K100RS Backfires, runs rough at low RPM, rough idle

[JanekW]

Hey guys, having an issue with my '89 K100RS

I have the following symptoms:

• Runs rough at low RPMS - sounds like its not running on all cylinders up until about 2-2.2K RPM, at which point it suddenly sounds smooth and I have a lot more power
• Idles low and rough, often dying. Sometimes surges at idle.
• Sometimes (and temporarily) will run rough and lose power at 3-4K rpm, but only for just a moment before it starts running smooth again
• Is sometimes tough to start
• Backfires on deceleration (not excessively, but more than I feel would be normal)

These problems seem to get much worse when the bike is warm.

So far, I have:

• Replaced the rubber vent tube
• Replaced all rubber caps on throttle bodies
• Checked injectors for leakage/spray patterns (one injector replaced, rest look good)
• Verified fuel pressure (hovers around 35PSI, locked to 37PSI if vacuum hose disconnected from regulator)
• Replaced fuel filter and pump intake screen
• Inspected condition of boots which join airbox and throttle bodies (appear to be in good condition, though I haven't tested with propane yet)
• Checked spark plugs (all appear to be in good condition)

I'm starting to run out of ideas here. I'm thinking it may be something to do with ignition or fuel/air mixture, but don't really know what exactly would cause my symptoms. Any ideas?

Thanks!

[Laitch]

Welcome, Janek!

Here's an idea. Do the propane test on all hoses with the engine at idle. A visual inspection is almost worthless. Here's some other ideas. Clean the battery connections, the frame ground connections, the battery ground strap connection on the transmission, the ignition switch connector under the tank and check that the starter connections are clean and tight.

We're in the process of elimination here and diligence is necessary.

Tell us how long you've had the moto, the odometer reading, and anything else about its history. Post a photo of it.

[frankenduck]

Look here - http://www.motobrick.com/index.php?topic=14323.0

[JanekW]

Thanks Laitch,

Great ideas, I'll clean all electrical and try propane on all hoses tonight.

Unfortunately in it's current state I would be quite embarrassed to post photos of the bike as it is someone's half-assed attempt at a cafe racer. Welds on the frame look like they were done by a first timer, angle grinder marks still visible on cuts, cheapo amazon lighting, zip ties used to hold damn near everything together, cheap crimp connectors used all over the electrical, etc. I bought it a couple of months ago on the cheap with the intention of fixing it up and hopefully turning it into something worthwhile. I don't have a reliable odo reading as it has a custom Tenet gauge, but I don't think the bike had an easy life!

I'll update when I do the things you have recommended.

[JanekW]

Look here - http://www.motobrick.com/index.php?topic=14323.0

Hmm I thought I had searched through posts with similar symptoms but I missed this one. This is good. I'll do what has been recommended so far and follow up.

Thanks!

[Laitch]

Great ideas, I'll clean all electrical and try propane on all hoses tonight.
Unfortunately in it's current state I would be quite embarrassed to post photos of the bike as it is someone's half-assed attempt at a cafe racer.

Telling us it's a failed project is useful. Try to curb your vanity and post several photos shot at seat height. Somebody here might notice something that you've missed. Photo posting guidance is on the site or PM me for clarification.

[The Mighty Gryphon]

My first impression is that you have a rich mixture at low rpm. 

What do your spark plugs look like after idling for a couple minutes? 

If they are black and sooty, I would look for some information on the air flow sensor and the coolant temperature sensor.  One of them may be calling for too much fuel, and the rising rpm pulls more air to correct the mixture.

Make sure the door in the airflow sensor moves freely, and that the wires haven't been damaged.

[frankenduck]

What do your spark ugsplugs look like after idling for a couple minutes? 

If they are black and sooty,

This might not be such a great indicator.  If K bikes idle too long then they foul plugs to some extent. I suspect this is why the owner manual recommends that you ride off right after starting.

(And for some reason K75s do this more than K100s.)