I have tried to tune the engine after a complete re-build. A second hand trigger unit has been fitted, but as soon as the engine is warmed up and I blip the throttle the revs stay high for a while and then sometimes the revs will reduce to near their normal level.
I have stripped the carbs, but have not found a problem and the slides are working OK. There is plenty of fuel reaching them and now I am at a loss as to the problem.

I would appreciate any helpfull input.

Sorry to hear - common problem.

You did:
- adjust the valve play
- adjust the timing
- change or clean the airfilter
- checked the coils
- checked secondary resistance
- checked or changed the plugs
- double checked the HT-lead
- checked throttle and choke wires for damages
- choke and throttle return
Right?

You have air leakage.
- the stubs does come loose
- the rubber hose does crack -internally!-
- the steel lid at the top of the carbs is an electrical element corroding away the aluminum => getting loose and letting in air.
- butterflyvalves can become loose...
- diaphragm can get damaged
- jets can become loose...
- if you have the crank-vent through the carbs you will get gooey jamming up the main jet and atomizer
- chokehouse gasket can break...
- gas and choke wires does get splitted endings causing jamming of the wire return

Hook up a set of vaccum gauges - watch what happens => you will see the underpressure drop or rise suddenly and forcefully as the carbs heat up => airleakage.

__________________
Drive Safely,
Albert

Sounds like an air leak on the inlet manifold of the carburettor, or the carbs are out of balance. You will need vacuum gauges to check the balance. Make sure that the ignition timing and valve clearences are set correctly before you balance the carbs. It is unlikley but its worth checking the ignition advance/retard to make sure that the timing is retarding when the motor is running at slow revs. Check the pilot jet/air screw is set about right. You will need a strobe light to check the timing.
Hope this helps,im not a BMW expert,but ive had 50,000 mls out of a Ural! Andy.

Made my own wiring harness, as it would be far superior to a bought item, new Ht cable and NGK caps, tested the coil, control unit, Eduralast EDL450 alternator fitted, foam air filter, carbs stripped and cleaned new 'O' rings diaphrams and gaskets, adjusted and balanced using mercury guages. I have checked that the slides seat properly by removing the inlet pipes and using finger power which they do. So I can discount the carbs as the problem.
As the engine is warmed to working temp and I try to strobe the timing correctly, as soon as I advance the ignition to align the S_ the engine carries on advancing and only reduces when I rotate the trigger unit to retard the ignition then it sometimes settles down.
I still think it is the second hand trigger unit I fitted which is causing this problem but they are too expensive to buy and try out to disprove, although the man at Devon BMW doubts this.

You have a faulty ITU.

Impressed with all that you have done with your bike!

Take a stock multimeter gauge.
Check the three connections in the socket to the ITU. There should be resistance in only one (1) way between only two (2) pins - any other readings and the ITU is faulty. (see my thread on busted ITU below in the list http://www.horizonsunlimited.com/hub...oken-itu-41532 ;here are some pics and if you read swedish you could read this as well Svenska BMW MC Klubben • Visa trÃ¥d - Trasiga ITU - Tändimpulsgivare /7 1981-1995 ).

__________________
Drive Safely,
Albert

and it turned out to be crap in the carb (mine was a single, only one carb)

http://www.horizonsunlimited.com/hub...-problem-45363

it took my four tries, to clean the carb properly and solve the problem. I would recommend completely stripping both carbs, spraying carb cleaner everywhere, and blowing through every hole with compressed air.

Also, something I just learned. When removing the carb from the bike and before stripping it, make sure you keep it completely vertical until you remove the float bowl. This way any dirt that's in it will remain in the bowl and not get washed up into the workings of the carb, and you can see it in the bowl when you remove.

Good luck...

Did you strip the replacement can to check that the timing weights are swinging away as expected?

I have cleaned the weights on two GS's so far and both needed it.

__________________
\"I empty a bottle I feel a bit free\" Mick Jones

To be honest I havn't touched the bloody thing for over a week now, (decorating my house).
I have checked the replacement unit and the weights work OK?.

How accurate does the butterfly in the carb need to be positioned, as I removed them to fit new 'O' rings on the shafts so perhaps it may be the cause of the problem especially as I am thinking once the heat reaches them. Although they move freely within the carbs.

To be honest I would prefere to fit a pair of Dellorto's basic slide type' as I feel they would be far less troublesome.

Ooops... they should close the carb completly... all you may see is a slim strip of light next to the shaft. You did lock the screws with normal or weak Loctite ?!...
If the valves get loose you are in for a nasty surprice... hit me once as I was about to exit the town Norrköping southwards as I closed in on the last stoplight... let's say I've experienced better moments...

Dell'Orto PHM 36mm would be perfect for you. This is ok as well:
Rocky Point Cycle, vintage motorcycle parts
Rocky Point Cycle: Mikuni carb kits
I have a set of Dell'Orto PHM 40 to my summer bike - what a difference!
http://www.dellorto.it/Media/Prodotti/PHM1.png
http://www.dellorto.it/prodotti.asp
Perhaps does CC still sell their carb-kits... ask Chris at:
SJ BMW Racing


Tested a R100RS with Krauser cam and Mikuni 38mm flatside... now... a Beemer with a lower lift cam is not supposed to do wheelies... that's how different it proved to be...

Even Keihan slides would make life more agreeable than the ding Bings...

I get a lot of flack for my opinion on Bings... It is ok since the Bings are -very- good carbs, just so damned sensitive to tune and maintain! On my Winterbike I am stuck with Bing CVs since the rubber straps to the skiis rests on top of the carbs making it impossible to use Dell'ortos, Mikuni, Keihan or Bing slides...

These are the differences you will notice:
- response like a japanese four!
- very calm and stead idle
- about 15-20% better milage
- hell to install less you have an old R75/90 with dual long throttle-cables... the split under the tank needs to be modified a lot and you still will not get full lift of the slide... it is worth the trouble!

__________________
Drive Safely,
Albert

Hallo Albert, thank you for the carb info. You recommend 36mm sized PHM's, 4mm larger than the Bing's, would they work OK sraight off. Do you have any idea of the settings for the DEll's, eg. slide cut-away, jets. etc.
If as you say performance and fual economy is improved, which I can well believe it is a wonder you don't see more Dellorto's fitted.

Just to go back to the Bings, I reset the butterflys, but when looking through the venturi with the choke fully closed I can see a thin line of light either side of the butterfly's, that is to say on the shaft side, and no matter how much I try I cannot improve the position. Would you say that is normal. I always use use loctite on the screws, just the thought of them little buggers jigging about the combustion chamber doesn't bear thinking about.

Almost certainly sticking advance retard weights. change the springs and make sure all the pivot points are free and not binding.

Hm... I -did- reply to this... it did apparently not stick to the string...

First:
- ooops. My appologies - you should have 34mm since you have an 800cc.
You could get the carbs from R90S - I think they are 32mm but I'm not sure.
Yes - fits more or less right off. You may need to have a new manifold made though... and if you have the wire-split and not the two long gaswires, you will have to modify the split a lot, or make a new one from scratch, since the split stops throttle lift to about 50% lift... The split is exlusively made for the Bing CVs... you can use the throttlegrip from the R75-90, but that creats some headache getting the brakes to work since those grips had a wire to the main brake cylinder which was placed under the tank instead of on the handlebar... Small/easy tasks to fix though.
As for jettings... I still have to learn that, but it is easier than the bings according those I have spoken with over the years.

Jettings is individual for each carb, not cc -
You may need to fine-tune the carbs in accordance with the engine on which they are installed; this is best done at a shop that knows how to do this "by the book" (i.e. racing tuners).

I do agree with you that it is odd that so few have converted to slides/thottle carbs and stuck with the CV's... I do too (!) but only on my winterbike and that only since physics forces me to do so...
The wire-split hassle may perhaps be the cause...


By your description of the venturi on your bings - you got it right! -

Start looking for an option. Scrapyards do get a lot of rise-cookers - they tend to go faster than their operators brains can handle... you should be able to pick-up a nice set of Mikunis or Keihans at a very fair price. Making manifolds to hook them up on your beemer is a small/simple task for any worksshop -

800cc runs best on 34mm venturi, but anything from 32-36 is ok - note that the 64/32/xxx are merely 27mm... I have used 94/40/yyy on my 800cc, made it run much smoother, lost a bit of lowend torque but gained in "kindness". Accelleration was much better once 3000rpm was passed, and it pulled a lot better from 3000-red! Bing 40's have a venturi of 36mm - hence I know this is a bit too large. You can also use the formulas in Graham A Bells 4-stroke performance tuning - you will get the same result with mathematics as I did with trial and error -

__________________
Drive Safely,
Albert

If you stripped the carbs, did you get the choke discs mixed up? They are handed.
Also check that the chokes are fully closed - disconnect the cables at the carb and see if it cures it. This was the problem I had with the motor not returning to idle.

John

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Nostradamus Ate My Hamster

I agree with a few others. A new trigger unit would cure it. Yeh they are expensive. Any secondhand ones around now are at least 13 years old.
Made a huge difference to my bike when I finally stumped up the cash. Never had to touch the carbs other than maintainance in 23 years.

Glynn

Well I cured the problem by replacing the Bings with a set of 34mm Mikuni's.
What a difference they make, much more responsive and ticks over beautifully now.
So does any one want to buy a set of 32mm Bings?.