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MG MGB Technical - Starts revs up stalls

Ok, now I am stuck. I checked manifold leaks, gas tank venting, plugs, air supply. This just started I think my itself. I try to start engine with manual choke. Engine starts just fine. Revs up to 2000 rpm with pedal apply then quits. Repeats. Sometimes I get a backfire when I pump gas pedal. Does the same when no choke is used after warm up. Also have had it several times quite while driving , 30mph and 50mph usually will restart and take off after sitting for a couple seconds. Sometimes just apply pumping pedal will get it coughing and running while still rolling down the road... fuel pump makes pressure when help back and pumps lots of fuel in bucket when disconnected. Found only clean fuel in tank. Have Super Charger (Moss) and it has been running great for two years. Electrical???, Fuel????
help
Ronald Milostan

Regarding the fuel system: I assume you also checked the fuel filter?

Rick
Rick Stevens

The last time this happened to me it was the Lumenition ignition having a great laugh at my expensive. Very difficult to trace. That said, the first time it was a bit of dirt in the float bowl inlet blocking the flow just before needle valve (HS4s). I'd go through a troubleshooting sequence (try Paul Hunt's "Pages of Bee and Vee") or at least clip on a timing light to see if you have spark to begin with.
Steve Postins

yes, I have put in new fuel filter. I will add that earlier this year I had a chipmunk, (now dead)eat all my ignition wires (replaced all) and many, many other which I assume I have found and re-soldered. Is there anything in the spark side that could be a very small wire that maybe could be loose or half eater thruough.???
Ronald Milostan

I have also replaced all vacuum hoses. My vac gauge shows 20 at idle. And when running I pull 6 lbs on my boost.
Ronald Milostan

Another thing to check, since you're running the Moss blower system, is the vacumn operated pulloff that operates the blower bypass system. If you develop a leak in this system, it will give the problems that you have decribed. The line running to the pulloff is larger than the fitting on the intake manifold, which is held in check with a largish hose clamp, and can fracture and leak. Something happened on my system and it took me a while, eliminating all the other usual suspects, to find the problem. Ray
RAY

Just a thought....But an inexpensive one that can't hurt, replace all rubber fuel line sections. Should'nt cost more than ten bucks.....if they are cracked they may be allowing air into the lines .....particularly when hot. This may not be the problem, but it is cheap to do, and certainly won't hurt anything.
Best of luck.
Jim
Jim Kelly

Monday will replace all rubber fuel line parts. Checked Blower bypass system looks good. Tightened clamps to be sure. Did not see any cracks
Ronald Milostan

This sounds very like it could be the distribitor earth wire which is fitted between the baseplate and the body. Worth a look ??
Iain MacKintosh

Check if the distribuitor 12v supply wire is ok, and if there is no bad connetion there, i had that problem before, and 12v was intermitent to the contact points.
alf
alfredo

Going out right now to check all wires on distributor and coil......I sure appreciate the interest and help in this.
Ronald Milostan

Just checked all wires to coil and battery. Re connected after cleaning all. Vehicle still starts, revs up and then stops running.
Ronald Milostan

Ok, I guess its time for some R & R repairing. The coil is inexpensive and then the fuel pump.....
I think that is cheaper then a tow to a garage....
Ronald Milostan

Ron,
Any luck? As I said may not be the lines.....but it certainly can't hurt.....however, that said, I would defer all further inquiries to Mr. Macintosh.....he may not remember me, but he has saved me ( and more importantly my "B" ) a couple of times.
Jim
Jim kelly

Mr. Macintosh???? I not sure what you mean Jim
Ronald Milostan

I think he is referring to Iain MacKintosh suggestion about 6 posts up. He suggests the flexible wire inside the dizzy that flexes and wear could be faulty. A fairly common, but difficult to spot problem!

If I were you I would find a friend and borrow their dizzy and/or coil and see if the problem goes away with one of those parts fitted.

Iain C
I D Cameron

What year? If electronic tach rather than rev counter does it suddenly drop to zero when the engine starts to die? Or does it only slowly drop as the engine spins down?

If the former it is electrical in the coil primary circuit. Distributor ground wire is a case in point, but the points wire as it goes through the distributor body also flexes as the vacuum advance changes and this can fracture the conductors inside the insulation. If it won't start afterwards diagnosis is relatively easy, although it varies with year and market. If chrome bumper check for 12v on the coil +ve or SW (white) with the ignition on, and with the points open track this through the coil to the other LT terminal, the moving contact of the points, and the fixed contact of the points. If no voltage on the white check back at the fusebox and the ignition switch. If on there but not on the other coil LT terminal, then remove the wire from that terminal. If still no 12v there the coil is open-circuit. If you see 12v with the wire off the terminal but not with it on the wire is shorting to ground, could be condenser, or points wire insulation damaged. If 12v on the other coil terminal with the points wire on but not the points moving contact the points wire is open circuit. If on there, *and* the fixed contact with the points closed, then the distributor ground wire is open-circuit.

If the tach only slowly drops then it is either HT (coil, rotor, cap, or plugs) or fuel.
Paul Hunt 2

Ok, replaced condenser just in case while checking other parts of distributor. I pulled the distributor and really checked it out. Problem the same. Starts rev's up and stalls out. The vehicle again is 1975B roadster, with super charger and has been running great since sc installation. Problem was intermitent starting a month ago, now is always. Fuel flow is constant and builds pressure when stopped with finger over hose end. Tach show's 2200 rpm and goes down as gradually as engine dies.
Ronald Milostan

replaced coil, all the same, rev's up stops///
Ronald Milostan

Sorry,
Mackintosh....( lain )
jim

Thanks or your kind comments. We always try to help but it can be difficult at arms length. Iain Cameron's suggestion about fitting another distributor is a good one and certainly worth trying as I still feel that there is a problem with the baseplate flexible lead or the thin LT lead leading to the points. Rermember that both of these leads are covered in a very fine sleeving which makes disgnostics difficult.
Iain MacKintosh

As an alternative diagnostic route, can you lay your hands on a gunson Colo(u?)rtune? It's a transparent spark plug and is perfect for this sort of fault as it lets you see whether you have fuel and/or spark in each cylinder at the instant of the problem.
Steve Postins

i have a spare coil and distributor with leads for cases like this, its easy to replace them just to check...
alfredo

"Tach show's 2200 rpm and goes down as gradually as engine dies"

OK, nothing to do with an open or short in the ignition LT i.e the switched current flow through the distributor i.e points, ground wire etc.

If this is easily reproducible with your head under the bonnet/hood I'd clip an inductive pickup-type timing light onto the coil lead and each plug lead and watch the flashes.

If the timing light stops flashing *suddenly* as the engine starts to die then the problem is HT, i.e. coil (replacement noted), rotor, cap, HT leads, plugs. If it does this on both coil lead and plug leads it is almost certainly the coil. If only on the plug leads then the rotor or cap.

If you still get HT flashes as the engine is spinning down then on plug leads 1 and 4 point it at the crankshaft pulley and see if the timing suddenly goes way off as it starts to die. If so, then it is something to do with the timing, like points or cam suddenly shifting somehow, another distributor (reference number doesn't really matter for the purposes of diagnosis) would help.

If the flashes remain on time then it must be fuel. Can't remember the year, but if it has the full emissions kit including anti-runon valve then disconnect the hoses from the carb float bowl(s). If *that* cures the problem, then you have a blockage in the emissions system somewhere. The anti-runon system works by sucking the fuel out of the jets to stop the engine, some of the vacuum from the inlet manifold is fed back to the float chamber vent ports, when the anti-runon valve operates, to achieve this. With the valve not powered, or not connected, there is insufficient vacuum to make this happen. However with certain things blocked, or partially blocked, you could be developing enough vacuum at the float chamber vent(s) to cause it even when the valve isn't electrically operated.
Paul Hunt 2

Ok, before I address the anti-runon valve stuff. First I get confused with the LT and HT designations. I am american ignorant please define. Ok, replaced, coil, fuel pump Higher output for my Super Charger,(which I intended to anyway),fuel lines, rotor and distributor cap. Vehicle now will idle at 1000 But when gas applied it will stall out as said above. When I install the super charger most of the air pump and stuff was removed. The charcoal canister with all its hoses is still connected. I will check the anti run-on valve and wires tomorrow. Alos will check those hoses on Canister. Do I need this anti-run-on valve with the super charger. Do I need this absorption canister. Do I need the piston in the carburator damper. It was suggested by Triumph racing engineer that I really no longer need it. Soon I will have a 'new everything'....
Ronald Milostan

ok, its Oct. 1st opening of Bow hunting season for Deer. Tomorrow opens Turkey season. I will be gone camping on hunting property for three days. Please solve my car dillema while I am gone. Oct 7th starts Duck season and Nov. 15th starts rifle season for deer. I really don't don't know when I will get a chance to work on car again. Iam just about over-dosed. Tam and I just love fall riding in it but that is not happening. See ya possibly in 4days for a short look at the run-on valving...hopefully that is the real problem and all the rest will just make the car better...
ron
Ronald Milostan

LT is Low tension i.e. the 12v side of the coil, two spades one usually with white (chrome bumper) wires or white/light-green (rubber bumper) on one side and one or two black/white wires to the distributor and tach (73 on) on the other.

HT is High Tension and is the high voltage output of the coil through the distributor cap and rotor to the plugs.

The anti-runon valve opnly comes into play when the engine is switched off so the question about whether it is needed with anything else is irrelevant.

"Do I need the piston in the carburator damper. It was suggested by Triumph racing engineer that I really no longer need it." Words fail me! YES it is still needed, and was one of the things I was going to ask about. If it is not fitted, or there is insufficient oil in the damper cylinder, then you will get stalling (or at least heavy stumbling) when you go to accelerate ...

But you really need to do the timing light tests as the problem could be either ignition or fuel.

But I'm sure your a better hunter than me.
Paul Hunt 2

This should be the end of this thread. Thank you everyone for the suggestions. The last email I received suggested that the run-on-valve could be sucking the fuel out of the float. Normally this valve only comes into play when the engine is shut down. Therefore I assumed that this is a closed valve untill shutoff and get a momentary open. I only assume this. Anyway I removed the hose from this valve and plugged it at the carb. side. My engine started and ran great. Took it for a ride. Pulled more vac and had more boost then ever. I assume this came from all the new stuff, coil, higher pressure fuel pump and 10,000 wire contacts cleaned. I have not had a problem without the non use of the run-on-valve. Perhaps with the SuperCharger we really do not need this valve any more. I am still hooked up to the canister.
ron
Ronald Milostan

'Tother way round actually. The valve has three ports - one open to atmosphere, one connected to the charcoal canister which is at a slight vacuum, and one to the inlet manifold which also has slight vacuum. With the engine running the port to atmosphere is open, and hence the other two ports are as well. With the valve operated (i.e. when the ignition is switched off with a running engine) the port to atmosphere is closed off which allows the vacuum from the inlet manifold to be applied to the chacoal canister as well as its own connection to the inlet manifold. The two sources of vacuum combine to apply a greater vacuum to the carb float chamber(s), which sucks the fuel out of the jet and stops the engine instantly.

Disabling the anti-runon system can cause the engine not to stop at all when the ignition is turned off, but to run normally for a period. This is due to a factory wiring design error, which wasn't noticed at the time as it is the anti-runon valve which stops the engine, not the ignition, but of course to the 'user' the effect was exactly the same.
Paul Hunt 2

This thread was discussed between 24/09/2006 and 06/10/2006

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