Weber DCOE 40 151 Fuel Level

[alfadave]

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.

[ericbushby]

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

[alfadave]

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

[alfadave]

PS
Like you Eric, I won't add the jet set to my shopping list, as I've never been a member!

[alfadave]

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

[mbell]

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.

[Grizzly]

Out of interest, do you know where in the rev range your car was running rich with the 25mm fuel level?

I have changed my set up to Keiths VF Tube's, W jets, 36mm chokes, 25mm fuel level and it runs great (between 12-13 afr all the way through the rev range and pulls like a train with no stumbles or flat spots) the problem i have found is cruising in a high gear and low rpm drops right into the 10 afr range. It was suggested the problem may have been down to the idle jet being too big so swapped it for the size down, now i have a cruise in the 12's but my on throttle afr is touching 14 at times (before the mains tip in).

I think my next step is to refit the W50 jets and drop the fuel level, the only problem i have with that is the VF tubes are made to run at 25mm.

[prezoom]

I have been running the 25mm fuel level in my S2 with 18's, 30mm chokes, since 2006 with good results. Also running 25mm on my 152G's, 5 progression holes, 38mm chokes, in the Plus2 with even better results. The Plus2 will be getting the VF tubes and kit, with a visit to Keith during the West Coast Lotus Meet. If everything works as expected, will be bringing back a second package for the S2.

[mbell]

Out of interest, do you know where in the rev range your car was running rich with the 25mm fuel level?

This was at least ~9 months ago (possibly 21 months..) so not really. I think it was across what you call the cruise rev range, so 2k to 4K sort of range. I was able to tune it out by adjusting the air bleeds on my hyper jets that got me back to better AFR.

From memory I am running H20 idle/main jets (i think these are 50 thou equivalent), 125 main jets with his O3 Emulsion tube I think. With 34mm chokes, what ever pump jets came in the carbs. I originally had H18 jets but couldn't get rid of transition stumble with them. The H20 set up allows me to get tune that out but seems sensitive enough that I have to re tune between summer and winter fuels! I have to lean them out in summer to avoid rich AFR but then the stumble comes back with winter fuels. It might be there is bigger change in the two fuel mixes in this part of the world due to the high summer temps.

Have you tried adjusting the air bleed on the idle jets? The air bleeds should allow you to trade idle mixture v off idle mixture, so hopefully lean off the off idle mixture and use idle adjustment screw to keep idle mixture right. I tend to just lean them out until the stumble is present and back them off one. Checking with AFR gauge that they are about right.

[Grizzly]

To be quite honest i'm only just really getting into setting the w jets and VF tubes up (VF tubes were a doddle), the W50 jets currently have all the holes open which gives me a solid 12.5afr wot and slowly accelerating (progression mixture is almost seamless into the tip in with no detectable stumble). I have just bought a set of W45's but not really had chance to play with them, the little i have done fixes the cruise but at the expense of the stumble and progression mixture.

The reason i asked about the fuel level is there are quite allot of posts regarding 240Z owners struggling with the 25mm level, i think it's due to the angle the carbs are fitted at but one of their symptoms is a rich cruise with a decent progression afr under throttle (like mine).

I'm sure i will be able to dial it out with the W45 jets.

[prezoom]

When I purchased the intake manifold from Dunnell Engineering for the Zetec, I didn't check to see if the manifold mounted the carbs at the same angle as they are with the twin cam. Turns out, they mount the carbs horizontal, rather than at an angle. The Zetec has absolutely no hesitation from idle to 7k. I can drive the car down to below 1500 rpm in top gear in town and pull away without dropping a gear. Not quite so lucky with the S2, It has a very minor stumble around the 1500 to 1800 mark, but it is so minor, I think I am the only one who notices. The 240's carbs are also mounted horizontal, but have only driven one equipped with 44mm Mikuni's,which had no hesitation or stumble. Makes me wonder if the carbs they are using do not have the 5 progression holes. I wish it was as easy to set the float level on the Webers as it is with the Mikuni's. Keith's original white paper had using the acrylic rod to set the levels. Mikuni as a very similar tool to do the same, though trying to find one is like trying to find frog hair.

[ericbushby]

It is simple to check the fuel level on the Webers if you do it this way.
Remove the jet cover and one large jet stack.
Use a dipstick with a mark on it at the correct distance from the rim of the jet cover.
Lower it in watching with a small torch and the fuel shivers as the stick touches it.
On mine 25mm from the top of the fuel chamber is 41mm from the edge of the jet cover.
I use a depth gauge as found on the end of an ordinary Vernier caliper to do this but a cocktail stick with a mark at 41mm works almost as well.
Eric in Burnley
1967 S3SE DHC

[Grizzly]

Don't get me wrong i'm becoming a master at setting the fuel level (digital vernier caliper with a very small led torch taped to it and watch for the circle as it starts to break the fuel surface tension, that's my method of choice) i just wanted to know if the fuel level made a difference to the cruise without having to spend an hour resetting it afterwards.

[ericbushby]

Yes Chris I agree.
It doesn`t take long to measure the level that way. I just wish there was a quick way to adjust it for experimenting.
Eric

[mbell]

I just wanted to know if the fuel level made a difference to the cruise without having to spend an hour resetting it afterwards.

As ever with weber there isn't a simple answer. It depends what "phase" the carbs are in when cruising, if they are in the transition phase (both circuit providing some fuel) then lowering the fuel level might be right. As it will delay the main circuit engagement, so should lean it out a little. Another approach is to lean off the idle jet at higher RPM using the air holes.

An easy test is probably to remove the main stacks and go for a little drive. If you can cruise at same or close to engine speed and mixture is better or too lean, then you know you are in the transition phase. If you can't anywhere close to cruising RPM, then you know you were on the main circuit and should look at that. If you can go a higher RPM before it leans out then your probably still on idle circuit and should look at the idle jet air holes.