Bing carb adjustment.

Nin · Mar 23, 2023

One for the cognoscenti.

I’ve got 32mm bj gs on my R100 Siebenrock with R80 heads and a squish band.

The carbs have been completely rebuilt/serviced.

Cables all have 2-3mm slack

But

On balancing the carbs, I’ve wound out the LH throttle stop so far it’s no longer actuating the throttle spindle, and I can’t get a balanced tickover below 1200 rpm. I can drop it by adjusting the rh carb but then it’s out of balance.

Float levels are fine, chokes seem fine, I Sparta carb cleaner on intakes when it’s running, no increase in recession so assuming no leaks.

Im going to go right back to basics, but before it do thought I’d check in here that I’m not missing something obvious?

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Gelandestrasse · Mar 23, 2023

Right back to basics means starting with valve clearances and timing, then balance carbs. (source: Haynes)

Then there's checking the chokes for correct assembly internally, and that the cables allow the shaft to return all the way to the stops (which have been correctly set after any rebuild).

You could also check if a butterfly securing screw has dropped out....

Nin · Mar 23, 2023

Cheers Tim, good procedural discipline.

Valve clearances are good, cables are free, new air filter, compression equal on both sides.

But haven’t checked choke stops and assembly - good shout.

mikeyboy · Mar 23, 2023

What have you got the mixture screws set to?

mikeyboy · Mar 23, 2023

As Tim says above,
Check the choke discs to make sure they’ve not been mixed up.
From the inside of the assembly they are marked L +R on the brass spindle.
Then it’s worth checking that the throttle butterfly discs are correctly located on the spindle and are closing correctly,
Most 32 mm have a dot punch mark that should face engine side and be at the top.

Nin · Mar 23, 2023

mikeyboy said:
What have you got the mixture screws set to?

To just keep the cylinder running smoothly when the other is not sparking.

It’s about 1-1.5 turns out.

mikeyboy · Mar 23, 2023

Nin said:
To just keep the cylinder running smoothly when the other is not sparking.

It’s about 1-1.5 turns out.

I’d check the above 1st off,
Then take the screws in to about 3/4 to 1 turn out.
At 1 1/2 out the revs will hang and you’ll not get the idle down

Nin · Mar 23, 2023

Got it ta - so I’m chasing the wrong end of the problem.

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Steptoe · Mar 23, 2023

Has this problem suddenly started, or since you’ve done the other work you’ve mentioned ..

Nin · Mar 23, 2023

Steptoe said:
Has this problem suddenly started, or since you’ve done the other work you’ve mentioned ..

It’s a setup thing, so have systematically worked through everything else and this is the last one to get right.

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Carts · Mar 24, 2023

I had a similar problem found out its was the disc in the carb that regulates the air input ( don't know the correct term ) this wasn't sitting square and letting in air. I loosened the 2 brass screws and let it settle

All good after that

Wierpig · Mar 24, 2023

Air leak from the throttle spindle o ring?

Nin · Mar 24, 2023

It shall be in pieces tomorrow for a check over, I shall report back.

Nin · Mar 25, 2023

Getting there.

Starter jet on LH was properly knackered - left and right compared.

I now know why it never starts on the LH side when cold - has the wrong (new) disk fitted. Luckily I’ve got a pair of 40mm Bings in the roof so can steal one of them.

RH side

LH side

Float levels redone, let’s see how that works …

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SteveHop · Mar 25, 2023

Are your butterflys shutting nicly when you screw out the throttle stop screws ?

Nin · Mar 25, 2023

Yep, and they’re the right way round too!

SteveHop · Mar 25, 2023

I found that after checking all that stuff it makes things a lot easier if i first set the throttle stops accuratly using a feeler gauge.

Mad Hatter · Mar 25, 2023

That disc on the left hand side is identical to the discs on my R80GS basic (both left and right) with two out of the four holes blank. Was scratching my head thinking they were blocked at first but apparently they’re supposed to be like that. Mine starts fine on choke on both sides

Nin · Mar 25, 2023

Yep I can get that. Having different one on each side is not so good. I swapped in one from my 40mm carbs and have identical on both sides now. It started better.

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Nin · Mar 26, 2023

All sorted and running smoothly now. Ign timing is just a wee bit advanced which isn’t helping, but that’s easily sorted.

Bing carb adjustment.

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What a waste.

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