Not the distributor - but what?

Background: '72 MGB with later 70's engine, starts and runs fine cold, but once warmed up stalls out and may or may not start again, only to repeat the problem. I thought it was a shelled distributor (see previous thread), and after getting it rebuilt by Jeff at Advanced, reinstalled it this morning and it fired right up. Set the timing as Jeff indicated, things looked and sounded good, so I took it out around the neighborhood for a spin. Once I got it warmed up (maybe 10 minutes of driving) and as I was driving down a long hill it started to fade as if it was getting no gas. Rolled to a stop, tried to start it again (which it did but not for long), and acted like it was starved for gas. Wound up towing it home - again.

I have eliminated the dizzy as the problem, so all I can think of is carburetor or coil. I've heard of coils going bad once they heat up, but don't know the symptoms. This car has a Weber 38/38 side draft with electric choke. I bought the car last summer with that setup, and know nothing about Weber carbs. Could this be a needle valve or float issue? There is plenty of gas to the carb, as I've tested the fuel flow, but not the fuel pressure. I have a see-though filter that sits next to the carb, and it is always full.

Any ideas? I'd sure like to drive this thing before the snow starts to fall and I have to wait another six months.

Tim

Not having "boots onna ground" this can be a process.

Does the coil seem too hot after it runs and konks out?

You said "side draft"... somehow that seems incorrect. And if you measured fuel flow and it doesn't flood fuel down the throats, the pressure is likely okay.

What is the pump? A standard SU or an aftermarket one?

...another thought: is there a catalytic converter on the car? They can plug up partially and cause short run times.

I had a similar issue (though it would stumble not just die) turned out to be that the manifold nuts had loosened. when the car was cold the gap was small enough, when warm the metal expanded and opened the gap, the engine leaned out and died. The real easy test is to hold your hand or something in front of the air cleaner (restricting the air) if the engine picks up, it is getting air from somewhere else.

Since I do not recall your last post how about the coil being wired backwards or the fuel cap needing to be vented? Maybe trash in the fuel tank and getting sucked up to the screen in there at random? Good luck hunting this problem. I would take tools and ideas with me for a ride. When it happens pull the fuel supply line off the carb and turn on the key and see if its really still getting fuel.Be sure you are prepared and have enough fuel line and a can to catch the fuel and not have it running all over the exhaust manifold. Take a spare proper balasted coil and swap it at the side of the road. It does not need to even be boltd down. And be sure the negative terminal is the one going to the distributor. (normally white with black tracer??)keep the triple A card handy and stay in cell service areas! Bob

As Bob says, fuel tank venting would be my first guess.

When the car stops, remove the fuel cap. If you hear a whoooosh, you've found the problem.

Dr. E,

I believe I should have said down draft, not side draft, since I am talking about what I believe is a 38 DGES carb. At least that is what I have come to conclude by visually comparing it to photos and drawings online. The two throttle venturies are the same size, not different like a progressive carb.

The coil has always seemed warm to me, so maybe a quick swap with a good one would help eliminate that from the possible options.

The fuel pump is a standard SU, and has been redone, although how long ago I have no idea.

There is no catalytic converter on the car, but if I really want to work by a process of elimination, I could remove the current muffler and run it a bit to see what happened. I've had other old American cars that had that happen, and it can really drive you nuts trying to figure it out. Living on a nice quiet street with friendly neighbors sort of keeps me from wanting to try that option.

Tim

Thanks for that tip - I had not thought about air gaps. I'll check that today.

Tim

I've not tried the fuel cap test, but have noticed the PO has disconnected the return tubes to the tank for ventilation. That could certainly cause a vacuum if not vented properly. I'll check that out. I suppose one way to do this is to run without the cap on and see if the problem repeats?

Tim

Tim_Creger said:

I've not tried the fuel cap test, but have noticed the PO has disconnected the return tubes to the tank for ventilation. That could certainly cause a vacuum if not vented properly. I'll check that out. I suppose one way to do this is to run without the cap on and see if the problem repeats?

Tim

Click to expand...

run with a plastic bag over the hole (elastic band) and a pin hole.

Not that this is the problem, but I had a similar problem on a gas engined truck once and after a long process, we found out that it was a connection problem in a spade lug. When cold the outer part would make contact, but when hot it broke contact and the truck ran on the alternator until shutting down. It would not start again until cold. Very odd and rare. I know that bullet connectors, due to age, have connection problems if not cleaned. Just a hint. PJ

If no cat-converter, I doubt a muffler would clog to the point of stopping the engine. That just rules out one thing, we no longer need to consider the exhaust...

Coils are relatively cheap :wink:

The gas cap "test" is free :laugh:

I have the impression the fuel pump may be getting hot and stopping. Checking it when the stalling occurrs may yield evidence. Flow should be about a pint in 30 seconds if memory serves (that may be sideways info, as now I seem to need to leave-and-return to the house three times before I have all the "stuff" I should have on the first departure).

If I was inclined, I'd check the diaphragm in the PCV valve :wink:

:lol: If it ~had~ a diaphragm in a PCV valve. :shocked:

I started it yesterday with the gas cap off, still faded and died soon thereafter. I suspect it is internal to the carb or something to do with fuel pump/pressure. Please next post.

Tim

When it dies, have you tried restricting the air?

Sorry,

I got interrupted at the end of my last post. What I meant to say was "please see next post", then life happened and it is now 6 hours later.

I am attaching a photo to show the carb setup. I don't have a manual for the carb, so excuse me if I sound ignorant, but shouldn't there be a pressure relief line headed out of the carb back to the tank?

Tim

No, I have not tried that. If I can get it started tomorrow I will give that a try.

Thanks,
Tim

OK, I think I've found the problem. After studying a schematic of the carburetor I noticed there was a small screen filter on the inlet side of the float bowl. I pulled the top of the carb off and found no gas in the bowl. That easily explains the symptoms I've experienced, so I thought "AHA!" it's the filter. I pulled the filter and find it to be clean, and all ports that carry gas to be free of clogs and debris. So, back to the drawing board. I decided to try the fuel volume flow of one pint in 30 seconds, as Dr. E suggested. Fixed up an empty quart glass jar on the end of the line and turn the ignition on and.. and... and......... no pump clicky. Fuel pump must be working intermittently, since it filled the jar a few weeks ago, but now doesn't run at all.

So, I can say I've eliminated everything down to the fuel pump. I know I reworked that pump last summer when it wasn't working, so it's probably time for a new one, or would anyone suggest something else, like a rebuild? Anyone do a price comparison on new vs. rebuilt lately?

Tim

Pumps are easy to rebuild. In fact, if someone rebuilt it and didn't set the points to "tickover" properly, it may just need some adjustment.

Been a while since I've fiddled with one, but I seem to recall you want the points to "flip" near the end of the pump stroke. Doc will chime in if I'm wrong.

The good ol SU fuel pump. I can't tell you how many times those things left me stranded. I finally got sick of it and bought an aftermarket fuel pump from Autozone. It had a lifetime warranty and lasted 11 years before I recently had to replace it (for free).