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#553257 - 11/01/19 04:43 PM Won't idle.....
TBM Offline
Learner Plates Off!

Registered: 06/11/18
Posts: 463
Loc: Northants, UK
Now I seem to have a solution to the poor starting, I've got another interesting problem.

On the last three occasions, after I filled the petrol tank (I usually go almost 'brimful') she refuses ti idle. Starts fine, but when I get to a roundabout/junction/trafficlight etc the revs drop and then she stalls.

Starts again fine, and runs OK until the next junction etc. This happens for about 20 miles, and then she cures herself.

Initially I was thinking that tank debris was being stirred up and blocking the idle jet, but the problem is far too uniform and self rectifying for that.

My next thought is a slightly blocked breather vent in the fuel cap. My thinking is that there is much more chance of a vacuum occuring in the tank when she's full, and this affects the fuel flow. At high revx the draw of the fuelpump is strong enough to overcome the vacuum, and when she stalls, it gives the tank a second or two to equalise pressure so she starts OK. Once a couple of litres have been burnt off, the space is too great for a vacuum to occur.

Does that sound plausible? Any other suggestions? I will give the fuel cap a blow through with the airline, although am tempted to buy a lockable one....
1972 4/4 4 seater
Too many ratty motorbikes

#553291 - 11/01/19 06:57 PM Re: Won't idle..... [Re: TBM]
Paul F Offline

Talk Morgan Expert

Registered: 20/08/10
Posts: 2320
Loc: Costock, South Nottinghamshire...
Does the carburettor have a solenoid operated anti-run-on valve?

Experienced similar symptoms on our 1985 car with a loose connection on this valve - but cannot see why this would be linked to filling the tank as in your case. A zero cost option is to check the wire to the carburettor - if there is one.

Do you get a whoosh of air when you remove the cap? If not, unlikely to be your blocked breather option (stress on unlikely - so still possible).

The self rectifying bit is mystifying.
Costock, UK
2014 4/4 Rolls Royce Garnet Red
Disco 4
Teddy - 17h1 Irish Draught cross

#553296 - 11/01/19 07:07 PM Re: Won't idle..... [Re: TBM]
TBM Offline
Learner Plates Off!

Registered: 06/11/18
Posts: 463
Loc: Northants, UK
No, carb is a basic twin choke Weber - nothing electrical on it.

The 'slightly blocked' breather idea came to me on the way home, and by the time I got home, she was back running perfectly.

However, if it was blocked enough to get the 'whoosh' I think I'd struggle to get her running at all - I once had fun with a bike - on the way to a bike rally, she cut out. Took the tank bag off, got my tools out, checked plugs, fuses etc and all seem fine. She'd then start, I'd get 10 miles down the road and same problem. This happened four times until I realised the tankbag was blocking the cap breather......
1972 4/4 4 seater
Too many ratty motorbikes

#553328 - 11/01/19 10:28 PM Re: Won't idle..... [Re: TBM]
Graham, G4FUJ Offline
Salty Sea Dog
Member of the Inner Circle

Registered: 03/07/07
Posts: 22171
Loc: Cheltenham, Glos. UK
It does sound like a breather problem.
Graham (G4FUJ)

D8921 L44FOR '93 4/4 Giallo Fly 2 seat smile
'90 LR 90 SW
'07 MINI Cooper

#553373 - 12/01/19 09:30 AM Re: Won't idle..... [Re: TBM]
John V6 Offline

Member of the Inner Circle

Registered: 21/07/07
Posts: 13175
Loc: Suffolk
+1. It does to me too.
2006 Indigo Blue Roadster S1

#553377 - 12/01/19 09:35 AM Re: Won't idle..... [Re: TBM]
Gambalunga Offline

Member of the Inner Circle

Registered: 03/09/11
Posts: 11248
Loc: Mandello del Lario, Lake Como,...
It is not going to be your problem but in general terms it is not recommended to "brim" the tank when you fill it. Most recommend to stop at the first cut of of the bowser.

In the case of cars that are fitted with carbon canisters the overfilling of the fuel tank can cause raw fuel to contaminate the carbon canister. This can cause problems. In theory the fuel should be blocked from entering the canister by a valve but this may not always work or it may jam. The raw fuel in the canister may provide too rich a mixture until it is purged because the canister is normally purged of fumes by a connection to the intake manifold.

#553443 - 12/01/19 01:48 PM Re: Won't idle..... [Re: TBM]
Richard Wood Offline
Talk Morgan Enthusiast

Registered: 03/02/16
Posts: 1672
Loc: East Harling, Norfolk UK
Had this on a small capacity single cylinder bike in freezing conditions. The petrol cap breather hole was blocked by ice.

2018 Roadster - Red/Magnolia - "Morton"
1967 Land Rover series 2a SWB
1960 Velocette Venom

#553444 - 12/01/19 02:14 PM Re: Won't idle..... [Re: TBM]
IvorMog Offline

Talk Morgan Regular

Registered: 17/04/17
Posts: 789
Loc: Staffordshire
I might have missed some of your earlier posts and threads so forgive me if you've already covered this.

Have you had the carb rebuilt or at least thoroughly cleaned since you've had the car

There are all sorts of things that could cause those symptoms in the carb such as sticky floats or gummed up jets etc.

At least if the carb has been rebuilt you can rule out carb issues.

Edited by IvorMog (12/01/19 02:15 PM)

1999 4/4 1.8 Zetec

#553447 - 12/01/19 02:53 PM Re: Won't idle..... [Re: TBM]
TBM Offline
Learner Plates Off!

Registered: 06/11/18
Posts: 463
Loc: Northants, UK
Yep, carb was cleaned out by Allon White when it first happened.

The uniformity of the problem and then the 'self repair' points me away from the carb,
1972 4/4 4 seater
Too many ratty motorbikes

#553460 - 12/01/19 04:14 PM Re: Won't idle..... [Re: TBM]
britmog Offline
L - Learner Plates On

Registered: 13/12/17
Posts: 181
Loc: Colorado, USA
Rule number 1:

Never fill the fuel tank beyond the first shut off of the fuel pump on any vehicle of any age.

Such a common mistake to get an extra half gallon into the tank.
Bruce Marshall
1964 4/4 Series V Comp (Megan)
1994 +8 (Maurice)
2013 M3W (Olga)

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