BMW K bikes (Bricks)

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1Back to top Go down   K75 Back firing Empty K75 Back firing Sun Jul 12, 2020 4:45 am

jjefferies

jjefferies
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Took my restored K75T out for its 2nd spin in the past 6 years. Shaking out the bike, I've got a small issue, backfiring. Also noticed that the fuel mixture must be running rich as the stink of unburned hydrocarbons follows us everywhere. The cylinders were synced and the throttle advance a.k.a. choke was also advanced so the idle is just above 1000 rpm. Now the bike has new spark plugs and the valves are within tolerance. So what do you suggest checking that could be causing excess backfiring.  My other K75 doesn't have a similar problem. It will backfire with a trailing throttle but that's expected.

Any suggestions?
It's late and I've been up since 6AM. Sorry if I'm not quite coherent here.

best regards
J.Jefferies

    

2Back to top Go down   K75 Back firing Empty Re: K75 Back firing Sun Jul 12, 2020 4:59 am

Arlina

Arlina
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Adjust the TPS.
Loosen the 2 allen bolts a bit, turn it counterclockwise, slowly clockwise till you hear a click.
Tighten the bolts up again and check with the throttle if you did right.


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K75 Back firing Eu-log10  K1100RS/LT - R1200RT - R1100RS (RIP) - Cagiva SST 350 Ala Verde - K75LT project - K75 Schurgers - K75S
    

3Back to top Go down   K75 Back firing Empty Re: K75 Back firing Sun Jul 12, 2020 5:59 am

MartinW

MartinW
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There are a few reasons 75's backfire. Intake air leaks, exhaust leaks at the header to muffler junction, overly rich idle mixture, incorrect injector spray pattern, TPS adjustment incorrect.
Regards Martin.


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K75s Hybrid
    

4Back to top Go down   K75 Back firing Empty Re: K75 Back firing Sun Jul 12, 2020 7:23 am

mike d

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"and the throttle advance a.k.a. choke was also advanced so the idle is just above 1000 rpm"


That doesn't sound correct. As others have suggested, I would start with a TPS check and air leaks.


Mike

    

5Back to top Go down   K75 Back firing Empty Re: K75 Back firing Sun Jul 12, 2020 7:29 am

MartinW

MartinW
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Engaging the choke will effect the TPS adjustment and can contribute to backfiring.
Regards Martin.


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K75s Hybrid
    

6Back to top Go down   K75 Back firing Empty Re: K75 Back firing Sun Jul 12, 2020 8:18 am

Point-Seven-five

Point-Seven-five
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Engine temperature sensor, air temperature, and MAF can affect the mixture.  What is the fuel pressure at the rail?


__________________________________________________
Present:
1994 K75RT
1991 K100RS


Past:
1988 K100RS SE
1994 BMW K75S
1992 BMW K100RS
1982 Honda FT500
1979 Honda XR185
1977 Honda XL125
1974 Honda XL125
1972 OSSA Pioneer 250
1968 Kawasaki 175
    

7Back to top Go down   K75 Back firing Empty Re: K75 Back firing Sun Jul 12, 2020 1:31 pm

jjefferies

jjefferies
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Point-Seven-five wrote:Engine temperature sensor, air temperature, and MAF can affect the mixture.  What is the fuel pressure at the rail?
How do we measure the fuel pressure? I mean what kind of tool. I've got several air pressure gauges but nothing that I would put gasoline in to.

    

8Back to top Go down   K75 Back firing Empty Re: K75 Back firing Sun Jul 12, 2020 2:54 pm

Laitch

Laitch
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jjefferies wrote:
Point-Seven-five wrote:Engine temperature sensor, air temperature, and MAF can affect the mixture.  What is the fuel pressure at the rail?
How do we measure the fuel pressure? I mean what kind of tool.
Fuel pressure is measured with a fuel pressure gauge. O-100 or 0-140 psi or the Bar equivalent.

Click on this site's Portal tab at the top of the page. In the Portal, click on the Tech pages. In the Tech pages, click on Troubleshooting the EFI in the second column from the left. In Troubleshooting the EFI, scroll down to the last article in the document which explains fuel pressure testing with a gauge. That will show you what type of gauge to use, how to couple it to the system and the values that are appropriate in a working system.

You go to AutoZone in Alameda then ask them if you can borrow a fuel pressure gauge with a T-coupling and clamps. If they have one to loan, you'll pay a refundable deposit. If they don't have one, try NAPA.


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1995 K75 81,000 miles
K75 Back firing Usa-lo10
    

9Back to top Go down   K75 Back firing Empty Re: K75 Back firing Sun Jul 12, 2020 3:09 pm

jjefferies

jjefferies
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Thanks

    

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