Duck, it's possible that having multiple throttle body vacuum connections might unload the vacuum quicker when the throttle is snapped open allowing fuel pressure to rise a little faster.  This might have some positive effect on the throttle lag on some bikes like my K75RT.

I am going to try it, but will just do it with tubing and tee fittings at first.  The idea being that less volume will help response.

There should not be any lag at all. If you do have some lag then there must be something else wrong. I would check the second switch is working on the TPS. This switch notifies the board to enrich the mixture. Would be a good thing to look at. The vacuum response will increase the pressure for a moment to ensure initial demand is there but then TPS telling the board to hold the injectors open longer enriches the mix to keep it screaming. I'm on a early 85 model k100. Can only speak confidently about the early 2valves. Burt had posted some great info on all this years ago.

Point-Seven-five wrote:Duck, it's possible that having multiple throttle body vacuum connections might unload the vacuum quicker when the throttle is snapped open allowing fuel pressure to rise a little faster.  This might have some positive effect on the throttle lag on some bikes like my K75RT.

I am going to try it, but will just do it with tubing and tee fittings at first.  The idea being that less volume will help response.

T fittings I was thinking the same, supercheap or maybe Bunnings, I will be interested to see how you fair

Some k75's have a backfire issue on overrun (dunno if this is at mid throttle range, guessing so) 

Could it be that this modification also helps with this problem, by reducing the fuel pressure more quickly on overrun?

A fuel pressure test before and after might help. 

Porto, I haven't done  fuel pressure, but have tried to get a vacuum reading, and as you say the needle was jumping all over the place, couldn't get a reading at all.

Not shore about the overrun. But If you put a fuel guage on test the pressure you will see that the needle steadies out when all the vacuum ports are connected.
When testing vacuum of balancing throtle boddies i use vacuum guages that have a flow tap on them. The flow tap is screwed in to reduce the swing on the needle.
With the over run problem it's common for the balancing to need doing. If you boddies aren't balanced well you will get hang time.

I've already got the "T"s and hose in stock left over from another project. I Might give it a go later in the week.
Regards Martin.

My throttle bodies and TPS are properly set and carefully balanced.  I check them twice a year.  Valves are checked for clearance and have been set for uniform gaps on all cylinders.  New spark plugs and air filters have no positive effect.  I have checked fuel pressure many times and it always read properly at steady throttle, but due to the gauge I was using could not verify what it was doing when throttle opening were changing.

Timing is set to factory specs.  Some playing with advance had no positive effect.

Problem is only evident on my K75's, the 16V K100's don't do it at all, and the 2 valve K100 I owned briefly had some lag, but nowhere as bad as the K75 engines did.  Whatever it is, it appears to be related to the Jetronic and not the later Motronic.  That is the reason I have suspected it may be the flapper in the airflow meter which is going to introduce a mechanical inertial lag in system response.

MartinW wrote:I've already got the "T"s and hose in stock left over from another project. I Might give it a go later in the week.
Regards Martin.

That would be interesting.  The way things look around here, it could easily be at least a couple months before I can get the bike out of the garage.

I am curious as to why linking the the vacuum ports isn't done consistently on engines to this day.  I'm sure the BMW engineers would likely have considered it when developing the K but did not implement this until later.  Earlier this year I bought a '22 Kawasaki Versys 650 and the throttle bodies do not have linked vacuum ports but this is a common up-grade for many owners.  Is there a mechanical downside to doing this?

Woodie wrote:I am curious as to why linking the the vacuum ports isn't done consistently on engines to this day.  I'm sure the BMW engineers would likely have considered it when developing the K but did not implement this until later.  Earlier this year I bought a '22 Kawasaki Versys 650 and the throttle bodies do not have linked vacuum ports but this is a common up-grade for many owners.  Is there a mechanical downside to doing this?

My first thought is that tying the vacuum ports together will violate emission regulations.  From the factory the butterflies are new and have been set on a flow bench and all the passages are squeaky clean.  Valves, cylinder walls and piston rings are new.  All the mechanical conditions are identical to the conditions of the engines that qualified for emission certification.  That way the manufacturer can certify that mixtures are optimum for minimum emissions. 

Manifolding the vacuum ports changes the manufactured configuration that met the emission requirements, and thus, would make certification impossible.  Based on how vacuum configuration diagrams are posted on most if not all vehicles for some time now it seems to me that vacuum is pretty important to emission control as well as proper operation.  Tying the vacuum ports together adds a variable that was not part of the certification process.

Woodie wrote:I am curious as to why linking the the vacuum ports isn't done consistently on engines to this day.

Because, according to Yamaha about their YICS system, the cost of the system outweighs any gain. You will need to bear in mind that the statement was made at the time when each of the Big Four were trying to out-do the others in speed and acceleration, so if any of them dropped a system that promised to give some form of induction gain then you can bet that it probably isn't really worth doing. Why did BMW do it? Again, bear in mind, BMW did it in the same time period as Yamaha and it probably continued long past the sell-by date because it was easier to leave it in production than change it. Or maybe they just wanted to use a stockpile up

The vacuum hose to the fuel pressure regulator reduces fuel pressure when the vacuum is high. I'm going to guess the computers have enough inputs to work out what the vacuum is, so doesn't need a mechanical item to reduce the fuel pressure. One item less, to go wrong.

Just had a look on eBay at a fuel pressure regulator for a K1100 (with the more modern motronic computer) it doesn't have a barb for a vacuum hose

Chi-Com vacuum block from Amazon installed. I preferred sealing the threads with yellow teflon tape as it was thick and stiffer so easier to wrap. In addition I used aluminum crush washer on inlet nozzles.

4mm hose ~4ft in length is needed. Small rubber pads were added at the contact surface pressing on the fuel rail (see pic below, encased in yellow square). 2 black "go fast" zip ties used.

Did I notice a difference, yes, at start up and idle but alas the roads are covered with salt water after the winter blizzard so no test drive to check throttle response. So happiness in the under $20 upgrade has been achieved for now.

Bricki I'm so happy you noticed a difference. It looks good too. When the weather comes good and you have a run and re tune, I'm certain you will never go back to how it was. Also mate in your climate it's crucial that the the water temprature sensor is reading well. I used the map that Burt posted on here years ago and I tested my original and two new ones. The original was way out, not to bad here down under but would have been s problem where you are. After installing a new unit there was a noticeable change. With the age of our bikes it's extra important because the components inside the jetronic no longer read as the should because of age. Capacitors and resisters especially, so important that temp sensor is keeping it as close as possible to what it needs. Happy new year from OZ brother brickers

G'Day!  Temp sensor was tested and well within spec range. I have heard many a capacitor "pop" in my day and hopefully they built my ECU on the day the union got a raise. May head out in the arvo as it'll be 15C.

Yes Happy 2023! Last 3 years have been a drag as I have turned into a caregiver for me mum.

I did the conversion using 5mm plastic Tees and 4mm vacuum hose using bits I had in stock. I did a price check with Supercheap and a Champion 2 pack of 5mm Tees is $8.00 and 4mm vacuum hose is $8.00 a metre. Conversion for a K75 required 2 Tees a K100 needs 3 Tees and under 300mm of hose. No rocket science involved quick and easy job of around 15min. I did not do any changes to  the TPS at this stage maybe later. I was sceptical about the conversion achieving anything but I was proved wrong. 

1/ Initial starting from cold, albeit Queensland cold no longer requires throttle advance just an initial blip to steady the idle and cut the charge light. 
2/ The jerkiness associated with slow maneuvering with on and off throttle manipulation seems to be eliminated. 
3/ Rapid throttle movements no longer result in hesitations.

The only negative thing at this stage is the occasional popping from the exhaust on throttle overrun.  My 75 is not prone to backfiring and only does it if you don't fully close the throttle on overrun. On the workshop day I might run some tests with the vacuum gauges and try to record any differences. I also maybe disconnect the TPS. I am also trying to get one of the K Kronies to do the conversion on a K100.
Regards Martin.

Good to know, thanks for the update. Did you use the existing hose from the throttle body to the fuel regulator, or is that also new? I'm thinking that might be time consuming to replace.

Vacuum hose to the FPR was in good condition and as you stated a pain to replace. Although I can replace it by removing the air filter element and then removing the lower filter housing. I can just manage to squeeze my hand into the housing to undo the bolts. Replacing the lower housing is made easier by taping or gluing the spacer/washer in place as they prove hard to get into position otherwise.
Regards Martin.

Righto, a bit of a long bow, but here she goes. Spose there's a restricted spot, or weak spot in that hose, and it temporarily collapses with the vacuum suddenly applied. Yeah it's a long shot, but not a lot of jobs for your upcoming workshop by the sounds of it.

The amont of vacuum hasn't been increased it has just been evened out. And my hose has not been long replaced.
Regards Martin.

Goodo, not worth replacing then.

#2 on your list of observations is really interesting to me. 

With the top heaviness of my RT fairing, front-end dive, and the seat height, that jerky throttle has often had me puckering when required to do tight low speed maneuvers.  And I was a half-ass trials rider a long time ago.

If indeed the slow speed handling is improved, that is all I need to make the change to my K75.  

Monday I'm going out to get some vacuum line and tee's.  Then I'll wait for a warmer, dry day.

Gryph once warm snapping open the throttle from idle is also virtually instantaneous. I've only had two test rides since the modification but so far I'm impressed. However I'll continue to monitor just in case the testing was an anomaly. I definitely want one of the K Kronies with a 2v K100 to do the mod. Further testing to come.
Regards Martin.

Martin, I've been thinking about this mod for a while now, and I suspect that by having all the vacuum ports connected together, there is more(less?) vacuum to the fuel pressure regulator to make fuel pressure response to throttle opening better.

I am really looking forward to doing that mod.

Gryph I'm on the same wavelength, as at this stage it sort of makes the only sense. However I might be able to do a bit more testing, if and when I get the time. Who says retiring gives you more time.
Regards Martin.

ricklayer wrote:Chi-Com vacuum block from Amazon installed. I preferred sealing the threads with yellow teflon tape as it was thick and stiffer so easier to wrap. In addition I used aluminum crush washer on inlet nozzles.

4mm hose ~4ft in length is needed. Small rubber pads were added at the contact surface pressing on the fuel rail (see pic below, encased in yellow square). 2 black "go fast" zip ties used.

Did I notice a difference, yes, at start up and idle but alas the roads are covered with salt water after the winter blizzard so no test drive to check throttle response. So happiness in the under $20 upgrade has been achieved for now.

Where does the 5th tube go? One per cylinder(3), one to the FPR, and then???

duck wrote:Where does the 5th tube go? One per cylinder(3), one to the FPR, and then???

Bricklayer has an '85 K100RS; the K75 is just an unclassified memory.

Laitch wrote:

duck wrote:Where does the 5th tube go? One per cylinder(3), one to the FPR, and then???

Bricklayer has an '85 K100RS; the K75 is just an unclassified memory.

My bad. I mistook his avatar for a K75S - upon closer inspection it's OBVIOUSLY a K100RS. DOH!

After another month or so I have concluded that the 75.
1/ Starts better no longer needs the fast idle just a quick rev to start the alternator charging. 
2/ Better smoother throttle response when slow maneuvering no longer snatchy.
3/ Better throttle response when snapping open the throttle no longer hesitates.
The only slight downside is it tends to pop and gurgle a bit on throttle overrun but not a full backfire, which hasn't been a problem on my bike unless to fail to close the throttle on overrun. On our last workshop day we also did the modification on Dans (Standard K's) K100 with not much feedback other than easy starting no longer requires fast idle. As the benefits outweigh the very very slight negative which may or may not be able to be eliminated I will be keeping this modification.
Regards Martin.

Thanks for the update.

I may try this on my 91 K75S with a Staintune exhaust. It always starts reliably but with the Staintune is just a tad slower to start than my K75s with OEM exhausts.

I have a Staintune also using the ChiCom vacuum block. Actually got it for free. The darling mechanics at the BMW shop I worked at would swap parts off bikes that were brought in for repair estimates that ended up considered salvage. So before many a wrecked K bike was off to the junkyard, I got many free upgrade: voltmeter, temp gauge, K1100 center stand, the Staintune exhaust and the newer footpeg mounts for my '85 KRS. Lucky days have long gone.

I don't believe using the block/reservoir is necessary and just using the nylon "T"s makes a cheaper, easier and more a compact installation. Due to the fitment of a heat shield on the injector rail you can't see the extra plumbing on mine. And on Dan's K100 naked it is hardly noticeable.
Regards Martin.

I had the "T" setup  but I am hoping this will simplify doing my occasional "hot soak".

What problem is the "Hot Soak"?
Regards Martin.

ahhh, the Hot Soak. Using a can of America's favorite cleaner, Sea Foam, and taking an end of one of the vacuum lines in the can to suck up the liquid which then goes through all the vacuum lines on my ChiCom vacuum block.. Not enough to let it stall put to soak all the intake components. Do this after the engine is hot and the after a half can or so has been ingested into the motor,let it sit over night. The next day when you start it it will look like someone dropped a smoke grande. After the smoke clears, an oil change ensues. I  used to do it with a 50/50 water/transmission fluid mix into the carb of my Chevy small block.

Arrrr the fog lifts.

Also known as the "mosquito fog".  Do it on a Saturday morning before a neighborhood barbecue.

After running this modification for over 5 Months the initial results still hold. I will not be going back to the  OEM set up. Not much feed back from Dan's K100 conversion. However he may have to suffer severe interrogation on the up coming service day. From the little information extracted from him so far just better starting. Bruce's K75s is the next conversion and we might get around to doing more testing. Has anyone else done the conversion? .75 did you do yours?
Regards Martin.

Martin, as luck would it life has put bike stuff on the rear burner.  The best laid plans and all that.

It has been years now since I did my one. Mine had the filter in the tube as well. The filter rotted out years ago and now it is a tube of four into one as you see in picture. It works as well now as it did then. I get the occasional comment on how well it idles and runs. Starts perfect even in sub temperatures. Will never go back.

Even BMW went in that direction with the K1100, except that the vacuum links are on the rear of the tb block. You soon know when you've knocked one off when refitting the airbox...

One of my K100s is running K1100 throttlebodies and I did exactly that recently during an overhaul. I'm telling you that a bag of Irish nails sounded more pleasant than that engine - ! Because the linkages are out of sight, it took ##@@##!! hours to find the problem.

Dai wrote:Even BMW went in that direction with the K1100, except that the vacuum links are on the rear of the tb block.
. . .! Because the linkages are out of sight, it took ##@@##!! hours to find the problem.

According to the K1100 service manual and the attached photo of some attractively Ceracoted K1100 throttle bodies, the vacuum ports are in the same location on each 4V body as the 2V models. What is in the rear is the return air manifold receiving crankcase blow-by recycled through the cyclonic oil separator in the air box; separated oil gets dumped back into the crankcase elsewhere.


I've been obsessed with this mysterious array since it showed up in another thread earlier this year and now I believe I might understand it enough to actually be able to own a K1100 and deal with it except the prospect of dead-lifting a Brick that might weigh 100lbs more than the one I now own because I was distracted at a stop while watching the graceful landing of a white admiral butterfly on a pile of dog squat just isn't appealing.

  Still makes the engine run like a bag of Irish nails when it falls off

Similarly though, it could still take a pounding.

As laitch says in post 42 - the clue is the rear ports are above the butterflies, so no vacuum there guys, sorry. Been digesting the advice in this thread - recently installed a fuel pressure gauge and an adjustable fpr and found much fluctuation in fuel pressure on tickover and low revs. Was more worried about wear in the fpr than smoothness. Taking vacuum from all three stubs (K75) reduces the fluctuation by a lot. No effect on tickover, dunno how I'll cope when it's time to balance the brass screws though?

Disconnect them and refit the original caps Chris. Or something that will do the job.

Too much time on my hands and not enough spare money...

It's 25mm aluminium rod drilled to 95mm deep x 16mm diameter. The plug end (left side) is drilled to 18mm diameter x 12mm deep - the plug itself is 10mm deep. I wasn't at all sure that once tapped home it would be completely airtight, so I filed off the edges to form a grove when it was flush with the main body. Once I'd had fun beating the bejeezus out of it, I capped off the plug with a dome of lumiweld making sure that it ran into the groove properly - the curve is just about visible in the pic. It's definately airtight now. I bored it 2mm off centre to get the maximum possible wall thickness for the inlets - 6mm where the inlets are and 3mm on the opposite side.

The inlet ports (for want of a better word) were made from 6mm diameter brass tube with a 3mm bore hole, threaded to about 10mm. I countersunk all five holes and fitted a o-ring under each washer which squashed into the countersink. All five brass pipes were glued in with high strength threadlock. Again, I'm pretty sure that all five are airtight where they should be airtight.

It's held to the mounting bar with two 6mm dia x 10mm deep blind threaded holes. By the time I'd finished the volume of the manifold was just about 18cm cubed.

It seems to work okay and it fits behind the fairing. Just.

Hmmmm...  Dai, you have my mental gears engaging.

I am seeing a small piece of copper pipe with several chunks of brass tubing and a pair of copper end caps.  All soldered together.

Hopefully on tomorrows K Kronies workshop day we might be able to run a few more in depth tests. I have a single automotive vacuum test gauge that we can attach before and after the conversion. However we are just fitting nylon "T's" and 4mm hose to a K75s.
Regards Martin.

Point-Seven-five wrote:Hmmmm...  Dai, you have my mental gears engaging.

I am seeing a small piece of copper pipe with several chunks of brass tubing and a pair of copper end caps.  All soldered together.

The only thing I was not happy with is the tube at the throttlebody end. It's 9mm od/5mm id and feels a bit too loose for my liking, so I've orded a pack of these in 8mm:

https://www.ebay.co.uk/itm/272913390831

We finally had our K Kronies WorKshop. Dan reported back on the vacuum mod to his K100 and the only thing he had to say was that he thought it started a bit easier. Dan did the conversion on Bruce's and we put a vacuum gauge on it before and after with interesting results. Before mod large erratic needle swings and with restrictor reads around 35 KPA, after mod mild needle swing and with a restrictor 30 KPA. Bruce after his second road test due to his throttle mechanism being fouled by one of the new hoses in the first test. Reported he was satisfied with the modification. He reported that throttle lag had gone and believes it might be better at slow speed stop go. He will do more slow testing later in the week. He reckons the mod was worthwhile. Jury is still out on whether it's worthwhile in K100's. I might see if another K Kronie with a K100 is willing to be a victim.
Regards Martin.