Float chamber levels in twin SU carb setups

[phatgixer]

If they are not level (by say, 4mm or so) how much will affect performance?

Have leaky banjo bolts, too. How tight should they be against the fibre washers?

Something not right with fuelling on my 1966 Alvis TE21.

Rebuilt the pump, so not that.

[Steve_D]

phatgixer said:

If they are not level (by say, 4mm or so) how much will affect performance?

Yes it would matter but why would you not set them to the correct level?

phatgixer said:

Have leaky banjo bolts, too. How tight should they be against the fibre washers?

First start by fitting the banjo without the washers to make sure the banjo bolt screws right in without bottoming in its thread. Check all faces for flatness and no damage. Replace the washers.
Tightening is a touchy, feely, thing and impossible to describe. If you have done the above there is no other reason for it leaking so if it does then tighten a bit more.

phatgixer said:

Something not right with fuelling on my 1966 Alvis TE21.

Rebuilt the pump, so not that.

Can't help on that.

Steve

[stevieturbo]

phatgixer said:

If they are not level (by say, 4mm or so) how much will affect performance?

Have leaky banjo bolts, too. How tight should they be against the fibre washers?

Something not right with fuelling on my 1966 Alvis TE21.

Rebuilt the pump, so not that.

And are the fibre washers sound, and the banjo faces flat ? Fibre washers would just be nipped up, definately not TIGHT

[phatgixer]

Thanks guys

I have ordered new washers and banjo bolts from the (excellent) SU website. Hopefully the leaking will stop. They are done up much too tight at present so change is needed. All faces are flat and clean, but the washers are very old....

The reason the float levels differ is that at some time in the past one of the carburettors has been rebuilt using different parts for the needle valve actuator arm and the are set at differing heights. I am not sure which is the correct one! The car idles nicely sometimes, and lumpily at others (all new plugs / leads / points / cap / coil and timing is right) and when cruising will happily accelerate for a few seconds then suffer what feels like fuel starvation almost as if there is too little fuel in the chambers?

It also pinks quite badly when hot, but I put that down to needing a jolly good de-coke. I was hoping there is a slim chance that one of the carbs is running lean due to float level being too low and causing the front or rear three cylinders to run a bit hot, exacerbating the problem?

Or have I got a vacuum leak?

Gawd, I like things to run nicely!

[Steve_D]

If things are old then I would check/replace any rubber fuel hoses particularly on the suction side as I have seen many instances where the inner skin of the hose collapses when the flow increases.

Steve

[phatgixer]

Steve_D said:

If things are old then I would check/replace any rubber fuel hoses particularly on the suction side as I have seen many instances where the inner skin of the hose collapses when the flow increases.

Steve

oooo, there's a thought!

[Pumaracing]

What you are after achieving is the same OE specified level of the fuel in the float chamber in both carbs. Different components might well make a difference to the height of the top of the float relative to the top of the carb flange but this isn't what you should be measuring if things are non standard. Set the standard carb up to spec and then set the other one up so the fuel is at the same height before the needle valve opens. By definition both carbs must then operate in the same way.

Easiest way to do this is take the carbs off and mount them level, connect a can of fuel sat a foot or two above the carbs to the inlet pipe and let both chambers fill until the needle valve shuts. Measure the fuel height from the carb flange with a vernier caliper and adjust things until both are the same.

Or just fit standard components in both carbs and save yourself the buggering about.

[phatgixer]

So the height in the chamber is critical to good running?

[Steve_D]

phatgixer said:

So the height in the chamber is critical to good running?

Yes very much so.

Steve

[perdu]

Steve_D said:

phatgixer said:

So the height in the chamber is critical to good running?

Yes very much so.

Steve

And the condition of the needle valve is important too

If it is grooved it will alter the fuel level at which the float shuts its flow off and may even affect the amount of fuel going through each carb if it is allowing fuel to dribble instead of shut off

Not common but if one has been bustardised the needle valves may not be similar for wear

Haynes do a good-ish manual for SUs, no 299, has lots of info for various versions

[Pumaracing]

phatgixer said:

So the height in the chamber is critical to good running?

Seriously? What do you think the OE float height setting specification is there to ensure? The height of the float itself or the level of the fuel?

[bemused, wanders back off into his lair]

[460ci]

Blimey, stop faffing about, the fuel level is important but not that critical.
I have set up more of these things than I can remember and it is simple, you have answered the possible problem yourself, different components fitted in each.

Get a spindle service kit for both, ( I assume they are HS 4’s) but there isn’t much difference to the novice till you get to HIF’s, get a jet and needle kit for both and fit.

You now have a level platform to work from.

To set you floats just try this
Remove the float bowl covers, clean the float bowls, remove old sediment, turn the cover upside down, and get a 1/8" in drill bit, set the drill bit across the cover, the float tab should just touch the bit. Make sure the needle is moving and seating properly. This is just like *most* floats. Replace the cover. On some old floats they have a small bras tab you can gently bend, some are plastic and don’t have any adjustment. If the fuel dribble out of the overflow when the pump is running then you may need a little more adjustment

You can check for matching float settings, after setting the mixture, by removing the pistons, and peering down at the jets. The fuel level should be about the same on both carbs, a little below the top surface of the jet. (After car has been run only). Wash out the old in the dash pots and fill with the correct oil.

[davepoth]

phatgixer said:

Thanks guys

I have ordered new washers and banjo bolts from the (excellent) SU website. Hopefully the leaking will stop. They are done up much too tight at present so change is needed. All faces are flat and clean, but the washers are very old....

The reason the float levels differ is that at some time in the past one of the carburettors has been rebuilt using different parts for the needle valve actuator arm and the are set at differing heights. I am not sure which is the correct one! The car idles nicely sometimes, and lumpily at others (all new plugs / leads / points / cap / coil and timing is right) and when cruising will happily accelerate for a few seconds then suffer what feels like fuel starvation almost as if there is too little fuel in the chambers?

It also pinks quite badly when hot, but I put that down to needing a jolly good de-coke. I was hoping there is a slim chance that one of the carbs is running lean due to float level being too low and causing the front or rear three cylinders to run a bit hot, exacerbating the problem?

Or have I got a vacuum leak?

Gawd, I like things to run nicely!

Float chamber level wouldn't hurt the idle surely? the fuel pump would be easily be able to supply enough fuel in that case. I'd look at getting the two carbs into a matching specification as much as possible.

[duncancallum]

It does. If its incorrect it will flood it.

[davepoth]

duncancallum said:

It does. If its incorrect it will flood it.

Fair enough. So in that case, it could well be that the issue is on the "other" carb - if the carb that was rebuilt was done because the needle valve was sticking, perhaps the unrepaired one now has the problem.

Definitely worth building the carbs up to the same specification anyway, you'll be wasting your time trying to tune them otherwise IMO.

[phatgixer]

Got new needle valves ready to fit from Burlens (SU) and have already set the float chamber height using the 3/8" drill.

New fibre washers for the banjos in pack as well and have checked fiel line on low pressure side. Pickup in the tank had lost its gauze filter so that has been repaired and the reserve solenoid made to work. Car driving appreciably better, but still lacking a bit of tune.

Hopefully it is the needle valves

Thanks for the help. Appreciated :thumbsup:

[phatgixer]

Done all the needle valves, float hieghts, tank pick up, line, filters, pump refurb and it is still missing a bit.

The only part of the electrics I haven't changed is the coil and I touched it today and it must have been 70 degrees C or more. Is that usual?

[chard]

Sounds a bit hot, worth swapping you can always keep it as a spare if it makes no differance.

Chard