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MG TD TF 1500 - Carburator Burp

I have a K & N conical shaped air filter on my SU H4 carburetor. The carb sits on the rear end of a Judson supercharger. I have some tune up issues I am working on, like my supercharger stopped supercharging, but I wonder if anyone has some thoughts on my burping problem. Whenever I turn the engine off, there is a burp of gasoline that spits out of the carb. You can see it in the short video. The carb has been refurbished by Dave Braun and we have discussed it without a definitive answer.
Any thoughts?

The engine is off at about 6 sec and burps at about 11 sec.

Mort 50TD (1851) Mobius

Hmmmm could just pack that funky old Judson in a box and send it to me. I send you back a pair of carbs with K&N filters in Volks air cleaners.
Problem solved!
(Somehow I don't think this is going to happen.)

Nice set-up!
David Sheward

Mort, I think a phone call to George Follci would be in order. I'll bet he has the answer. Not sure about the spelling of his last name. I have some guesses, but that's all they would be.

Gene Gillam

In discussing this with Mort:

The original carburtor did not have an overflow pipe, instead it had a short nut and solid washer to fasten the float lid to the float bowl. We added an overflow pipe. It is oriented correctly with the larger diameter facing the scalloped washer on the floatbowl lid.

The original carburtor depended on the tickler pin to vent the float bowl. The tickler pin was quite worn. We replaced the pin with new when we added the overflow pipe. It fits better but could still vent.

The carburetor sits straight on the Judson, hence the float bowl is vertical.

The floatbowl is further from the exhaust manifold than the standard set up.

There are no obstuctions in the transverse tube from the bowl to the jet.

The float level seems lower than one would expect in the jet after allowing the bowl to fill. Mort bent the float arm with not a lot of change to this.

The fuel pump has not been verified as to its pressure or capacity.

Ideas welcome!


Dave Braun

note due to the throttle and the fuelbowl being in close proximity, the throttle comes in from the back side and the disk rotates the opposite way, with the throttle adjusting screw at the bottom of the stop.

Dave Braun

"my supercharger stopped supercharging..."
I don't know squat about superchargers but if isn't working maybe the fuel has nowhere else to go? Duh?
efh Haskell

"my supercharger stopped supercharging..."
I don't know squat about superchargers but if isn't working maybe the fuel has nowhere else to go? Does it do this when the blower was working?
efh Haskell

LaVerne, Gene,
I have talked to George Folchi. He could not think of any way the supercharger would cause the problem whether it was working or not. It has a positive displacement. Besides, why the burp 6 seconds after the supercharger has stopped?

When the blower was working there was always a wet spot at the bottom of the filter. I have just recently been focusing on it. I just happened to notice it a while back when I had the air filter off and the engine stalled while I was tuning it.
Mort 50TD (1851) Mobius

Just a guess, but it sounds like a carburetor problem, not the blower. I would certainly check the fuel pressure, might even lower it a bit if you have a regulator. Even if new, the float valve could be bypassing a bit until the static pressure drops. PJ
P Jennings

Ok Mort I'll throw my site unseen guess work here based on what you have said. The key thing being the statement " it isn't super charging".

I'm guessing that the vanes aren't turning for what ever mechanical reason. Stripped gear? Broken shaft? sheared pulley key? Maybe even broken vanes. What ever it is I'd say that's the culprit. Your'e getting a rather large plenium sitting there full of fuel drenching all the now non spinning parts and when you shut it down it's draining to the low point. In this case your carb. I'm guessing it takes a while for it to fire when first started also. I think you have blower issues not carb problems. I'm rather suprised that George didn't offer up some thoughts...mostly about whats up with the super charger.

What you say makes sense. But..... Before this winters worth of work on Mobius the supercharger was giving me about 5 psi boost. At the same time I always noticed some gas at the bottom of the air filter. I don't think it was as much as I am now seeing. It could be that the supercharger was working but not completely up to par, so I was getting boost and a smaller amount of blow back of gas.

Now perhaps, the supercharger has failed to the point of not giving me boost and there is even more gas in the chamber. There may be some internal component in the supercharger that opens and releases some of the compressed air/fuel. There may be some kind of gate or one way valve designed to keep everything flowing toward the engine. If this has failed that could explain the symptoms.

I am going to drop Mobius of at Bill Thomas, who does a lot of classic restoration and mechanical work, this weekend. He will analyze it next week and we shall see.

Mort 50TD (1851) Mobius

Not sure there's anything internal to the Judson that could cause it:

BUT...looking at your installation...the Judson, although not compressing the air is still drawing gas and some amount of air through the carburetor...enough to let the engine run. When the engine stops the unused gas will run out of the Judson, down into the intake/carburetor and out into the air filter.

I can't imagine what could have happened internally to the supercharger though...surely all the blades didn't suddenly let go at once? Unless, of course, the pulley is just spinning on the shaft because the key sheered?
Gene Gillam

Mort; As PJ says-really need to know about fuel pressure and volume. Looks like uncontrolled fuel level in bowl is pushing fuel out the jet. New needle and seat and float level changes are not curing the issue so see if the pressure is high enough to push past the needle? Dan
Dan Craig

The Judson is very easy to take apart and put back together. I would check and see if maybe one of the vanes has broken or chipped off a piece. The vanes are made of some sort of old type plastic or that black stuff( can never remember what that is called. Anyway take it apart, easy to do.
Tom Maine (TD8105)

Been a lot of years since I had a car with a Judsen on it ...but thinking back recall simular problem long time ago. Culpret was a sheared pulley key.
i.e. No boost / srong odor of gas and "burp" on shutdown.
As I recall ...fairly easy fix.
Hope that's all it is for you.
David Sheward

After reading the responses and the link Gene sent I took the belts off. I rotated the pulley by hand and listened for the tap tap of the vanes. The tapping should be steady as you rotate the pulley and you should hear all four taps with no gaps. I get three taps and a gap. Not sure if one bad vane would give me the symptoms I have described (burping gas and no boost).

I also slide the pulley off to check the key but it is ok. So hopefully a rebuild on the supercharger should get me on the right path.

I'll post when accomplished.

Thanks for all the advice,
Mort 50TD (1851) Mobius

Once again... many thanks to you all for the various pieces of valuable advice. It was comforting to here Dave had a similar burping problem and it wasn't only me. I also have had a few conversations with George Folchi(Happy Birthday George). He had me check for a broken shaft internally but it turned out ok.
I did pull the vanes and have a new set on order. All four of them had 2 inches worth of chipped edges at the rear end of the charger. I will wait till they are replaced and see if the symptoms go away.
I'll post more next week.
Mort 50TD (1851) Mobius

This thread was discussed between 04/04/2012 and 06/04/2012

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