Bypass Screws

[Detox]

I recently carried out a throttle sync on my 1100 GS. I managed to balance and set correct idle speed with the bypass screws set half a turn out (from lightly seated) on both bodies.

My question is this:

What is the THE recommended setting, I have seen figures of 1.5 or 2.25 turns out? I appreciate all bikes will setup differently but is .5 of a turn out abnormaly low?

 

[BiG DoM]

I recently carried out a throttle sync on my 1100 GS. I managed to balance and set correct idle speed with the bypass screws set half a turn out (from lightly seated) on both bodies.

My question is this:

What is the THE recommended setting, I have seen figures of 1.5 or 2.25 turns out? I appreciate all bikes will setup differently but is .5 of a turn out abnormaly low?

I just did the famed Zero-Zero reset (despite all the caveats). I could not get a TB balance and concluded they were not opening equally. Anyway the bike has never gone better and the method calls for the screws to be lightly seated for the procedure prior to Twinmax sync. The RHS side one is now maybe 1/4 turn out if that and other still lightly seated. Bike idling beautifully, balance spot on etc. My instructions said the ideal is not to have to adjust them out much at all. So I would say do not panic.

 

[The Mechanic]

Also depends upon.....

Many other things can affect the air by-pass settings for instance:

TPS setting
Valve clearances
General health of the engine...compression etc
Type of spark plug can also influence idling and pick up
Fuel line pressure, fuel filter condition, injector cleanliness
Air filter and housing
Crankcase breather problems

In the end these by-pass screws are only a means to enable some form of idle speed control and to some extent throttle balance just off idle ( say upto quarter throttle) after that the by-pass screws are literally by-passed. Withthe by-pass screws fully seated (closed) the idle will be slightly richer as there is less air being allowed in and this will give the impression of smooth but possibly too slow an idle speed.

You could try (with normal temp engine) letting the screws out slowly and listen to any change in engine note speed (rev. counter) and repeat screwing back in. You will not hurt the engine if you run with the screws fully home or half way out (provided they are balanced and you certainly will not hurt it in the current positions you have them in according to your post.

The most important thing is to ensure that both throttle plates are set accutrately as imbalance here will cause vibration which could be damaging

 

[(RIP) Kaister]

Agree with what Tony has said.

Also - the correct cylinder head mixture is determined by correct and most efficient fuel burning. - to this end the screws should be adjusted for maximum engine speed - best done with a digital rev counter.

These screws are the equivalent of the small brass air screws on japanese type engines.

Too rich, effectively 'damps' the engine at idle, giving a false impression of smoothness.

If you have a gunson carbtune - put one in to the head - if you can - i've not tried - and check the flame colour compared to screw position.

out = weak = blue
in = rich = orange

i used to do my lawnmower with a colortune