weber what if ?

[jayh]

These are cheap enough for experimenting with and the fuel pumps look like a good alternative to the holley pressure regulator too.Postage was only about 8 pounds for 2 x cams,and fast service !. J.

http://www.webcon.co.uk/shop/shopexd.asp?id=601

http://www.webcon.co.uk/shop/shopexd.asp?id=48

[higgy]

So the morale of the Weber and the Paso is

First start out, before spending a fortune on jetting solutions....try the Pump cam mods

Especially if your only issue is the typical part throttle stumble at low engine rpm

Also.. the weber can be made to work cheaply and effectively which really is the only thing I wanted to prove. Jay has found a solution that works for the cost of a file


So don't throw that weber in the bin and before you go to the lengths of Romus and I Take a little metal off you pump cam

One last comment..at one time there was a second pump cam availible from weber don't know if it can still be had but they are cheap enough and easy enough to modify all you need is a file and a little time.


Great job Jay

[paso750]

So don't throw that weber in the bin...

Who would do that? After seeing for what amounts some went on ebay I`d rather sell it than throw it away
(I`m keeping mine)

G.

[higgy]

do a search and see how many offer it up as a fix

[jayh]

Chuck em in my bin ! I'm not to proud to scrounge

[higgy]

apparently, neither am I Sold 2(cheap to other pasochists) and kept a spare

[Mc tool]

Chuck em in my bin ! I'm not to proud to scrounge

Me neither ! I want 4 of them ,906 ones ( something to do with a ford capri and a rover v8 )

[jayh]

I have finally got around to trying a new profile cam on the weber.The cams I received from Webcon (the link posted earlier),are aluminium and a pretty chunky profile,so they are a perfect candidate for modifying.



The 3 cams pictured are from left to right, Webcon as purchased,Webcon my new profile,Weber plastic cam after modification (I have been using the bike with this one for over 6 months now).

Just before I describe what I'm currently doing,It should be noted that the bike has seen a really big
improvement in lower rev operation over how it was before I started messing around with the pump cam.These faults that I'm going to describe are far easier to live with than the missing and sluggish behaviour from before.The hot climate that I live in makes rich running problems a lot worse than a cooler zone would,so If I can get it near to perfect here,It should work that way anywhere.Anyway there is room for improvement,so:

Starting with the plastic cam :
After using the modified cam for a period of time,one of the limitations of the profile became apparent.
The long period at the begining of the cam with no lift,while perfect for reducing fuel supplied at low revs,created another problem,the start of the throttle was delayed.The best way to describe it is you have idle at zero degrees of throttle rotation,then a period of several degrees,perhaps even 5 degrees,where there is no change,(just idle rpm) and then the throttle starts to operate as per normal.
This can be a bit difficult to judge at times especially when reapplying the throttle after slow cornering,there can be some snatch if you are not smooth as the throttle starts quite abruptly,and if you have the revs up a bit with compression braking,the throttle can come on hard enough to give you a fright if you are not expecting it.Fine if you are in the mood for sports riding but a little too much if you are just cruising.

As the cam is supplying no fuel during these few degrees of throttle rotation,I can only come to the conclusion that the accelerator pump is supplying all of the fuel,(or lack thereof) for acceleration at this point of carby operation !!

So to remedy this problem,I have decided to try a stepped profile cam (the middle one).The problem being that supplying too much fuel during this period is going to put me right back where I started,with a lot of missing and flooding at low revs on acceleration.It is a catch 22 situation.So I will supply a little at the beginning,then none for a period,then continue as per the existing cam profile,which is quite aggresive,and should ensure plenty of fuel when accelerating under load conditions.

It is quite hot at the moment here over 40 degrees celcius for the next few days,so it is perfect for testing the modification.A short ride showed that even with the little step at the begining,that there is too much fuel being supplied,just a tiny stumble that wasn't there before,but the delayed throttle is gone,I'm not sure how much more I can remove without the delay creeping back in, but I'll give it another try tomorrow and keep you up to date with my progress.

The lines on the cam are measured from the right hand side ,6mm ,10.3mm,16mm.A flat was filed parallel with the bottom surface of the cam down to the point where the 10.3 mm line intersected the ramp,and then the other end of the flat was smoothed back to match the rest of the ramp at the 16mm mark.

I will try filing a little more off the shoulder for tomorrow and see how it goes. J

[higgy]

Perhaps what is needed is a combination of a new profile and a different nozzle.

Burned out the stator on my 906 so I'll have to solve that issue before I get back to the weber.
I think I will order a size or 2 smaller on the nozzle just to have them here when I do get back to it.
Plan will be to go back to the stock weber set up and try going your route and work on the accelerator circuit first.
instead of the other way round

[jayh]

I think that the 035 pump jet that is fitted stock to the 750 paso is as small as they make them Higgy,and I have also read that the smaller jets tend to clog easily but that's why we run good filters inline isn't it ,so you could probably tig and redrill one smaller.

In hindsight,I wish that I too could start fresh with a stock carby and just play with the pump circuit... :banghead

[jayh]

Ok,I modified the cam some more by filing the flat spot even lower and reducing the little ramp in size.took the bike for a good ride and no luck,the stumble is back,not only at low revs in 1st and 2nd but also at about 3000 rpm when accelerating slowly from cruise in 4th.

As it is too hot outside now to continue,and I have nothing better to do on my last day of holidays,I'll bore you all with my analysis of what is happening.

Have a close look at these two cams.



The thing to keep in mind here is that fuel can only be pumped when the pump lever is in contact with an upward sloping surface of the cam.

The skinny end of the plastic cam has no upward slope for the first 15 mm or so of its rotation and pumps no fuel whatsoever for this area of throttle rotation.It also causes no stumbles at all.

The skinny end of the aluminium cam has a short upwards ramp about 5 mm in length,to try and overcome the throttle pause off idle (this works as intended) but that short little ramp causes
most,if not all, of the 'weber stumble' that many of you are familiar with.

I know that no fuel is being pumped on either side of the ramp and the plastic cam causes no stumble,
so all of the stumble I am now experiencing is being caused in the first 5 - 6 mm of cam rotation,and this is where my efforts will now be concentrated.

stay tuned for more riveting information j.

[jayh]

Discovered something interesting the other day while removing the pump diaphragm cover on my weber,(this has to come off so that the pump cam can slide off the throttle shaft).The cover has two positions for the actuating lever pivot rod,one position gives more pump stroke than the other.The positions are labeled 1 and 2.Mine was in position 2 which supplies more fuel (gives a longer stroke).I don't know if they are all the same from the factory or not but I know that it had never been in the position 1 hole as the pivot pin has little splines that retain it,similar to a car wheel stud and the hole was still smooth so the pin could not have been fitted.This may allow people that only have a tiny stumble to eliminate it,without playing with the cam ?.I wonder why there is no information about this feature ? could have saved me some work if I had noticed it earlier I swapped it over anyway,it's hard to guage the effect it's had at the moment,because I've changed my float setting and main jets,but it may be just enough to eliminate the delayed throttle if I modify a cam to suit. J.

[paso750]

Jay, could you make a picture of that ?

G.

[jayh]

Pic 1 a close up of the square diaphragm cover showing pivot holes numbered 1 & 2 :




Pic 2 a little further away,showing actuating lever with small rod on end contacting a very rough looking circular cam



I'm not sure yet how great an effect moving the pin will have,but it must have some or they wouldn't have bothered putting it there. J.

[Mc tool]

Just wondering ....as you do I have a book by some guy called John Passini' Weber carburettors : 1. theory. In this wunnerful book there is talk of accelerator pump bleed jets/orifices . These orifices feed back to the float bowl, well below the fuel level. The idea , I believe is to feed some portion of the accelerator pump discharge back to the float bowl ( or the bottom of the main jet/emulsion tube well ) rather than the pump nozzles . Because the pump nozzles are well above the float level and the bleed orifice is below the float level the tendancy is for the majority of the pump discharge to flow thru the bleed rather than the pump nozzles.Because the bleed orifice is much smaller than the pump nozzles ( remembering that the bleed is below the nozzles gravity comes into play here ) depending on the duration of the pump stroke more or less fuel will make it to the nozzles,IE if you apply the accelerator veeeery slowly all of the pump discharge will flow back to the float bowl via the bleed circut rather than go all the way up to the nozzles, If the accelerator is applied quickly the vast majority of the discharge will make it to the pump nozzles as there is not enough time to squeeze it all thru the bleed circut. This would suggest that the actual volume of fuel discharged thru the nozzles depends on how fast the throttle is applied. The second pivot that Jayh has shown us is a bit of an enigma as yes the other ( 2 ) pivot will supply a greater volume but it would also supply it faster too ( relative to the speed at which the throttle is applied ) ..... just wondering