MGB Classic Gold fuel tank drain plug

[wkilleffer]

A couple of months ago, I installed a Classic Gold fuel tank from Moss on my 1974 MGB. The old tank appeared to be original, showed alot of rust, and seemed to be allowing quite a bit of fuel vapors into the boot. While I was at it, I replaced all the rubber lines for fuel and evaporation, and replaced the fuel filter. I no longer smell fuel in the boot and the car seems to run better, at least when it's running...

Trouble is, I've since had a couple of instances where I've had to be towed home because insufficient fuel was making its way to the carbs. The pump's a points SU that's a little over a year old, and its connections are solid.

I'm of the belief that the fuel I poured into the new tank might have had some contaminants that weren't visible, and that they've settled near the tank's pickup tube and may have clogged the "sock" on the tube's end. Part of what may support this notion is that the time before the last time I had to be towed, I used a bicycle pump to blow backwards through the fuel line. I could hear it bubbling in the tank. After doing this, my SU pump would fill a 1-pint container in about 45 seconds. It pumped good and strong, and I thought I was home-free. But the same thing happened again. The fuel pump gets a kind of squelchy sound, and I soon find myself bogging down and at the side of the road. Removing the gas cap makes no difference.

So, I decided to drain the tank and get the bad gas out. There's a plug in the tank's bottom where a 10mm hex key fits snugly. I thought this would be a piece of cake, but it's been anything but. I've tried vice grips to no avail, and even violated a cardinal rule of tool treatment by sliding a length of pipe onto the key. All that served to do was twist the key a bit.

Has anyone else had this kind of trouble? I knew it might be tight because we don't want leakage, but I really thought it would come loose with some gentle persuasion.

Any assistance would be appreciated.

Thank you,
-William

 

[dklawson]

Unless you have already contacted Moss to confirm a replacement plug is available, I suggest leaving the tank plug alone. If it is this hard to turn, you will likely ruin it getting it out of the tank.

You said that you blew backwards through the fuel system into the tank. Was that AFTER or BEFORE the fuel pump? You should not be able to blow backwards through the fuel pump. It has check valves which should prevent backflow.

The amount of debris that may have been in the old gas should not plug the pickup tube screen in the new tank. However it won't hurt to drain the tank, blow it out with compressed air, and try again.

You said the pump was new and a points type. When the car dies next time try the following two tests: 1) leave the ignition on and go back to the fuel pump and notice if it's ticking, 2) remove the gas cap and pay attention to whether or not you hear air rush into the tank. The pump will tick away as long as its points are in good shape and it can move fuel to the float bowls. If you don't hear ticking, take a chunk of 2x4 and strike the metal body of the pump a few times. If ticking resumes, you have a pump points problem. If you heard air rush in when you removed the fuel cap you have a plugged tank vent line and/or need to fit a vented cap.

 

[wkilleffer]

Unless you have already contacted Moss to confirm a replacement plug is available, I suggest leaving the tank plug alone. If it is this hard to turn, you will likely ruin it getting it out of the tank.

You said that you blew backwards through the fuel system into the tank. Was that AFTER or BEFORE the fuel pump? You should not be able to blow backwards through the fuel pump. It has check valves which should prevent backflow.

The amount of debris that may have been in the old gas should not plug the pickup tube screen in the new tank. However it won't hurt to drain the tank, blow it out with compressed air, and try again.

In the interest of not seeming too long-winded, I didn't mention that I had taken each part of the fuel line apart piece by piece and blew through it. I didn't try to blow through the fuel pump because I knew that wouldn't work.

I'm going to go blow through the evaporative line. I didn't do this last time because the place where there's a fastener in the right rear fender well was clear of rust and debris when I opened it, and I was told that was an area that could cause trouble.

Now, I've been told that about removing the gas cap and listening for a whoosh of air. I don't think I've heard it, and don't think running the engine without the cap has made a difference. The pump has also continued to pump during these problems, but instead of a steady pulse that would fill a pint in 45 seconds, it would only send out a weak pulse every couple of seconds.

Thank you,
-William

 

[wkilleffer]

I just got through blowing air through the vent line and the tube that runs from the tank to the pump. Did both things with the gas cap removed.

While blowing through the vent line, I couldn't hear anything from the rear of the car, but I could see some shimmery air coming up from back there. I didn't get any backpressure, so I'm surmising that the vent line is clear.

I then blew through the line that runs back to the tank from the pump. I could hear "underwater" bubbles. Did that four times and put everything back together. I disconnected the fuel line at the carbs, and was getting a nice, steady, strong jetting that had I been measuring, it would have easily filled a pint in less than a minute. I collected that fuel and poured it into the lawnmower can.

I haven't yet tried a drive, mostly because I can't afford another towing bill. The AAA membership is lapsed and I haven't yet signed the old girl up for a classic car insurance policy that includes towing. Another story for another day...

Any further thoughts?

Thank you,
-William

 

[dklawson]

I'm sorry if I overlooked things you said you had already checked.

Based on what you observed the vent line is open. Likewise, if you never heard a whoosh of air going into the tank when removing the cap, that also says the vent is open.

The bubbles you heard indicate there is no blockage between the tank and pump.

If you had a lot of dirt, scale, and rust in the old fuel that you put in the new tank there is a chance that debris is blocking part of the pickup tube. However, pickup tubes are typically not touching the bottom of the tank and they are typically covered with a coarse screen. I anticipate you would have had to pour a lot of trash in the new tank to block the pickup. I really doubt that happened. You would have seen it.

You said the pump was new and that it seemed to benefit from blowing backwards through its supply line. Is this the AZX1307 pump or something else? Most SUs have a screen on their intake check valve. You could partially tear down the pump to inspect its input screen. If the pump is still very new you could also take some 400 grit wet/dry paper and pull it through the pump points (once or twice only), then spray the points with alcohol to wash away any debris or coating Burlen may have applied to the new parts.

The pump should tick away nicely until the float bowls are full then slow and stop ticking. I have not heard other noises from the points type pumps.

 

[wkilleffer]

No need to apologize. Just because I may have checked something doesn't mean that it's 100% right after I'm done with it.

After draining the old tank, I let its fuel sit for a day so anything in it would settle, knowing that I wouldn't be able to use all of it. There was quite a bit of rust dust in the bottom of the bucket. I was careful when pouring it into the new tank, but that didn't mean I did it perfectly. There was a small amount of sand grain-sized rust in the fuel filter by the carbs, and that's quite a bit less than what I used to see. I have looked inside the pump and didn't see any signs that it's getting clogged.

After I poured the previous gas in the tank, I went to the VIP for some ethanol-free 93 and filled it up. This place also sells some racing fuels, but that's another story for another day...

The reason I thought draining the tank through the plug was a good idea is because the pickup tube is right there. Any crud would drain right out.

Edit: In my previous reply, I mentioned not being able to hear a whoosh of air and that the pump was only pumping every couple of seconds. What I meant by that is that whenever one of these incidents has taken place, I have unhooked the fuel line from the carbs and switched on the ignition. What happened was very weak pump pulses that were around two seconds apart. After blowing out the supply line to the pump, I would get strong, steady, high-volume pumps that would fill a US pint in less than a minute.

Could water have made its way in and settled down at that low spot in the tank? I haven't seen anything like that in the fuel filter.

Yes, the pump is an AZX-1307. It works pretty well like you said. Pumps until the bowls are full, then all but stops pumping. It was trouble-free in the brief time that it was installed with the old tank.

 

[dklawson]

Water won't affect the delivery or sound of the pump so I doubt that is your car's issue.

Since the pump worked well with the old tank and not the new one... it does suggest an issue with the tank.

 

[wkilleffer]

Ok, the first time there was an incident, I was very close to home but had been doing a bit of local errand-running. That's when I went piece by piece through the fuel lines and blew them out, as well as replacing the fuel filter. I drove the car several times, including to work on at least one occasion, without issue.

The most recent time took place on 08/20 when I was five miles from home. I guess what gets me is that all will seem well, then the car will start acting fuel-starved and I'll end up at the side of the road. This required a tow home. As stated in my original post, I once again blew back through the line from the tank to the pump, and this time, also blew back through the evaportative vent to the pump.

I've also checked the electrical connections, but I was checking them when these incidents happened as well. The connections at the pump are clean, the trunk grounding point is good and clean, and the white wire connections at the fuse box and where the harnesses come together is also clean. The main harness is about a year old, and the rear harness is a little older. The fuse panel was replaced at the same time as the main harness. Those connections are also clean.

Today, I've ventured a little further afield, taking a drive of around 18 miles without incident, and will have the car out again this evening.

 

[DrEntropy]

The "vapor separator" and the evap canister can load up with fuel and cause a similar issue. Check to see if the canister has liquid fuel in it. Unfortunately, that requires removing it from the car.

But you have said there was no "woosh" and no change with the no-start condition when removing the gas cap. Just trying to cover the bases.

 

[dklawson]

Thanks for the summary.

In two previous posts you mentioned checking the fuel delivery rate and found the pump capable of providing 1 pint in less than a minute. Have any of your tests shown a delivery rate lower/slower than that?

Your signature suggests you are dealing with your '74 MGB. Please refresh my memory. Did the '74 have the dual HIF carbs or did it still use dual HS4 carbs? If the car has HIF carbs, did you check the float valve? Some HIF float valves have a built in screen filter (like the fuel pump). They can plug up.

 

[wkilleffer]

I think I mentioned two different occasions where I blew through the line running from the tank to the pump. I thought to do that because when I ran a pump test, the pump seemed to be giving an anemic effort. It would click every couple of seconds, and deliver a small amount of fuel. I can best say that it seemed rather unenthusiastic. After blowing back through the line from tank to pump, it would resume its usual rate of a pint in less than a minute.

I probably should have been more clear: my car is a '74 with an 18GK engine, and is equipped with twin HS4's. Now, shortly after I replaced the tank and some of the lines, before it even had a chance to play up like it is now, there was a no-run situation that resulted from quite a bit of junk in the grose jets in the float bowls. That has not been a problem recently, however. Last time I had a problem, while waiting for the tow truck, I had a look into both float bowls and the grose jets, and they were all clear.

Thanks for the summary.

In two previous posts you mentioned checking the fuel delivery rate and found the pump capable of providing 1 pint in less than a minute. Have any of your tests shown a delivery rate lower/slower than that?

Your signature suggests you are dealing with your '74 MGB. Please refresh my memory. Did the '74 have the dual HIF carbs or did it still use dual HS4 carbs? If the car has HIF carbs, did you check the float valve? Some HIF float valves have a built in screen filter (like the fuel pump). They can plug up.

 

[dklawson]

If you had an open discharge line and the pump was only cycling every couple of seconds there are only a couple of possible problems. The first is a blocked suction port and the other is damaged/burnt pump points. I suppose it could also be an out of adjustment pump toggle.

I believe in trying all the cheap fixes you can before spending money. You have tested and measured everything I can think of. The only remaining cheap thing I can suggest is to carry a piece of 2x4 or hammer handle in the boot and drive the car. If the car loses power and the pump sounds slow and weak... whack the metal pump body with the wood. If it resumes normal operation, work on cleaning and adjusting the pump toggle and its points. Alternatively, you could buy an inexpensive, 3 PSI Facet type pump on eBay and use it in place of the SU pump. If you want to use an original SU pump, consider the Facet a stopgap part until you service the SU pump. Later you can carry the Facet in the boot for emergencies.

Using Grose jets can be a hit or miss situation. I no longer use them. I have had them stick closed and stick open.

 

[wkilleffer]

Well, I've put a grand total of about 25 miles on it today without any issues.