Topic: Weber carb set-up

[Tintop]

Just wait Bear, once their right, they are magic.  It's the accel pump that is gold.  For you there is lots of historical data running the Webers on modified 750's.  For me, I'm charting unknown waters.

Iwahples - not looking for a smooth idle (race cam), just something consistant at around 1100rpm.  Part of the problem is the  big chokes.  Once you get on the throttle it's fine, but rich as the last dyno run showed.

[scottly]

I'm getting the sidecar ready for the coming season, and want to try to make some more jetting improvements.  Figured I'd post here and see what the collective minds suggest.  For anyone not familiar, engine is a 750 twin, Wiescos (10/1), running 110 race fuel, 4.5lb max fuel pressure.

The following is from the last race, where there it seemed the work OK at upper rpm's, but idle was cr#p, and I did get an rpm related bog ( three times in 10 laps) coming out of 5C at Mosport (slowest corner). 

Choke - 34 (class allowed max)
Air - 185
Main - 140
Tube - F11 (was improvement over F16 used in practice)
Pump jet - 50
Idle - F945
Discharge (unknown, not marked )
screws turned out 1+

Current thoughts are to go to 135, or 130 main, as the race fuel seems to run richer than pump gas.  Smaller pump jet (40, or 35?), and also change idle jet (F940??).  As the current discharge isn't marked, will replace with new, thinking probably around a 50.

comments??

Regarding the idle jets, I would go up to 50s. I'm running 45s as 40s were too lean. If the screws have to be more than one turn out just to get it to idle at all, you need bigger jets.
For the accelerator pump jets, I would try the 35s if you have them on hand, and leave the discharge alone for now. Try to measure it, so if you do need to fine tune the pump, you know which way to go.
For the mains, I would try the 135s before jumping all the way to 130s.

[Tintop]

Thanks scotty.  According to the dyno it was way rich at idle (10/1), cleared to about 12/1 as the throttle opened, then went rich again all the way up.  That was with - 175 air / 140 main / F9 tube / same pump & idle as now.  Seems to like the F11 tubes (much cleaner everywhere), and I have 130 & 135 mains to work with.  I've got to order pump jet(s), so figured I might as well get a new discharge, so I know what I have (no guessing).  I will measure the existing discharge.  A 50 is .5mm correct?

[scottly]

Yes, in this case means .5mm. The switch to .35mm squirters will make the discharge jets act larger, plus you will get better atomization of the squirt.

[Tintop]

Thanks, I'll measure it tomorrow, and see what I have.   It's the one thing I have very rarely had a reason to change.

[johno]

trust the dyno !     the dyno man will start with the mains,  and then mid throttle,  then idle. Presto smooth runnin momma, smoke it out that slow corner baby.

[johno]

  i'm talkin full lock powerslide Bear , you are gunna love it, 

[bear]

i'm talkin full lock powerslide Bear , you are gunna love it, 

Hmm........I've had that slapped out of me over the last couple of years.
Apparently it's not the go in this new  fangled road racing caper.
Love a wet track though.

Cheers,
Brian

[scottly]

the dyno man will start with the mains,  and then mid throttle,  then idle.

That's bass ackwards, mate. You start with the idle and work upwards with Webers.

[scottly]

not looking for a smooth idle (race cam), just something consistant at around 1100rpm.  Part of the problem is the  big chokes.  Once you get on the throttle it's fine, but rich as the last dyno run showed.

The size of the chokes doesn't affect the idle, but does have an effect once the throttle blades have moved past the progression slots. I think the rich sniffer readings at idle may be erroneous; if the mixture is too lean, to the point of miss-firing, unburnt fuel will be passed on to the sniffer. If you were to monitor the spark voltage with an oscilloscope, you would see a higher voltage, which would indicate a lean mixture.

[johno]

Listen here Tintop, to make it easier to tune the carbies just put in that SOHC honda engines and all your probs will be gone     go on do it 

One of the greatest engine sounds I have heard in my life was a Honda sohc chair run by a great outfit man Stan Bayliss,  He was warming it up in the pits and had an awesome sound , the crowd all gathered around and then near the end of his warm up session he just give it big mobs of #$%* send heads spinning and hands on ears,     I love that stuff man.

[johno]

Bear ....straight is fast , sideways slow.............................. more fun sideways though bro

[johno]

Captain Webber !     I got this theory !   whoever pays for the jets can do what ever they like 

[Tintop]

the dyno man will start with the mains,  and then mid throttle,  then idle.

That's bass ackwards, mate. You start with the idle and work upwards with Webers.

+1 scotty, that's how all the Weber books I have say to tune, idles 1st.

No misfire scotty.  3 twists of the throttle from dead cold (no choke), hit the starter button, and it's running.  Carb is off now, so I'm going to check plate position to make sure the progression holes are not getting uncovered early.  The big chokes do effect idle because they create low/poor air velocity, and thus poor fuel atomization.  Not expecting it to purr like a kitten at idle, just hold a higher idle without loading up.  Hard to settle yourself on the bike at the start if one hand has to keep pi*sing with the throttle.

johno - the Fiddaman will be getting an SOHC, so things will be easier then.  However class rules for this bike call for a twin.

[scottly]

Your chokes aren't that large, relative to the displacement per cylinder. Your chokes are only 28% larger than my 30mm, but you have 80% more displacement.
I suspect a lot of your issues are due to the .50mm accelerator pump jets....

[scottly]

Wondering? Your are at what idle speed with 1 turn out? Is it crap just off idle or while sitting at idle?I think the 50F9 will be giving you more fuel.
 
                                                                              Larry

Welcome to the discussion, Larry. Please share your set-up so far.

[Tintop]

+1

[lwahples]

OK,
     The motor came from a Chop I bought three years ago.Had it running then but only took it around the block  before parking it. While on the engine stand I noticed that it has the Deep oil pan,125 PSI on all four and
punch marks on the cam towers. Making me believe it's been apart.
      After installing the Webers,it took awhile to get the bike to idle. Went
to 50F9's but the mixture screws are at 1 3/4 and the idle screw is turned
up. Still need to fine tune,but so far really like Webers!
                                                                                       Larry

[bear]

One of the greatest engine sounds I have heard in my life was a Honda sohc chair run by a great outfit man Stan Bayliss,  He was warming it up in the pits and had an awesome sound , the crowd all gathered around and then near the end of his warm up session he just give it big mobs of #$%* send heads spinning and hands on ears,     I love that stuff man.

Stan was indeed "The Man."
Steve (son of) has been a great help to us over the years and a sidecar legend in his own right.
Back on topic.
The carbs I have seem to have a bit of wear in the shafts.
I don't think they are a matching pair either.(different serial numbers)
I may be better of with new carbs, thoughts?

Cheers,
Brian

[scottly]

Hey Brian, I'm not sure what you mean by wear in the shafts? The throttle shafts ride in ball bearings.

There are both proper serial numbers, and a "model" number on the carbs; the model number would refer to the OEM jetting for a particular fitment. Are they both marked as made in Italy?

[scottly]

      After installing the Webers,it took awhile to get the bike to idle. Went
to 50F9's but the mixture screws are at 1 3/4 and the idle screw is turned
up. Still need to fine tune,but so far really like Webers!
                                                                                       Larry

Do you mean the idle speed screw is tightened up as far as it will go, or close to it? Those 50 idle jets are pretty big, especially with 1 3/4 turns out on the mixture screws. I'm guessing you've either got a vacuum leak, or very black spark-plugs.

[bear]

Hey Brian, I'm not sure what you mean by wear in the shafts? The throttle shafts ride in ball bearings.

There are both proper serial numbers, and a "model" number on the carbs; the model number would refer to the OEM jetting for a particular fitment. Are they both marked as made in Italy?

Hey Scottly,
I haven't pulled these things appart yet but I can move the throttle valves (butterflies)? up and down in the throttle bodies. So I assumed they had a similar arrangement to the CRs and the shaft or bush had worn.
It would appear to be a model number it's above the serial number.

Cheers,
Brian

I suppose I'm going to have to bite the bullet at some stage and pull these things appart.

[754]

Another great feature of Webers,you can change the bearing, rather than bore out and bush the body..

[johno]

 Hi Bear,
Try this web site www.weberperformance.com.au/         they are in Melbourne and have the best stock of carb parts in Aust.    Cheers     Johno

[bear]

Hi Bear,
Try this web site www.weberperformance.com.au/         they are in Melbourne and have the best stock of carb parts in Aust.    Cheers     Johno

Thanks Johno,
Can these things operate at 14deg mate.

Cheers,
Brian