1979 R45 - 860cc Siebenrock upgrade kit. Any experiences ?

[tedmagnum]

Hi all.

Firstly, thanks to admin for welcoming me to this forum.   I hope to contribute as well as learn from it.

I'm Ted. I'm a motorcycle technician with 20 years experience and eight years experience with a BMW main dealer.  Although that was from 2012 onwards so as you can imagine, airheads were mostly unseen.  I now work for a Kawasaki/ Suzuki dealer. I also run my own independent workshop at home for older bikes. Mainly from the 90's and 00's.  I also work on BMW's from this era too. Although the R45/R65's are a new beast to me. But I'm learning quick.

Well, to the point.  I bought an R45 from a customer who had a problem. He took it for a service at his local shop and after it came back the the engine would race away at high RPMS. Like a stuck throttle. He took it elsewhere where they rebuilt the carbs and it still did the same thing. 

It came to me.  I re-checked their work and they had re-build the throttles badly (choke parts mixed up etc).  But after correctly rebuilding, that wasn't the issue.   They also massively over-filled it with oil. Like 3x times the amount it should have (More on this later).

The old guy didn't want to spend a penny more on this bike so I bought it off him as project....


It has the old mechanical bean can. I suspected sticking bob weights so converted to an Electronic ignition.  But still the revs still ran away. 

Perplexed, I decided to pull the heads and barrels and measure up.  Even though worn and a bit smokey, they wouldn't cause the run-away engine condition.  However, there was a lot of burning and scoring on the pistons. 

My theory is that the engine was so over-filled that the crank case breather was chugging oil into the carb manifolds and into the cylinder. Like It was running like a bloody diesel.  This burning has ruined the pistons and cast iron cylinders. They're 40,000 miles old anyway.



The price of a re-bore and new pistons for the R45 is around £500.   Nikasil cylinders from the later 1980 models aren't compatible with my 1979 due to a machined 'step' on the motor.

I could buy older cast iron R65 cylinders but they'd probably need a re-bore and new rings too. All expense.

SOOOOOOO.

I was considering spending £950 and buying the Siebenrock 860cc big bore kit.   In for the penny, in for the pound

I was hoping folk on here would have experiences of the kit for the R45 ? Can she take it ?  The con-rods run very smooth with no knocking. 

I'd love to hear your views.

Cheers, Ted

[wilcom]

Hi Ted and welcome to the group!

There is 860 info stashed under the search button and I'm sure a few folks that have already gone thru the process will chime in shortly

[dogshome]

Hiya, I did the post 81 650 to 860 conversion here https://bmwr65.org/smf/index.php?topic=5525.0

The 450 isn't such a leap, but should convert your (slow as a RE single) into something much more useful. As a 650, my bike was like a modern 250. As an 860 it's a modern 600 / retro 750.

Apart from the brakes.....   that on dual Brembo with SS lines and all rebuilt. "250" = I never noticed. "600" = ah, well, could do with a lot more actually.

Tony is the man for this and will likely drop in soon.

[dogshome]

Ps not sure oil would cause rpm increase at all. I think you have another cause there.

Siebenrock advise checking rods, but my engine was obviously good and felt that way. Even valves didn't need grind at 62,000. Bores had another 20,000 in them.

If it's been horribly overfilled, blowing by and running hard and you have the barrels off already, it's not a leap to check big ends.

[Tony Smith]

Twins, if the rev runaway wasn't the adv/rtd sticking it is most likely airleaks in the rubber intake trunks. Leaks here can be very subtle however there is an easy test. With the engine doing it's thing run a garden hose over the rubber trunk. If the engine suddenly drops revs you have found the leak. If not then I have no idea.

Re the 860 kit. If the bottom end is in good nick then just do it, it will transform your bike.

[tedmagnum]

Hi. Thanks for the replies guys.

Yes. I'm not 100% convinced that's the actual issue. As is often the case with these older machines, you have all sorts of issues compounding together. And it's often difficult to find one problem without first ruling out the other. And the further you dig, the more you find..

But the bores are worn and the pistons are burnt. So I'm kind of stuck at this phase. The right pot was always smoking anyway. 

I did leak test the manifolds with mists of brake cleaner when it was idling. The revs stayed the same which indicates to me that there was no leak.

These carbs don't have springs. So maybe the plungers are sticking. Or one of them at least.  I will go down this route eventually I'm sure.

Ted

[tedmagnum]

Well I've bitten the bullet and ordered the 860cc kit from Motoworks.  It's a bloody big dent in my credit card but hopefully it will be worth it.

I'll report back.

[dogshome]

Are you sure your carbs should not have springs ??   If you search snowbum, he mentions fitting them to those that didn't have them on early models. It wouldn't take much for them to stick without springs.

[georgesgiralt]

Correct me if I'm wrong, but....
Suppose the butterfly is closed and the piston is up in the bore. The fuel mixture would be rich (because the needle is way too high) and the air is very very restricted in the passage by the butterfly .... So the bike will choke with too much fuel and won't run too high because it is choked to death.
I suspect a spring problem, but a weak return spring not allowing the butterfly to close after acceleration and perhaps a sticky throttle cable.
Could you please report back to prove one of the explainers theory once the bike had been sorted out ?
TIA.
P.S. : IMHO, the spring above the piston serve to augment the apparent weight of the piston AND help it return to base if somewhat sticky

[dogshome]

The SU carbs on my minis and umpteen other cars worked like this. I must have had a sticking piston at some point - cracked cylinder heads, errant flywheels, errant rubber doughnuts, distributer tops arcing, dynamo lights like glow worms, battery cable to the boot catching fire, subframes rotting, floors rotting, headlights rotting and falling out, driveshaft joint seizing, brake slave cylinder perpetual leaks.... But yes you would think a stuck carb piston would just run really rich.

So the contenders:
1. Air leak on inlet somewhere. Favourite.
2. Ignition timing advanced (is your flywheel on in the right spot, mine wasn't). 2:1
3. Oil chuffing up bores and dieseling. I'm giving 10:1 on this one.

 

[KeithN]

Hi Ted

im considering the 860 kit for my R65...and then to reuse the R65 barrels on my 45...i do know its not a straightforward thing to do in my case.

ive only heard good things about the 860 kit though.

are you UK based ?

[tedmagnum]

To clarify the issue. This isn't just a high idle. This is a scenario where the bike would rev itself up to it's redline after being warmed up and blipping the throttle a few times.  And it wouldn't always do it. Maybe 7/10 times.  The right pot was always smoking.

An air leak was my first thought.  But I'm certain I've exhausted this theory as I've tested it upteen times with a very fine mist of brake cleaner around all the joints.

I also considered the bob weights were sticking in the bean can even though no advance should make it rev up to it's redline if it was stuck.  I changed it to a full electronic ignition to rule this out all the same.

This bike does not have carb springs as standard. But fitting them is possible. And if I still have this problem after fitting the new 860 kit, I will do

The burning oil (run away) theory is a long shot. But I have to change the bores and piston anyway due to burning/scoring and the 860c kit is the best option for a cast iron early model (later Nikasil bores aren't compatible)

Would sticking carb pistons create an over-rich scenario ? Undoubtedly.  But when the engine is reving it's arse off, the throttle butterflies are both in their idle position. (I physically held them there).  Would this bog down the bike or make it run like mad ? I don't know. You would think so.

So I guess the only way enough air could get through was though the choke circuit or air leak (both which I'm certain are correct)...  I have checked, re-built and cleaned the choke discs and checked their alignment (using snowbums guide and pics)

I'll kick myself if it turns out to be something simple.

One thing I'm annoyed with myself for not doing is narrowing down which side was the offender.  I could have shorted out the plugs when it was running to see which side was causing the run-away.

And yes. I'm in the U.K.  The Wirral to be specific.

[dogshome]

The engine has to be getting air (as well as fuel). This carb rebuild that was wrong:

* Butterflies on right way round and definitely closing? You looked in through the engine side and shone a torch through from the filter side?
* O rings on butterflies not missing?

I know that leaving the vacuum port open has little effect on idle speed, so there must be a fair amount of air getting through somewhere. I can't think a normal wear leak would do this. A big gap around the butterfly or it's in backwards/upside down might do it though.

There isn't something really daft like a jet left out or the choke plate not closing down due to its spring missing? Butterfly, inlet rubber and choke are the only big enough air paths. Beyond a cracked head or absolutely mullered intake valve stem letting air in that way. I think you'd notice those though!


You KNOW it IS going to be something simple don't you? 

[tedmagnum]

The engine has to be getting air (as well as fuel). This carb rebuild that was wrong:

* Butterflies on right way round and definitely closing? You looked in through the engine side and shone a torch through from the filter side?
* O rings on butterflies not missing?

I know that leaving the vacuum port open has little effect on idle speed, so there must be a fair amount of air getting through somewhere. I can't think a normal wear leak would do this. A big gap around the butterfly or it's in backwards/upside down might do it though.

There isn't something really daft like a jet left out or the choke plate not closing down due to its spring missing? Butterfly, inlet rubber and choke are the only big enough air paths. Beyond a cracked head or absolutely mullered intake valve stem letting air in that way. I think you'd notice those though!


You KNOW it IS going to be something simple don't you? 

Carb is assembled perfectly. This ain't my first rodeo    Although I did triple check with all sorts of manuals and re-build guides. I got it right. Butterflies correct, all jets in place. 

The choke plates move nicely. Although one never knows when it's installed. 

I hope it's something simple. I just hope I don't spend a fortune before I find it. 

[jp9094]

Came across this from a local specialist shop. You may be interested!  https://motoborgotaro.com/motorcycles/1982-bmw-r65ls-custom

FWIW