Smoother idle....tickover, better low down

The Mechanic

Try this mod its relatively quick and simple to carry out and should cost nothing (assuming you have the odd length of tubing laying around.

Remove both injector body vacuum take off blanking plugs (the little rubber caps underside of the throttle body).

Obtain a length of vacuum rubber hose (same dia. as the brass pipes that you've just exposed. You will need enough to pass from one side of the bike to the other. An alternative could be metal pipe such as brake pipe and some shorter pieces of rubber vac. hose.

The mod.... You are going to join the left vac take off pipe to the right. I routed my mod pipe through above the transmission following the petrol pipes and breathers (pull the right side panel off and you can see straight (relatively) through to the other side.

Super glue the hose to any connections made but not to the brass stub pipe (the one fixed to the throttle body) and use cable ties (zip ties) to secure the mod pipe safely.

Start up your prized possession and listen to the difference, go for a ride and feel the difference...this is low rev stuff just off idle and traffic trickling.

If you feel no benefit then just take off the mod pipe and return the bungs onto the brass pipes.

On mine (2003 1150GS still under warranty) I noticed a distinctly settled idle, all the sync stuff and Zero zero done and dusted using manomitor and digital volt meter etc. before this mod.

I make no claims, but try it for yourself; its not rocket science and my old 1968 Bonneville had a similar arrangement fitted as standard by the factory.

Good luck and keep out of the bushes.
Tony, does the idle speed change?

Sounds like an interesting experiment or thing to do, sort of like the crossover pipe that joins the header pipes on the 69 - 94 twins.

Think I'll give this one a try and see what happens.

Did you do this as a one off, or, was it done in conjunction with Zero = Zero and that kind of stuff?

i remember this as being a common mod to mid eighties jap fours - particularly z650's. Then you joined 1 and 3 and 2 and 4.

Dont know if it'll make 'that' much difference, the pipe is fairly small bore and the length is quite long for pressure pulse equalisation.

Might give it a go though - what can possibly go wrong ?? :)

In reply

Hi Mick,

I did this mod after all the other adjustments, I even went so far as removing the throttle bodies and measuring the throttle plate gaps for comparison, as a point of interest the gap -(measured with a tapered feeler blade at the base of the body to plate)- on both was exactly 0.002 in. (I'm a feet and inches man {imperial})

Incidentally even considering the possible difference to induction flow between the two cylinders, I wouldn't be at all surprised to find a similar dimension in the Dealer BMW w/shop manual. Having worked with a load of other Bosch injection set-ups the measurement is identical, plus the system on the GS is quite an old design now.

The idle speed was smoother in my opinion and when testing especially in low speed traffic I got the distinct feeling (bum dyno) that the just off idle was much smoother.

True about the point Motomartin makes, the pipe might benefit from being larger in diameter but then you may find that the idle is no longer as stable due to the pulsing of some very large pistons. Of course on the other hand further up the rev range there may be more improvements to be had with such a mod. Must go and take another look perhaps 2 systems are called for.

The zero=zero /throttle pot (TPS) adjustment can be made without breaking the BMW (blue paint on the TPS screws) or the sealing on the throttle stop screws, you need the bodies off the engine (hanging carefully) operate the throttle a few times and you will see two ways of obtaining small adjustments, three if you are to brutalise certain parts. If you your mechanically minded you'll easily spot what I'm referring to.

As my GS is still under warranty I do not want to reveal my full name as BMW could be monitoring this site (who knows).

When I had finished my hand-held voltmeter registered 0.310v (TPS red/white wire)now watch 55,000 people reply saying its too low! Well I know you are supposed to read between 0.370-0.400v but if you check with Mr. Lentini he does give some alternatives.

Anyway mines blue and white so its faster than the other colours.

Popping and banging on the overrun may be caused by too much air through the air bleed screws. The manual on this site states that for the 1150 GS the screws should be set (to start with) at two and a half turns out from full in, this is not the case when the idle is stabilised and finally set, one and three quarter turns out from full in is more like it (I know that their suggesting a starting point but its too much)

Elsewhere someone suggested that it was wrong for me to use a vacuum gauge, well bugger me my £124 vacuum/pressure gauge
is marginally more accurate than 'Carbtunes' manometer and therefore I'm satisfied; proved to be more accurate than 'carb synch' device and far more accurate than BMW dealer apparatus. Used with skill and care the results are always the same, alot of feel comes into these types of adjustments especially with a long stroke slogging engine like the GS.

Hope you have success with your mods guys. (oops..and gals)
Tony, I found that my own system which is 1100 without a Cat, works best at or nearest to 3.7V mine is set at 3.69 V which is the closest to 3.7 I could get.

I have set it at 3.9 V and I didn't like it, neither did the bike. When I set it close to the original factory setting which was 3.4V it was a dog, so I reverted back to what this particular machine likes and it's now permanently set at 3.69V.

Having had a mate change a worn out R/H throtle body, I am familiar with the innards of said units, and yes, there are other ways to skin a cat, as they say.

As far as idle settings are concerned the TPS unit setting does change the brass screws influence on idle and on a friends 1100RS bike we had to adjust the TPS so that we could get the idle down to an acceptable level.

These settings do of course alter the fuel consumption as well. My own particular machine is now using about 4.8 to 5.7L 100Klm's depending on load and speed. This is almost exactly what the factory says it should do. Until I fiddled with the TPS using Rob Lentini's articles as a guide and set everything as I think it should I was getting about 5.6 to 7L 100Klm's. I was extremely happy with that as it was pretty similar to what my old R100CS was getting. However I now know better and I think that the endless ways in which we can tune and/or alter the machines gives us that tinker, real satisfaction, especially when accompanied by a nice ride after some successful fiddling on the bike.

Well it's Saturday morning, I've had a fiddle and I'm off for a ride!!!!


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