Tuning Carb flooding woes (26imb 10)

[Albuzz]

I’ve just replaced the head and gasket after the head gasket blew and at the same time fitted a new carb. On firing the engine up for the first time I noticed that fuel was squirting out of the drip tray nozzle. I tried adjusting the mixture with no effect. After some web research it seemed possible that the float or valve may have been stuck but that seemed unlikely as the carb is new. I removed the cover to check the float to find its plastic and not brass. I then read on a website that the float to gasket gap should be 8.5mm if plastic rather than 6.5 for brass. It was actually set to 6.5, so adjusted it accordingly to 8.5 and refitted it and hey presto it no longer floods, but there’s a flat spot when I open the throttle.

I have 2 questions if someone could help me with.

1. Is it correct that the float gap should be 8.5 for the newer plastic floats. I wonder if it it’s maybe a little too big so there’s a slight starvation of fuel hence the flat/sluggish spot when opening the throttle.
2. Can someone tell me definitively whether screwing in the mixture screw is making it richer or weaker. I’ve seen both on the web but most descriptions just talk about the process of setting the mixture without saying which direction makes it richer. I’ve literally spent hours searching!

Many thanks for any information

Alan

 

[bugsymike]

I’ve just replaced the head and gasket after the head gasket blew and at the same time fitted a new carb. On firing the engine up for the first time I noticed that fuel was squirting out of the drip tray nozzle. I tried adjusting the mixture with no effect. After some web research it seemed possible that the float or valve may have been stuck but that seemed unlikely as the carb is new. I removed the cover to check the float to find its plastic and not brass. I then read on a website that the float to gasket gap should be 8.5mm if plastic rather than 6.5 for brass. It was actually set to 6.5, so adjusted it accordingly to 8.5 and refitted it and hey presto it no longer floods, but there’s a flat spot when I open the throttle.

I have 2 questions if someone could help me with.

1. Is it correct that the float gap should be 8.5 for the newer plastic floats. I wonder if it it’s maybe a little too big so there’s a slight starvation of fuel hence the flat/sluggish spot when opening the throttle.
2. Can someone tell me definitively whether screwing in the mixture screw is making it richer or weaker. I’ve seen both on the web but most descriptions just talk about the process of setting the mixture without saying which direction makes it richer. I’ve literally spent hours searching!

Many thanks for any information

Alan

I don't have specific data, though I am sure someone will help, but in general terms and which has stood in good stead since 1969 as a basic starting point for measuring floats I aimed for the thickness of a Biro pen around 7mm for many carbs.
Some Japanese carbs had a glass sight hole with a dot in the middle so you could actually see the level with engine running.
The other thing I always did was with the top off and a clean tube on the fuel inlet, blow through it and turn the carb to over slowly to see when flow stopped and started and if it sealed correctly, as even a brand new carb, needle jet may not do that.
Not specific re your carb. but generally if mixture screw is low down near the throttle spindle, it is a fuel screw as in screw in to weaken, whereas if higher up on the carb body is often a "volume" screw controlling air flow, so screwing in would "richen" if that is a word. Also as you unscrew a volume screw engine revs often increase.

 

[Albuzz]

Thanks for the insight… I’ll probably try reducing the float gap as I’m sure the valve is operating correctly.

Your help is much appreciated

 

[the hobbler]

I’ve just replaced the head and gasket after the head gasket blew and at the same time fitted a new carb. On firing the engine up for the first time I noticed that fuel was squirting out of the drip tray nozzle. I tried adjusting the mixture with no effect. After some web research it seemed possible that the float or valve may have been stuck but that seemed unlikely as the carb is new. I removed the cover to check the float to find its plastic and not brass. I then read on a website that the float to gasket gap should be 8.5mm if plastic rather than 6.5 for brass. It was actually set to 6.5, so adjusted it accordingly to 8.5 and refitted it and hey presto it no longer floods, but there’s a flat spot when I open the throttle.

I have 2 questions if someone could help me with.

1. Is it correct that the float gap should be 8.5 for the newer plastic floats. I wonder if it it’s maybe a little too big so there’s a slight starvation of fuel hence the flat/sluggish spot when opening the throttle.
2. Can someone tell me definitively whether screwing in the mixture screw is making it richer or weaker. I’ve seen both on the web but most descriptions just talk about the process of setting the mixture without saying which direction makes it richer. I’ve literally spent hours searching!

Many thanks for any information

Alan

The Weber instructions (for most Weber carbs) says that if the engine needs the mixture screw out 3 or more turns you will need a larger slow-running jet and if the mixture screw needs to be in 2 or less turns you need a smaller slow-running jet. Therefore my interpretation of the Weber instructions is that screwing it out makes it richer and screwing it in makes it leaner.

 

[bugsymike]

Online search:-
"What is the difference between fuel screw and air screw?
The air screw is on the side of the carburetor and meters the amount of air that makes it to the pilot-jet nozzle. A fuel screw is located underneath the float bowl and meters the amount of fuel that makes it to the carb's main body"

As an apprentice most vehicles used mixture screws that I worked on BMC, Ford, Vauxhall etc. and were often older Solex carbs I seem to recall, for me it was only later on cars like Citroen GS 1220 Clubs etc. that I came across volume/air control screws on their later twin choke Solex carbs.
So as @the hobbler mentions most Weber carbs use a mixture screw and screwing in will weaken/make leaner the fuel/air ratio on idle.
I did own a 1967? Peugeot 607 V6 Automatic with a twin choke Solex and a extra single barrel Solex carb for the choke/cold starting before the advent of injection on later versions. It was a lovely car to drive, but on short journeys evil on fuel.

 

[Albuzz]

Thanks to bugsymike and the hobbler for your help.. it does confirm my thoughts but I had previously found conflicting views and I couldn’t find anything definitive! SU documentation by contrast is quite clear about which direction is required to make the mixture leaner or richer, where Webber’s is not! Thanks again.

I’d be grateful for any further views from anyone re: the plastic float gap )