Hi everyone! I posted a few questions on Flyingbrick, but havent managed to sort the problem, so i thought i'd try here instead!

Back story:

In September last year i bought a 1985 K100RT on Ebay as a non-runner with the intention of a quick fix and tidy as i went along. Sadly when i collected the bike i discovered it wasnt quite as good as the pictures made out (along with a semi rolling chassis came 5 boxes containing the rest of the bike...)

Over the winter i've TOTALLY rebuilt the bike as a naked K100, and after managing to unsieze the engine (stuck solid) i'm most of the way to getting it back on the road.

Now comes the problem. Between me and my stepdad we've managed to get the bike to start twice. The first time was a fluke with loads of easy start, and it ran for about 10 minutes. The second time was after 45 minutes of tweaking, and i have no idea what made it come to life.

Now, she absolutely does NOT want to play.

The engine turns and turns, and fires a bit, but doesnt ever really catch.

I have already done a LOT of troubleshooting, so bear with me!


It has:

New Plugs, New HT Leads, New fuel pump, new fuel hoses, new fuel filter, new oil, new oil filter.

The injectors have been ansu tested and cleaned, and are good.
I have replaced the fuel regulator, and am getting 2.8bar consistently.
All four plugs are wet after cranking.
Good spark at all four plugs.
The plugs ARE on the right coils.

We have tried with one new, and one known good battery.

I have checked the air, and water temp sensors, both read withing known good values. I have resistance checked the air flow meter, and it meets the results for a known good one.

The cam timing is correct, and the ignition timing is as close as i can get it without having the bike started.

*Breathes out*


The only concerns i have any idea about now are a) that the starter doesnt seem to crank very fast. It was stuck when i tried to test it to start with, so i dismantled it, and cleaned and unjammed the brushes, and re-assembled and it works well. I have never had a BMW before, so i dont know what the cranking speed is supposed to be.

b) If i take the lead off the starter, and turn the ignition on i get 12v at the feed for the starter. Without pressing the starter button. In my mind this is NOT right, but if i then touch the starter lead onto the contact, the relay clicks, and the voltage drops back to pretty much nothing.

Press the button, and the starter cranks, and the voltage jumps right back up again. Sick Relay?


Anyway, i really want to get using the bike ASAP, so any ideas you've got that help me narrow this down would be well appreciated!

Cheers!

ToGGoT

Hi ToGGoT,
What you are seeing with the lead off is the unique (as far as I know) load shedding relay design used on these bikes. Basically the load shed relay is earthed through the starter so when the starter is not running the relay operates and non essential (to the starting process) loads are energised but when the starter is running those non essential to starting loads are disconnected.

OK on first review you look like you have checked all the standard items but what are the compression readings from the four cylinders. The basic rule is fuel + air + compression + ignition = BANG take away any of the first three = no bang so what pressures are you seeing?
Regards,
K-BIKE

My compression tester is bust. Have borrowed another one from a local mechanic - will post results tonight!

ToGGoT

Cylinder #1: 152psi
Cylinder #2: 62psi
Cylinder #3: 68psi
Cylinder #4: 160psi


Hmmm...

ToGGoT

I am sorry to say this but I don't like to look of that.The way I do it is to open all throttles fully and turn the motor 5 times. Watch the pressure build with turns, in my case it got to max in about 3.However, I had to do it a few times until I got used to my new gauge.

Yeah, to be honest i reckon thats new rings.

I tried with the throttle open, and closed, same results. Whats the figures for a 'good' engine? On my kwak it was much higher (180 psi) so i was expecting similar.

Oh well, i knew everything else was going too well!lol!

ToGGoT

I would have been reasonably happy with 160+ I think that my readings were around 180.

Can you squirt a bit of oil (teaspoon full) through the spark plug hole and see if the pressure changes.. No change = leaky valves.

looking at them rates i think head warped or head gasket blown, if u remove ur coolant cap and crank dose it gush water out at u. and was the bike stored with coolant in it, motors that sit a long time with cooland suffer from head gasket corrosion, enginge sat with no coolant have seals dry and crack its a loose loose situation, water in enginges no coolant rusts them out,, y was the motor seized b4, sounds a lot like to me that the middle to are getting water in them, with lots of easy start u get enough volitlity to ignite, and the fluke adjustment u just might have diluted the water enough,, with crank and crank crank ect ect,, so i recomend u check the colour of the plugs and see if the middle 2 are clean like new or near new, and the outer 2 dark and have some soot on them, and remove ur rad cap and see when cranking that it dosent spray water out of there,
buy the sounds of it it will, i belive u have a blown head gasket, warped head aswelll probably, but u should have the head skimed when the head gasket is replaced any way to get a good seal, and if u do a head gasket dont put coolant in for 800kms, the gasket needs to cure,. hope this is of some help

I am sorry that what I suspected was right, the compression is so low on two cylinders they wont fire and you have to tear down to find why. I don't think the teaspoon of oil trick will work with our engines because they are on their side so the oil would not seal the rings anyway.
Regards,
K-BIKE

K-BIKE wrote:I am sorry that what I suspected was right, the compression is so low on two cylinders they wont fire and you have to tear down to find why. I don't think the teaspoon of oil trick will work with our engines because they are on their side so the oil would not seal the rings anyway.
Regards,
K-BIKE

Yeah. I updated my project thread and they asked if i'd done a wet test. Not knowing what that was, i looked it up and got the same advice Ned's just given, so thumbs up for that.

Gave a generous squirt of 10/40w into each cylinder (Thank god for long spout oil push cans), and retested.

Got:

1: 155
2: 130
3: 122
4: 160

So IMHO that'll be rings.

The head gasket has been replaced very recently (The bike has only run for two short bursts since), and i KNOW i torqued it up properly when i did it, so i doubt its blown. But i can easily check when the head comes off again to do the rings.

Sodding bikes.

ToGGoT

kool sounds good, member to check for scores and ur ring gaps and piston to bore clearence ect if it needs a hone i find a bead hone dose the best job, more expensive but the hatch is much better,

Alex, for my own information, I must ask this: Can we hone K100 cylinders? Somewhere, perhaps in the OLD forum we talked about this and I don't think that anyone has given a definitive answer.

The problem is that the K100 as well as other high perfmance german engines are made using a particular technique of tougheneing the surface of the cylinders to reduce wear. I forget the actual name of the process, but I think that it affects only the surface. Can you actually hone it without removing it?

BTW the head gasket thing may be correct because the leak could be between two cylinders and the oil (doubt this, but..) may provide a temporary seal.

To my knowledge, you cannot hone the cylinders. They are plated but the plating is very thin. I remember reading about a K100 with over 500K and the plating was gone. The rings were riding on the aluminum of the cylinders.
Somewhere I have the thickness of the original plating and will try to find it for you.

see there is anther car mechanic blipo for ya , its good to have these bmw pros arround. the cyclder hone dosent take much off tho, mainly cross hatches the surface again, i am not talking boring if thats whatu think i am,, but i supose it would scrtach up all the plated surface.

Well ... unfortunately the finishing (plating) of cylinders is in the order of 0.003" ... 3 thou is not much. I can't imagine how it lasts as long as it does. Not me, but one of the gurus has written up this ... here it is: 
http://bmwmotorcycletech.info/cylinders.htm 

Holy moley!

There is a shite load of information on that site...some for the K100 too!

Click on the "Return to Technical Index page" and be prepared to be amazed on the info in there!

yep, that is a good one. btw your moly tube is on its way back thank you for the "loan".

Right guys, dismantled the engine again this morning, and sure enough the front 2 rings on 2 and 3 are stuck almost flush into the piston, whereas they pop out neatly on 1 and 4.

The bores look good, and i can still see the original honeing marks. Is it best to just leave the cylinders as they are and fit new rings? Because it'd been suggested that should at least glaze-break when fitting new rings, and this is not a job i've done before...

ToGGoT

the info link ned posted up said u can hone with a ball hone or bead hne same thing, but only a certin type and grit, if u can see the marks i would leave it alone, if it aint broken dont fix it. the valleys to catch the oil will still be there,, great insight thanks ned

dose any 1 have an idea of how long the plating will last ie kms/miles. with std use

blakey wrote:
Click on the "Return to Technical Index page" and be prepared to be amazed on the info in there!

I just put a link to the index page on the portal. I was assuming that people clicked on "home" to see all the post. Maybe not!

CF