My K100 is backfiring sometimes on deceleration. Any ideas where I should start?

I am running on 98 octane. New (ish) plugs.

I ran some injector cleaner through the old girl about 500klm ago.

Runs like a swiss watch, pulls like steam train. Should I drop the octane back?

More than likely the exhaust system has a leak. Possibly a Gasket at the head or a cracked header pipe or leak where the pipes join the muffler just like my K1100.

Thanks Rick, I can live with it.

Can lead to burnt exhaust valves also.

Al

Bugger, better get the spanners out then.

Can I ask how much you had to change the ignition timing for running on 98 octane?

Hey Bob, did not think I had to do anything. In Australia we had lead in our fuel and a higher octane when the my bike was delivered, 1986. I have just been putting the premium fuel in it as I figured it would give me a cleaner burn. I am happy to be corrected as it would result in a cheaper running cost.

95 or 98, neither is better than the other, one burns faster than the other - the reason that the ignition timing is changed if the fuel is not what the factory specify. 
The most efficient time for the maximum expansion of the fuel is when the piston is half way down the bore, if fuel burns quicker then that happens too early and if the fuel burns slower then that happens later. That is why some car could use a plug that altered the timing to suit the fuel used.
I was wondering if you had found the ignition timing figure for a K to use 98. It is certainly inefficient to use a different fuel to what was specified for the bike as there is no gain to be made and a slight disadvantage in doing it without altering the timing.

Thanks Bob, we have three fuel options here in Australia. 98, 95 or 91. Which should I be using?

K100 Rider's manual specifies minimum 91 octane RON. I use 95 but get the same mileage from 91. The odd time i've filled up with 98 I've got slightly less mileage.... probably because it detonates a little later being a higher octane.
K1100... the minimum is 95. I think that goes for all 16V.

Re backfire. I'd do a compression test and check valve clearances just to be on the safe side.

Bigsax wrote:My K100 is backfiring sometimes on deceleration. Any ideas where I should start?

I would start with something very simple: Check the throttle position switch (TPS).
If it's set correctly, it will cut the injection when decelerating.

Yup doesn't hurt to check the valve clearance

Bekman wrote:Yup doesn't hurt to check the valve clearance

I don't disagree with this statement, but when troubleshooting you normally start with the easiest. Even if the valve clearance is OK, the K bike will backfire if the TPS is not set correctly. There is a good reason why BMW installed a TPS.

I agree as well...  to the original poster, does your bike still have the heavy engine braking when letting off the throttle?

Many K100 owners disconnect or disable their TPS so that fuel is not cut on deceleration. I've not heard of reports that this produces backfiring. If there is any popping or backfiring it generally means there's probably a leak in the exhaust system as Rick mentioned above or the exhaust valves are too tight as was the case with my K100RT when I first bought it. In any case, as I said above.... just to be on the safe side. A compression test to compare all four is best and quick to do. If the valves aren't closing properly, the last thing you want to do is run the engine for too long. Just my thoughts. Cheers

Thanks for the help guys!

I have been busy on other projects (why does everything break down at the same time?) car boat even my pushbike!

Anyway the engine bleeds off speed when I decelerate perfectly so I dont think I have a valve problem.

If someone can tell me where the TPS is and how to adjust it, I will try this first.

I have also dropped the fuel back to 95 with no discernible change in performance. The next fill will be 91 and I will report back.

Bigsax wrote:Thanks for the help guys!

I have been busy on other projects (why does everything break down at the same time?) car boat even my pushbike!

Anyway the engine bleeds off speed when I decelerate perfectly so I dont think I have a valve problem.

If someone can tell me where the TPS is and how to adjust it, I will try this first.

I have also dropped the fuel back to 95 with no discernible change in performance. The next fill will be 91 and I will report back.

Its located at the rear end of the throttle bodies, a small screw either side locks it in place.  

To adjust:
With the bike idling and running correctly (ie, adjusting the TPS should be done after all settings have been done on the throttle bodies)

Bike turned off
check the setting by listening to the switch by ear while just cracking the throttle open, you should hear a light click as the TPS switches.  

If it doesn't click when just opening the throttle, it will need to be rotated until it does, loosen the clamping crews and rotate either direction till you get the required result.

Al

Thanks AL.

The TPS is fine, I hear it click when you twist the throttle one millimetre each way.

I wil lower the fuel octane and take her out for a belting this week.

What plugs are you using? and how do they look?

I dropped the octane back to 91 and I don't know if that or if the warmer weather did the trick, but my backfire has all but gone.

I have not noticed any change in performance, the only real change I have noticed is less $$$ to pay at the pump. happy days!

If your bike is a 16v, and the picture looks like a 16v, then also look at the exhaust headers where they are welded into the silencer. They crack and let a little air in which produces a backfire on overrun.