weber what if ?

[jayh]

I have been messing around tuning my weber carby for the last few months,and prior to that have done a lot of research on various forums on the net,trying to get to the bottom of the tuning issue.The information I was gathering was conflicting and very confusing,it just was not making sense.Anyway I decided to go down the line many people have gone down in the past by enriching the idle circuit and the main circuit to try and cover the lean hole at transition.With a selection of most mains and air correctors in the 150-180 and 160-220 range I began experimenting with nearly every combination,some were good others were shocking,but none eliminated the stumble.I also tried many combinations of holes in air correctors with the same results.I got to a stage where the bike started and ran beautifully,No lean hole whatsoever,but after a good run 1/2 hour to 1 hour when the bike really warmed up (it gets quite hot over here this time of year 35 -40 celcius is not uncommon) the stumble appeared again.This made no sense to me at all as it was a sign of a rich condition rather than lean.Anyway after messing around for a while I noted that the idle wasn't tuning as well on the front cylinder,and some carbon on the choke vent,a leaky choke piston was the culprit,and it was soon fixed,You beauty,I thought,problem solved,so off I went for another test ride,but after 1/2 hour or so the stumble came back,nowhere near as bad but it was still there when I gassed it on cruise.This got me thinking,What if this is a Rich stumble not a lean stumble.All of a sudden it all started making sense,I reduced my mains and Idles back to stock and then some more and lowered my float level back to stock.I then blocked all of the holes at full float level on my air correctors,(these are the ones that start emulsifying first) and added a couple above float level to delay the start of the main circuit.The result was a much more responsive bike in all low throttle positions,and no noticable effect on top end performance,with the added benefit of the carby responding to tuning as it should,I still have a tiny stumble,but I'm extremely confident of tuning it out.I believe that it is being caused by the additional fuel of the pump shot,when the throttle is first cracked open during a still slightly rich transition. I have since run a LM2 wideband on the bike and this verified that I was going rich just before the pump added the additional fuel which I believe causes the stumble.

I hope this information helps,but please don't lean your bike up too much without some way of measuring the changes,as this can lead to engine damage.

In a colder climate I don't believe the condition would have occured,as the bike would never get to the temperature where it really started stumbling,could this be the reason some pasos dont seem to have to much of a problem ,while others do ?

I will post my final jet sizes and emulsion tube holes when I'm happy with it

all the best Jay.

[higgy]

Quite a few 750 owners have been searching for a fix for the weber
When you get the time take a look here

http://mysite.verizon.net/vze2hw9t/


perhaps you would have an interest in contributing your final settings to a 750 section of our document

[jayh]

The new page on meccanica looks terrific Higgy,I hope to have some more testing done over the next week or so and if all goes well , I will post the full specs. of my jetting.You are quite welcome to use them. Regards Jay

[jayh]

Just a quick update, 50 idles appear to be about right,they eliminate the rich bog when accelerating and give about the right AFR at cruise,but open up a lean hole at 5300-5500 rpm,the Weber is really starting to sing now,and the carby is responding to my tuning changes,something it wouldn't do when too rich.I'm going to bring the mains back up to 155 to try and cover the hole and I can still play with my AC's a bit as Wide open throttle is still pretty rich.The bike feels much crisper now ! Jay

[907pasonut]

Jay, I get the feeling you know a fair bit about carbs...its all double dutch to me

[higgy]

Its really not that complicated Claude.
Time and patience are all that are required. Do things one step at a time and verify your results.

[giocast]

Thanks for taking the time to post in detail Jay - looks like all that perseverance has finally paid off.

[jayh]

I hope I'm on the right track Giocast,most of the information available tells you to do the opposite to what I'm doing,but I found a very helpfull site called the Samba.com for VW fanatics ,the aircooled engines have a lot in common with the paso 750 ,cylinder capacities,operating temps etc. and they run Webers as a tuning modification.These guys tune leaner to run cooler and slightly retard their ignition timing,rather than running extremely rich and advancing the ignition to compensate for the slow burning rich mixture.They reckon that it makes for much more responsive and fuel economic running as well.So far so good,fingers crossed I'm on the right track. Jay.

[ducapaso]

Be careful abour lean running, because it maker temperatures go very very high and this IS DANGEROUS for your engine! I've seen Cilinders melted in the proximity of the spark plug because of this!

[jayh]

I'm not running lean - just leaner than the factory rich which is also damaging to engines,washing the oil from the bores and accelerating engine wear.My Wide open throttle is still low 11's which would cause a rich stumble at lower revs,and my cruise is in the low 14's which is on the balance between rich and lean or 'stoich' as it is called.This is actually a hotter air fuel ratio to run at than 15's and 16's at cruise.At any time I accelerate my AFR drops to 13's which is still on the rich side.The factory Idle jetting for the 750 is too rich,this seems to be the common 'cure' for a transition stumble on the weber carbys,but there are other ways of fixing the stumble without richening up the idle jet (which you do most of your driving on).

The 55 Idle jet that I was running supplied fuel all the way up to 5500 rpm.By dropping to 50 I have uncovered a lean hole between 5300 and 5500 rpm,This is where the main jet takes over.

I don't know how you guys ride your bikes ,but I don't like to spend to much time above 5500- 6000 rpm if I don't have to,which means that the only time I'm riding on the main jets is during hard acceleration.This is when it is important to not be to lean.Nearly all of my riding is done on the idle jets,which were too rich and unresponsive. Jay

[ducinthebay]

If you don't spend much time above 5k then you are missing a good amount of what that engine will do. I spend most of my time above 5k, unless in town. Redline is 9k, so you are using only half of the powerband. Everything smooths out and works lovely at speed and the engine loves to sing.

[higgy]

I spend the majority of the time on the 906 and the 907 in the 2500 to 4500 range. On the street most likely you do to. The track is a different story, on the track the standard weber jetting works very well for most

[jayh]

Don't think my paso hasn't been ridden hard,it gets it's fair share of revs,I just find that it's got a lot of torque and doesn't mind less frequent gear changes and less revs.My pantah 600 ncr replica on the other hand must be ridden a lot harder as it lacks the low down torque and spends most of its time above 5000 rpm,maybe your carby tune is too rich down low and thats why you find the need to rev it hard all the time ? Jay.

[jayh]

For anyone interested here is a link to some of the information I found usefull.
It is quite a long thread but if you skip to the posts by john@aircooled,you will get the gist of it,there are some useful graphs on lean vs rich running and power,cyl temps etc. as well as an alternative way to tune out transition stumbles,and also some interesting stuff on ignition timing vs mixture.
http://www.thesamba.com/vw/forum/viewto ... sc&start=0
Please note that I don't intend to lean my bike up as far as these guys go at cruise,and they DONT try and run lean under acceleration only on transition and cruise. Jay.

[jayh]

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.