Good Day Fellas,
I have a 1988 K75S which has been running terrifically all season. This weekend out for a ride and out of the blue it started losing power a bit. I gave it a little throttle and it really bogged down. Tried to downshift and give it some gas and it was just popping, sputtering, and backfiring. I pulled over and topped off the fuel (it was a little under 1/2 tank) thinking maybe last fill up could have been a little off - even though it was a trusted station). I pulled the plugs to see if they looked fouled or anything unusual, but they seemed ok. When I started it up again it sounded a little off, but ran ok for a mile or two, then started to lose power again and the same popping and bucking. I pulled over and shut it off for about 5 minutes and started again fine for a mile or so, then again same issues. if I pull over and stop it seems to idle fine. It starts up just fine, but I'm not sure where to begin the troubleshooting. You guys have been remarkable with all the tips given for other issues before, and I hope you know how much novice wrenchers like myself appreciate it. Thanks much, Tim

Tried a new fuelfilter?

Even after replacing sticky injectors and a bad fuel pressure regulator this exact same issue happened to my 1985 K100RS. I tested my ignition coils (original) and they were bad. My recommendation is based on your bike starting and as soon as it gets a tad warm the issue commences. When I tested the resistance on my coils the resistance would slowly and continously charge.

TimK100 wrote:I have a 1988 K75S which has been running terrifically all season.

It doesn't seem like it was running terrifically in June of this year when you posted about a stalling issue, so you must be referring to Fall.    You installed a new fuel filter some time in mid-June and maybe cleaned the 4-pin fuel pump plug. You didn't followup that thread with which treatment made it run terrifically after that. Is this the same Brick? How does the stalling now differ from the stalling then?

If the fuel filter was installed correctly; if the tank interior fuel lines are the correct type, in good condition and firmly connected; if the screen pre-filter at the bottom of the fuel pump is not clogged then to consider next would be checking the z-tube connecting the crankcase to the air box, the vacuum caps on two of the throttle bodies and the vacuum hose on the third for slits and cracking; and checking the rubber manifolds of the throttle bodies for leaks.

Checking the coils, as has been suggested already, could be last in the line because unlike the 1985 K100RS coils, the K75 coils have a history of being reliable.

Thanks for tips, your right Laitch, regrettably I didn't follow up that last thread, which I will. It ended up being a fuel pump issue. I will have to check the rubber parts this week

It was a fuel pump issue previously that was resolved in  mid July. I was able to look at the rubber vacuum caps and the 3rd vacuum hose all seemed in good shape. however when I inspected the z-tube it was very cracked at the front elbow. Would this cause that issue? I will have to look into getting a replacement regardless. I am attaching a photo of what it looks like. Thanks again

TimK100 wrote: when I inspected the z-tube it was very cracked at the front elbow. Would this cause that issue?

It could be contributing to this condition. That's why it was mentioned in post #4.

I replaced the Z tube, but that didn't seem to have any affect on the issue. I was considering replacing the plugs and wires, because it really seemed like a misfiring type issue.

Did you replace the fuel filter when you replaced the pump? 

There have been problems like yours that were cured by replacing the filter in the tank.  The filter chokes the flow of fuel so that as the engine runs the fuel delivered can't keep up with what is used.  There is enough to sort of run the engine at low rpm, but not enough to generate good running power.

Make sure that the spark plug leads are FIRMLY seated on the spark plugs. What I do is grab the tab on each lead with the factory toolkit pliers and then give the pliers a hearty smack with the other hand to make sure that they are all of the way on. Then I try to wiggle each lead by hand. They should not wiggle.

I did replace the fuel filter earlier in the year when I was having the previous issues. I don't think it would be an issue so soon, but if this sounds like it could be the issue, I wouldn't hesitate to do it again. I believe I have an extra around here somewhere. I have ordered and gotten plugs and wires also, but haven't had a chance to install them yet. Maybe not until next spring, I'll keep you posted, but until then, as always, thanks so much for everybody's input. This forum is invaluable!

Easy to test the fuel filter. Take it out and try to blow through it in the direction of the arrow. You should be able to blow through it fairly easily.

Not my K but I had similar symptoms this Summer on the Busa. Turned out it was the tank bag causing an airlock. I guess any other cap breather blockage might do the same (esp. with the apparent self healing after a stop or fill-up)

In general excessive backfiring while loss of power means it's starving for gas . It's difficult to get to the return line connection on the front of the tank so I usually attach it then push the tank fwd into position, but you have to be careful not to kink the hose. 

if its a problem with spark it  loses power and sputters but doesn't backfire like the 4th of july or if you are in texas somebody is shooting at you.

another place to check for kinked hose is the connection to the fuel pump where there's not enough  space between the pump and the top of the tank so you have to push the pump down further into the mounting assembly.

unlikely but quick to check.

It sounds to me after topping up the fuel it runs ok for a while then loses power. If a new filter doesnt fix it, maybe the tank vent isnt good. when it loses power open the fuel cap then close it again. if its then ok for a while before losing power again, the vent might be blocked.

Thanks for the options. It didn't respond any differently after topping off. I may be a vent issue, I'll check into that further, maybe in spring. It does seem like a starving for gas type issue. I may replace the filter again just to rule that out. After that I may investigate the pump again. I replaced it in July. I purchased an aftermarket pump on Amazon from All Balls Racing. It looked the same as the OEM and worked fine until this issue. I don't know if there's a way to check if the pump is working properly at all levels of throttle. It starts and idles pretty well, as soon as you give it some throttle it seems to bog down or stall. Could be a kinked hose issue I suppose, also worth looking into. Although nothing was done in between riding fine daily and all of a sudden in the middle of a ride, this issue came about. Again probably not until spring. Thanks again so much for everybody's input. Issues are so much easier resolved with so many great mind working on it, and this forum is amazing!

my guess, spark plugs

One of the last things I replaced in November were the plugs and wires. It didn't change anything.

It might be a good time to check the fuel pressure, it might be a lousy pump. It should be 36psi.

Thanks, that will probably be one of the first things I check come spring.

Finally got around to getting to it. After having a fuel issue with my 85 K100RS and straightening that out, I thought about the injectors on this one. I pulled the fuel rail with the injectors still attached and gave it a few hits on the starter and sure enough it looked like they weren't all putting out equally. I also have a 93 K75RT that runs very well, so I thought I might just swap out the fuel injectors, and sure enough, it started and ran great when I gave it throttle.  I have ridden it several times, and since sent the injectors to Mr. Injector in Idaho who tests and cleans them. I got those back and put them in. and we're back to running like a new bike again.

 Thanks again for all of your input and suggestions. This forum is invaluable.

glad this ones fixed , if it happens again look at the charging and battery condition , sounds like a problem i have experienced

Lo-tech, but a quick way to check fuel delivery is to pull out the fuel rail complete with injectors, hold it over a piece of your favourite brand of paper towel and crank the starter. The injectors bank fire (i.e. in unison) and a quick crank giving three or four squirts should produce four neat, healthy, equal-sized fuel circles about 20mm in diameter.

Note that low pressure in the fuel rail insufficient for the engine to run will still produce circles of fuel, but experience will tell you if they're not big enough.

I said it was lo-tech!