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Run On - With a catch

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Run On - With a catch
#1
Philip8794 Philip Schmid
Zurich, Switzerland   CHE
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Hey Guys, something weird is happening;
My 66 MGB Roadster runs on - Buuuuuut here's the catch
If I drive it for maybe 15 mins on normal roads - mostly straight - at around 60kmh slowing down and accelerating for traffic as I go, and then pull over, wait 10s and shut her off, she runs on mildly. Now, If I drive for 30 minutes or more, and drive her "hard" (run it up to 4.5k or somewhere around there) and then pull up and shut it off nice and hot, also waiting a few second I don't get any run on whatsoever.
How do I fix this?�
I've checked cylinders for coking etc, they're clean as can be.



Edited 1 time(s). Last edit at 2024-04-11 09:02 AM by Philip8794.

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Re: Run On - With a catch
#2
Ahmed Avatar
Ahmed Ahmed A
Calgary, AB, Canada   CAN
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1958 MG MGA
1972 MG MGB
1979 MG MGB
Most likely the fuel. Do they have Ethanol in the gasoline in Switzerland?
I had similar issues, and tried adding Lucas Octane Booster and it cured the run-on. Give it a shot and see (or any similar product in your locale)


My 1958 MGA is not a car, it's a CAREER..!! ... angry smiley angry smiley



Edited 1 time(s). Last edit at 2024-04-11 10:18 AM by Ahmed.

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Re: Run On - With a catch
#3
Philip8794 Philip Schmid
Zurich, Switzerland   CHE
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I checked, legally allowed to add up to 5% ethanol.
But why would it run on if it's warmed up but not hot, and not run on if it's hot and been run hard? That's the part that confuses me...

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Re: Run On - With a catch
#4
B-racer Avatar
B-racer Jeff Schlemmer
Shakopee, MN, USA   USA
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1950 Willys Jeep Pickup "Ratrod"
1964 Land Rover Series IIA
1971 MG MGB
If idling helps create more heat that causes run-on, this is a mixture problem - either the wrong needles or a small adjustment away from ideal. Try adjusting the mixture 1 flat richer per day to see if it improves. Fuel is a coolant that quenches the hot spots that cause run-on.

More base timing can also help. The higher your timing is at idle, the further closed your throttle plates will be to maintain the same idle speed. Less air coming into the carbs also reduces its ability to run-on. 14 BTDC at idle should be enough.


jeff@advanceddistributors.com

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Re: Run On - With a catch
#5
Philip8794 Philip Schmid
Zurich, Switzerland   CHE
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Hey Jeff, thanks a lot for the info!!
I thought that advancing timing makes run on worse, no?
I'll try making it a tad richer and see what happens...
There shouldn't be many, if any hot spots due to coking, I checked the pistons and they were about as clean as can be.
Currently my idle sits at approx. 800rpm, since much lower than that and the ignition lamp comes on.
I swapped condenser, points, rotors, plugs and leads recently, which made it a little better but it still does it from time to time.
Could it be that a slightly bad coil causes it? It runs perfectly after 1000rpm, no hiccups no nothing but doesn't sound perfectly uniform at idle, but pretty close.
What would you recommend first, richer or more advance?

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Re: Run On - With a catch
#6
Watauga Avatar
Watauga William B
Richmond, IN, USA   USA
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1974 MG MGB
Philip, I wonder if it is a carbon issue? 30 minutes of driving her "hard" might blow out the carbon that has built up from driving on slower roads?

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Re: Run On - With a catch
#7
RAY 67 TOURER Avatar
RAY 67 TOURER Ray Marloff
Fort Bragg, CA, USA   USA
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1967 MG MGB "My Girl"
The early B heads have a sharp point where the heat can build up and cause running on. It acts as a sparkplug. To completely rectify this you'll need to remove the head and remove this spot from each combustion chamber. RAY

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Re: Run On - With a catch
#8
Philip8794 Philip Schmid
Zurich, Switzerland   CHE
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Thanks for the advice. Is it realistic for that sharp edge to have a lot of carbon deposits if there's essentially none on the pistons?

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Re: Run On - With a catch
#9
RAY 67 TOURER Avatar
RAY 67 TOURER Ray Marloff
Fort Bragg, CA, USA   USA
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1967 MG MGB "My Girl"
Philip, It's not a build up of carbon issue, but a spot in the combustion chamber that comes to a sharp point and glows hot enough to detonate the fuel mixture on its own. The later heads had this sharp edge removed from them during production. RAY

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Re: Run On - With a catch
#10
Philip8794 Philip Schmid
Zurich, Switzerland   CHE
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Thanks, Ray. I see. Soooo, when does that happen, approximately in relation to mileage? Can it be remediated or minimized by running rich, or increasing timing and decreasing how far the throttle is opened to maintain idle?

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Re: Run On - With a catch
#11
Brazzle99 Avatar
Brazzle99 Brian Aslett
Cape Town, Western Cape, South Africa   ZAF
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1970 MG MGB MkIII "Millie"
2010 Honda Accord "Alison"
2014 Toyota Highlander 4WD "Magnus"
Ignition timing also affects run on. If I set my timing at 13` BTDC at 1 000rpm - the spec for my engine, it runs on badly. If I set it at about 10-11`BTDC and allow it to isle for about a minure, it does not.

Remember that these engines were designed for 98 Octane (RON) when they were built.

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Re: Run On - With a catch
#12
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B-racer Jeff Schlemmer
Shakopee, MN, USA   USA
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1950 Willys Jeep Pickup "Ratrod"
1964 Land Rover Series IIA
1971 MG MGB
In reply to # 4784059 by Philip8794 Hey Jeff, thanks a lot for the info!!
I thought that advancing timing makes run on worse, no?
I'll try making it a tad richer and see what happens...
There shouldn't be many, if any hot spots due to coking, I checked the pistons and they were about as clean as can be.
Currently my idle sits at approx. 800rpm, since much lower than that and the ignition lamp comes on.
I swapped condenser, points, rotors, plugs and leads recently, which made it a little better but it still does it from time to time.
Could it be that a slightly bad coil causes it? It runs perfectly after 1000rpm, no hiccups no nothing but doesn't sound perfectly uniform at idle, but pretty close.
What would you recommend first, richer or more advance?

Richer first. That should help the most. Then more timing AND richer hand in hand. I stand by my first post word for word.

Hot spots are caused by sharp metal - the top edge of the combustion chamber, sharp edges on anything from the plug electrode to the valves. It doesn't have to be carbon. A clean combustion chamber indicates its a bit lean. Add fuel.


jeff@advanceddistributors.com

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Re: Run On - With a catch
#13
Philip8794 Philip Schmid
Zurich, Switzerland   CHE
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Added two flats and a degree of timing advance. Runs okay-ish, wasn't run on seems to have gotten better, but traffic while hot out is really hard on it - rough idle and wants to die along with 95° C water temp. Any idea how to solve that? I'll go ahead and give it another degree of advance and 1 flat of fuel later today.

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