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TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. => The Motobrick Workshop => Topic started by: acyclicsalmon on January 08, 2022, 04:10:13 PM

Title: K100 knocking / ticking sound. Surging at low fuel.
Post by: acyclicsalmon on January 08, 2022, 04:10:13 PM

Hello crew, I'm back for another edition of "Whats wrong with my K100?".  Hope the experts here can help suggest some starting points of inquiry.  Model: 1986 K100RS. 

My symptoms (In order of most concerning to least):

• There is a knocking / ticking sound coming from my motor, I've made a video documenting it which I've attached down below
• I get a lot of rattles / vibration coming from "somewhere" in or underneath my gas tank when I accelerate around 3000-3500 RPM
• I have some concerns about my fuel pump, but the whole fuel system in the tank is relatively new (sans fuel pump).  I feel like I replaced all tubes inside the tank as well as the fuel filter (bosch unit), mesh screen, and a new rubber fuel pump holder.
• I get terrible surging as soon as I get the red low fuel light indicator.  I've seen the PDF service bulletin for this issue, but I don't think it applies as I see gurgling fuel on the left front side of the tank (which I assume is the fuel return line)

Here is the sound the engine is making. Here's the link, https://youtu.be/EjJ0hnwkrW0, and here's the hyperlink: https://youtu.be/EjJ0hnwkrW0 (https://youtu.be/EjJ0hnwkrW0)

Here is the view inside the tank, it doesn't look concerning to me but I'd love any feedback if this looks weird.  Here's the link, https://youtu.be/E88FOVyvYxc, and here's the hyperlink: https://youtu.be/E88FOVyvYxc (https://youtu.be/E88FOVyvYxc)

I have some theories on work I'd like to do next:

• Check the fuel pressure, but I've been putting this off as I don't have a fuel pressure checker
• I installed new OEM headers after an accident, but I forgot to put in new header gaskets.  I can't tell if there's a header gasket leak, but I don't feel any surging air coming out from underneath, tho it is hot
• I don't think its a valve thing, I had my valves checked 8k miles ago
• My right-most plenum boot looks wet, maybe its an air leak?
• I had a throttle sync about 8k miles ago, but it was like 15 years ago.....

Thanks again for the help, this forum was instrumental in helping me fix my bike last year

Title: Re: K100 knocking / ticking sound. Surging at low fuel.
Post by: Laitch on January 08, 2022, 05:05:08 PM

There's a spring-loaded ball check valve in the fuel return line assembly screwed into the tank. When the spring gives way, it makes a noise like that. Many owners simply remove both the ball and the spring but that makes removing the return line slightly problematic when the fuel tank is full—nothing that can't be overcome by resourcefulness. While the fuel level is low, remove that valve assembly to inspect it and verify, also, that it isn't clogged.

A fuel pressure gauge assembly is likely available in Oakland for only a refundable deposit at O'Reilly's, NAPA or AutoZone. Verify that the return line from the fuel pressure regulator isn't pinched.

An air leak in a plenum bushing/boot shouldn't make the bushing wet. Something else is going on there.

What's your concern about the fuel pump?

Title: Re: K100 knocking / ticking sound. Surging at low fuel.
Post by: blackie1 on January 08, 2022, 05:15:39 PM

I would sort out those missing gaskets  112350

Title: Re: K100 knocking / ticking sound. Surging at low fuel.
Post by: Laitch on January 08, 2022, 10:13:10 PM

Quote from: blackie1 on January 08, 2022, 05:15:39 PM

I would sort out those missing gaskets  112350

Tell me it isn't true! Missing?   177381

Title: Re: K100 knocking / ticking sound. Surging at low fuel.
Post by: milq on January 15, 2022, 10:42:14 PM

My first thought was exhaust gasket leak. Definitely more noticeable as you moved the camera down.

Title: Re: K100 knocking / ticking sound. Surging at low fuel.
Post by: acyclicsalmon on January 21, 2022, 04:56:19 PM

My goodness all, I've wanted to post a response for over a week but I get a Your session timed out while posting. Please try to re-submit your message each time I post here.  Turns out I had to clear my cookies for this website, and it works now.

Appreciate the advice all, I ordered the exhaust gaskets and am looking forward to installing them soon.  Also they weren't missing! I just reused old ones. @Milq, I see what you mean about the sound being more noticeable when you move the camera down. 

As for the fuel pressure -- I rented a tester at O'Reilly's (Super easy to use!), and I was seeing 32 PSI at idle, and 34-35 PSI with minor throttle.  I know this is a hair low but I think its probably because the tester caused the fuel line to bend pretty hard.

I'll bump this thread when I get the gaskets in, which looks like it will probably be in a month or so because I ordered them to my midwest address accidentally...

Title: Re: K100 knocking / ticking sound. Surging at low fuel.
Post by: Laitch on January 21, 2022, 07:43:51 PM

Quote from: acyclicsalmon on January 21, 2022, 04:56:19 PM

I know this is a hair low but I think its probably because the tester caused the fuel line to bend pretty hard.

Are you indicating you tested fuel pressure with a kinked delivery line to the injectors? What was the gauge's reading when you revved the engine? What was the reading when you measured directly from the delivery port to the gauge?