dual carbs

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Re: dual carbs

#61

Post by wayno » Thu Oct 17, 2013 9:15 pm

My dual SU setup also has a return line, I used the emissions line as my return, I bypassed that stuff in the fender well and piped it directly to the tank.
The bowl vents go directly to the airfilter on my truck, the back of the airfilter housing has connections just for that, maybe you can add fittings for that on the back of your airfilters.

You also must remember that you have 4 holes on the face of each of your SUs, two have threads to bolt the airfilters on, the other two are to vent the piston, or at least one is, you have to drill a hole for that vent hole, otherwise the piston will not move up and down properly, if it don't move up and down properly, it will never run right. :)
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Re: dual carbs

#62

Post by datmo » Thu Oct 17, 2013 10:00 pm

I'll just leave it as is then.
I doubt they'll float a lot anyways so I hope I'll be fine.
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Re: dual carbs

#63

Post by datmo » Thu Oct 17, 2013 10:02 pm

wayno wrote:My dual SU setup also has a return line, I used the emissions line as my return, I bypassed that stuff in the fender well and piped it directly to the tank.
The bowl vents go directly to the airfilter on my truck, the back of the airfilter housing has connections just for that, maybe you can add fittings for that on the back of your airfilters.

You also must remember that you have 4 holes on the face of each of your SUs, two have threads to bolt the airfilters on, the other two are to vent the piston, or at least one is, you have to drill a hole for that vent hole, otherwise the piston will not move up and down properly, if it don't move up and down properly, it will never run right. :)
Drill the hole where?
My air filter is connected to both of those holes that are for the air.
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Re: dual carbs

#64

Post by Laecaon » Thu Oct 17, 2013 10:18 pm

There are 5 holes. 2 mounting, 1 intake (big), 2 vents. Per carb

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Re: dual carbs

#65

Post by Converted_to_datsun » Fri Oct 18, 2013 4:56 am

There is also two holes near the top for the valve cover to vent to or the overflows

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Re: dual carbs

#66

Post by datmo » Fri Oct 18, 2013 6:58 am

Laecaon wrote:There are 5 holes. 2 mounting, 1 intake (big), 2 vents. Per carb
I'll drill the holes in the air filter to make sure all are accessed correctly.
Converted_to_datsun wrote:There is also two holes near the top for the valve cover to vent to or the overflows
speaking of that, I always see people puting an air filter on the little spout on the valve cover but it spews OUT air?
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Re: dual carbs

#67

Post by draker » Fri Oct 18, 2013 10:08 am

But when the car is not running it's could allow dirt into the engine.

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Re: dual carbs

#68

Post by datmo » Fri Oct 18, 2013 1:28 pm

draker wrote:But when the car is not running it's could allow dirt into the engine.
Oh... see the air filter keeps it clean...
alright. I'll pick one up for the little spout.

Or where you talking about those return fuel lines?
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Re: dual carbs

#69

Post by wayno » Fri Oct 18, 2013 4:27 pm

datmo wrote:
draker wrote:But when the car is not running it's could allow dirt into the engine.
Oh... see the air filter keeps it clean...
alright. I'll pick one up for the little spout.

Or where you talking about those return fuel lines?

I think you will find out that at an idle air comes out of that hole(valve cover vent), it's called blowby, but when you rev the engine, mine sucks air in, that is why the manufacturer put that line into the air filter, the air blowing out gets sucked back into the carb, but the air getting sucked in is filtered air. :)
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Re: dual carbs

#70

Post by datmo » Fri Oct 18, 2013 11:24 pm

wayno wrote:
datmo wrote:
draker wrote:But when the car is not running it's could allow dirt into the engine.
Oh... see the air filter keeps it clean...
alright. I'll pick one up for the little spout.

Or where you talking about those return fuel lines?

I think you will find out that at an idle air comes out of that hole(valve cover vent), it's called blowby, but when you rev the engine, mine sucks air in, that is why the manufacturer put that line into the air filter, the air blowing out gets sucked back into the carb, but the air getting sucked in is filtered air. :)
So it sucks in filtered air.... gotcha!
so ill get a filter lol
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Re: dual carbs

#71

Post by Converted_to_datsun » Sat Oct 19, 2013 8:03 am

Actually the best thing to do is to buy PVC line and connect it to the back of your air filter

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Re: dual carbs

#72

Post by datsunmike » Sat Oct 19, 2013 9:01 am

The overflow fittings should have hoses that direct any spilled gas AWAY from the hot exhaust manifold. Remember they are only there in case the float should stick or sink and the carb over fill. This may never happen, but you want to keep gas and hot manifold separate.
Converted_to_datsun wrote:A return is for excess pressure, those vents are in case of overflow. Wouldn't work properly
The return line is not there for excess pressure. The L fuel pump is regulated at (about) 3 to 3.5 PSI. L20Bs tend to run hotter under the hood than earlier Ls. This can cause the gas in the fuel line to the carb, and the gas in the carb to boil. This is particularly bad when the motor is shut off in hot weather. Under hood temps soar to 200F or more and the gas in the lines will boil. Vapor in the hot lines prevents gas from flowing into the carb and the gas in the carb has boiled away.. starting is difficult to impossible. To reduce this, a return line is added and cool fuel is constantly circulated past the carb and returned to the tank.

Now simply adding a return line won't work, as there is no reason for the fuel to push past the needle valve and into the carb when there is an open easier path back to the fuel tank. To increase the pressure on the carb side a restriction is placed in the return line. The fuel pump pushes fuel against this tiny pin hole and pressure builds and now is enough to force it's way into the carb normally.

Return lines are a great idea on any car as the fuel in the tank is constantly recirculated through the fuel filter. Think about this... dirt and rust do not accumulate in the tank. The filer collects dirt all the time and keeps the tank cleaner.

Restriction at the tank end of the hard line above the fuel pump.
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Re: dual carbs

#73

Post by datmo » Sat Oct 19, 2013 9:40 am

Converted_to_datsun wrote:Actually the best thing to do is to buy PVC line and connect it to the back of your air filter
what's the difference?
Going to the air filter will just filter it just as much as its own smaller air filter.
datsunmike wrote:The overflow fittings should have hoses that direct any spilled gas AWAY from the hot exhaust manifold. Remember they are only there in case the float should stick or sink and the carb over fill. This may never happen, but you want to keep gas and hot manifold separate.

Return lines are a great idea on any car as the fuel in the tank is constantly recirculated through the fuel filter. Think about this... dirt and rust do not accumulate in the tank. The filer collects dirt all the time and keeps the tank cleaner.
I think that it will be a good idea to get some hoses and just point the excess gas to the ground because you're right. gas + hot manifold = terror.

I do think the return lines are also a great idea but I feel like if it'll rarely happen, I shouldn't need to worry about making a whole return line for maybe an ounce of gas? more or less.

I'll put in my carbs tomorrow.
I'll definitely put some hoses and point the excess gas downwards or something along those lines.
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Re: dual carbs

#74

Post by datmo » Sat Oct 19, 2013 9:40 am

Converted_to_datsun wrote:Actually the best thing to do is to buy PVC line and connect it to the back of your air filter
what's the difference?
Going to the air filter will just filter it just as much as its own smaller air filter.
datsunmike wrote:The overflow fittings should have hoses that direct any spilled gas AWAY from the hot exhaust manifold. Remember they are only there in case the float should stick or sink and the carb over fill. This may never happen, but you want to keep gas and hot manifold separate.

Return lines are a great idea on any car as the fuel in the tank is constantly recirculated through the fuel filter. Think about this... dirt and rust do not accumulate in the tank. The filer collects dirt all the time and keeps the tank cleaner.
I think that it will be a good idea to get some hoses and just point the excess gas to the ground because you're right. gas + hot manifold = terror.

I do think the return lines are also a great idea but I feel like if it'll rarely happen, I shouldn't need to worry about making a whole return line for maybe an ounce of gas? more or less.

I'll put in my carbs tomorrow.
I'll definitely put some hoses and point the excess gas downwards or something along those lines.
Just a kid with a love for datsuns and no money.
4 doors riding slow and hangin loose.

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Re: dual carbs

#75

Post by datsunmike » Sat Oct 19, 2013 11:36 am

datmo wrote:
Converted_to_datsun wrote:Actually the best thing to do is to buy PVC line and connect it to the back of your air filter
what's the difference?
Going to the air filter will just filter it just as much as its own smaller air filter.
The difference is that when the blow by becomes more than the PCV can suck into the intake the flow reverses. Fumes blow out under the hood and stink up the cab. In addition the 'cute l'il blue filter' gets wet with oil and drips. When connected to the stock air filter the fumes (and any oil fog) are sucked down into the carb. This is a way better idea.


datmo wrote: I do think the return lines are also a great idea but I feel like if it'll rarely happen, I shouldn't need to worry about making a whole return line for maybe an ounce of gas? more or less.
An ounce or two?? Gas is flowing through the return all the time the motor is running. Imagine 3PSI squirting through that small hole. On a long trip gallons and gallons would be circulated. The whole point of the return is to provide cool fuel from the tank past the carbs. SUs even have a heat shield. I wonder why???????
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Re: dual carbs

#76

Post by izzo » Wed Oct 23, 2013 7:45 am

Plus your 510 should already have a return line. Maybe not 68? But it should. It's a matter of hooking up a rubber line... Not hard.
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Re: dual carbs

#77

Post by datmo » Thu Oct 24, 2013 10:16 pm

izzo wrote:Plus your 510 should already have a return line. Maybe not 68? But it should. It's a matter of hooking up a rubber line... Not hard.
From what I know, I don't have a return line.
I don't know how to check either.

But as i read, i thought it was rarely that the gas leaked out.
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Re: dual carbs

#78

Post by wayno » Sun Oct 27, 2013 10:06 pm

See the black hose on the bottom, and see the spring above the round ridged nut, that is the fuel mixture round nut.
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