weber what if ?

[higgy]

Are you using the 3.5 or 4.5 tall auxilliary venturi ?

One more question,well actually 2
Is your serial number on the carb top or the side?
How thick is the phenolic block used fro the base gasket ?

[ducinthebay]

Sounds all like good work so far.
In general, the first rule in jetting is to start with the mains, and work you way down. Second is to make only one change at a time.

I agree that a lean condition is just as difficult for me to diagnose as a rich condition.

Cheers,
Phil

[jayh]

Higgy, I'm using the 3.5 aux vents,I know you suggest the taller ones,but they are rather difficult to acquire so I decided to try working with the ones that most people already have,as a last resort I'll switch,I'm a glutton for punishment.

the model no.is on the front side of the carby, 44 DCNF 107 6F,I havn't noticed a serial no.but there may be one under the air cleaner somewhere ?
the phenolic spacer is the base of the air box, trimmed to match the air horns
the distance being 21 mm between the top of carb cover and top of spacer,add another 2 mm for a gasket and air cleaner base and you have 23 mm between carb top cover and base of air horns.The spacer block protrudes down into the carby a mm or 2,restricting the air flow a bit,I thought that this may be important so I left it on there. Jay.

[jayh]

Just found this,havn't read it yet but I reckon it will be very handy for the Weber guys. Jay

http://www.xs4all.nl/~godelind/SM/Weber42.pdf

[Duc906]

Just found this,havn't read it yet but I reckon it will be very handy for the Weber guys. Jay

http://www.xs4all.nl/~godelind/SM/Weber42.pdf

That's out of the Haynes Webber Manual, I have it saved as a pdf. If you are interested I can email it to you. I also have the Webber Tuning Guide by Pat Braden. Pm me.

P.S. Awesome work Jay.

[higgy]

In my case,with the mods I have made the 906 runs rings around the 907 where I ride. From sea level to a little over 5000 feet.Pulls smoothly from idle all the way up,only one issue left to sort out and not that I get a lot of chance to ride in excess of 120mph. The 906 easily outruns an Aprillia 1000 mille a much more "modern" setup.
It still tends to go lean at road speeds above 120 at full throttle. I have a few ideas on a solution,just need the time to sort it out along with finally doing my cam timing.

[jayh]

Hi Higgy, I'm still trying to get the tune spot on,I have several setups that I'm trying, the best of them still has a niggly little rich miss at about 2000 rpm,depending on gear selection/throttle action,but is awesome all of the rest of the way up,and smooooth.None of that jerky on / off behaviour that makes the bike a handfull mid corner as you screw the power back on.A 2000 rpm miss is insignificant as you rarely spend time at those revs except just after taking off, It is really difficult to get the right balance of correct idle jet without opening up the lean hole further up the rev range.It would appear that a 53 or 54 idle is about spot on for the 750 as far as air fuel ratios and performance go ,but they both fade out just a little too early creating a lean stumble.On the other hand a 55 will cover the lean hole but create a rich stumble.I'm playing with emulsion tubes and various ac main jet combos,(some rather unlikely ones that work).I have a tune that I would be happy to use but I'm sure it could be better. Jay

[higgy]

Keep in mind Jay,you can vary the plug gap for those minor issues...sometimes it helps

[jayh]

Is there a method to the plug gap tuning ? or is it trial and error? I've been toying with the idea of advancing the ignition timing slightly to overcome the rich patch,I know that my 600's timing wasn't stock when I last checked the pickup clearances,but I can't remember which way it was advanced or retarded.I assumed that it had been adjusted to compensate for fuel octane and left it where it was as it goes hard,but that may also be contributing to the lack of torque it suffers down low.

I'm finding some interesting things as I play with the weber,for example there are many different combinations of jets and settings that will give you the required air fuel ratio's at the three main stages of jetting but they vary in the way they transition between the stages ie smoothly or harshly.I'm finding that main jet size is not all that important (as long as it is large enough to supply the required fuel at wide open throttle),the carby will only use what it needs and has the emulsion wells to draw from,so the main can be juggled around a fair bit to change lean and rich spots.Also the air correctors although they are supposed to mainly effect High revs actually have influence over nearly the whole rev range and as mentioned before the idles influence much higher up the rev range than you would expect,. almost to 6000 rpm.

One thing that I am finding a little confusing is the rich stumble down low
It isn't the mains coming in early as I can quite clearly see them coming in much further up the rev range on my data log,and I can see the idle transition circuit leaning off just prior to the mains coming in.I thought it was the accelerator pump squirt,but I can push it up and down the rev range by adjusting Idles mains and ac's.The only things I can think of are 1) the ignition pack adjusts timing somewhere around this time and starts advancing,(but that should help a rich mixture burn) or 2) the transition holes start to release too much fuel for the volume of air passing the butterfly,which is the most likely explanation as I can eliminate the stumble by decreasing the Idle jet to 53,but this opens up the lean hole up around 5500 rpm,which is not at all welcome.

I'll see if I can upload a couple of plots to show you what my mixture is like and the difference the idle jet makes. Jay.

[higgy]

Is there a method to the plug gap tuning ? or is it trial and error? I've been toying with the idea of advancing the ignition timing slightly to overcome the rich patch,I know that my 600's timing wasn't stock when I last checked the pickup clearances,but I can't remember which way it was advanced or retarded.I assumed that it had been adjusted to compensate for fuel octane and left it where it was as it goes hard,but that may also be contributing to the lack of torque it suffers down low.

Two things you can do,both trial and error. Adjust the gap in steps to its widest gap You can look this up on the manufactures website but you may have to dig a little.
Also you can do what is called indexing. Indexing uses different thickness copper washers to align the plug gap in relation to the valves. Also a trial and error process.

I'm finding some interesting things as I play with the weber,for example there are many different combinations of jets and settings that will give you the required air fuel ratio's at the three main stages of jetting but they vary in the way they transition between the stages ie smoothly or harshly.I'm finding that main jet size is not all that important (as long as it is large enough to supply the required fuel at wide open throttle),the carby will only use what it needs and has the emulsion wells to draw from,so the main can be juggled around a fair bit to change lean and rich spots.Also the air correctors although they are supposed to mainly effect High revs actually have influence over nearly the whole rev range and as mentioned before the idles influence much higher up the rev range than you would expect,. almost to 6000 rpm.

Yes that is exactly what I have found as well. There are many theories on whats is going on. I have found the most helpful info on Innovate's forums. Your Vw guys also have a lot of useful info.
To me it has come down to a more refined placement of holes for the ET in combination with the proper length/port size for the auxilliary venturi. The jets all have influence but it is my conclusion the relationship between the Aux venturi, air horn( choke) and ET is where the final solutions will come. I do believe you are on the right track. I was planning to do the final configuration on a dyno where I work, make that worked as it would be less time intensive, not having to take the body works off and on for each run. Now however I will have to do it on the street,longer process but more accurate in the end

One thing that I am finding a little confusing is the rich stumble down low
It isn't the mains coming in early as I can quite clearly see them coming in much further up the rev range on my data log,and I can see the idle transition circuit leaning off just prior to the mains coming in.I thought it was the accelerator pump squirt,but I can push it up and down the rev range by adjusting Idles mains and ac's.The only things I can think of are 1) the ignition pack adjusts timing somewhere around this time and starts advancing,(but that should help a rich mixture burn) or 2) the transition holes start to release too much fuel for the volume of air passing the butterfly,which is the most likely explanation as I can eliminate the stumble by decreasing the Idle jet to 53,but this opens up the lean hole up around 5500 rpm,which is not at all welcome.

Yes, tuning the timing I agree will help,lots of guys here have added a restriction to the vacuum supply successfully to delay the advance,but I still think it comes down to regulating the vacuum at the aux
venturi by getting the port size correct in conjunction with the right placement of holes on the Emulsion tube
Also some have added holes opposite the transitions on the butterfly to delay the effects of the transition holes. I have a spare set of throttle plates,just so far have lacked the courage to try it out.

[jayh]

Ok, so here are a couple of data log plots from my 750 paso
they are both taken from the same piece of road at about the same pace
for comparison purposes.Rpm is shown at bottom air fuel ratio on left.
all of the pink spots are different moments during the particular ride.All of the spots above and to the left of the top yellow line should be disregarded as they are from deceleration.

the first one is pretty much standard jetting
57 idle,150 main,170 air corrector and custom e-tubes
the things to note here are the yellow line bottom left showing the air fuel ratio dropping to below 11 and closer to 10.5,at 10.5 the engine will be stumbling particularly at higher loads and lower revs.
the other upper line shows the air fuel ratio approaching a lean miss,anything above 16 could cause a hesitation,particularly in the 5000-6000 rpm range,this plot doesn't show many in that range .The main problem here is the rich stumble caused by the 57 idles being too big.

the second shows the effect of dropping to 55 idles with a 165 main and 160 air corrector and custom e-tube.This is one of my better combinations.There is not a lot of difference between these two plots,but you can clearly see the
air fuel ratio no longer spending much time down around the 10.5 stumble zone
a few times the air fuel ratio has crept up to 16 whilst accelerating slowly,this can be seen near the centre of the top line,this is the lean zone between the transition to main jets and the engine only spends a split second there but this is where it can stumble if you place too much load on it,especially if it gets above 17 afr,with this combination of jets it only happens while the engine is warming up.It should be noted that the above combination behaves the same.


the difference between the two setups to ride is quite significant.Because of the leaner idle jet I find myself selecting a gear higher than with the stock setup,with plenty of power to accelerate out of corners without feeling like you are labouring the engine,the only drawback is I keep reaching for 5th now when I'm already there

This third one is a 55 idle 170 main 160 air corrector combination which also works very well.It is not from the same section of road as the other two.A rich miss is quite noticeable bottom left,but no hint of a lean problem,I will be working on this combination trying to eliminate the rich problem and lift the whole mixture just a bit leaner by .5 to 1 AFR. I will probably try a 54 idle first and then increase the main by one size if needed.Another scenario would be to
try a main in the 167,168 range,I have a drill about the right size,but don't want to bugger up my 165 main jets until I'm sure I've tried everything else.

As I said earlier a rich problem down this low in the rpm range isn't a real big deal,as it is a bit high in the revs to effect taking off from standstill and a bit low to effect general riding,it is only usually encountered in first and second gear so would probably be a pain in the ass in traffic conditions though.


one other thing that I have found helps a fair bit in smoothing out the on off throttle response is to set your idle speed adjust screw,so that just a tiny bit of the first transition hole is exposed.You can tell that this has happened by slightly adjusting the Idle speed up until when you rev the engine it takes just slightly longer to return to idle,(I mean ever so slightly longer),if you go to far you will lose too much engine braking.the sweet spot is at about 1400 rpm on my bike.I know that this goes against what the books say,but it works well.I know it takes away that really lovely sounding lumpy idle,but selecting neutral as you slow down also becomes easier !,and I hate the sudden surge of power as you accelerate in low gears mid corner.

I'll be doing some more testing this weekend. Jay.

[jayh]

Just a quick update,I tried the 54 idle 170 main 160 ac combo out and it was very good right through the rev range until wide open throttle where it went rich and caused a stumble,the obvious cure being to increase the ac to 170.If only it were that simple ? I will try next weekend.I changed the plugs after this run and for the first time they were a lovely tan colour,not jet black with soot !
Higgy's advice on plug gapping was tried on the 55 165 160 combo.First with new plugs I decreased the gap to .58 (.60mm is stock) and the stumble got noticeably worse,so I increased the gap to .62 and this helped a fair bit,although did not eliminate the stumble entirely.I may try larger.

It's been so long without the fairing on the bike that I got the urge to put it all back together again,the experimenting will have to wait a week or so now.
Jay.

[desmodave]

Still going at it,I havn't given up.The lean hole that I uncovered by reducing the idle from 55 to 50 is proving very stubborn to remove without excessively richening up the main jetting,so I've decided to tackle it from the other direction.Somewhere between 55 (rich stumble when gassing it around 2500 -3000) and 50 (lean hole or flat spot between 5000 -5500),I'm hoping is a happy compromise,so I've ordered the idle jets in between.

Who would have thought that the small step between idles would have such a dramatic effect on mixture, and at different ends of the spectrum.None of the information I can find describes the idle jet having any influence above 5000 rpm,but that is all I'm changing to get the effect,either too rich down low or too lean up higher.The average operating mixture is at 12 (bit rich) on 55's and 14 (about right) on 50's.The 12 is just a bit low at the lower revs,to accomodate the squirt from the fuel pump,which drops the mixture down to 10's for a second and causes a mild stumble,right at the speed where you almost have to drop down a gear to power out of a corner,or feel like you are labouring the engine.Drop down to 50 idles and theres no stumble and she powers out of the corner nicely (thus saving two gear changes in the process).I must be getting lazy in my middle age,cos I'm starting to enjoy riding with less gear changes. Jay.

INTERESTING!KEEP US INFORMED WHEN YOU GET THE BEST RESULT JAY -DESMODAVE. WEBER FAN.

[jayh]

Will do Dave, I'm going to order some more jets,and formulate my plan of attack while I'm waiting on them,I had a sort through my miniature drill bits and found one half way between the 54 and 55 idle jets so I'm thinking about turning my 54's into 54.5's or there abouts,I know this sounds like I'm getting a bit carried away but if it stops the rich stumble and still supplies enough fuel to carry through to when the mains kick in then I will have won the battle.

Here is a recap on what I have found to date:

57,56 idles too large cause rich stumble down low
55 idle border line too rich down low needs about a 165 main to cover the lean hole at transition.
54 idle good down low needs about a 170 main to cover lean hole which is too rich at wide open throttle.Might try 167,or 168 main here.
53,52,51,50 idles all supply enough fuel for normal running but fade out progressively lower down the rev range during transition,causing a lean hole that gets harder to cover.
There doesn't appear to be any easy cure for the rich stumble other than reducing idle jet size.

My thoughts on the above information are that the paso being such a light vehicle with a large motor (fairly high power to weight ratio),is able to run much higher up the rev range with less throttle butterfly opening than a car,this means that it spends much more time during this progression phase of the carburettor than a car would and what would just be a fleeting moment during the acceleration of a car,turns into a second or so of this critical period of carburettor operation thus magnifying the effects of any lean period.To simplify it, a car being accelerated through this transition phase would not notice any drop in performance from such a brief lean period whilst the paso spending longer at this phase of operation will develop a noticeable drop in power or miss.Ducati's answer to this problem was to increase the idle jet size to supply fuel even further up the rev range,to the detriment of low speed operation.

I have many pages of plots and other data that I will run through over the coming weeks and try to correlate the information into something meaningful
.I would like to be able to say exactly the effect on air fuel ratios a change of air corrector will make and it's effect on moving other areas of carb operation up and down the rev range.likewise for main jets.I think that I have the idles sorted out.The main problem in trying to diagnose the weber on the paso 750 is that it simply does not behave anything like what it would on a car.So most other information available is totally misleading.

I'm not giving up on the 'simple', 'perfect' tune just yet Jay.

[higgy]

I'm not giving up on the 'simple', 'perfect' tune just yet Jay.

The one big issue with the Weber is its lack of a compensator/power valve. The DCNF series was an all or nothing type of carb designed for maximum performance.