MGB & GT Forum

I noticed my fuel filter was only about 1/4 full recently. I wanted to fill it as I still have a small stumble ( rough running ) when out for a ride. The fill up went well and I took her for a spin. After about 20 minutes, I had the stumble again and when I got back to the garage noticed the filter was again only 1/4 full.
I was thinking that if I prefilled it, it would stay full as the fuel pump would be reacting to fuel supply on a full filter.

Any ideas as to why this would be happening?

Dave.

The fuel filter does not fill up in regular service. Your problem is somewhere else.

Btw. The first rule to keep in mind is that most carb(fuel) problems are ignition problems.

Does the issue always occur after the car is fully warmed up like it did this time?

It's no fault of the filter.

Pull your plugs and check them.

Pop your dizzy cap and ensure your contacts are clean as well as the rotor. I pull and zip them clean with my Dremel's wire wheel at every other oil change when I check the point's(which I run a point's file through). As previously mentioned; Most fuel issues end up being ignition...

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"One test is worth a thousand expert opinions"--Alvin "Tex" Johnston...Boeing test pilot.

"Who do you think you are? I am."...Pete Weber

73 MGB. Tires: Round, black, hold air. Oil: Sometimes old, sometimes new...always slippery. Oil filter: Yellow, usually full of oil. Carbs: 2 SU HIF. Distributor: Yes. Headlights: Not that bright but bright enough. A bunch of other stuff most cars have but not really important enough to itemize. Oh, wait...it has a cool sounding exhaust with stickers on the chrome tips. Really slays the ladies...

Never had a full filter on any of my Bs. If I did I'd be wondering why

I always wonder why people look at the fuel filter, presuming that it is either clear or transparent, and become concerned that it is not full. The carb fuel bowl is what needs to remain full to supply the demand of the intake.

Your '73 has twi8n SU's right?
are you sure your choke (enrichment device) is 'in' (open) all the way? Your air filters are clean?


stumble could be
old fuel
incoret timing
bad wire
bad spark plug
cracked distributor cap
failing condenser
worn points
intake manifold air leak
vacuum leak
weak fuel pump

What are you OTHER symptoms?

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Michael J. Caputo
'79, '77, '76, '74.5 (rubber dual SU), and '73 owner. Extensive experience in 12v Audio System design and installation. Vendor of Regalia and Promotional Products since 1993. Supplier of Accessories to MOSS. Forum Member since 2009; with a warped sense of humor since birth. Publisher of the annual MGB & GT Calendar, mailed worldwide.

Never mind the bubble in the filter. If you romp on it, the bubble will get bigger when the carb's needle/seat open and draw fuel in. If you have a stumble with your HIFs, the first ting to check is whether the floats are still full of air or if they've filled with fuel. That would leave the needle/seat open all the time and fuel could be pumping right up the jet. Just a thought. And it could be coli or condenser too :-). Basil 707.762.0974 basiladams@yahoo.com

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Basil C. Adams
1956 MGA Coupe (Show Car)
1957 MGA Roadster (Driver)
1958 MGA Coupe (Racecar)
1959 MGA Coupe (unrestored)
1960 MGA Coupe (unrestored)
1960 MGA Roadster (Driver)
MKIII Elva Courier (E1056)
1967 427 Cobra
1972 Alfa Romeo Montreal
A coupla late MGBs
1960 Austin Healy BN7
More Cars than Brains

Thanks everyone for the replies.

To Barry - if fuel bowl is what needs to be full, and I started with a full filter but then it dropped to about 25% full, isn't that saying the carbs are drawing more than the pump can supply? I did notice, when I primed the filter full, that I had some air being pumped into the filter from the fuel pump. I will check that again.

To Michael;
old fuel - fresh fuel to fill the tank in December
incorrect timing - possible but I had an MG mechanic "fix" my carb issues in December and not other than the stumble the carb runs great.
bad wire - 2 years old
bad spark plug - 2 years old but I will check them
cracked distributor cap - 2 years old but I will check
failing condenser - electronic ignition
worn points - see above
intake manifold air leak - I'll check with carb cleaner trick
vacuum leak - see above
weak fuel pump - how do you check for this?

Coil sound like a question mark? The coil in this car looks like it has been in the car for quite a while.

If your manifold is warped, the leak will be so small as to not be readily detectable.

Ensure your carb mounting nuts are tight and you have good seals in the throttle shaft.

I'll almost guarantee the manifold is warped on both surfaces. I've yet to find one that's not. They draw up toward the studs so finding the leak via traditional methods is virtually impossible because the leak is so small in one spot but taken over the entirety of the area equals a significant amount.

Also, check both ends of the "log" where it's plugged.

It's more than likely not the coil...

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"One test is worth a thousand expert opinions"--Alvin "Tex" Johnston...Boeing test pilot.

"Who do you think you are? I am."...Pete Weber

73 MGB. Tires: Round, black, hold air. Oil: Sometimes old, sometimes new...always slippery. Oil filter: Yellow, usually full of oil. Carbs: 2 SU HIF. Distributor: Yes. Headlights: Not that bright but bright enough. A bunch of other stuff most cars have but not really important enough to itemize. Oh, wait...it has a cool sounding exhaust with stickers on the chrome tips. Really slays the ladies...

Think of the filter as as can of beer. As long as there is beer in it, you'll keep drinking, even if it's down to 25%. Really, the bubble in the filter has nothing to do with anything. I've driven cars tens of thousands of miles with a huge bubble in the filter.

Beer can analogy understood, but if a filter is always nearly empty when the engine is idling, with just a dribble a fuel from the pump, is that necessarily an indication of a problem? I'm fearing that's a sign of a carburetor about to flood.

No, in itself, there is no problem with a nearly empty fuel filter. There should not be more than a dribble of fuel when the car is idling. I can't think why these conditions might indicate a hightened possibility of flooding.

If air bubbles are coming out of the fuel line, when you prime the filter, you have a leak in the fuel line between the tank and the pump. RAY

I'm thinking that if the carb was flooding there would be no build-up of fuel in the filter; everything would be flowing into the carb and eventually out the carb through the overflow. It just turns out the most recent time I had the complete empty filter, the idle speed was stumbling, engine was hard to start and there apeared to be some excess gasoline in the carb throat.