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MG MGB Technical - Was running fine - now won't start

'73BGT
Has been running well, but yesterday it would not start.
Acted like lack of fuel. Removed the output of the fuel filter and turned the ignition key - fuel pump engaged, and gas was exiting the fuel line.
Re-attached the fuel line to the front carb and tried again - no start.
Tried spraying a little carb cleaner in each carb. It started for a brief time, then quit (so I assume I'm getting spark).
The carbs look dry, even though it appears that fuel is getting at least a far as the entry to the front carb.
Can the anti-run on valve cause this type of problem, if it is malfunctioning?
Any ideas most welcome.
Thanks,
Pat
Patrick Callan

If you have fuel then you still need spark. check for spark at the plugs.
SANDY SANDERS

Patrick,
When you check the appearance of the plugs, see if they are clean and dry or black with soot. This will give you an indication of your fuel delivery. Make sure you are not loosing fuel through the overflow pipe(s) due to stuck needle valve in a float bowl(s). If you still have the emission control charcoal canister, the float bowl overflows are probably attached to a pipe leading in and out of the cannister. Sometimes you will not notice the loss of fuel, since it will be running out a hose under the car on the right side of the engine.

Eddie
Eddie Haynes

The anti-runon valve *can* cause this problem, if it is operated when it shouldn't be. Disconnect the carb overflow ports at the Y connector where the hose goes to the charcoal canister, and if the engine now starts and runs then the valve or its wiring are the problem. With the hose reconnected but either wire on the valve disconnected, if the engine still starts and runs then the problem is in the wiring, if not then in the valve itself or one of the emissions hoses round it. With twin SUs a problem with one carb would normally allow the engine to run, albeit badly. This sounds like a problem common to both carbs.
Paul Hunt 2010

Definitely not a flooding problem - had one of those before, so I know what to look for there. Everything dry, so the fuel is being cut off somewhere - and since I got good flow at the input to the front carb, I started thinking about the run-on setup.
Disconnected the hose between the carbs and the charcoal canister, and it starts fine. Next step, I guess, is to check the valve and wiring.
When I used the text book check for the valve (grounding), I got a good click, which the book says means that the valve is ok. But I'll continue my checks through that part of the system.
Thanks Paul - looks like that puts me on the right track.

Pat
Patrick Callan

The implication is then that the charcoal pack in the canister is blocked, or at least the bottom port of the canister. The anti-runon system works by the valve closing the bottom port of the canister, which allows crankcase ventilation vacuum acting on the carb overflows to pull the fuel out of the carb jets. It *could* also be the valve faulty, the flap stuck in the closed position but the actuator still clicking, or indeed to port at the bottom of the valve (which should be open to atmosphere) blocked. If disconnecting the hose between canister and valve also allows the car to start (with hose between canister and carbs reconnected!) then the fault is indeed in the valve or its bottom hose. If not then in the canister.
Paul Hunt 2010

Paul,
After re-connecting everything, now it starts. I did blow air through most of the hoses, and the line to the separator (disconnected near the tank first, then shot some air through - got a little crud out of that).
I will check out the canister and the rest of the plumbing, but may have to wait and see if it happens again.
This occurred after a drive on a hotter than normal day - could that have resulted in something akin to a vapor lock?
At any rate, thanks for the additional info - I'm on the diagnostic trial.
Patrick Callan

"could that have resulted in something akin to a vapor lock?"

Aaaargh!

Vapour lock is often mentioned, but very rarely occurs. It's almost impossible in an MGB, any vapourisation that occurs in a fuel delivery pipe say due to heat soak on a very hot day is simply going to be pushed through the open float valve and the float chamber vent into the atmosphere to be replaced by liquid fuel. It would have to be vapourising at the rate of nearly two gallons per minute to defeat the pump, which would be chattering away like billy-oh. It can't occur in the pipe between tank and pump for the same reason, as the pump is more than capable of priming the whole system from dry. Even if it occurred in the pump itself again the symptom would be rapid chattering as the one-way valves don't seal when they are in air or vapour. The only time it *might* occur on an MGB is when the heat-shield is defective, but even then really only on HS carbs in the pipe between the float chamber and the jet. Remember these cars have run in desert states for 40 years without it.
Paul Hunt 2010

Thought that was the case - guess I'm grasping at straws, since it is not really clear yet what single (or combined) thing caused the issue.
Yesterday, I dismantled the charcoal canister, and found that the filter in the bottom was a bit filled - didn't look like it was completely clogged, but could certainly have contributed to things.
Will have to be patient and hope that clearing the plumbing, etc. did the job.
Patrick Callan

Problem re-occurred after running fine for a couple of weeks.
Removed the hose between the carbs and the canister - still wouldn't start.
Removed one of the wires from the anti-run on valve, and the car started right up. The switch still clicks when testing by the book method.
So, I guess that narrows it down to either the valve or the switch.

Pat
Patrick Callan

Not the valve, if removing one of the wires fixed it. Sounds like the oil pressure switch has failed with the contacts stuck closed - or has been replaced with an oil pressure warning light switch which are *designed* to have the contacts closed with no oil pressure.
PaulH Solihull

But then again if removing the hose didn't allow the car to start (which it should do if the valve is operating when it shouldn't do) then it looks like something else altogether, intermittent, and it was just a coincidence that removing the wire seemed to allow the car to start.
PaulH Solihull

Thanks Paul,
Tough one to troubleshoot. After getting the car started (with wire disconnected from ARO valve), reconnecting it had no impact.
I'll try a couple more combinations of connecting/disconnecting, but it seems like I'm up against an intermittent issue.
Are there any ways to check the oil pressure switch itself? Any ways to free up a stuck switch or ARO valve?

Pat
Patrick Callan

Paul

I'm curious as to how you know so much about the workings of the US emissions control system?
Dave O'Neill 2

Dave - a dangerous level of curiosity! But partly because running-on with my 73 became a serious issue after the change to unleaded, the after-market valve made no difference at all, but I knew the American system was 100% effective. Investigated it, modified the after-market system to emulate it, and also got 100% success.

Pat - I now don't think it *is* anything to do with the valve, or anything in the emissions plumbing come to that, or the engine would have started as soon as you disconnected the canister hose from the carb overflows. Either that, or you have two intermittent problems, one of which *is* associated with the emissions/ARO system, and another.

A 73 doesn't need the ARO system to be functioning to kill the engine like the 78 and later models do, so you can leave one of the wires off the valve for a while and see just what happens. You *may* get a bit of running-on i.e. Dieselling though.
PaulH Solihull

Paul,
Ran today with the wire to the valve disconnected. Started fine, turned off fine.
Puzzling, but running.
Patrick Callan

Leave it like that for a bit and see what happens. If you find you don't get any Dieselling then strictly speaking you don't need the ARO system anyway, but it would be nicer to find and fix it, whatever it is that is wrong.
PaulH Solihull

This thread was discussed between 01/07/2010 and 29/07/2010

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