B21A SU Carb Flooding

Dpr05 · 2025-10-01T09:43:33+00:00

Hey all, currently attempting to get a 78' 245 back on the road. Current Dilemma is an SU HIF6 that draws heaps of fuel. For context this is the first carbur...

nh2868

When you put a kit through the carb - is the float needle a brass one or one with a rubber coating ? As others have said the main cause of flooding is a sticking or non-seating float valve. I haven't played with the SUs on a Volvo but have been driving Triumph cars for 40 years now with HIF6 carbs and the aftermarket parts can be rubbish. I've often had to mix and match old needles and seats before getting a combination which will work properly.

They are also very sensitive to fuel supply pressure - you only want the pump putting out 1-2psi, really just to keep fuel available at the carb.

Dpr05

nh2868 Yes it does have a rubber coating on its tip, feeling it may even be fully rubber, old Needle also has this coating.

Had another play with the car today. Car will start reliably on fuel with the bowl empty, and will run at part throttle with audible pops and bangs in the exhaust. However, directly after this run the motor will refuse to start again, and will only start again when fuel has been disconnected and 15~ seconds of cranking to clear all of the unused fuel, motor also starts in this process before choking out when all fuel is gone. After performing this, the car will start again with fuel connected. This was all done with the Jet super lean.

Had a go with swapping things out, tried the old jet, needle, and shut off valve in various configurations. All performed similarly, but the old jet definitely way too rich - had a puddle formed in the carb after its run.

This was also all done with a Holley type fuel reg, modified to suit lower pressures. No idea if it actually did perform as intended, but a proper reg is on the to buy list.

nh2868

Certainly sounds like it's flooding. The spring is installed in the dashpot correctly ? The piston is free to rise and fall ? The piston has oil in the top ?

Dpr05

How does one install the spring correctly? I have just been placing it in without much worry. Have ticked the other two boxes.

nh2868

Dpr05 I recall you saying that this is the first carb that you'd played with. The SU is a really clever contraption which is quite simple but not intuitive as to how everything works.

The dashpot is under vacuum when the engine is running, and without the floppy spring on top of it would go to the "full rich" position on the metering needle. Yes it doesn't need any care to install it, but the spring really does need to be there and the piston and cover need to be oriented the correct way meaning the ports in the bottom of the piston face the correct way and the piston moves freely.

jamesinc

Maybe just lower your float height and see what happens.

I feel like this has to either be the float valve not closing properly, or the float height being high enough that excess fuel makes it out the jet. Your testing seems to rule out the valve itself, so that leaves the float.

Dpr05

Attacked it again this weekend with not much more luck. Plugged in a proper pressure regulator that had it down to 2psi, Timing light showing it had 30 degrees worth of timing so lowered that down to ~10 degrees. Also had a play with a vacuum gauge that showed low vacuum, so replaced the intake manifold gasket with a substantial improvement there. A compression test showed 100psi minimum on cylinder 3, with the rest ranged from 110-120.

Car will run better now with what I've done to it, but still no where near a driveable point. Can now start it around 3~4 times without needing to clear the excess fuel out and still wont idle. Current thinking is going carnut222 route and finding a few carbs or one that just works.

Also everyone's suggestions are much appreciated, cheers all.

carnut222

Dpr05 Good luck! I swear this sounds identical to the problem my car had…wish I knew what the actual problem was to help out!

Dpr05

To update, I got it running. Have thrown out the SU and went the weber conversion route. Took a few hours to swap everything over, only unexpected quirk was having to remove the Lower inner two studs for bolts.

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Revs quite happily, and always returns to idle. Currently have it idling at 7~800rpm. I have a manual choke installed now aswell rather than the electric type shown.

Car is not ready to be driven yet, so it has not been subjected to a proper stress test.

Now in terms of getting it ready for a blue slip, I have been told that I will need to cover the filter, and make sure all of the pollution gear works. Lucky there doesn't seem to be much.

Current idea is to hack up the original airbox plate to suit the weber:

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The airbox clears under the bonnet, and only real modifications is resizing the Hole for the carb inlet, and plugging the bolt holes from the SU.

Onto the Emissions side of things,

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I believe the car is running the second vacuum/charcoal canister setup for a B21A.

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Unfortunately this is the only photo I have of the canister itself.

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It seems I will have to drill and tap into the carby to get the bowl vent to connect to the charcoal canister. My question is where does that bottom line run too? I would imagine the top two run to the tank vent, and one to the carb.

My car also has both advance and retard adjustments on the distributor, where on the weber there is only the advance port. I was not able to sight how the original system worked, as it was not connected when I bought the car. Is there away to retain both functions? Or is it not worth the hassle.

This was my best guess with the SU following the below photo. Don't mind the cut heater hoses, I have replacements ready, The fitting snapped on the hard line when replacing the water pump.

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Another problem is the original coolant routing flows through the inlet manifold, I have plugged the head port with a 12mm welch plug. Is there any reason why I cannot just plug off the inlet back into the block? Or could it be used to return the coolant from the heater into the block using it, Instead of taking the exhaust manifold off to replace that coolant pipe. My worry here is stripping or breaking the studs.

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This is also an old photo, the car is still dirty, just not this dirty. Crazy how much dirt sitting in a barn for 20 years can produce.

I am debating opening up a build thread, if that is more suitable than just continuing this one let me know, as it is now slightly off topic.

Thankyou everyone for having a read, any advice is much appreciated. Cheers.

jamesinc

Dpr05 My car also has both advance and retard adjustments on the distributor, where on the weber there is only the advance port. I was not able to sight how the original system worked

Some (most) distributors are vacuum-advance and just reference manifold pressure, but others also have a retard input that is taken from a venturi by the throttle butterfly.

It might not be possible to configure the weber to suit it (I don't know honestly).

Alternatives would be to get an advance-only distributor or (imo the right choice) get a 123ignition, you'll be able to set it up to suit your Weber.

dmc

Charcoal canister: There will be 4 pipes,

a drain at the bottom (ignore),
one to the tank (ignore)
a 'switch' hose that operates a valve
a connection to the intake manifold to suck the gunk out of the canister.

Generally the switch/valve is a smaller hose and will connect to the vacuum operated switch on top of the canister, the vent hose will be the bigger one.

The image above looks like it has a separate valve which just makes things weirder. The big hose goes to the main chamber of the intake manifold, it is for sucking the fuel/fumes/gunk out of the canister once the engine is up and running. It should be manifold vacuum. The pic with the SU at the end looks like the big hose that arcs across the photo.

The little hose that opens the valve (for the big hose) goes to a vacuum line port before the throttle disk. At idle it should be no suction, at 1500 RPM it should be vacuum. There will be a port on the Weber that can do this, it is the same as the vacuum advance hose on the distributor (which you can see in the pic).

The mention of the fuel bowl vapour vent in the carb diagram is for connecting the fuel bowl to the charcoal canister as it is technically a fuel tank, just a really little one. Do not worry about this.

The big breather pipe from the cam cover needs connecting too, should be:

one hose to the manifold vacuum via the metered screw in fitting (it has a specific diameter hole in it, it is the long one on the manifold) - this is visible in the 'before' pics

-a vent to atmosphere - which was via the air filter box.

Easiest way for the filter end is to get a little filter from a parts shop similar to a rocker cover filter from an old Holden/Ford. Pop it onto the fitting. It is for sucking air into the crankcase that is then sucked into the engine and burnt off. It NEEDS to be there, otherwise you are sucking dust into the engine - which is bad. This is common to almost all Volvo engines of that era, I expect the install kit deals with this. Otherwise the fitting from the old manifold can be unscrewed (11mm spanner?), and fitted into the main plenum of the new intake manifold via a drilled and tapped thread. It is probably a pipe thread.

Also, are you aware the brake master cylinder is missing? :-)

Dpr05

dmc This is awesome, exactly what I needed.

dmc The big hose goes to the main chamber of the intake manifold, it is for sucking the fuel/fumes/gunk out of the canister once the engine is up and running. It should be manifold vacuum.

dmc one hose to the manifold vacuum via the metered screw in fitting (it has a specific diameter hole in it, it is the long one on the manifold) - this is visible in the 'before' pics

So, seems I will need to find a space for these two fittings somewhere, as there is only one standard vacuum fitting on the manifold, then its all plug and play.

And yes, I have had the master cylinder off for a reseal, getting very close to the brakes being done.

This is awesome, thankyou.

Dpr05

jamesinc

Haven't seen these 123 distributors before. Seems like an option if I definitely cant get this distributor to work properly. Car has already blown through what I expected to put into it, so a little more cant hurt.

jamesinc

123/TUNE is a lot of fun, and can give you a much more sophisticated advance curve than what a mechanical distributor is capable of.

Anyway, this is how your distributor would have been setup originally:

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Note there is a one-way valve on the advance hose, I believe it allows air to be drawn into the engine, but not the other way around (don't hold me to the direction but, if I recall, the valves are usually marked "DIST" on one side to help you orientate them correctly.

The advance hose gets vacuum when the throttle is open, at idle it sees no vacuum. The retard hose is just manifold referenced.

If you don't have a good place to connect the advance hose, IMO just run the retard hose to the manifold, it should still drive fine I think.