Carb flooding problem

[nospark]

I've got 1 3/4" Su's fitted. Before the winter layup I was getting an infrequent flooding of the rear carb. Since its been back on the road after winter its much worse. It can happen at tickover but its more likely after a bit of revving.

I've got the float at the right height. The needle and valve are tested and sound. Its as if the float sometimes gets overwhelmed by the amount of fuel fed into the rear carb float bowl. Two thoughts:-

1)The mechanical fuel pump is bad. The in / out valves in the pump are supposed to give a constant fuel feed no matter what the engine speed (according to the works manual). I wonder if the ethanol has got to them and made them go bad

2)The fuel return rail/pipe is blocked and therefore putting extra fuel pressure on the rear carb.. Is this realistic ?

Any thoughts would be welcome

 

[jonbills]

I know you said the needle valve is tested and sound, but I think it's the needle valve. or float level. something specific to the rear carb anyway.
The back carb won't consistently see a different pressure to the front one, so both of your scenarios would result in flooding on the front too.

 

[nospark]

Yesterday I fitted a brand new lid/float/needle/seat (came as one unit) from Burlen to try and sort this problem. The float height was set correct. It flooded. I have just fitted a spare old style float with steel needle that I have as one of several spares. See how that goes but I will be surprised if that resolves it..

The fuel return rail "turns back" to the tank at the rear carb. If its sometimes blocked then the rear carb is going to get this added pressure and so releases the pressure by flooding the rear carb. This pressure release happens before it has a chance to feed the pressure back to the front carb. (I am just guessing and might be talking c**p. Not the first time....)

Thanks for your obs

 

[Rob Gaskin]

Check your float for a minute hole - had that problem myself.

Edit - Ok we were typing at the same time!

 

[jonbills]

I don't know the SUs so well - can you swap the needle valve/float assemblies front to rear & rear to front? if your theory is right the rear will still flood, whereas if I'm right the front will then flood.
or something else will happen to prove we're both talking cr*p.

 

[Farmer42]

This has got my interest.

I posted yesterday about a carb problem following the completion of my engine installation. Every time I revved the engine slightly, it would cough and then die. At first I thought it was fuel starvation but then Jonbills gave me a thought that it could be the other way round. My problem was also the rear carb.

I disconnected the choke cables in case they made the choke stick on. I watched closely with the engine on a fast idle and when I moved the throttle, shed-loads of fuel bypassed the needle and flooded the carb, You guessed it, the engine died.

Checked the floats and needle valves and even swapped them round but still flooded. I then isolated the rear carb and ran it on front only and the engine revved without cutting out albeit a bit sluggish as expected.

I therefore reckon it is a worn needle and jet assembly on mine letting too much fuel through. I did try altering the mixture but that didn't work. I have now swapped carbs over for the ones on my old 2 litre engine which are really too small but at least it runs and drives.

Perhaps it's the same problem and we both need to buy a carb rebuild kit.

 

[nospark]

Farmer
Two things to check on yours.
1. The float level is not set too high. Too high will flood the engine because the petrol level in the float bowl will be above the jet opening within the carb body.

2. There is an air vent at the top of the float bowl on SU, s. This is also an overflow hole but it's main purpose is to release air pressure above the float to allow it to rise up and close the needle. Make sure this hole is not blocked

 

[Jimbo]

i have 1 3/4 SUs as well and when i changed the float needles from a service pack the rear one leaked and over flowed the bowl.
when put the old one back in problem was solved.
these needles are a wear item but even new ones can be no good.
have you set the float heights? how did you do it?

 

[Jimbo]

can you take the return pipe off and feed into a bucket or if you have a long enough bit of hose back in through the fuel flap? this will see if you have too much back pressure in the return pipe.

 

[nospark]

Jimbo
you set the float height on SU's by turning the lid/float unit upside down. Take a drill bit (i use 4mm) and adjust the float so the gap between the lid and the edge of the top of the float is correct.

A good book is Haynes "SU carburettors" workshop manual.

The modern kit is the black "stay up" floats and Viton (rubberish) needle. There are reports on the web about the reliability of the viton needles. Ive just put an old style steel needle back in and white float and will test

Plan to take a srynge and inject fuel through the return pipe if necessary

 

[Farmer42]

i have 1 3/4 SUs as well and when i changed the float needles from a service pack the rear one leaked and over flowed the bowl.
when put the old one back in problem was solved.
these needles are a wear item but even new ones can be no good.
have you set the float heights? how did you do it?

The link below gives a good description of how to set the float level. I'm going to give it a try although I thought mine was set correctly using the method NoSpark described.

http://datsunzgarage.com/engine/