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Weber 45 over rich cruising

14K views 38 replies 11 participants last post by  slowcreek 
#1 · (Edited)
I thought it was a good idea to question the collective wisdom of the bb in order to understand if what I observe is a inevitable/expected issue or if there is a fix that has been already found by others.
I checked in many bb threads but I didn’t find a straight answer.

I run a modified ‘fast road’ 1.8 litres carburetted twin spark on my GT Junior step front.

The carbs are the 45 DCOE 152 ‘Spanish’ Webers with the 36 standard venturis. They are brand new and carefully balanced.
I checked all the car’s settings, advance, cams timing etc and the engine pulls strong with no high end problem whatsoever.
I have installed a AEM UEGO meter to keep things in check.

With several years of practice and long readings of the expert’s texts I learned how to get rid of the infamous stomping just off the idle, the flat spot at 2000 rpms that we usually have with the modified/high lift/longer duration cams.
In my case this involved the drilling of the forth progression hole, and the use of somehow rich idle jets, like 60F9 for example, and a lot of road testing.

Now the issue is that if I set the car for strong acceleration with no ‘flat spots off the idle’ I get over rich mixture at cruising, for example at 120 km/hour.
This is around 4000 rpm with the gas JUST opened. The car runs at 10 ratio and it shows the typical hesitations we see at over rich steady driving.

If I fix the issue, for example by going to 55F8, I have a nice driving experience, no over rich mid range cruising and by the way, better mileage. But the flat spot in acceleration just off the idle is back.

I understand Webers are not designed to distinguish between a status where the butterfly valve is just opened because the light became green in town or because you are calmly driving on the highway.
Nevertheless I wonder if someone has found a fix to this issue that could be circulated here.
 
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#30 ·
Thank you Ed.
This is very similar to my setting.
I cannot go leaner than 60F9 at present, not that I haven't tried all sort of tricks.
This gives very torquey acceleration at slow opening of the gas (no lean spot) but at the same time is rich at cruising.
I also always found impossible to tune my engines with F16 emulsion tubes for street driving.
Take care
 
#32 ·
There’s a guy named Keith Franck who has undertaken the Herculean task of rethinking Weber tuning. While there have been several false starts and blind alleys, some of what he’s done works very well.

I don’t have a link at hand, but you can probably find him via a search. He’s over near the Bay Area.

Most of his work is focused on refining the control in the transition zone. He also works on progression holes, and has developed a tunable idle jet.

My first effort used his “VF” tubes with remarkable results. I’ve been too busy to get up to the dyno for further tuning, but it’s in the cards. My first effort produced a 50% improvement in slow speed economy with a noticeable increase in mid-range torque.

The poor economy of Webers around town is largely related to a broad and poorly defined transition, resulting in a rich mixture right in the 25-50 MPH range used for city driving.

Keith is now up to, I think, four different replacements for the classic E-tube, depending upon the ratio of engine displacement to Venturi size, plus how strong is the engine’s manifold vacuum.
 
#34 ·
I don't know much about the fine tuning details of weber's but dellorto idle jets go up in increments of one. 55 56 57 58 59. Also they have a comprehensive selection of idle jet carriers. Such small adjustments have helped me fine tune a few of my own engines. Listen to alfaparticle's advice. I think he's the best source of advice here when it comes to webers.
 
#35 ·
It would be great to fit a throttle position sensor to one of the carbs and connect it to a data logger along with a wideband O2 sensor and a tach input. That way you could see the throttle position for the lean spot and open up the corresponding progression holes a little at a time. My Data logger will accept such an input but I don't have a way of fitting a throttle position sensor since a previous owner of the carbs milled off the potential mounting points. I tried to install a linear transducer on the linkage but I was unsuccessful. I know that Jim Steck has logged Weber throttle position during dyno testing.
 
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