Brilliant. Thanks for the thorough explanation.

the hobbler said:

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.

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portland_bill said:

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.

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I have seen it described elsewhere that the 126 and 500 pumps work differently. This is apparently because the 126 has a conventional tank location so the pump has to lift fuel a greater distance. The 500 fuel supply is gravity assisted in the first instance. Haynes says the 126 can deliver 25 litres per minute hour! but I can't find a spec for the 500 pump.
There are suggestions that you should use a 500 pump on a 126 engine if you want to cap off the return.
It sounds logical to me but maybe just complicates the conundrum, especially since I think that the 500 pump is physically a problem when used with the alternator from a 126.

I know the last post on this was two weeks ago but I discovered something regarding this when putting my 650 engine in recently. I decided to T piece the return line in before the pump but it was a real pain to get the fuel to flow from the tank by siphoning because you have the return line from the carb as well, so by trying to suck petrol through the rubber hose before the pump meant you were also sucking air through from the carb.

To solve the problem I disconnected the fuel line from the tank at the pump, connected up a new rubber hose fed from a fuel can raised higher than the pump and attempted to start the engine. Effectively I was priming the fuel lines, pump and carb from an outside source. I left the return from the carb connected to the T and had the original fuel line from the tank just hanging there disconnected. To be honest I had forgotten about the fact that it was likely to start pumping fuel out when the engine started. When the engine did start it did exactly that, the fuel from the can was being pumped to the carb and back out the return line at the same time as then engine ticking over perfectly happily. I immediately put my thumb over the fuel line from the tank to stem the flow but I was quite taken aback by the amount of fuel that was being returned by the return line. Obviously once you are on the move it’s going to be reduced but at tickover there is a considerable amount being returned from the carb.

In summary I do think that ideally a T piece is probably the preferable way to go as it is going to ultimately keep the fuel cooler by allowing it to flow better which I should imagine was Fiats thinking in introducing a return line?

It also raises the question where does all the excess fuel go at tickover if you have blocked off the return line at the carb?

Also later cars had the additional flap on the fan housing in front of the spark plugs that you can manually open and it directs a blast of cold air directly onto the carb, so it suggests that Fiat recognised that their were issues with fuel vaporisation that they were trying to improve.

I am not making a judgment on whether one way is better than the other and can see issues with both. i.e additional joints on the T piece etc...

Vitesse said:

In summary I do think that ideally a T piece is probably the preferable way to go as it is going to ultimately keep the fuel cooler by allowing it to flow better which I should imagine was Fiats thinking in introducing a return line?

It also raises the question where does all the excess fuel go at tickover if you have blocked off the return line at the carb?

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You raise a very interesting question and one that has not cropped-up before.:worship: Presumably, the answer is that the pump keeps rotating fuel around that circuit until it's all nicely warmed up.

Where the carburettor has no return spigot, the back-pressure at the pump outlet caused by a full carb-bowl cutting off via the needle-valve will prevent the diaphragm-spring in the pump from forcing more fuel to the carb; the operating lever will idle for this time. However, when a return line is fitted the pump never has the need to stop pumping; hence your healthy fuel return flow. If you block the return properly there's no surplus fuel being thwarted; whatever technique you use to block the return it will only ever be subject to a fairly low pressure.
Just today I was looking at this and decided that using the "T" method simply allows the fuel more time in the engine bay and the recirculating fuel is likely to warm up the fresh, cooler fuel coming from the fuel tank.
The tank in the 126 for which this system was designed was in a more conventional location under the floor at the rear and it was relatively easy to arrange some neat return plumbing and I am sure the gain was that the fuel had a chance to cool down. But there may also have been an intention to reduce "chatter" at the fuel pump and to keep the pressure at the carb. more constant. I think that the story that the fuel gets "cycled" and has a chance to cool down with a "T"-piece before the pump is mistaken and real gains will only be made by piping the returned fuel all the way to the front tank of the 500. :bang:

fiat500 said:

The tank in the 126 for which this system was designed was in a more conventional location under the floor at the rear and it was relatively easy to arrange some neat return plumbing and I am sure the gain was that the fuel had a chance to cool down. But there may also have been an intention to reduce "chatter" at the fuel pump and to keep the pressure at the carb. more constant. I think that the story that the fuel gets "cycled" and has a chance to cool down with a "T"-piece before the pump is mistaken and real gains will only be made by piping the returned fuel all the way to the front tank of the 500. :bang:

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Thanks for explaining how the fuel pump acts when the carb is capped off that makes sense now.

I don’t disagree that ultimately the best solution would be to return the fuel to the tank as per the 126. I agree there is potential for the fuel pumping around the engine bay is going too warm up but it is circling around the engine bay in probably the coolest part, so I don’t think there is any risk of vaporisation in the fuel lines and I don’t believe that Fiat were trying to alleviate the problem there. I would have thought the whole point of cycling the fuel through was to stop vaporisation in the carb bowl when the car was stationary after the engine having run for a period of time? Despite having the Bakelite spacer the carb does get pretty warm.

I can’t think of any logical reason other than vaporisation why Fiat decided to cycle the fuel back to the tank? It must have been a problem that was identified over the years that they were trying to improve? I can’t imagine they changed things if there wasn’t a problem to fix, it would have been a lot cheaper to keep things as they were. It would also explains why Polish built 126’s added this flap to divert air onto the carb on top of the tinwork on later cars.

You and all previous are right in saying that the primary objective of the return was cooling and it also reduces pressure at the carb inlet. Combined with constant movement of the fuel, this in itself delays vapour-lock in the supply pipe from the pump. My 900e has the system too..... another hot-running, rear- mounted engine. Part of the reasoning for it and your little flap...[emoji33] will be that whilst power output has been increased to a stupendous level with the 650 engine, the cooling system stayed unchanged. I've wondered if the 126 has a greater area of cooling intake aperture (or exits.)

Has anyone tried a direct comparison on the same car?
trying a carb with capped line, then trying it with a return line to compare the results?


I must admit I am guilty of fitting a blanking plug but never researching the reasoning behind it.


I've had mine in place since 2006, I've always fitted 126 fuel pumps and imb28 carbs and have had no issues that have caused me to investigate if it is correct or not.


I've been playing around with carbs recently and I may run a bit of a trial.


I know how it runs blanked but it would be interesting to see how it runs with a return installed, without changing anything else.


I have a slight fuelling issue that needs addressed but once I've sorted that out I'll give it a shot.

About two years ago. Before all this talk about the cooling effects of the fuel return line, I decided to cut off the brass fitting so I could block up the tiny little return hole with a small bolt tapped into the hole and loctited in.

Has anyone done this with success or do you think I’ve destroyed the top of a perfectly good carby?

I was aiming for a neat solution that didn’t require extra plumbing.

1500500 said:

Has anyone done this with success or do you think I’ve destroyed the top of a perfectly good carby?

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As I said above, mine has been blocked off for over a decade. The longest journey has been 566 miles in 2 days, never had an issue. (With the carb!)


So blanked off works, but I'm curious to see if a return line would improve performance or efficiency, but it won't change reliability for me.

MOD500 said:

As I said above, mine has been blocked off for over a decade. The longest journey has been 566 miles in 2 days, never had an issue. (With the carb!)


So blanked off works, but I'm curious to see if a return line would improve performance or efficiency, but it won't change reliability for me.

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When you say blanking plug in your post is that a brass fitting pressed in, plug bolted in or a piece of fuel hose with plug held in by clamp?

1500500 said:

When you say blanking plug in your post is that a brass fitting pressed in, plug bolted in or a piece of fuel hose with plug held in by clamp?

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I used a fuel hose with the end of a 6mm drill bit (trimmed to suit of course.) with 2 clamps. not as permanent as yours, but the effect is the same.

MOD500 said:

I used a fuel hose with the end of a 6mm drill bit (trimmed to suit of course.) with 2 clamps. not as permanent as yours, but the effect is the same.

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Ok thanks

I will keep going with the idea of tapping the hole and inserting a bolt. Once it’s done I will Use an electric fuel
Pump to check for leaks and report back.

I suggested previously that if you have the fuel recirculating from the carb return to the pump supply line, all that is happening is that the fuel has another 600mm or so of pipe in which to cool down, heat up or stay the same temperature. Whatever it does, any change will surely be marginal. But in addition you have increased the potential for damaged pipes and loose connectors. I've now done 7,000 miles with the blocked 28IMB and I've had a total of zero cold-starting, hot-starting or running problems......(touches wood).

fiat500 said:

I suggested previously that if you have the fuel recirculating from the carb return to the pump supply line, all that is happening is that the fuel has another 600mm or so of pipe in which to cool down, heat up or stay the same temperature. Whatever it does, any change will surely be marginal. But in addition you have increased the potential for damaged pipes and loose connectors. I've now done 7,000 miles with the blocked 28IMB and I've had a total of zero cold-starting, hot-starting or running problems......(touches wood).

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I don’t disagree with what people are saying and ideally as we have discussed before the best method would be return the fuel back to the tank as Fiat intended. They clearly introduce the return line to fix a problem they they had identified and it was possibly something that occurred in hotter climates that we don’t experience. On a 126 if I remember correctly the tank was under the rear seat, so to reach it fuel pipes would still need to run through the engine bay but obviously it went straight back into the tank which would have made sure that the returned fuel would drop down to the ambient temp of that inside it.

I don’t think it would have been done to cure a problem with fuel vaporisation in the actual fuel pipes themselves. It would have been much more logical that any vaporisation is likely to take place inside the carb bowl or the fuel pump once the engine has been switched off after a drive. A greater flow of fuel cycling through the fuel lines would almost certainly assist in getting fresh cooler fuel into the carb and pump.

As I posted on the other thread with that link to the VW website that suggests that modern fuel tends to vaporise at a much lower temperature than the old leaded fuel our cars were designed for and that inline fuel filters are probably detrimental to the running of older air cooled cars as they can restrict the flow of modern fuel.

To be honest I was always pro fuel filters thinking that they couldn’t cause any problems and could only assist in keeping crap out of the carb but I must admit I am pretty much converted to the other train of thought now that they are probably a potential hindrance.

1500500 said:

Ok thanks

I will keep going with the idea of tapping the hole and inserting a bolt. Once it’s done I will Use an electric fuel
Pump to check for leaks and report back.

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Here is the tiny little hole of that needs blocking off. Has anyone ever used lead solder or similar to block a hole in a carby?

The drilled holes for jet passages are blocked with something that appears to be jammed in the holes.

Would fuel tank repair handle the heat of the engine bay?

Any thoughts?

If you are going to go down the route of blocking the 'return' outlet on the carb's top-plate, I would have thought that the best material to use would be "J-B Weld". This is a 2-part epoxy metal, which is resistant to most things. My old marine engine had part of one of the barrels repaired with this stuff about 15 years ago--it is still water-tight and regularly is at 140-160F. It does, of course, have a MUCH higher working temp than that, and is impervious to petrol.