R80/7 1978 carb balancing issue

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Suili
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R80/7 1978 carb balancing issue

Postby Suili » Thu Dec 15, 2016 12:00 pm

Hi all,
I have a 1978 R80/7 that was running really well, the idle was not 100% and a friend of mine tried to play with the idle screws in my absence to get the idle right. Basically it is now completely out, I have tried to get the idle back again but cant get it right. The idle is revving really high or dropping away completely. I am not great at setting these things up so any info on how to get this right again would be greatly appreciated!

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Tincan3
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Re: R80/7 1978 carb balancing issue

Postby Tincan3 » Thu Dec 15, 2016 11:12 pm

These days I use 5 metres of fine bore clear PVC tubing from Homebase. At the ends of the tubing I have a short length of slightly larger tubing that fits the carburettor pressure take off tubes (can't remember their proper name - they are stopped with a screw that is removed to measure the negative pressure). The middle two metres of tube are stuck to a metre of wood that is hung vertically (take the tube in a gentle curve around at the bottom - not a shap bend). About a metre of the tube is filled with water - colour it if you can to make it more visible. This makes a very effective manometer. My bike has maximum vibration around 2500 revs so this is the engine speed I maintain when balancing the carbs by using the throttle setting.

So, the sequence roughly is that you check the tappets, engine cold. Clean plugs etc.
Engine warm, throttle screws slackened off.
Connect the manometer
Carb screws - air and throttle screws adjusted to give balance of the column of water - menisci level with each other
Adjust throttle screws to have one or one and a half turn from cable take-up.
Maintain revs at maximum vibration and adjust appropriate cylinder to give balance of the manometer.

This last bit is tricky 'cause I always forget that it is the other way round than expected - the cylinder putting out more effort will depress its side of the manometer.
Maybe I had better stop there before I get myself confused - again. For sure though, this is the best method I have found so far and very cheap to set up.

Suili
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Re: R80/7 1978 carb balancing issue

Postby Suili » Sat Dec 17, 2016 11:19 am

Thanks for this!
I will give it a go next week, really appreciate the help on this, I will let you know how it goes!

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Re: R80/7 1978 carb balancing issue

Postby windmill john » Wed Dec 21, 2016 8:52 am

Hi Suili, if you post again, make sure it is under the Airhead section, the right area for your bike and it will also ensure it is read by all the Airhead guys and gals.
The other thing, when you balance your carbs, make sure the engine is warm and you have travelled at least 15 miles to get it to temperature.
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SteveD
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Re: R80/7 1978 carb balancing issue

Postby SteveD » Thu Dec 22, 2016 2:56 am

Suili...this might help. It goes thru the prerequisites and has links to alternatives.
The standard setting for your 80/7 mixture screw is 0.5 turns out from closed....ie cw in.

http://rossmz.blogspot.com.au/2008/12/t ... r-bmw.html

As written above, it's vital to have the bike adequately warm before starting the carb tune. Otherwise you'll end up with a high idle when you go riding.
Cheers, Steve.
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Tincan3
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Re: R80/7 1978 carb balancing issue

Postby Tincan3 » Thu Dec 22, 2016 9:31 pm

Suili,
Here is a photo of my manometer set-up complete with untidy garage in the background.
Maybe two photos if it takes it.
In my description of the procedure it is the throttle stop screws you adjust (from their standard setting) on the carburettors with the throttle cables slackened off. Then, when all is well with the tickover, set the cables to one or one and a half turns back (slack) from the point where the cylinder picks up as the cable is tightened.
At maximum vibration revs adjust appropriate cable to balance the manometer. All this is assuming the engine is warm, the carbs are clean, the choke cables are reset when the choke is closed, the cables are not strained when the handlebars are turned (& maybe more I have forgotten).
manometer1.JPG
manometer2.JPG
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Re: R80/7 1978 carb balancing issue

Postby PhilO » Thu Mar 09, 2017 1:48 am

Hi Tincan3,
I was very interested that you do your carb balance at 2,500 rpm. Does this help to get rid of the vibes?
I was under the impression that all airheads vibed a bit around this speed (mine do)and it was a characteristic of the rocking couple harmonics or some other phenomonem of balance which is inherent in the design. If you manage to tune it out then I'll try your method of balancing at those revs.
My personal method of having a vibe free engine is to ride a K75!

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Re: R80/7 1978 carb balancing issue

Postby chasbmw » Fri Mar 10, 2017 1:09 pm

there are 2 aspects of carb balencing,

1) at tickover to ensure throttle stops and mixture are providing the same power from each cylinder.

2) at higher revs to ensure that the throttle cables are balanced, so that each carb starts to come off the idle at the same moment.

I would suggest that No 2 is more important for riding, but back in the day people competed to see how low they could get their BMWs to idle, it was seen as an indicator of what a brilliant BMW mechanic you were.

when doing 2, I find it helps to get both roughly equal, then lock down one carb and finish off on the second, its very sensitive on a CV carb and even tightening down the locknut on the throttle cable can alter the settings a bit
Charles
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Tincan3
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Re: R80/7 1978 carb balancing issue

Postby Tincan3 » Sat Mar 11, 2017 4:22 pm

Hello PhilO,
I was very interested that you do your carb balance at 2,500 rpm. Does this help to get rid of the vibes?
I was under the impression that all airheads vibed a bit around this speed
My thinking is that if the balance is done at the maximum vibration range then that is the most sensitive part of the rev. range to work at. No, the vibrations there do not go away but can be reduced and my thinking is that if you tune there then the balance above and below that maximum will be better. All I can say is it seems to work, is easier to do than listening (if you don't have the ear) or a doing a plug chop comparison.
My last long trip was 5200 miles and I didn't touch the engine until I got back (apart from checking oil level).
Many thanks for the technical term - "rocking couple harmonics". At last I know what to call it! :grin:


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