K75c Backfires

[mjanulis]

Very new to the K75 world and as a new owner ('87 C Model) I am a little surprised by all of the hype surrounding the bullet-proof nature of these bikes if there are so many problems......am I missing something?

I really like the bike and have been looking for the exact bike I wanted for over a year. Finally got it and it backfires when I downshift and let off the throttle. I noticed a crack in the air inlet tube (right side at the air box), could this be the issue or should I start from the beginning and consider a full service tune-up??

I keep my bikes for a long time, so I am committed to this one good or bad.

Thanks in advance.....Mark J
Other bikes: K1200RS, R90S, Duc 748, R125RS, 74 CB200, 66 Duc 350, HD 350 Sprint, Yam 360 Enduro.:

 

[DEANCOX]

I think they are pretty trouble free. I have an 89 K75c and it really hasn't given me any significant problems. Part of the fun is going through and maintaining them.

I would fix any obvious stuff, just because.

My personal experience and I think others will second this, that on the older K75's it is easy to think you have rolled off on the throttle, but in reality you holding it slightly open and this will cause backfiring under deceleration and downshifting. Try to consciously twist the throttle closed hard against the stop when decelerating and see if the backfiring doesn't diminish dramatically.

Have fun and welcome to the community!

 

[tjtraver]

Backfiring .... is "normal "

K75s backfire upon deceleration if the throttle is held even partly open .
They all do it ; it's "normal" .
If the throttle is completely closed when decelerating they will not backfire .
There is a micro switch on the throttle body that's set to click just above idle .
With your bike off , try turning the throttle and listen for the click of this switch ( near where the throttle cable hooks to the fuel injection ).
I don't recall what that switch does , but that's what makes the difference between backfiring and not (perhaps it's misadjusted ? ).

The K75 is a very nice bike , timeless design , I'm sure you'll like yours . I've ridden lots of other "better and higher end" bikes , but for my money , the K75 will run with any of them . My '95 K75C is a "keeper" I have no desire for anything else ( esp after I read about the final drive failures on the newer bikes, no way ! ).

Todd

1995 K75C

 

[deilenberger]

The switch is the throttle position sensor - it tells the FI the throttle is fully closed. The FI turns off injection on it's own below 3K on decelleration, then when the switch closes on fully closed throttle - it turns it back on so the engine doesn't stall.

To check the adjustment - put your ear next to the switch while opening/closing the throttle. You should hear a click from it just as the throttle closes and one from it just as you reach full throttle (which tells the FI you want to GO and it richens the mixture a bit..;.)

As others have said - it's sorta the nature of the beast.. if you hold it slightly open, expecially on downhills with engine braking being used - it will backfire. This can be useful sometimes if you have a tailgater.

And DO replace the crankcase breather hose (the Z shaped one you mention is cracked.. it's letting unmetered and unfiltered air into your intake.. which will make the backfiring worse and dirty air isn't good for the engine.

 

[cayuse60]

.. if you hold it slightly open, expecially on downhills with engine braking being used - it will backfire. This can be useful sometimes if you have a tailgater.

And DO replace the crankcase breather hose (the Z shaped one you mention is cracked.. it's letting unmetered and unfiltered air into your intake.. which will make the backfiring worse and dirty air isn't good for the engine.

I love when my K75 gives a little "heat" through the exhaust...I like to leave the throttle open just a hair when I have an HD in tow

 

[ted]

When I first got my K75 it backfired as well, I soon realized I was as has been mentioned not fully returning the throttle. Once I figured this out the only time it ever backfired was when I wanted it to (mainly when being tailgated at night )

 

[mjanulis]

A big THANKS to everyone for responding to my backfire concerns. I will certainly take everyone's advice and begin getting calibrated with my newest favorite bike.

One thing that I did notice this weekend (first real riding since I picked up the bike) is that the K75 is very balanced and handles great! I noticed that the previous owner lowered the triple clamps a little, but the bikes feels extremely solid (low CG) and really feels nice.

I am smiling a lot more today........glad to be here. MJ

 

[ted]

If I could make a suggestion to look under the rear of the tank and make sure your prior owner did not mess with the air accumulator. The tank vents overfill on the K75 out the bottom rear of the tank into a cup clipped to the frame under the tank (see this page, #5). From there a tube carries it down and dumps raw gas overfill behind the footpeg. This is especially important to have if your flapper has been removed (as mine had been.)

When I first bought my '88 K75S the prior owner for some insanely stupid reason had removed this cup and tube. During the first day of my very first big motorcycle trip I had stopped for gas at a station at the start of the Spaulding Turnpike and I-95 in New Hampshire. It was full serve only and I stupidly did not watch as the attendant filled my tank all the way to overflow (on the center stand) and about 200 yards from the station, going up the curved onramp, I looked down to see my entire leg and right side of the bike bursting with flames. Big flames. I immediately stopped and put my leg and the bike out with a handy 1-liter water bottle (fits perfectly between the bike and handle, flipped up, of BMW system cases.) Very little damage, just a slightly singed Aerostich and a melted ground wire that a passing Harley rider helped me cover with tape.

I've seen one other K75 with this removed - now I always recommend new owners take a peek to make sure it is there.

 

[sheridesabeemer]

I'd forgotten that the backfire was anything but normal! Enjoy! You've got a great bike there! Pictures please!!

 

[st3ryder]

Yes, but...

Yes, they all do that, but, I changed the push rod boot at the back of the Getrag the other day, second one in 7 years, and had to slip the cannister off so the rocker arm could rotate backwards far enough to allow access to the boot. When I was buttoning everything back up, I noticed the thermal gasket/collar in the cannister was flaking very badly, and when I checked for leaks at the joint afterwards, sure enough it was leaking. I slipped the cannister off again and wrapped a swath of aluminum duct tape around the end of the header pipe, and slipped the cannister back on. No leaky, and, after sevral rides since, using a lot of trailing throttle, no sonic boom, just a little bop-bop once or twice, not even pop-pop from the exhaust. So, like other bikes, exhaust leaks will cause or exacerbate after fires.

 

[deilenberger]

I slipped the cannister off again and wrapped a swath of aluminum duct tape around the end of the header pipe, and slipped the cannister back on.

I would be very surprised if the duct tape lasts for very long - temps at the output of the headers probably exceed 500F.. but you're right - an air leak into the exhaust makes the backfires worse. The backfire on a K75 is related to a rich condition in the exhaust system. Add some more oxygen (from air leaking past the header/pipe joint) and it fires off in the muffler. BANG!

I rather enjoyed it actually.. people riding behind me didn't.. but I did..

 

[scootrp125x]

backfire help

My 92 K75RT was popping and backfiring when I bought it. My local dealer solved the problem. Looking at the receipt I see that they replaced the Z Breather Tube and rubber vacuum caps. They also syncronized the throttle bodies. That was 3 years ago and I havent had any more problems although I have read that the Z tube and vacuum caps will probably need replacing every few years or so since they are rubber and prone to cracking from heat and age.

Mark