Power loss question

[steve hawkins]

Good luck with it.

Steve H

[nhmaf]

As for checking the float levels - so how much fuel is in the bowl (either depth from the bottom, or distance to top of fuel level measured from the top of the bowl with the bowl carefully removed and set on flat surface)?    If the fuel level is either way too high (floats may have degraded and become too heavy) or too low you may have a starvation issue, or it may be running too rich at partial throttle opening which could also cause ti to "bog" - but I don't initially suspect that you are running too rich, but maybe..

How far out have you adjusted the air/fuel mixture screws at the bottom of the carbs?   You might try turning them about 1-2 screwdriver blade's thickness counterclockwise (richens mixture) and see if it has any change on your partial throttle (1/8th to 1/2 throttle) performance.   If the sputtering situation slightly improves then it maybe a a lean/starvation problem - if it gets worse then you're really into an over-rich situation, which could be too high of a fuel level in the bowl or saggy floats.

I think (cannot recall) that the target for float bowl fuel level is somewhere around 6mm below the top of the bowl, though I may be thinking of carbs on another bike...

[Bob_Roller]

You mentioned that you are using a fuel filter, what kind is it, specifically for motorcycles, or is it an automotive filter ?

[Tv]

I got the filter from the local BMW workshop so I assume it's OK.  I will adjust the mixture screws and measure the fuel level in the bowls.
Thanks, see how I go this time
REgards
Tv

[Tv]

I've tried (nearly) everything.  Adjusted the float levels and readjusted the idle mixture screw with no change.  Out of curiosity I changed the carb needle jet position from 1 to 3.  The breakdown started to occur around 5000 revs and I had no top end power.  Set needle to position 2 and back to where I began at the start of this thread. I've got bottom end power and the most usable power range at position 2; So needle position 1 dies at 3000 rev, 2 at 4000 and 3 at 5000.
Would this indicate the carbs are working OK and the poblem is elsewhere? (I have been following clousea's thread about the same problem)
I found I could get to 5500 travelling about 100k in top gear and as soon as I tried to get more power (either going up a hill or giving it the gas) the bike started to blop-blop-blop-blop as if it was running on one pot and seemed like it was misfiring.  I've changed coils, leads, plugs, ignition module. If anyone else has encountered this symptom I'd be interested in your cure.

Tomorrow I'm buying a new timing light. Will any type of automotive timing light be OK?
REgards
Tv

[Justin B.]

I have always used just a regular inductive pickup light.  You probably don't need the fancier models with the advance knob, just a regular one.

Quite often when the bike just sorta runs outa beans at a particular RPM a diaphragm is at fault.  But, if all of that stuff has been addressed...

[Bob_Roller]

Is the left cylinder the one that is misfiring when you develop this problem ?

Or can you even tell which one is giving you the problem ?

Have you tried running the bike without the fuel filter installed ?

[Tv]

Today I re-checked the float levels including measuring the fuel in the bowls and they seem Ok. Both sides are the same.
Got a new timing light and ckecked the ignition timing. The timing seemed to be set correct but I advanced it a bit - test ride with no change to my existing problem, retarded it a bit with the same result.  
Tommorow I'm changing the bean can - let you know what happens
Thanks

[montmil]

I've read each post in this thread. Early on, I had a very similar problem with my '81 R65. Very similar. So...

Have you replaced the diaphragms and the floats? What technique are you using to set the float levels?

Monte

[Tv]

It's not the bean can.
Monte - I have only replaced the diaphrams. To check the float level I turn the petcock to 'off', remove the float bowl, push the floats right up, turn the petck to 'on', slowly let the floats down until fuel starts to flow and at that point, check whether the top od the float is parallel with the bottom of the carb body.  Then i let the float go so petrol pours out and slowly raise the floats til the fuel stops and that point looks to be the same spot the fuel starts as described above.

[nhmaf]

I'd recheck your diaphragms to make certain that they are in there properly and haven't popped out of position - there is a small tab on one edge that must fit in its little slot properly or all bets are off.

Our brit friends have also discovered that the moto-bins replacement diaphragms (not Bing brand) are much thicker and heavier and more stiff than the stock ones - this has sometimes caused issue with getting things running right.   The thread on that is relatively recent in this tech section somewhere..

[Tv]

I just checked all the fuel fiters and lines from tank to carbs and all ok. I am confident I have the diaphrams in correctly as I have had those carb tops off at least 20 times during this exercise.  The locating tags are in the right spot. Now that the timing is set, tappets done, I'm going for a ride to warm up then re-calibrating the carbs and that's about it for this session.  I've scored a pile of work so the bike goes in the shed for a while, however, I'm going to seal the carb cap metal inserts with epoxy before I put it to bed so next tiem I ride - who knows - maybe that's it.
Thanks and best regards
Tv

[Tv]

PROBLEM SOLVED!
Whilst researching all the possibilities I was reading an article by Snowbum about 'Intake and Exhaust tuning' and the intake theory pointed me to the airbox.  There was another article concerning the restricted flow of air . One thing led to another and I checked the two air box lids I had and the one fitted had smaller snorkel holes than the spare so I fitted the spare and BINGO, no more mid range flat spot.

The difference in size of the snorkel tips, measuring across the oval dimensions are 25mm x 15mm as against 35mm x 25mm.  Thats a fair bit more air getting sucked in.

It looks as it may be running a bit lean now but at least I've overcome the problem issue and I can get down to tuning it more accurately

Thanks to all who helped me through the problem and I'll try to help others in the future also

Here are the links to the articles
http://bmwmotorcycletech.info/InExTuning.htm
http://bmwmotorcycletech.info/R80,%201980+,%20Airboxmods.htm

Thanks again - I'm going out riding - Yeeeee Haaaarrrrrrr!!!!!

[Justin B.]

I don't recall anybody here ever having that issue before!  Glad you finally figured it out and maybe this might help somebody else down the road...

[Bob_Roller]

Just did a quick search on RealOEM parts catalog, and there are a number of different airbox tops for the later style airboxes that use the flat air filter, what the differences are, I don't know, but they have different part numbers, something to keep in mind, if you buy one second hand .