Hello,

I'm currently working on a project bike and firstly i noticed a high fuel consumption. however since I don't have any consumption indicator, I calculated the consumption from full to full based on the km.
For this bike as I understand the avergae fuel consumption was about 5l/100km (this was my dad's bike and this is what he used to get on average). Well, I'm getting more than 10 on an usual ride, measured from full to full..

The bike stood still for a while and I am trying to fix it as it was some time ago. 

One other thing that I have are some explosions on the exhaust when decelerating. For example, after the engine wateer temperature reaches the normal usage (90 degrees), when decelerating, basically no matter the gear i'm in, at about 2-3k rpms something explodes in the exhaust, producing a single "boom". Basically everytime the engine reaches over 3-4k rpms, when decelerating, this explosion is heard. I believe this is related to the high consumption, i think some gas "escapes" into the exhaust and explodes. It doesn't do the 'usual' popcorn sound, as in a car with a modified exhaust, it only does a single boom.

And another thing that I've been seeing is it dripping gas when standing still. Now, of course it is not a lot of gas, but after standing still running for a few minutes, some drops of gas can be seen underneeth. I don't see them always. I'm seeing them from time to time, I don't think it is continuously dripping, but I'm not exactly sure of this.

From what i understand there could be either the fuel pressure regulator, or maybe a valve ?

I'm not sure. Can anyone help me?


Edit: The bike is a '91 K100 LT (2V).

Later edit: The problem was the fuel pressure regulator, read comment #23.

Thanks, 
Andrei

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Could a leaking fuel injector be the reason?
Remove them and check.

bad boy wrote:Could a leaking fuel injector be the reason?
Remove them and check.

Leaking fuel injector = one that is stuck open.

You don't indicate what year your bike is, so I am going to offer my response based on it possibly being a 2 valve of the 80's.

First, regarding the explosions.  The throttle position switch needs to be correctly adjusted to help reduce them.  Also, when coasting, it is necessary to fully close the throttle.  This was something that took me over a year to learn, but it goes a long way toward reducing backfiring explosions.

As far as fuel leakage, I would suggest that it will be quicker to find the leak if you spend a little time with a good light inspecting the engine rather than counting on the speculations of people who are thousands of km from your bike.  Most leaks are usually found at hose connections, where the rail connects to the injectors, and from holes in the lower parts of the tank that are caused by corrosion.

Can you post up the model and year?

91 k100 from his profile. So its 4V, not 2V

K100 RS or LT?  RS would be 4V, but if it is an LT, it will be a 2V.  I think '91 was the last year for the K100LT.

First order of business is to address the fuel leakage which was pointed out as possible stuck injector.  Then see if the explosions still happen.  The explosions are after-firing which is usually caused by air leakage in the exhaust header area.  You could renew the copper gaskets in each of the exhaust ports that seal the header pipes.  It could also be leakage where the header pipes enter the collector.  Or air infiltration at the joint between the header and muffler.

Point-Seven-five wrote:K100 RS or LT?  RS would be 4V, but if it is an LT, it will be a 2V.  I think '91 was the last year for the K100LT.

You are right about the LT, mine is one fo the last 50 or so. Late 91 manufacture date.

Hello,

Thank you for the comments.

The model is a 91 K100LT (2V).

Regardig the fuel leakage, if it might be a stuck injector, how can i check that? 

Also worth mentioning is that a while ago, before changing the spark plugs, i had a bad one which was broken and the engine sounded quirky as if it was running in 3 out of the 4 cylinders. However, now that's not the case, the engine sounds good, it doesn't sound like one of the cylinders is not working (stuck injector)..

One more thing i noticed today.. the leakeage comes from the joint of the exhaust at the middle of the bike (just below the engine where the 4 exhaust tubes from the engine joins the final exhaust). let me draw you


engine exhaust
 |   |   |   |
c1 c2 c3 c4
 |   |   |   |-------| (dripping)-------
 |   |   | ----------|------------------
 |   |--------------|------------------ final exhaust
 |-----------------|-------------------
                       joints


so not from the engine body, but from that joint on cylinder number 4. Only from the joint of cylinder number 4 is dripping gas when the engine is running.
 
sorry but i don't know the terms in english

katsawa,

you probably mean 'exhaust manifold'

Could you make a photo and upload it here?
Would make it easier for us.

bad boy wrote:you probably mean 'exhaust manifold'

What I believe Katsawa is trying to convey is that gasoline is leaking from the joint where the #4 cylinder exhaust pipe attaches to the muffler. "Engine exhaust" means cylinder head. Final exhaust means "muffler" or "silencer". The right-angles in the dotted lines are the pipe bends. "Joints" are the clamps where the pipes are joined to the muffler.

As has been indicated previously by robmack, it could be that the #4 fuel injector will not close so fuel leaks through its exhaust pipe pipe then detonates when enough of its vapor enters the hot muffler.

Let's find out. 
 

OK, so:

Here is the leakeage (as @Laitch also showed above):



From what I can tell, it can come from 2 'problems', either a fuel injector (as you've mentioend), or from a bad valve..

However, regarding the injector, maybe i'm asking a stupid question, but if one of the injectors is bad, shouldn't it suppose to stop working (stop spitting gas)?  Can it also remain open like "forever" and spit gas without stopping?

Also regarding the valve.. I might think this is a more probable cause, since I am also noticing some bad acceleration times, in my opinion. I mean I think the bike can pull harder and faster to 100km/h than 8 secs, and I believe it might be the exhaust valve on the 4th cylinder that doesn't close properly, causing the cylinder to not make pressure (and implicit power) and causing bad performance of the engine. However, the engine seems to run good at idling, it also revs up to maximum ~8/9k rpms, so no problem here.

Here is a video of the bike pulling to 100km, with the acceleration at max.
https://streamable.com/l8araa

What are your thoughts? 

Thank you!

If it is stuck open it remains open like forever because it is receiving fuel under pressure until the engine is turned off. In this case forever is relative short compared with the history of the Milky Way but it feels like forever to a fuel injector, or to somebody mopping up leaking fuel day after day.

If the cylinder had a bad valve, it wouldn't have enough compression to make the engine run as well as you indicate. The engine would run like it had only three cylinders.  

The injectors are filled with fuel the moment the engine starts cranking. They open when they receive a simultaneous signal from the fuel injection control unit beneath the seat. If they don't receive the electronic signal to close, or they are stuck open, fuel leaks like the one you are seeing can happen, and sometimes fuel even can leak out the end of the silencer.

Gawdamitey, I can't stop!

When an injector keeps dumping fuel into a cylinder non-stop, it is changing the fuel:air ratio and limiting the amount of air for good combustion. If want to prove your bad valve hypothesis, do a compression test  of each cylinder with the throttle held wide open when the engine has  warmed up. That should take you less than an hour and will provide an answer.

Ok then, how can i test the injectors to see if they are running properly?
And what about the acceleration?

Because it's 2v I'm thinking it's got a vacuum hose to cylinder 4 for the fuel pressure regulator. It might be leaking fuel into cylinder 4. A quick simple check, pull the hose from throttle body 4, if it's damp inside (there could be other signs of leaking) excessive fuel is entering cylinder 4 and you want to replace the regulator.

katsawa wrote:Ok then, how can i test the injectors to see if they are running properly?
And what about the acceleration?

If you cannot release your mind from thinking a "bad valve" is causing poor acceleration, please do a compression test with the throttle wide open so we can both quit thinking about it.    I swear that I already wrote that somewhere; maybe I was in a dream state. 

I mentioned the my reasoning for poor acceleration in Post #16—the post just before your last post.

daveyson wrote:Because it's 2v I'm thinking it's got a vacuum hose to cylinder 4 for the fuel pressure regulator. It might be leaking fuel into cylinder 4. A quick simple check, pull the hose from throttle body 4, if it's damp inside (there could be other signs of leaking) excessive fuel is entering cylinder 4 and you want to replace the regulator.

My dad also thinks the same.. (this was his bike for the last 15 years or so and he claims he knows it very well , however he hasn't drove it in a while before i got it about half an year ago).. I already ordered a new fuel pressure regulator, however not from bosch since i cannot find one where i live and to order one from abroad would just be too expensive, but i ordered an identical new one based on BMW part number and waiting for it to arrive to replace it. I will also check with the method you said and provide an answer

Laitch

katsawa wrote:Ok then, how can i test the injectors to see if they are running properly?
And what about the acceleration?

If you cannot release your mind from thinking a "bad valve" is causing poor acceleration, please do a compression test with the throttle wide open so we can both quit thinking about it.    I swear that I already wrote that somewhere; maybe I was in a dream state. 

I mentioned the my reasoning for poor acceleration in Post #16—the post just before your last post.

) please let me know how can i do a compression test ? Do i need a manometer gauge to insert in a sparkplug place or is it another way?

Regarding the acceleration, i understand now, so if the bad injector is the cause, then a wrong fuel:air mixture would cause the poor acceleration.

katsawa wrote:
) please let me know how can i do a compression test ? Do i need a manometer gauge to insert in a sparkplug place or is it another way?

Dragă Katsawa

Deoarece pari foarte tânăr și s-ar putea să nu fi auzit niciodată de el, încearcă acest lucru:

Serch internetul

Ați găsi imediat ceva de genul:

BMW K 100 - compression test

there are english subtitles

good luck

PS sorry for this probably very bad romanian. I am not a native speaker and had it therefore translated by :
the internet

A failed fuel pressure regulator [diaphragm holed] can dump neat fuel into cylinder no 4, through the vacuum line.

92KK 84WW Olaf wrote:A failed fuel pressure regulator [diaphragm holed] can dump neat fuel into cylinder no 4, through the vacuum line.

Yes, that's correct, that's exactly what was happening. 
I changed the fuel pressure regulator, not with an OEM one from bosch which is about $120, but with a brand new one from Ridex, which was $10.

After the change, the leakeage stopped and the motorcycle seems much more agile, the acceleration from 0 to 100 went from 8 secs to about 5.5-6 and the whole motorcycle seems to run a lot smoother. Also the bangs from the exhaust stopped, so it's clear that gas doesn't reach the exhaust anymore.

I would have to change also the vacum line from the fuel pressure regulator in the foreseable future since the rubber seems very old and breakable, but overall this problem is solved.

Glad you got a relatively simple solution in the end.

Change the vacuum line and also the vacuum caps.

I found the easiest way to do the vacuum line was to attach the fuel pressure regulator end first then feed it out to the front and only cut it when you are connecting it to the throttle bodies. Also slip a thicker pipe as a sleeve over it if you dont have the original sleeve and spring. If you havent changed the outside tank fuel lines do them too, you can get the pipe in a honda shop as they use the same spec.

katsawa
I am happy to read that you found the problem