Fuel starvation only when moving

Hello everyone, I'd really appreciate any wisdom on this - the dreaded intermittent fault.
My  1991 clipper with a webber carburettor repeatedly appears to be starved of fuel and stutters then cuts out when moving. From a standstill it will start again after a couple of turns and run fine for about half a mile before the same thing happens again. It never does this in tickover, only when I'm moving. It doesn't do it all the time, some days it'll run fine all day long, other days I'm stopping all the time. It had a new engine in last year but the fault was there with both the old and new engine.

Up to now my husband has tried the following (he's knows his way round an engine so has done all the usual stuff e.g. plugs, leads, distributor cap):

 - New fuel pump
 - Carburettor rubber mounting changed
 - Carburettor cleaned out
 - Tank checked (it's clean inside)
 - Sender unit checked and looks fine (not replaced because it looked ok)
 - All fuel pipes look in good condition - all rubber fuel lines replaced
 - In line fuel filter is clean

Could it be anything to do with the fuel vapour separator? Or does anyone have experience of this happening and could suggest what else to look at?
Much appreciated.
Clare
 

do you know if there is a restrictor in the fuel return line?, I have heard of welding tips being used as restrictors. you could try and bypass the separator and plumb fuel straight to the carb and plug the return line, is there fuel in the clear filter when it cuts out?

Thank you we'll try that - yes there is fuel in the clear fuel filter when it cuts out. The only thing is, we've never done anything to it and it ran for about 10 years without any problems. Could it suddenly start doing that?

Have you made sure no carb jets are blocked?   How far will it move before it stutters out?  will the engine rev up with the car not moving?

Hello there,
Yes the jets are clear.
The car can go for variable amounts of time before it stutters, from 30 mins to an hour.
It'll rev fine when it's not moving.
Today my husband has removed the thermostat because a mechanic friend said he had heard of incidences where the engine gets hot and the fuel vapourises in the carb, causing a stutter. Therefore by removing the thermostat the engine will run cooler and stop this happening - we'll see if that makes any difference.
Thanks. 

Vaper lock is a thing on aircooled engines as the fuel hardline can get over hot but never heard of it on a water cooled car.

I did have a similar problem to yours on an old aircooled vw engine, turned out the fuel pump did not supply much fuel, so car drove fine but then under high demand like hill climbing with foot down the engine would use more petrol that the pump was putting into the carb. it would then stutter and stall out.  i would check fuel pump pressure.

which new pump did he fit, some of the new stuff is total garbage.

Removing the thermostat will make no difference to a fuelling issue so please don't remove the old one unless the car is due an coolant change.
The thermostat needs to be in the car to regulate the coolant temperature.
 
The problem you are having may be down to to much fuel getting pumped back into the fuel tank so the carb is starved of fuel under load.
To test you can remove the return to tank fuel line and plug up the end and plug up outlet on th carb, the carb needs some sort of return so only try is short term.

Fitting a 7mm welding tip inside the return to tank fuel hose from the carb seems to be a popular choice of re stricter.

It seems some Webber carbs have this problem when fitted so there is a few threads on the web about this problem.

mark1gls said

Removing the thermostat will make no difference to a fuelling issue so please don't remove the old one unless the car is due an coolant change.
The thermostat needs to be in the car to regulate the coolant temperature.

The problem you are having may be down to to much fuel getting pumped back into the fuel tank so the carb is starved of fuel under load.
To test you can remove the return to tank fuel line and plug up the end and plug up outlet on th carb, the carb needs some sort of return so only try is short term.

Fitting a 7mm welding tip inside the return to tank fuel hose from the carb seems to be a popular choice of re stricter.

It seems some Webber carbs have this problem when fitted so there is a few threads on the web about this problem.

Do they really have a return line, carbs normally have a float in the bowl that shuts of pressure, never worked on a carbed car with a return before. (only really worked on aircooled for years)

I did hear once of one with similar problems and that turned out to be the electronic ignition module was coming away from the heat sink but it wasn't obvious from looking at it.
I would have thought that the basic fuel system was okay if it's been running for 10 yrs, it would be better to look for a fault rather than modifying it.

Yes the carb cars have a fuel return line, that is why there is 2 fuel pipes going to the tank and there is 2 fuel lines going to the carb.

More info here.

http://sciroccoregister.co.uk/carforum/viewtopic.php?t=15397

hi cabriolady did you get to the bottom of the fault as my cabrio has got exactly the same fault as yours. and i have changed all the same things as you and still no joy,and like you said it will tick over on the drive all day long. 

cedar said

I did hear once of one with similar problems and that turned out to be the electronic ignition module was coming away from the heat sink but it wasn't obvious from looking at it.
I would have thought that the basic fuel system was okay if it's been running for 10 yrs, it would be better to look for a fault rather than modifying it.

I had a peugeot 106 once that had a bad electronic ignition and it also had this kind of intermittent fault.

hi keith is the ignition module the square black thing held down with 2 bolts and a big electrical plug?

On my 1982 GTI it's on the passenger side next to the bonnet hinge, as you say a black box attached to an aluminium heat sink. I think there is some kind of heat conductive paste they use that is probably available from the likes of Maplins or RS.

lukecab said

hi keith is the ignition module the square black thing held down with 2 bolts and a big electrical plug?

Don't take my word for it that this is 100% the problem, I would  ask people in the main forums if there is a way of having the one you have tested before you spend money replacing it.

https://www.vwheritage.com/191905351b-tci-ignition-control-unit-t25-mk1-2-golf-without-points

As Keith says don't spend money replacing it, just make sure it has a good contact with the heat sink.

lukecab said

hi cabriolady did you get to the bottom of the fault as my cabrio has got exactly the same fault as yours. and i have changed all the same things as you and still no joy,and like you said it will tick over on the drive all day long. 

Husband is on nights at the moment but he's going to have a look - I'll let you know how we get on.
Thanks for your advice everyone.

mark1gls said

Removing the thermostat will make no difference to a fuelling issue so please don't remove the old one unless the car is due an coolant change.
The thermostat needs to be in the car to regulate the coolant temperature.
 
The problem you are having may be down to to much fuel getting pumped back into the fuel tank so the carb is starved of fuel under load.
To test you can remove the return to tank fuel line and plug up the end and plug up outlet on th carb, the carb needs some sort of return so only try is short term.

Fitting a 7mm welding tip inside the return to tank fuel hose from the carb seems to be a popular choice of re stricter.

It seems some Webber carbs have this problem when fitted so there is a few threads on the web about this problem.

He took it on a long run after he took the thermostat out and it ran great with no hiccups?

mark1gls said

Yes the carb cars have a fuel return line, that is why there is 2 fuel pipes going to the tank and there is 2 fuel lines going to the carb.

More info here.

http://sciroccoregister.co.uk/carforum/viewtopic.php?t=15397

We've had a look and there is one pipe from the pump to the vapour separator - one pipe from the vapour separator to the carb and one return pipe to the tank from the vapour separator.

mark1gls said

.

It seems some Webber carbs have this problem when fitted so there is a few threads on the web about this problem.

The thing is it's had the webber on for years without any problems before this started?