K75 stumbles below 3K and above 6K

[Bull Gear]

So this '88 K75 that I recently revived for a lap around Lake Superior ran flawlessly at first. Two days into the trip it developed a really annoying driveability problem. It runs great between 3000-6000 rpm but it kind of spits and sputters at lower rpms and then lays down on the top end. It still idles but not as nicely as it did. I'm guessing something has gone flaky in the fueling. I rode it about 3900 mis that way and it never got any better or any worse.
I finally got motivated to troubleshoot.
- Fuel pressure looks OK
- No vacuum leaks
- Checked throttle position sensor
- Checked throttle body balance
I guess I'll pull the air flow sensor and look it over. What do I look for?
What else do I need to be looking at?

[bocutter Ed]

Did you check your filter?

[Bull Gear]

Air filter is new.
FYI I basically stripped it down to a motor and frame to prep for the trip... new air filter, new fuel lines, new upper TB boots, check TB balance, new plugs, valve lash, etc, etc.
It basically happened overnight. One day it ran great, the next it didn't.

[Laitch]

etc, etc.
I guess I'll pull the air flow sensor and look it over. What do I look for?

etc, etc. differs from person to person, Bull Gear. It's more helpful to be specific for diagnosis. Check all ground connections, electrical connectors and starter connections because you've stripped it down.
 
The airflow sensor vane should move freely. Gently push it to see if it's stuck somewhere in its range.

[kennybobby]

Take it for a ride above idle and below 3k where it is acting bad, then kill the motor and pull the plugs.  Are they dark and sooty, or dry/white and ashy, looking down into the insulator around the electrode. 

i suspect it is running rich based upon your description.  Did you check/replace the vacuum tube from the #3 port to the fuel pressure regulator (enclosed in a metal spring).  If that tube is leaking then you have full pressure all the time and will run rich below 3k, run okay in the mid range, and lean at the upper end.

i assume you would have replaced the Z-tube between the engine breather and the intake plenum if it were cracked, although that would make for a lean mixture.

The answer will be found in the plug chop.

[bocutter Ed]

Air filter is new.

I meant the fuel filter ...

[stryder]

Coils?

Separately, they may pass a "field test" but together, may be too weakened.

Do your plugs have the caps screwed on the tips, or just the exposed screw tops?

Do you smear gobs of dielectric grease on/in between all things electrical?

If so, the coils may have been working overtime overcoming the resistance/gaps and maybe are on their way out. 

Best way to check is to replace them with coils off another similar bike and see how it runs.

[Bull Gear]

New fuel filter and all the hoses are new. Nubs are on the plugs.
I don't have any spare coils, but I can check the ignition with a spark gap checker. That should give me a pretty good indication.
The reason I'm leaning toward fueling issues is because the problem is so consistent and repeatable.

[Chaos]

sometime water settleing in the gas tank can cause erratic running.  a bottle of HEET usually takes care of it.  Injectors usually benefit from an occasional dose of Techron, also.

[Motorhobo]

I guess I'll pull the air flow sensor and look it over. What do I look for?

I remember the AFM to be easy to get out but a major PITA to get the screws to line back up when reinstalling into the air box. Plus, it was all for naught because the problem turned out to be a vacuum leak -- bad throttle body connecting hoses. So maybe do the propane torch test to search for leaks before you pull the AFM out of the box...my $0.02...

[kennybobby]

It seems fuel-related to me, can you swap out the Jetronic unit with someone nearby just to see if that could be the issue--rule it out or point the finger...

[K1300S]

New fuel filter and all the hoses are new. Nubs are on the plugs.

if you are referring to the vacuum ports....only two should have covers.  the third(aft) one should have a hose on it which controls the fuel pressure regulator.

[Bull Gear]

I was referring to the nubs on the spark plugs. The fuel tank was spotless and dry when I put everything back together.
All great suggestions to follow up on, though.
Unfortunately, I don't know of anybody around that i could swap parts with. I'll just have to keep working the problem until I can pinpoint it.

[Bull Gear]

Bringing this back to the top because I still haven't figured it out.
Using the troubleshooting guide (http://hrsbstaff.ednet.ns.ca/bvogel/K100/download/bike-wont-start1.htm), I've tinkered with it off and on over the winter with no improvement.
I checked out the fuel pressure, the idle switch, the air flow sensor, and the air and coolant temperature sensors, cleaned the under-tank grounds, the EFI connector, and the Hall effect connector. Plugs are sooty but all firing. Almost certain there are no vacuum leaks.
Thinking about sending the injectors to be cleaned and checked but really can't see how they could work flawlessly one day and then crap out the next.
Any other ideas?

[subforry]

New parts can be bad or fail after a short time.  Maybe consider testing/swapping the new bits.

[Bull Gear]

I dunno. I'm about ready to punt this bike down the road. Me and fuel injection just don't seem to get along.

[Bull Gear]

I think it's finally sorted out. I had previously used Deoxit on the ECM connectors but this time I took a fine emory board to the pins even though they looked bright and shiny. That seems to have done the trick.
Either that, or I WAY over-thought this problem and new spark plugs was the fix.  :hehehe  Yeah, yeah, yeah... I know.
However, I have a hard time believing that the prior new plugs fouled 500 mis into the trip, instantaneously, overnight, and then managed to carry me the next 3000 mis without getting any worse.
 :falldown: