Tuning Weber Carbs

[ian balson]

I recently had a problem where my car (VX) decided to work (poorly!) on 3 cyls. It turned out that the front cylinder was not firing.

 

After cleaning the spark plug, all was well again. I then inspected the remaining spark plugs I found the front two were rather

black in appearance in comparison with the back two (grey/brown in appearance).

 

I guess this means that the front Carb is out of tune? As you may have guessed by now, I'm no Keith Duckworth !

 

Can anyone recommend a good source of information (book/www) to help me learn how to tune them.

 

 

Many thanks

Ian (HPC150)

[oilyhands]

There is an article on tuning Webers on this website

 

http://members.aol.com/DVAndrews

 

follow the link.

 

There is also a jetting program which will get you close to the right jetting if working from scratch.

 

Des Hammils Weber/Dellorto book in the speedsport range is worth having as is the official weber book.

 

Oily

[Simos]

Hi Ian,

notwithstanding messing with the carbs to gain understanding, to tune it properly it really needs to be done on a rolling road where it can be run through the revs under load.

 

Roger King did mine 01992 421111 (Herts) and did a great job of mitigating the flat spot at 2k whilst getting the mixture right for 7-8k. There's no way you can do that by just looking at the specs and reading the books, you need to run it up and see what the combustion (emmissions) are actually doing.

 

Frankly its about the cheapest horsepower add-on you can do

 

Cheers, Simon G

[stevefoster]

RR is the best way.

Roger K is without doubt very good at doing RR etc.

 

 

[Graham Sewell]

Sounds as if the carb balance has gone adrift. To do the job properly, you can either practice with a piece of hose and listen to each barrel and adjust the balance screw until they sound the same, or by the 'synchrometer' from Demon Tweeks or good motor factor.

 

Once the carbs are balanced (at around 1000rpm), you can move onto the idle volume screws as per the build manual or Dave Andrew's web site. One tip htat I found useful for the idle volume, was get the engine speed to cause something in the engine bay to resonate. Then you can spot very small increments in engine speed by ear!

 

Cheers,

 

Graham

[I.Mupferit]

I found Des Hammils book absolutely invaluable and allowed me to get the jetting very close to ideal before final fine tuning on the rolling road. It can be a bit laborious to set up these carbs. properly but well worth it when you get it right.

 

Brent

[ian balson]

Thanks Chaps,

 

I met a friend on Sat night who was a ex Honda mechanic. He owns a set of air flow meters, so I now know all about Carb balance (and how far out mine were!).

 

HPC150 is now running much smoother. I think the mixture is still a bit rich, so I'm going to have a play with the mixture next.

 

Thanks

Ian(HPC150)

[STEVE GILBERT]

For Idle mixture use a Gunsons Colourtune, why more people dont use this 'window' is beyond me. You may be surprised at how small an adjustment on the Idle mix screw can make to the ideal (royal blue) colour. When all 4 are set then adjust the balance between the pair with your Vac Guage. The advantage of the colourtune is your not relying on any experience or dark art as you can see the combustion actually happen. Mind you it's no good for anything other than Idle but if your jets are right it will still make a big improvement

[Graham Sewell]

I found that a ColorTune would get me in the vague ball park, but because twin webers are not known for their even idling, the set point was difficult to get right. This was improved by using the idle resonance method I suggested earlier.

 

However, if you are using near standard idle jets, the mixture screws set to 2.5 turns out will be in the rough ball park and then can be fine tuned by ear.

 

Cheers,

 

Graham