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MG MGB Technical - Petrol Fumes

G'day. Just got my '67 back on the road after a restoration. I have a problem with petrol fumes, but it only happens on the overrun. There are no leaks anywhere, the carbies are not leaking and the smell is only there when decelerating. Happens with the soft top on or off.

Any ideas what it is or is this just a bit of blow back through the filters or a bit of unburnt fuel in the exhaust.

Attached is a picture of the latest addition to road going B's.

Cheers

Tony


Tony Oliver

If the mixture is very rich then the fumes could be there all the time but when you are driving they are just behind you and when decellerating they 'catch you up' and you can smell them. However I would expect a difference between hood up and hood down.

Forgive me, as you have just restored it, but is the top of the tank sound? They frequently rot through here and when the fuel surges forwards it could be slopping through. Should be visible as staining round the tank.
Paul Hunt 2

Thanks Paul. Mixture should be almost spot on, I took a lot of trouble to set the carbs up. Tank was good when refitted but I will check it out. I feel that any leaks would be noticeable all the time not just on deceleration.

Cheers

Tony
Tony Oliver

You might have a pressure leak somewhere between the pump and the carbies. When the carbies stop drawing so much petrol pressure builds a little. Is there a smell of petrol with the ignition on (engine off) and the fuel pump slowly ticking over every now and again?

PETER

I'll check that Peter. Shouldn't be too hard. I have two pumps fitted, one down the back as an emergency and the one, in daily use, sits in the engine bay next to the heater, so there is only about 12 inches of hose from the pump to the carbies.

Tony
Tony Oliver

The pressure doesn't really vary with an SU pump, apart from when the pump is energised when it is zero. Other than that the pump is applying pretty-well constant pressure of only 1 or 2 psi. If there is a leak then it will happen at idle, or with the ignition on but the engine stopped. In this latter case the pump shouldn't tick more than once every 30secs. If it ticks more frequently than that either the non-return valves in the pump are leaking, or there *is* a leak either in a pipe or float valve. However after-market pumps often apply higher pressure than this which can overwhelm the float valves, and they need a pressure reducer.
Paul Hunt 2

I had the petrol smell in 79 MGB. I discovered the line from the tank to the canister located in the trunk (boot) was crumpled and had disconnected from the canister. After repacing all the rubber lines to and from the cansister the odor went away.

Cheap fix and not an item many people check

cheers

Gary :>{D

79mgb
gnhansen

Tony,

just a question that came to my mind... are the hoses in your petrol system legal for petrol or are they just the right dimensions to fit?
I have seen and smelled it if diy restores, that, BTW, made a good job did not know the differences.

If your car has been restored (nice to see on the pic.) have you also retightened the hose clips after fitting them?

Just an idea but worth checking it, i think.

Hope this helps

Ralph
Ralph

Gary, thanks for the suggestion but my car doesn't have the cannister.

Ralph. All hoses are brand new proper fuel lines. Marked clearly as fuel line material, correct size and well clamped.

I can't see why faults such as those suggested would not happen all the time. I have driven about 500k since getting back on the road and only smell the fumes on deceleration.

Any thoughts on the carbies "misting" on the overrun. I have standard filters fitted.

I may be being a bit paranoid, but fuel smells worry me.

Tony
Tony Oliver

"but fuel smells worry me"

You certainly shouldn't be getting *fuel* smells and you are right to investigate them. The only time I have smelt fuel round my cars is when there has been a problem, although the V8 does vent more fumes from the carb vents/overflows after switch-off on hot days than the roadster. It's one of those relative things where different smells mean different things to different people. If there are definitely no physical leaks from the fuel system, and the mixture is correct, then maybe what you are getting is a normal 'exhaust' smell.
Paul Hunt 2

Tony,

Picking up on Peter's point - given the proximity of your second pump to the carbs - is there any overflow from the carbs through the overflow pipes down the left side of the block? You might need to be quick to detect this given block heat evaporating any fuel flow. Perhaps you could have a friend position to see any flow whilst doing a static reving/decelerating test.

Regards
Roger
Roger T

Thanks again to all you helpers. It doesn't happen when the car is in the garage so I am leaning towards the "normal" exhaust smell or mysterious misting in the filters.

Car will be back on the road tomorrow after replacing the rear crankshaft and front g'box oil seals. Wasn't sure which one was leaking so replaced both. Turned out to be the g'box seal, it was as hard as rock. I'll see what happens when I can get her mobile again.

Tony
Tony Oliver

This thread was discussed between 19/11/2007 and 24/11/2007

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