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'Post Ignition'?

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'Post Ignition'?
#1
OMGANMG Avatar
OMGANMG Howard McGiffin
Calgary, AB, Canada   CAN
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I have a 1977 MGB. It runs beautifully, however when I turn the ignition key off and the engine is warm. the engine 'runs on' for a few seconds. Is this called 'pre-ignition', 'run on', 'dieseling', 'post ignition', 'detonation', or some other mysterious term? I run the car on 91 octane and have no 'pinging'. Is this solved by retarding the timing and can it be done by simply adjusting the distributor by trial and error, or do I need a strobe light etc?

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Re: 'Post Ignition'?
#2
Rod H. Avatar
Rod H.
Amity, OR, USA   USA
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1964 MG MGB
1968 MG MGB GT
Commonly called dieseling or running on.

Other than making sure things are in tune, often it's exacerbated by a too fast idle.


Friends talking around a fire is the history of mankind.

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Re: 'Post Ignition'?
#3
lewk Avatar
lewk Silver Member Keith Lewis
Cambridge, ON, Canada   CAN
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1975 MG MGB MkIII "RBLUEB"
2010 Ford Explorer Sport Trac
2012 BMW 323i
Do you have an anti-run-on valve in the motor compartment and is it hooked up?


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Re: 'Post Ignition'?
#4
OMGANMG Avatar
OMGANMG Howard McGiffin
Calgary, AB, Canada   CAN
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Is there a run-on valve on a 77 B?

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Re: 'Post Ignition'?
#5
darlenne D G (Disabled)
no, address, USA   USA
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Yes, there is an anti -run-on valve on a 77B - unless someone removed it.

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Re: 'Post Ignition'?
#6
Mick from Scotland Avatar
Mick from Scotland Mick Rae
East Lothian, Scotland, UK   GBR
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1973 MG MGB GT
In the absence of anything designed, when I get this due to running low octane gas I turn off and press the accelerator which seems to work fine. Then I try to remember to fill up at a larger station with choices.

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Re: 'Post Ignition'?
#7
OMGANMG Avatar
OMGANMG Howard McGiffin
Calgary, AB, Canada   CAN
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I do have a run-on valve in the same configuration as your photo. Is it serviceable?

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Re: 'Post Ignition'?
#8
darlenne D G (Disabled)
no, address, USA   USA
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If you unplug it from it's 12 volt power source while the engine is running, the engine should stop. If it doesn't, than the valve may well be defective. $75 for a new one (Moss) - probably find one at a wrecker for 1/2 that.

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Re: 'Post Ignition'?
#9
OMGANMG Avatar
OMGANMG Howard McGiffin
Calgary, AB, Canada   CAN
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Thanks. I tried it, the engine did not shut off. I have ordered a new anti-run-off valve from Moss. We'll see!

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Re: 'Post Ignition'?
#10
curtis7420 Avatar
curtis7420 Curtis Wright
University City, MO, USA   USA
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1975 MG MGB "The "B"
2003 Toyota Highlander 4WD
2010 Jeep Wrangler
have there been any mods done to the engine bay? do you have the original carburetor, Zenith Stromberg? if you don't have the original set-up, it may be disconnected (and in some cases, you may not easily be able to use it either). do you have a picture of your engine bay to share?

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Re: 'Post Ignition'?
#11
darlenne D G (Disabled)
no, address, USA   USA
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Anti-run-on video:

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Re: 'Post Ignition'?
#12
lewk Avatar
lewk Silver Member Keith Lewis
Cambridge, ON, Canada   CAN
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1975 MG MGB MkIII "RBLUEB"
2010 Ford Explorer Sport Trac
2012 BMW 323i
The vacuum hose coming into the valve comes from a manifold vacuum port, so even if there was a carburetor change the valve should still be hooked up. Check and make sure it is hooked up as stated and that you have electric power going to it, then try Darlene's suggestion.

I should have said that if you have changed to a Weber the manifold will also have changed and you will have to drill, tap & fit a hose fitting to the new intake manifold.



Edited 1 time(s). Last edit at 2015-04-02 08:45 AM by lewk.

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Re: 'Post Ignition'?
#13
Winston Shift Right Avatar
Winston Shift Right David Crosby
Eagle, ID, USA   USA
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1953 MG TD "Martha Ann"
Like mister Twist stated in his video. The screen in the bottom of the charcoal cannister can and will rust away a little and allow some charcoal pieces to find their way to the anti-run-on valve and cause it to not completely close or shut off. If you just replace with new valve the condition may reoccur. Remove the charcoal cannisters and check for screen integrity. If you do decide to remove the cannisters, be careful not to break the nipples while removing the rubber hoses - THEY CAN AND WILL BREAK. Also be careful to not let too much of the charcoal escape. It can and will.

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Re: 'Post Ignition'?
#14
rpmerrill Avatar
rpmerrill Rich M
Easton, MD, USA   USA
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1980 MG MGB "Green Hornet"
I have an old valve. I think it works. (Checked it when I took it off.)
When I got my car it was dicsonnected. Apparently, because I have Weber carb.
I saved it in a box of used parts. I'd be happy to send it to you if it re-checks out ok.

Rich

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Re: 'Post Ignition'?
#15
maycontainnuts44 Avatar
maycontainnuts44 Gold Member Wayne Emery
Fairmont Hot Springs, BC, Canada   CAN
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1974 MG MGB
Thanks for the post Howard, I am now just a little wiser in regards to B's. Also I am sure I have everything you will need to fix the problem, give me a call and it's yours.
Wayne

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