Lotus Elan

Weber DCOE 40 151 Fuel Level

PostPost by: alfadave » Sat Oct 13, 2018 7:57 am

Looks like I need to go shopping!

Pressure regulator
Colour tune
AFR meter
Endoscope
Fuel pressure gauge
Acrylic rod

Being old fashioned, all I want to do, is bend the tabs to set the fuel level to a height where the car will run for 30 miles, without me having to take the plugs out, and wire brush them!
The carbs have been set up on the rolling road (twice)

I would buy an acrylic rod, but Webcons technical guy said they didn't stock them.
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PostPost by: ericbushby » Sat Oct 13, 2018 8:42 am

Hi Dave. Even though It was me who persuaded you to set the fuel level from the top in the first place, I also had the engine fouling plugs.
I changed the idle jets from 50/F8 to 45/F8 and got some improvement, but still not good enough.
I am now using 45/F9 and am now experiencing the onset of a stumble when lifting off idling. It is only very slight and not noticeable unless you are looking for it so I cannot go any further that way.
Back to Keith Francks advice, he says choose the weakest idle jet until the engine stumbles, go back one and then fit hotter plugs until they don`t foul up.
I am now using NGK BPR5ES and it is no trouble now from one month to the next. Plugs are dark brown and dry.
Also, Keith no longer advocates the acrylic rod method even though it was his idea. He now recommends a simple dipstick.
The other approach to the problem is to buy the jet sets from Keith which he says cure all the problems with Webers. I have insufficient courage to go that way.
Keep going and report back please.
Eric in Burnley
1967 S3SE DHC
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PostPost by: alfadave » Sat Oct 13, 2018 9:29 am

Hi Eric,
the guy on the rolling road told me it was set up as lean as he could get it.
He put a set of 5's in, but still didn't fix it.
Its a big valve head/711M block.

However I'll confirm the jet sizes, when I can.

As 2 adjacent plugs were more sooted up, I still feel I've got a fuel level problem
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PostPost by: alfadave » Sat Oct 13, 2018 10:14 am

PS
Like you Eric, I won't add the jet set to my shopping list, as I've never been a member!
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PostPost by: alfadave » Thu Feb 14, 2019 5:17 pm

I found some interesting on line info:-

" Weber DCOE Carburetor Reference,Theory,Configuration,Tuning,Modifications".

Relates to Datsun 240 Z etc it seems.

The paragraph above fig 55 talks about fuel level of 29mm below the cover, not 25mm.

I've set mine to 29mm, and seems an improvement in running, and no plug fouling
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PostPost by: mbell » Thu Feb 14, 2019 7:55 pm

Glad your making progress.

The fuel level is another one of the tuning parameters on weber carbs. It generally used to control when the main circuit starts providing fuel (during transition period between idle/slow jets and main jets). The main circuit works by using a venturi to create a low pressure area that "sucks" fuel up and out of the main jets. The more airflow though the venturi the lower pressure region is created.

When you lower the fuel level you are increasing the amount of vacuum (low pressure) required to "suck" the fuel up and out of the main jet. Therefore you are delaying the point that the main circuit starts working.

Normally the twink cam has problems with going lean during the transition stage, so raising the fuel level as high as possible and getting the main circuit working as soon as possible is the right thing to do. Hence most people set the fuel level it as high as possible (25mm) as it really helps avoid the stubble effect triggered by lean running during transition.

However depending on various factors (both carb settings and engine) a high fuel level could cause the main circuit to engage to soon and generate very rich mixture during transistion. Lowering the fuel is likely the right solution for this, providing that the mixture is right when running on the idle/slow jets (low revs) and when running on just main jets (high revs). I.e. the plugs only foul in the mid range.

If you still get fouling or running issues at other revs I think you possibly still have a un-diagnosed issue going with the carbs. Personally I think a Air Fuel ratio gauge is well worth the cost in these situation, if life permits. Being able to track see the mixture through the rev range and when the plugs foul will really help.
'73 +2 130/5 RHD, now on the road and very slowly rolling though a "restoration"
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