In order to simplify the fuel plumbing in my engine compartment, I want to try blocking off the return line on my Weber 28 IMB. I've read multiple posts that say the carb works fine without it. I'm struggling to find elegant solutions to make this happen however. Please see the picture below. Is that length of fuel line using a bolt (with a clamp over it) going to work?

My concern is obviously fuel leaking in the hot engine compartment.

Second, what are you opinions on the clear, green fuel tubing? The auto parts stores in Italy push that stuff because it's what Fiat originally used for these cars. I'm somewhat skeptical, and think the thicker, black line is more durable. Am I wrong?

Thanks for all the help!

Richenrygarcia said:

Second, what are you opinions on the clear, green fuel tubing? The auto parts stores in Italy push that stuff because it's what Fiat originally used for these cars. I'm somewhat skeptical, and think the thicker, black line is more durable. Am I wrong?

Thanks for all the help!

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As long as the fuel line is "Ethanol" resistant it should be ok...
I would go for clear if you can find a resistant pipe as it would remove the need for those plastic inline filters that (in my opinion) do nothing more than allow you to see the fuel...

However I have such a line that has hardened so any work on the engine may cause leaks...
I'm sure there is some newer pvc based pipe that remains flexible, but don't use the silicon based stuff as they are not suitable for Petrol..

The purpose of a return line is to keep the fuel flowing, which reduces the potential to overheat and vaporise in the pipes and also ensures there should be fuel always available when the float valve drops to allow more into the float chamber. Many carburettors never had this feature as it requires more plumbing, and therefore more cost.

The float valve should be able to shut to prevent flooding. The fuel supply pressure should be low enough to avoid forcing the valve off its seat, but plugging the return could cause the float chamber to flood if the pressure is too high. Best way to find out is to plug it and see.

I agree that the green lines get hard after a period. I made the switch this morning to black rubber tubes. We'll see how it goes!

Bigvtwin996 said:

As long as the fuel line is "Ethanol" resistant it should be ok...
I would go for clear if you can find a resistant pipe as it would remove the need for those plastic inline filters that (in my opinion) do nothing more than allow you to see the fuel...

However I have such a line that has hardened so any work on the engine may cause leaks...
I'm sure there is some newer pvc based pipe that remains flexible, but don't use the silicon based stuff as they are not suitable for Petrol..

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Thanks Portland Bill! I'm trying it now. Started her up and let her idle and there were no issues. We'll see how she performs after a road test.

portland_bill said:

The purpose of a return line is to keep the fuel flowing, which reduces the potential to overheat and vaporise in the pipes and also ensures there should be fuel always available when the float valve drops to allow more into the float chamber. Many carburettors never had this feature as it requires more plumbing, and therefore more cost.

The float valve should be able to shut to prevent flooding. The fuel supply pressure should be low enough to avoid forcing the valve off its seat, but plugging the return could cause the float chamber to flood if the pressure is too high. Best way to find out is to plug it and see.

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The 126 carb seems to one of the few that Fiat decided to have that return pipe but as I found out recently running an engine on the bench you get quite a bypass flow there. However should be no problem blanking it off as I have also done the same. I used an M4 taper tap to thread the inside of the ferrule then inserted a stainless grub screw. Kind of an invisible mend and easily reversable. The original Cavis fuel pipe does seem to petrify but even in the workshop manual it recommends pouring hot water on it when connecting or removing the pipe.
You might need a "Snap On" kettle though
I did have one maybe crazy idea that you could use an online petrol tap to shut off the flow then if you were suspecting fuel delivery problems the flow could be tested at the carb.

Toshi 975 said:

The 126 carb seems to one of the few that Fiat decided to have that return pipe but as I found out recently running an engine on the bench you get quite a bypass flow there.

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I suspect that the excess flow would be the same on the 26IMB carb. if it had the spigot, and that it exists simply to reduce the possibility of any excess fuel pressure bearing on the float-needle at times of reduced fuel requirement such as when going downhill. But I have always understood that when a carb is full the consequent backpressure temporarily immobilises the diaphragm and stops the delivery of fuel. Maybe this is not the case on the Fiat fuel-pumps?
I think that permanently blocking the return orifice is the best idea, but since some people have had problems with the brass spigot coming out it would be best to screw something directly into the carb body with the spigot removed.
I blocked-off the return spigot in a similar way to Richenrygarcia and it's been no problem. But having used the transparent pipe on the section between pump and carb for the reason Suggested by Bigvtwin996 I'm just about to change it for the modern rubber pipe because it has quickly gone hard and I suspect that it may put a strain on the brass spigots.

I pulled out the brass spigot and then tapped a thread into the hole. I then put in a button head screw with some threadlock and a dowty washer. No leaks and never had a problem with it.

Thanks for the insight everyone. After getting the car nice and warm, it appears I have a leak coming from the carb top cover. I’m going to double up on the gasket to see if that helps. I checked and double checked the float height before putting it together but I’ll check it again.

Incorrect float height could be a culprit in this case yes?

The fuel should not ever rise to the top of the float chamber. A leak shows the fuel is too high. Causes can be: the float valve not seating properly due to wear, dirt, or excess pressure; Float holed or porous, or pivot sticking; cracked top cover. (There may be others)

As all seemed ok until you plugged the return line, excess pressure would be the most likely cause. DO NOT add an extra gasket, as this will effectively raise the normal fuel level, causing it to always run rich.

As a rule of thumb, carburettors are normally fed with about 4psi of pressure. A pump mounted close to the engine will have a good suck, but only a light push, whereas a pump fitted close to the tank will usually have only a light suck, but a good push. This can cause problems when fitting an electric pump and using the wrong one.

A mechanical pump attached to the engine has a diaphragm with a spring under it. The lever is pushed by the camshaft to pull the diaphragm down to suck fuel in. The spring creates the pressure for the output, resisted by the float valve when the chamber is full. As the float drops, more fuel is moved, until the diaphragm reaches the top of its travel, and the lever contacts the cam again and pulls more through. In practice it only moves gently down and up near the middle of its range.

If the carb only has an inlet, pressure from the pump should not exceed about 4psi. But if a return line is used, often the pump will deliver higher pressure and flow, to circulate the fuel. The float valve is not stressed as the pressure is not bearing on it directly, as the flow rate is more important. The carb return outlet is usually a smaller hole as a restrictor.

If the pump is of the higher pressure/flow rate type, plugging the return can lead to the float valve being pushed off its seat, overfilling the float chamber, causing rich running and leaks.

Fixes:
Put a return hose back into the system.
Source a pump with a lower output. There may be differnt models without the return line, so their pump would be different, so might be a straight swap. If changing pumps, pay particular attention to lever position, length and travel, and the fitting of the pump to the block, as there are often spacers, and the thickness of these varies according to pump lever and camshaft relative positions. Get it wrong and levers can snap, cams can wear, etc. Or just no fuel delivery!

Just wishing to lose one pipe from the engine bay seems to create a lot of work. My advice, put it back to standard. Fiat probably knew what they were doing. The return pipe added cost, so there must have been a very good reason to have one.

Richenrygarcia said:

Thanks for the insight everyone. After getting the car nice and warm, it appears I have a leak coming from the carb top cover. I’m going to double up on the gasket to see if that helps. I checked and double checked the float height before putting it together but I’ll check it again.

Incorrect float height could be a culprit in this case yes?

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I think that most likely cause is a bowed carb top cover caused by over tightening probably with a poor gasket at some stage. I would guess that about 1 in 4 carbs I have fixed have needed the top cover straightening before fitting a new gasket.

Toshi 975 said:

I think that most likely cause is a bowed carb top cover caused by over tightening probably with a poor gasket at some stage. I would guess that about 1 in 4 carbs I have fixed have needed the top cover straightening before fitting a new gasket.

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A valid observation in many cases, but if you start at the beginning of this thread you might have a different view. The OP has blanked off the fuel return pipe, and now has flooding issues. Doubtful the blanking off of a pipe will distort the carb top.

I think the problem is fixed! I re-connected the return line and all is good. I stole the idea from nzboc (see the attached pic from his post) of placing a "tee" before the pump. After a test drive to get her up to operating temperature, there were no leaks.

Routing the return line is awkward thoough, due to the proximity of the Weber return fitting to the firewall. The hose makes a really tight bend there. I'll have to keep my eye on it, and maybe install some sort of chafing protection.

Thanks to all that helped me getting this mini project done.

I routed my 'return' pipe behind the big air-trunking (inside a section of small-bore water hose, for protection), secured with 'P' clips and had the 'T' piece secured, again with 'P' clips, to the side of the engine bay. Doing it this way made for a much fairer 'lead' for the fuel-pipe. Remember though--always use a METAL 'T' piece for the fuel-pipe connection. 'Cycling' the fuel helps keep it cool as well as preventing any possible over-supply to the carb. I have used my system for darn near 3 years, 1st of all on a 28IMB Weber and then on a Dellorto FZD--never had a supply problem. I also would NOT reccommend that a fuel-pipe connection is made over the alternator--a fuel leak could have serious consequences!

Paolo66 said:

I pulled out the brass spigot and then tapped a thread into the hole. I then put in a button head screw with some threadlock and a dowty washer. No leaks and never had a problem with it.

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OMG this is not original

Mine was blocked anyway but i put a piece of tube with a blanking screw. I have put that green pipe which is starting to getting hard so i will switch back to black fuel hose, more safe i believe.

If I had the energy I would copy Ricambi Fiat 500 and use this de-luxe method:

https://www.flickr.com/photos/ricambifiat500/albums/72157622038829612

fiat500 said:

If I had the energy I would copy @Ricambi Fiat 500 and use this de-luxe method:

https://www.flickr.com/photos/ricambifiat500/albums/72157622038829612

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not that hard.... all you do is take about 100g of Spaghetti per person and put it in boiling water with a little salt and let it boil for 11 minutes...
and a little bit of Pasta Sauce in the Microwave for 1m30sec...

Hobbler, where did you attach your “p” clips? Did you screw them into the sheet metal in the engine bay?

the hobbler said:

I routed my 'return' pipe behind the big air-trunking (inside a section of small-bore water hose, for protection), secured with 'P' clips and had the 'T' piece secured, again with 'P' clips, to the side of the engine bay. Doing it this way made for a much fairer 'lead' for the fuel-pipe. Remember though--always use a METAL 'T' piece for the fuel-pipe connection. 'Cycling' the fuel helps keep it cool as well as preventing any possible over-supply to the carb. I have used my system for darn near 3 years, 1st of all on a 28IMB Weber and then on a Dellorto FZD--never had a supply problem. I also would NOT reccommend that a fuel-pipe connection is made over the alternator--a fuel leak could have serious consequences!

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Simply put, the answer is---yes. The fuel line in the engine bay runs alongside what is effectively a chassis leg. I cut the fuel pipe, slid 2 short lengths of small bore water hose along each end of the (now 2-part) fuel pipe, slid a fuel-pipe hose-clamp over each end of the fuel pipe, inserted the METAL 'T' piece, did up the hose clips, slid the 2 lengths of water hose back to the hose-clips and fitted 2 'P' clips, screwing them too the chassis leg OVER the sections of water hose (which will protect the fuel pipeing from chafe). The 'return'fuel-pipe can be led behind the big 'air-intake' trunking, again in small bore water hose to prevent chafe and supported by 'P'clips, possibly modified to be secured on engine cowling bolts. I have also found that the plumbers 'foam-lagging' that you find in B&Q (and other well known stores) can be usefully used to prevent chafe, usually held in place by cable-ties.