# Weber carb deadhead or bypass regulator?



## krayzkrn (May 31, 2008)

I would like to run a bypass style pressure regulator but there are very limited options out there that actually regulate to a Weber recommended 2.5-3 psi. I have a Carter p4070 which is still ~5-6 psi output. Can anybody recommend a bypass regulator at ~3 psi?


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## candm (Apr 27, 2003)

filterking or malpassi make adjustable regulators. there are quite a few others as well


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## krayzkrn (May 31, 2008)

candm said:


> filterking or malpassi make adjustable regulators. there are quite a few others as well


I do not believe those are bypass style.


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## candm (Apr 27, 2003)

i forgot. you want a return to the tank i guess? there is no reason for a return line especially if you already have the carter pump.people will sometimes try and use the stock, high pressure pump and use a regulator with a bypass.


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## krayzkrn (May 31, 2008)

candm said:


> i forgot. you want a return to the tank i guess? there is no reason for a return line especially if you already have the carter pump.people will sometimes try and use the stock, high pressure pump and use a regulator with a bypass.


I wouldn't say that there is no reason for it. Most people just don't do it.


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## candm (Apr 27, 2003)

krayzkrn said:


> I wouldn't say that there is no reason for it. Most people just don't do it.


the reason they dont do it is because there is no reason for it. back when sidedraft webers were standard equipment on cars like alpha romeo, lotus and ferarri, there were no return lines. they only need from 2-4 lbs of pressure to operate properly and if your pump and regulator setup stay within those parameters, you wont have problems


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## krayzkrn (May 31, 2008)

candm said:


> the reason they dont do it is because there is no reason for it. back when sidedraft webers were standard equipment on cars like alpha romeo, lotus and ferarri, there were no return lines. they only need from 2-4 lbs of pressure to operate properly and if your pump and regulator setup stay within those parameters, you wont have problems


:thumbdown:


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## candm (Apr 27, 2003)

yeah. your probably right. i dont know what im talking about.


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## B4S (Apr 16, 2003)

krayzkrn said:


> I wouldn't say that there is no reason for it. Most people just don't do it.


No, there is no reason for one, if you're using a carb pump. This isn't BS you can ignore, a bypass regulator is useless on carb setups.


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## B4S (Apr 16, 2003)

krayzkrn said:


> :thumbdown:


Can't spell 'class' without 'a s s' I guess.


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## krayzkrn (May 31, 2008)

B4S said:


> No, there is no reason for one, if you're using a carb pump. This isn't BS you can ignore, a bypass regulator is useless on carb setups.


Why is it useless? I've seen quite a few setups that use bypass regulators for the purpose of cooling down the fuel via recirculation and to minimize the load on the pump.


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## B4S (Apr 16, 2003)

Sigh. Carb pumps have internal bypasses, so that when they deadhead, the fuel begins to bypass internally. Essentially, the pump shuts down.

As far as fuel cooling...lets not put too much thought into this. It's a carbed VW, not a 1000hp big block chevy running a filled block. There are no gains to be found with .1*F cooler fuel found by bypassing back to the tank. Especially since the fuel bowl usually transmits a fair amount of heat to the fuel before it goes into the engine . It all warms up to the same temp no matter how cool it is when it fills the bowl. A phenolic spacer works MUCH better. 

When I got into carbed VWs, I foolishly ran a bypass regulator on a carb pump. I had such problems with keeping a stable fuel pressure that the car was impossible to tune. Someone smarter in carbs than I told me I didn't need to do that...and I listened. Turns out they were right and my car ran AMAZINGLY well after that.

99% of OEM carb setups had no return line. Remember that carbs were used for the first 80 years of automotive technology, and the recipe for success is pretty much carved in stone .


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## RAB-817 (Dec 13, 2010)

every carb'd Mk1 ive had has had a fuel return line. 

It is T'd into the line from mechanical pump to carb. and the line is small to restrict fuel going back to tank.

Ive just finished putting new rubber lines and filter ect in one of mine. if it works, dont mess with it?? lol


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## B4S (Apr 16, 2003)

The difference being there is no FPR in that setup, so the smaller line acts like one. When the needle valve on the stock carb closes, the fuel has nowhere to go except back into the tank, which it does at a limited rate due to the smaller line. Electric carb pumps have internal bypasses, performing the same function when the fuel deadheads at the carb.

The return is needed on the stock carb because VW was awesome (as usual) and made their mechanical pumps put out more fuel/pressure than their carbs could handle. I'm sure there was a good reason for it, but if you had an electric pump on a car with a stock carb, you'd need an FPR or it would overpower the needle valve.


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## RAB-817 (Dec 13, 2010)

ahh awesome, 

i figured it was just something like that.


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## krayzkrn (May 31, 2008)

B4S said:


> Sigh. Carb pumps have internal bypasses, so that when they deadhead, the fuel begins to bypass internally. Essentially, the pump shuts down.
> 
> As far as fuel cooling...lets not put too much thought into this. It's a carbed VW, not a 1000hp big block chevy running a filled block. There are no gains to be found with .1*F cooler fuel found by bypassing back to the tank. Especially since the fuel bowl usually transmits a fair amount of heat to the fuel before it goes into the engine . It all warms up to the same temp no matter how cool it is when it fills the bowl. A phenolic spacer works MUCH better.
> 
> ...


I see that the Carter p4070 has an internally regulated pressure but I don't see anything about internal bypass? I am currently running the 4070 unregulated for a Weber IDF for testing purposes because I just got the motor built and running. Would the 5-6 psi from the Carter be too much for a Weber carb?


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## B4S (Apr 16, 2003)

Don't worry about the documentation, the bypass is there.

5-6 psi will definitely overrun the needle valve in the carb. Maybe not initially, but it will once the needle wears in. Having the pressure too high will also affect transition tuning, as the float level will be artificially high due to 'splash' from the higher pressure. It'll bring the emulsion tube/main jet into play sooner, which could make the car feel less snappy (it'll be rich, early).


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## krayzkrn (May 31, 2008)

B4S said:


> Don't worry about the documentation, the bypass is there.
> 
> 5-6 psi will definitely overrun the needle valve in the carb. Maybe not initially, but it will once the needle wears in. Having the pressure too high will also affect transition tuning, as the float level will be artificially high due to 'splash' from the higher pressure. It'll bring the emulsion tube/main jet into play sooner, which could make the car feel less snappy (it'll be rich, early).


That might be what I am experiencing during my testing. Off idle transition falls completely flat resulting in no acceleration.


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## candm (Apr 27, 2003)

why dont you post up what timing you are running and what jets, ckokes, etc. that you are running.


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## candm (Apr 27, 2003)

sorry. just saw you already posted the jetting in your other thread


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