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herbie1200
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Location: Rome - Italy
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 Posted: Thu Jan 15, 2015 2:20 am Post subject: Level of fuel in carb bowl: how to set & what to check |
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Never found a topic for those fundamental question.
My orig 1963 has its original 28 carb. My two main question:
1) how can I check (if I would know which is the correct value) the fuel level into the bowl? Removing half carb upper part is not useful because the 'real' fuel level is measurable only when upper carb half is present, so the needle pushes down the float and the fuel level grows until the level it mantains during normal running. Level without the float has no relation with the level when engine is running.
2) why fuel level should influence air/fuel mixture? From what can I see on tech doc all fuel during normal cruise has to go through the main jet, and the main jet is in the bottom of the bowl, no chance to suck air instead of fuel. So, why literature is so 'clerical' to recommend to check fuel level?
Thank you |
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[email protected]
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Location: Louisville, ky
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| Posted: Thu Jan 15, 2015 2:52 am Post subject: Re: Level of fuel in carb bowl: how to set & what to che |
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| herbie1200 wrote: |
2) why fuel level should influence air/fuel mixture? From what can I see on tech doc all fuel during normal cruise has to go through the main jet, and the main jet is in the bottom of the bowl, no chance to suck air instead of fuel. So, why literature is so 'clerical' to recommend to check fuel level? |
The deeper the fuel the more pressure on that bottom outlet and the more fuel is sucked in by the vernturi vacuum.
I will watch this post for a good answer to 1). I have always read to pull the top of the carb and measure from the top. I am assuming, which makes an ass of u and me, that the instructions telling how far from the top already took into consideration that the float is no longer in the liquid. But I will be more than happy to sit back and learn. |
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Aussiebug
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| Posted: Thu Jan 15, 2015 5:46 am Post subject: |
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1. With the top off the carburettor, the fuel should be 12-14mm below the lip of the float bowl (source - Haynes Manual).
As noted above, the fuel level is important as the height of it is balanced with the vacuum generated when air flows through the throat, fuel into the throat - if the fuel level is too high, you get a rich mixture, and it too low, a lean mixture.
the fuel height is controlled by placing washers under the needle valve. The normal washer thickness is 0.5mm. A thicker washer will lower the fuel level and a thinner washer will raise it.
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Rob and Dave's aircooled VW pages
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MacLeod Willy
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Location: Ontario Canada
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| Posted: Thu Jan 15, 2015 6:33 am Post subject: |
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| Aussiebug wrote: |
1. With the top off the carburettor, the fuel should be 12-14mm below the lip of the float bowl (source - Haynes Manual).
As noted above, the fuel level is important as the height of it is balanced with the vacuum generated when air flows through the throat, fuel into the throat - if the fuel level is too high, you get a rich mixture, and it too low, a lean mixture.
the fuel height is controlled by placing washers under the needle valve. The normal washer thickness is 0.5mm. A thicker washer will lower the fuel level and a thinner washer will raise it. |
x2 what Aussie says.
If the fuel is too high in the bowl it will cover some of the holes in the emulsion tube.
This tube introduces air into the mixture as it enters the venture that makes the fuel vaporize quicker.
Fuel does not burn, only vapors ignite. More holes covered the richer the mixture.
The perfect mixture is 14:1 air/fuel. This burns most efficiently and gives you the most bump.
Think of a hole in a straw above the level of pop in a glass, you get bubbles and required to draw very hard to get pop (Soda depends where your from  ) Imagine multiple holes in the straw you would get very little fluid.
Now if you raise the level of the pop above the holes you would get all fluid, no air = very rich.
You can lean out or enrichen any range of the throttle position by changing any of the settings of jets or tubes in the carb.
Now Im sure there are going to be guys chiming in on boosters and charges but Ive never been involved with them, YET 
Well that's my 2 cents
Believe it or not most of your driving is just off idle |
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herbie1200
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Joined: April 27, 2006
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Location: Rome - Italy
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| Posted: Thu Jan 15, 2015 7:54 am Post subject: |
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For number 2) your explanation is OK, air/fuel % depends on fuel level.
And I have to observe the needle is near the fulcrum of the bowl lever, so 1mm of washer thickness should modify fuel level of a 3-4 mm.
For number 1) I'm still in doubt: the 12-14mm have to be measured with the float IN or OUT of the bowl?
Needed value is different. I'm thinking about me and a full bathtube: with or without my body, water level is not the same.
The float is... floating, but a part of it is into the fuel, modifying the level. |
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MacLeod Willy
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| Posted: Thu Jan 15, 2015 1:33 pm Post subject: |
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| Without the float |
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bluebus86
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| Posted: Thu Jan 15, 2015 4:34 pm Post subject: |
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keep in mind that fuel pump pressure can have a big effect on bowl fuel level.
If you have more fuel pressure than your supposed to, the pressure fights the float pressure and over comes the float valve until the float sits lower in the fuel, exerting more pressure than before on the valve to stop the flow. Thus the float sitting lower means the fuel level is HIGHER than when the fuel pump pressure was correct.
Thus if you just replaced the pump and now find your bowl is over filling (or under filling) you should check the pump pressure to see if you should adjust it by shims or push rod length. With some aftermarket electric pumps, a fuel pressure regulator is required to get the pressure low enough for the stock VW float bowl pressure specification.
So check the pump pressure if in doubt before altering carb. as other poster mentioned, you can change the float valve washer thickness(s) to adjust the valve and thus fuel level height, add washers to reduce bowl level, subtract them to increase bowl level.
Also check float for leaks, dip in hot water and force it down with a tool (the water is hot) wait a minute and if bubbles appear you got a leaker, replace it.
Use hot water because the heat will cause the fuel that got into a leaked float to expand and push out as bubbles with more force than if cold water was used. Using cold water would cool the inside of the failed float, causing water to be sucked IN, and thus no bubbles would appear from the leaky float. The hot water soak will force a leak outwards so you can see the bubbles.
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Last edited by bluebus86 on Thu Jan 15, 2015 5:29 pm; edited 1 time in total |
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MacLeod Willy
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| Posted: Thu Jan 15, 2015 5:16 pm Post subject: |
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| bluebus86 wrote: |
| . it is a one to one ratio, 1/16th inch washer will change level of fuel 1/16th inch. |
Really?
With the pivot being so close to the needle contact there would be a greater travel than 1:1 I would think |
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bluebus86
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| Posted: Thu Jan 15, 2015 5:28 pm Post subject: |
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| MacLeod Willy wrote: |
| bluebus86 wrote: |
| . it is a one to one ratio, 1/16th inch washer will change level of fuel 1/16th inch. |
Really?
With the pivot being so close to the needle contact there would be a greater travel than 1:1 I would think |
You know you might be right, I am thinking of a British carb a Stromberg carb. It has a wing off the float arm that makes the pivot point centralized from the float to the valve, thus it is a 1:1 ratio. It is on my mind cause I am working with a friend on his 72 Lotus. Different animal, sorry.
I will correct it
I better watch what side of the road I drive on whilst I tinker with this Lotus
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Help Prevent VW Engine Fires, see this link.....Engine safety wire information
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MacLeod Willy
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| Posted: Thu Jan 15, 2015 5:32 pm Post subject: |
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Hey everyone likes a Lotus!!
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MacLeod Willy
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| Posted: Thu Jan 15, 2015 5:37 pm Post subject: |
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| Actually, that 1:1 would be so nice for adjusting the float level. Saves wearing out the top screws |
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kevhum
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| Posted: Thu Jan 15, 2015 5:48 pm Post subject: |
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heres A page from A factory workshop manual
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MacLeod Willy
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| Posted: Thu Jan 15, 2015 6:02 pm Post subject: |
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| kevhum wrote: |
| heres A page from A factory workshop manual |
That's cool! I see they have the float left in. Have to see next time if it actually changes the level when removed. I doubt it if its floating |
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herbie1200
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| Posted: Fri Jan 16, 2015 2:55 am Post subject: |
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OK now it's clear!
The value is 12-14mm from the fuel level to the top of the bowl, with the float still inside. This is logical: when engine is running, the float is there.
I can confirm your answers to my point 1) (fuel level affects air/fuel ratio) after adding a washer to my needle, making the mixture less rich, after 30-40 minutes of running I noticed:
- interior part of exhaust pipes now is of a light brown, like a nut; before the 'washer' it was almost black
- to have a stable idle I had to enrich idle mixture by opening idle screw of 20-30 degrees, so the fuel level has effect on idling too. |
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MacLeod Willy
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| Posted: Fri Jan 16, 2015 7:11 am Post subject: |
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| herbie1200 wrote: |
OK now it's clear!
The value is 12-14mm from the fuel level to the top of the bowl, with the float still inside. This is logical: when engine is running, the float is there.
. |
The float really should have little or no effect with the carb top off. The float sits on top of the fuel and is not forced down into the fuel by the needle to raise the level..
Seeing your in the process right now, please try it both ways float out and in NOT FORCED DOWN INTO THE FUEL
Will you post your findings? |
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highroller
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| Posted: Fri Jan 16, 2015 8:24 am Post subject: |
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| Would this information apply to a 34 pict3? |
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MacLeod Willy
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| Posted: Fri Jan 16, 2015 3:37 pm Post subject: |
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| Unless the rebuild kit says otherwise 34 as well |
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Aussiebug
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| Posted: Fri Jan 16, 2015 9:49 pm Post subject: |
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| MacLeod Willy wrote: |
| Without the float |
No No No.
To Confirm, from the Haynes manual...
"The level of the fuel - with the float in position - should be 12-14mm below the top edge of the float chamber."
When you remove the float the float chamber will be only about 1/2 full.
All Solex carbs for VW use the same fuel height.
A small change in the washer thickness will make a larger change to the fuel height, because as noted above, there is a leverage action... the needle valve is closer to the pivot pin than the float. If it were NOT so, removing the stock 0.5mm washer completely would make only 0.5mm difference to the height, and that NOT what happens - removing the 0.5mm washer completely would make about 2mm difference to the fuel height.
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Rob and Dave's aircooled VW pages
Repairs and maintenance for the home mechanic
http://www.vw-resource.com |
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MacLeod Willy
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| Posted: Sat Jan 17, 2015 6:55 am Post subject: |
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Bad call on my part on that one 
Last edited by MacLeod Willy on Sat Jan 17, 2015 4:14 pm; edited 1 time in total |
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MacLeod Willy
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| Posted: Sat Jan 17, 2015 4:13 pm Post subject: |
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| Aussiebug wrote: |
| MacLeod Willy wrote: |
| Without the float |
No No No.
To Confirm, from the Haynes manual...
"The level of the fuel - with the float in position - should be 12-14mm below the top edge of the float chamber."
When you remove the float the float chamber will be only about 1/2 full.
All Solex carbs for VW use the same fuel height.. |
How are you suppose to have the float in position without the needle???
Without the lid on, The float is just that floating , not submersed isn't it?
If you push down on the float how high does the level move to??
Push it all the way to the bottom and what happens? |
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