sync with the other three. All the slides moved smoothly, without any sticky spots. Since I got the bike, I completely rebuilt each carburetor, Adjusted all valves, synchronized the carbs, replaced the air filter, replaced the weak coils and dead ignition wires, replaced the plugs, and basically fine tuned everything I could.

I have a few ideas:

Since the diaphragm was torn before, it happened again... This would kinda account for some of this, but the bike runs smoothly, it doesn't drop a cylinder at high throttle, or any other time. This explains the missing horsepower, but it would be way obvious when riding (it's not)

The slides could be sticking... Even though, they were smooth after I rebuilt the carbs, the Uni foam I wrapped around the old filter frame may have spooged that sticky filter oil everywhere. The printout does kinda show delayed slide response.

The bike overheated from being thrashed on a dyno for ten minutes, and was pre-igniting... There is definately signs of progression with time. I know the bike runs like crap, and wants to stall when it gets hot (boils the fuel in the line). Maybe the carbs don't meter frothing gasoline very well.

And the final idea I have would be the fuel pump taking a crap on me... Maybe the engine is sucking the carbs dry, and the pump just can't keep up? Any more ideas, and any imput at all would be appreciated.

 

Could a clogged fuel filter do this maybe?? I just replaced a filter under the fuel pump that I never noticed before.

 

Yeah, I'm gonna dyno it again this weekend. I didn't really feel a difference, but I don't run it wide open in high gear up to redline when I ride (that much). I'm sure under normal riding conditions the difference would be negligable, but it was definately close to fuel starvation.

The thing is, This would definately cause fuel starvation at high RPM under load, but I would figure it would be a steady lean burn, not erratic like the printout showed. I'm still expecting a lean spike around 10K like on the green line, but more even up to that point. the clogged filter does explain the progressively worse performance each run, and the overall erratic AFR, but I'm still wondering about that nasty spike. The rich condition at low RPM is puzzling me as well, since these bikes are known for running lean... it could just be the sudden throttle application choked it a bit.

I removed the airbox, and the slides were all free, and all slide diaphragms were intact. There was no fuel where it shouldn't be, and a plug check turned up nothing. If it turned out to be nothing but this fuel filter, I invite everybody here to kick my ass for me.:first:

I'll be sure to get a video of the next dyno run. The sound is actually quite impressive, despite the stock exhaust (and rattling valvetrain). the fire that blows out the mufflers after acceleration is kinda neat too, comes out both sides at the same time... about two feet long... and really loud... I'm gonna blow up a muffler, huh?

 

My ancient recollection of the typical A/F ratio for optimum power is 12.8:1, give or take about 0.2 depending on engine type. Typical maximum lean condition used to be 15:1 for maximum efficiency, anything over that could be problematic.

I don’t know VFR’s yet, so I don’t know about their fuel systems. I can tell you that drag racers with gravity fed carbs replace the stock petcocks with substantially higher flowing ones, because even a ¼ mile WFO drag race can empty the float bowls on a mildly tuned 1000cc motor. So I guess what I am saying is that fuel flow that may be adequate for street use may not be adequate for a dyno run.

I would offer a guess that the fuel filter was a large part of the issue. I would also pull the spark plugs and check them before the next dyno run, because the high-load lean-out condition may have glazed them or severely overheated them. Use a magnifying glass and look closely to see if the ceramic insulator looks polished or slightly yellow. Over-heated spark plugs occasionally disintegrate and drop pieces of ceramic into the cylinders. If you’re lucky, the pieces blow out past the exhaust valve. If you are not, the cylinders walls and piston/rings can be damaged.

Forgive the slightly long-winded next paragraph, and forgive me if you are already well-versed in the subject…

The worst thing about high-load high RPM lean-out is, as you mentioned, detonation. Without beating this to death, the absolute worst thing about detonation is it disturbs what is known as the “boundary layer” of gases in the combustion chamber. The boundary layer is essentially a protective insulation blanket of gases that prevent the extreme combustion temperatures from directly contacting the piston crown, cylinder head and valves. When the layer is disturbed/destroyed from the violent shock wave caused by detonation, all of the aluminum parts get really hot, really fast, which makes the situation worse because high heat makes detonation more probable. When the aluminum is really hot, it starts getting soft, and the next detonation cycle starts hammering on all of the hot, soft, aluminum parts. The piston crown usually gives it up first and starts molding itself downward and freezing the top piston ring in its groove, so it can no longer expand outwards. When that happens, the combustion gases can now easily work their way down to the second ring and start overheating it. When the piston rings are no longer expanding outwards, the heat from the piston, which used to be partially absorbed through the rings into the cylinder walls and the cooling system, has nowhere to go, so the piston gets even hotter. You can see where this is going…

If you suspect a lean condition, address it ASAP. By the time you are aware of detonation on a high rpm engine, it might be heavily damaged.

And I do apologize for the dire warnings. I have disassembled a few high-perf motors that were just sad to look at, and I have also seen my share of the ones that have saved anybody the trouble of disassembling them.

I'm usually more cheerful than this... OK , I'll go away now!