Technical Help Requested - 1967 500 with a 650cc Engine

[jgc]

I'm new to the forum - hope this is the best place to post.
This vehicle cuts out after about 10 minutes of driving.
It now has a 650 cc engine.
The fuel return port on the carburator has been plugged by someone.
Fuel shortage seems to be the problem.
The 500 fuel tank does not have a fuel return connection.

Does plugging the fuel return cause flooding or other fuel problems.
Disclaimer - I normally work on large engines.
I am new to these small guys!

HELP - Please!

JGC

 

[phoenix1]

Hi. When you block the fuel return off you may get over filling of the float chamber.What should be done is to return the fuel to a T piece just before the pump.You can check the output of the pump by just taking the pipe off the carb and turning the engine over with a bottle over the pipe I would think it could be the tappets or points that need some adjustment Bryan

 

[IanEmery]

As the engine runs fine for the first ten minutes my money would be on the tappets closing up as the engine gets hot. This is a very common on these engines.

Welcome to the forum and I hope you enjoy your car.

 

[Ricambi Fiat 500]

https://flickr.com/photos/28386563@N03/sets/72157622038829612
You can click on my link & it will show you how to do the fuel return, it is very simple. As you can see in pictures I use copper pipe. Please contact me If you need any help.

 

[fiat500]

https://flickr.com/photos/28386563@N03/sets/72157622038829612
You can click on my link & it will show you how to do the fuel return, it is very simple. As you can see in pictures I use copper pipe. Please contact me If you need any help.

That's very thorough and a helpful set of images.
Is there any advantage in doing this rather than "T"-ing in before the pumps as is described here more regularly?
I can't get my head around how the flow works when you do that but a return to the tank is easy to understand.
If the return pipe went under the floor it would also cool the fuel well; I find the supply in the tunnel pre-warms it too much when the heater is on (most of time in Scotland!), although obviously it wold be a bit prone to damage in the places where I drive.

 

[Ricambi Fiat 500]

I just copied the design that the fiat 126 has done with the 28 webber with the return straight to the tank. Like i have done in many of my clients cars and no one has had problems. Like the fiat has done in the 126. The fiat did not do it with the t connection like you mentioned. I prefer that way i have never had any problems with my cars. We are all free to choose the way that works best. [emoji1417]

 

[fiat500]

I prefer that way i have never had any problems with my cars. We are all free to choose the way that works best. [emoji1417]

I like your way best. The other way is just relieving the pressure....at least it may be. The full return allows the fuel to really cool down whereas just cycling it back to before the pump keeps it in the hot zone.

 

[Paolo66]

Blanked mine off and never had a problem. Cant see the need to route another pipe

 

[fiat500]

Blanked mine off and never had a problem. Cant see the need to route another pipe

I think that's right as the 26IMB carb doesn't even have the facility and is otherwise a very similar design. But it seems that the intended purpose of the setup is to eliminate some of the perennial fuel issues many of our cars suffer from.
It is supposed to reduce the incidence of the petrol whiff you sometimes get from the carburettor overfueling. I don't think the 28 has the incredibly crude petrol dumping pipe at the base to alllow it to do that.
It is supposed to reduce the incidence of fuel vaporisation.
It is supposed to prevent some of the occasional hiccups you can apparently get when to much fuel is available and gets drawn into the engine.
It will reduce the strain that might otherwise be put on the needle-valve.
There are probably other benefits and the above may be rubbish or marginal anyway.
I doubt that it's a thing worth getting bothered about.

 

[Bambino]

This a photo of the offending pipe as fitted to the 28IMB. As you can see the return pipe is fed by a very small orifice. If your fuel pump and needle valve are performing correctly I can't see why it's required and usually causes no problems if blocked off.

I think the main issue with overflow happened when the choke was used when starting and I always had a minor issue with the way the excess fuel flowed out to the bakelite spacer and then down onto the engine shroud ...

The guy in the second picture seems to have triple returns fitted ...

Chris

 

[jgc]

Thanks everyone for weighing in on this.

I do have a question about these carbs.
Most of the carbs I work on have vented bowls and no fuel return line.
The float and needle & seat control the bowl level and the pump just keeps trying to push more fuel into the bowl.

Does the fuel return port drain excess fuel out of the bowl,
OR
Does the fuel return port bypass excess fuel BEFORE the fuel passes thru the needle and seat into the bowl?

(I need a tutorial on these carbs or at least to disassemble one and see the guts.)

 

[the hobbler]

Initially when I tuned the 650 engine that is in my 500, I retained the Weber 28 carb that it came with, complete with blanked-off fuel return! Initially I retained this system, but when I took the engine out (to change cam, port head, fit lighter flywheel and fit a sports exhaust) I took the opportunity to fit a simple 'fuel return' system. I fitted a METAL 'T' piece in the fuel supply pipe, before the fuel pump, and 'P' clipped it to the side of the engine bay. I then led the 'return' pipe up behind the big air-duct hose and round to the carburettor. Where I fed the 'return' round the back of the ducting, I fed it through a fairly small-bore piece of water-hose, to protect it from chaffing, and again secured it in place with 'P' clips. When I up-rated the carb to a Dellorto FZD, I retained this system, and so far I have not had any problems with it at all. Fiat obviously fed a return off the carb for a good reason, and I found this method the most cost-effective way to do it.

 

[Bambino]

I need a tutorial on these carbs or at least to disassemble one and see the guts.

Hi JG,

Have a look here - might help.

https://www.fiatforum.com/500-classic/355169-weber-26-28-imb.html

Chris