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MG MGB Technical - Carbon Canister

Hello,

Last weekend I had my mother-in-law stop at Victoria British in Lenexa Kansas and pick up about $850.00 worth of parts to restore my 1971 MGB. One of the parts that I was going to replace was the Adsorption canister. The car was running fine. As soon as I replaced the canister, it started, but would not idle, then eventually died and would not restart. I have put the old canister back on the car, but it still won't start. I have checked the fuel delivery and fuel is getting to the SU's. I am not sure what to check next. I can't tell if there is something wrong with the SU,s or there is such a vaccum leak that the engine will not turn over.

Thanks in advance for any help or suggestions.

Michael
Michael Cheek

Michael. There is some confusion here. You state, in the first part of your message, that the engine turned over and started, but would not idle. Later you state, "or there is such a vaccum (sic) leak that the engine will not turn over". Will it turn over? If not, that is a starter problem. If it will turn over, it is a problem with the fuel or ignition systems.

First, by removing the old charcoal cannister, you may have done something to plug the vapor line from the cannister to the fuel tank. This serves as the vent for the fuel system. Pull the gas cap off and try to start the car. If it works, check the vent line from the cannister to the fuel tank for blockage. I have had one go bad on me.

Second, you say that you have fuel to the carbs. How much and at what pressure? David DuBois has a tech article on the SU fuel pumps on my website. Go to www.custompistols.com/ and click on the MG section, then articles and read what the pressure and flow volume should be. If the vent is restricted (really all that has changed) you will have low fuel flow which might not be sufficient to allow the car to idle properly.

As to vacuum leaks, I do not know the exact pollution control system on your car. My cars are a 68 with a fuel tank vented to the atmosphere through the cap and a 79 with a system similar to yours. On mine, the large line from the cannister goes to the valve cover. The only illustration I have of the SU equipped cars shows that you should also have a line going to the fuel tank and another line going to the overflow vents on the carbs. Therefore, as I read it, you should only have a vacuum source at the rocker arm cover and it is a very slight one. None of this should cause a vaccum leak which would prevent your engine from starting and running.

If removing the gas cap does not allow you to get the engine started, examine the ignition system. Replace the spark plugs and make sure you have good spark at the coil lead and all of the spark plug leads. If not, replace and tune as necessary.

Remember the "ad post hoc" falacy. Just because something happens after something else does not mean that the first thing is the cause of the second thing. Your problem might be unrelated to the work you did. Les
Les Bengtson

Hello Les,

I am sorry about all the confusion. Maybe if I put it in a time line, I can do a better job of explaining what happened.

Last Saturday:
10:00 A.M. Car had started and was running fine that
morning.
10:30 A.M. I replaced the Adsorption Canister.
11:00 A.M. I started the car. The car started but would
not idle. I could keep the car running by
keeping the accelerator pressed.
11:30 A.M. I had my wife strat the car and try to keep
running while I checked some things under
the bonnet.
11:40 A.M I pulled the hose that comes out of the
bottom of the canister that goes over to the
other side of the valve cover next to the
SU's. That is when the car sputtered, died
and would not restart. I did this while the
car was running.

WHAT I HAVE DONE:
1. Checked the plugs.
2. Checked the fuel coming into the SU's. I have not
done a compression check, but I filled a mason jar
with fuel in a matter of seconds and I could hear
the pump just ticking away.
3. I put the old canister back on the car. The canister
on my car has 4 hoses, 3 on the top an one at the
bottom of the canister.

I just bought this car in December. I replaced the alternator, installed a new battery, changed the oil, and put new plugs in the car, all within the last 2 months. The car was running fine until last Saturday.
I hope this clears up a few things.

Thanks in advance for any help.

Michael
Michael Cheek

One possibility could be that you have knocked something loose in the ignition; like the wire from the coil to distributor or one of the low voltage wires. Sometimes some of the old wires will look intact, but have poor connections. I would check for any loose wires and wiggle all the connections in the area where you have been working.

You can pull one of the plugs or ground a spark plug wire to see if you have spark. Another thing I've done when trying to sort out problems is to get a can of starting fluid (spray can of ether) and give a squirt down each carb, then try to start. If it won't at least sputter with the starting fluid, your ignition is dead. If it starts to run and then dies, your carbs aren't delivering fuel.
Tom

As far as I was aware on a 71 the bottom hose on the canister was open to atmosphere, and the three on the top go to the fuel tank breather, carb overflows and rocker cover. Al the first two do is allow the adsorption canister to trap any fumes from the expansion of fuel in or filling of the tank or carb float chambers, the rocker cover pipe draws a small amount of air from outside through the canister to purge it if previously trapped vapours and burn them.

If the tank pipe is blocked you will get fuel starvation after running for a while. If the float chamber pipe is blocked you can get fuel pumping up through the jets and down the intake flooding the engine (poor idle/won't start?). If the bottom pipe open to atmosphere is blocked the suction from the rocker cover can suck the fuel out of the carb jets and prevent it starting or running. If the rocker cover pipe is blocked starting and running will not be affected, but in the long term you will get condensation and creamy scum building up in the rocker cover and internal corrosion.

Remove the hoses from rocker cover and carb overflow ports and remove the tank filler cap. If the engine will not run normally *now* there is something else wrong with the fuel or ignition systems. If it does start and run normally, make sure all the ports on your new canister are open and the charcoal is not blocking the through passage of air, as well as checking all hoses are clear.
Paul Hunt

I am such an idiot! I just noticed that I wrote in the first email that my car is a 1971 MGB. Yesterday was a hell of a day. My MGB is a 1972.
Michael Cheek

Michael. Same thing, I believe, except for the distributor advance curve. Have you done as Paul H. recommends and removed the lines from the charcoal cannister and the fuel tank cap, then tried to start the car? If it is a charcoal cannister problem, or a plugged line problem, that is what will confirm it. If not, it is time for a compression check and actual fuel pump and ignition system checks rather than the qucik checks you have made. Les
Les Bengtson

Hello everyone,

Thanks for all of the help. Here is what I have found so far.

I pulled the hoses that were mentioned in the previous posting and removed the gas cap. Car would not start.

I pulled the air filter covers off of the SU's to see if they were getting fuel. They are getting fuel.

I sprayed starter fluid into the SU's. The car did not even try to start.

I pulled all 4 plugs and tested them for spark. They all 4 showed spark.

I have checked all the wires that I could. I have a haynes manual with the electrical schematics, but it will take some time to trace all of the connections in that area. The car has had some bastardization done to it where the wiring is concerned. It might be time to haul it to the local MGB mechanic. If anyone else has any additional ideas I would welcome them.

Thanks

Michael
Michael Cheek

Michael, Did your engine backfire when it died? I ask because there is a plug in each end of the intake manifold, they have been known to blow out, the engine will not run if one of the plugs comes out. If you are getting spark at the plugs the wiring isn't the reason for the engine not starting. If the manifold is ok try cliping a timing light high tension lead on each plug wire and check the the light flashes for each cylinder. You said it was running good before you repaced the cannister so I wouldn't think the compression suddenly went away. Have you tried some new spark plugs? I wouldn't haul it to an MG mechanic yet, keep asking questions and solve one problem at a time. Good luck, Clifton
Clifton Gordon

What could you see when looking in the carb intakes to indicate that fuel was getting through?

Is the fuel pump clicking when you turn on the ignition after it has been off for at least several minutes? Over-night it should click quite rapidly several times.

What are the plugs like after cranking for a bit? Soaked with fuel? Bone dry and no fuel smell? Or a strong fuel smell but not soaking wet? The first symptom indicates flooding either due to a very rich mixture or no spark and cranked for some time on full choke. The second indicates no fuel getting to the cylinders. The third is what you should have.

Did you remove the plug leads at any time? Have you got the firing order as 1 3 4 2 *anti*-clockwise? Are you sure you know which terminal on the distributor cap should be connected to No.1 cylinder to git it a spark at the correct time?

How big is the spark? With the HT lead pulled out of the coil it should jump at least 1/4" and probably more - with the rest of the HT connected up of course.
Paul Hunt

CLIFTON:
The car did not backfire, but it did sputter and it would not idle. I had to keep the accelerator pressed so that the car would not die. I have not purchased a timing light yet. If one of these intake manifold plugs blow, is it easy to repair? Could I have a timing belt, or timing chain problem?




PAUL:
I do smell a strong fuel smell when I pull the plugs, but they are not wet with gas. When I checked the spark fron the plugs, the spark jump was less than 1/4 of an inch. I am sure the firing order is correct because the car was running fine before I installed the adsorption canister. I did not touch any of the spark plug leads.

Thanks everyone for all of the great suggestions. This is my 2nd MGB that I have owned, but it has been 25 years since I owned my first MGB (1970).

Michael


Michael Cheek

Michael,

If you have not found a fix by March 28th, contact me offline via email. I'm afraid that I will be out of pocket most of next week. I have a 1972 MGB with its original emissions equipment in tact. I rebuilt the charcoal canister on mine using instructions I got from this site and had no trouble.

I live somewhere near you and would be happy to help if I can.

glq Greg

Hello Everyone,

Once again I would like to thank everyone for all of the responses to my posting. I am learning so much. As for my problem, last Thursday evening I decided to take out and clean the spark plugs. I had tried all of the other suggestions from the postings, I figured I might as well try the plugs. After cleaning the plugs, the car nearly started. So, I decided to go and purchase some new plugs. I purchased a set of 4 Autolite Platinum tipped plugs. I gapped them and installed them and much to my surprise the car started right up and has been running fine. I did not consider the plugs earlier because I had just installed the previous plugs less than a month ago. The previous plugs did not have 100 miles on them. When I installed the new plugs, the car smoked quite a bit when it first started. I have a couple of follow up questions.

1. What would cause a set of plugs to become fouled so quickly? I believe that the car sat quite a bit before I purchased it.

2. I still have a strong fuel smell outside and inside the car. Is it gas, or exhaust. After you drive the car, everything on you smells of fuel and exhaust. I still have the old canister on the car, should I go ahead and install the new canister?

Once again I would like to thank everyone for their suggestions.

Michael
Michael

I've got the same model you'd got Michael, and had something similar happening to my car when I started running it, it ran rough until I corrected the timing, and would only run with the choke pulled out slightly, or with the pedal on the gas. it turns out on of the vacuum lines had gotten to the point that it had clogged up with bits of itself. After changing all of the vacumm lines in the car it ran perfectly. So check your vacum lines and replace them all if any of them seem hard.
CJD Dark

It may be time to carefully go through your ignition (if it hasn't been done recently). Perhaps new distributor cap, spark plug wires, points. Possibly a new coil as well. In any case, definitely check the point gap and all the connections.

If the car had done several years of sitting, there is definite potential for the carbs to have gotten a bit gummed up with residue from evaporated gas (or sediment from the fuel tank or fuel lines).
Tom

If you still have a strong fuel smell them maybe it is running very rich, which could well account for the fouling of your 'new' plugs.
Paul Hunt

This thread was discussed between 13/03/2004 and 22/03/2004

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