85 K100 runs fine one minute, unable to rev above idle the next!

[CCalms]

Hey there, new K100 owner here, hopefully I can solve this problem and be with you guys for a long time to come!

I recently bought a 1985 K100 that was sitting for twelve years. I've replaced many parts and I was able to get the bike running. It currently takes a few tries to get started and idling but it will idle relatively smoothly. When I ride the bike down the street it feels brand new! I slow down to the first stop sign about 200 meters from my driveway, and when I try the pull away the bike is unable to accelerate past ~2500 rpm. If I kick up one or two gears then I can cruise at around 30 miles per hour at around 1k rpm. If I do anything more than crack open the throttle, the bike bogs down and I need to drop to first gear to slowly accelerate back up to 25-30 mph. Periodically, the bike will wake back up and feel like it has 100% of its running power for a few minutes before bogging back down again (a few times while riding at speed, but I'm pretty sure the pattern is that the bike often rides well and bogs down after slowing back down).

The bikes fuel system was clogged with rubber from the fuel pump boot deteriorating when I got it. Injectors, fuel pump (Ford), fuel lines, and fuel filter were all replaced. I hooked up a fuel pressure gauge in line between my fuel rail and fuel pressure gauge and it read 30 psi for the first couple minutes of my ride and then when the bike started bogging it still read 30 psi! The injectors spray well and the fuel pressure is in working order. Air filter was replaced, but I did not assess the condition of the MAF sensor. Spark plugs were replaced, again I'm not sure if the plug wires are perfect.

If the bike is bogging and I let it sit for a few minutes, it seems to regularly start up with power again (I can only tell if the bike will rev by riding it, idle is the same) but drop off after just a block of riding. I've had rides last as long as 20 minutes and then finally bog down. I've had the bike bog for 10 minutes and randomly begin revving and riding freely for me. It is such a frustrating problem! This bike teases me so much all I want to do is ride it nonstop but this problem makes the bike unrideable! When I ride off with the bike cold in the morning, it almost always goes for 5+ minutes at least (again, my trials are limited here), and the bike seems to get worse and worse running the more I try to ride it.

I'm extremely anxious to hear what the good members here have to say about my problem. I've done some searches and found some similar issues, but I've given up and I really need some help! Thank you in advance!

Carter

[TimTyler]

How many of the rubber bits containing the air that goes to the cylinders have you replaced? 

Oil filler cap O-ring, S-shaped crankcase breather hose, throttle body intake and exhaust manifold tubes, muffler gasket, throttle body vacuum caps and hose, air cleaner hose? Any air leaks or cracks will cause big performance problems.

[rbm]

Engine related issues that appear when the engine is hot and disappear when the engine is cooler points to HES, in my book.  Can you try the HES diagnosis while the bike is on the bench - heat the sensors on a cold running engine with a hair dryer to see if the symptoms appear.

[beemuker]

Engine related issues that appear when the engine is hot and disappear when the engine is cooler points to HES, in my book.  Can you try the HES diagnosis while the bike is on the bench - heat the sensors on a cold running engine with a hair dryer to see if the symptoms appear.

hes???

[The Mighty Gryphon]

Hall effect sensor.  Two of them on the front of the engine trigger the electronic ignition.

[CCalms]

Thanks a lot for the suggestions. I'll make sure to attend to any other rubber parts that may be contributing to air leaks and I'll do the heat test on my Hall Effect sensor.

[chris846]

Hi,
I rebuilt my K after a layup. The rubber mount around the pump was gooey, the inside of the tank had dried fuel inside. I cleaned everything with steam and replaced the gooey rubbers. Built it all back up with a new pump, filter , hoses. All spangly. Ran great.
Did some engine work, started running lean top end - got an air fuel gauge fitted. Tweaked the air meter, tweaked the air temp sensor, adjusted the fuel pressure. Fitted different injectors. No joy. Did LOTS of work to sort out the top end fuelling - it got so it wouldn't rev out on open throttle.

Finally thought about the humble old fuel filter. It was gunked up after less than 2000 miles from new. All the old dried fuel from the nooks and crannies in the tank had finally found its way in there. Replaced the filter and my super-technical fuelling problem vanished. Laughed at myself for being such a t**t.

Check the fuel filter.