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SU carb plumbing - overflow

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mperdue

mperdue

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Hi guys,
I've posted here a few times with SU carb question and I've now got one more. I'm working on a carb setup with HS4 type carbs (made by Hitachi for a Datsun) and I'm trying to figure out what to do with the fuel vapor/overflow ports. I have asked my fellow Datsun enthusiests but none of them seem to know for sure, mainly because my setup is sort of a custom fitment that they aren't too familiar with. Anyway, my fuel tank has a vapor return line that I could use but I'm not sure exactly how to hook it up. Can I just run the overflows to a catch tank and then connect the vapor return to that, or do I not need the catch tank at all? Or shoudl I put a carbon canister or filter in the overflow lines? Or something else completely different? I guess I'd really like to know what you MG guys and others do with your SU carb fuel overflows. Thanks a lot for the help - I've learned a lot here.
Michael
 
well, I can tell ya that the stock '76 MGB with its Zenith-Stromberg carburetor runs a line from the float bowl vent/over flow goes to a carbon canister on the passenger side of the car.
 
well... if you are talking about the overflow on the top of the float bowl....

all the pre-emmissions ones that I've worked on just vent to nothing. No tubes, no nothing. TR's came with a little angle tube to point the flow down instead of spraying out to the fender. Beautiful design that points the stream in the area of the Generator in the front and the Starter in the back. I seem to remember on the TD that there was a longer tube that got the gas down below all the accessories and onto the road below.

I guess what I'm trying to say is that it depends on your safety comfort level and obsession with originality.

They should NOT be spraying out fuel unless you have a problem and the vapors are negligable to most. You are going to put more gas fumes into the atmosphere when you fill up the tank.

A catch tank in my opinion would be overkill. A couple of tubes that are long enough to get the overflow down past the manifold and all the electrical goodies and routed to spray out on the road if your float sticks would be the safest compromise I think.

But then mine still just have the little short 90 degree bends and if it overflowed it would run down the side of the fender and clean up my starter.... /ubbthreads/images/graemlins/smile.gif
 
Thanks guys. I guess I'm interested in both safety (no gas on the hot exhaust manifold) and environmental as well. I don't like the smell of gas that some older cars have, so if the carbon canister will reduce the odor maybe I will try that.

Does the carbon canister have both an inlet and an outlet or is it just closed and collect fuel and vapors? Does it act as a filter for the return vapor to the tank? Thanks again for the advice!
michael
 
On the pre-emissions standards cars the SU's had short rubber lines from the overflow vents on the float bowls to two steel lines running forward, close to the block and down to a "clip" on the block side of the motor mount. Ends were almost to the bottom of the oil pan in length.
 
Well Mperdue, on a '76 the canister vents from the bottom to the atmosphere under the car. The top of the canister on my car has 3 fittings: 1 for the valve cover breather line, 1 for the float bowl breather line, and the last goes to a line that's supposed to go back to the Vapor Separator.

hope this helps.
 
May I but in please?
I have HD8 SU carbs on a Healey BJ8 and when I switch on, without starting the engine, the pump ticks away to fill the bowls but eventually fuel comes out of the inlet manifold rear drain pipe.
Have checked all adjustments, good running order, tickover good.
What allows fuel through when not being fired up?
 
Cutrog1 said:
May I but in please?
I have HD8 SU carbs on a Healey BJ8 and when I switch on, without starting the engine, the pump ticks away to fill the bowls but eventually fuel comes out of the inlet manifold rear drain pipe.
Have checked all adjustments, good running order, tickover good.
What allows fuel through when not being fired up?
Click to expand...
First thing to check would be the floats. they can leak and fill up, but, most likely the float needle - especially if it has been replaced - the viton tips have a spotty reputation. Another might be just some grit or dirt holding it open. Or (with two) if the float levels are not the same presumably one could stop while the other continues to run. (not so sure if the last option is possbile)
 
Hit the float bowl with a wooden-handled screwdriver or similar piece of wood. That may cure the problem without having the dismantle the carb. It did for me.
 
Yup. A couple thumps on each float bowl lid first. If that doesn't "fix" it, the needles and seats need inspecting and cleaning.
 
Thanks for input. I did have to adjust float levers. However, is it not the case that even following adjustment procedures the needles may still be slightly open at rest? They seem to drop soundly using lifting pin. Should they be shut until either the choke is pulled or the engine is turned and creates vacuum to lift needle?
Still ticking!
Running very well, and all plugs look correct colour except for number 4 which is very sooty, even after swap.
 
Don't you love these old cars! If it doesn't act right, smack it with a stick to make it work! I think my pop read that somewhere when I was a kid and started acting up, sure sounds familiar! :censored1:
 
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