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Dynamic Ignition Timing

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Dynamic Ignition Timing
#1
Borotosh Avatar
Borotosh Andy C
Christchurch, Dorset, UK   GBR
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1973 MG MGB
Having just fitted a new electronic ignition kit and setting my timing with a strobe light, I found that if I went past my advised setting clockwise (advancing) I found that the revs increased, makes me think with all the variables on an old car would it be better setting the timing by ear/rpm?

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Re: Dynamic Ignition Timing
#2
ghnl Avatar
ghnl Eric Russell
Mebane, NC, USA   USA
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1961 MG MGA "Calvin"
At what RPM are you setting what timing?

Most MG engines are happy with ~ 32* - 34* BTDC at 3500 RPM (and the distributor doesn't advance more at higher RPM). Depending on the advance curve of the distributor the idle timing (~ 900 - 1000 RPM) will vary greatly.


Eric Russell ~ Mebane, NC
1961 MGA #61, 1981 Alfa Romeo GTV6, 1984 Alfa Romeo Spider, 1991 Honda ST1100

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Re: Dynamic Ignition Timing
#3
Borotosh Avatar
Borotosh Andy C
Christchurch, Dorset, UK   GBR
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1973 MG MGB
14 DBTDC at tick over as per book

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Re: Dynamic Ignition Timing
#4
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dickmoritz Platinum Member Dick Moritz
Philly 'burbs, PA, USA   USA
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If you care how the engine runs at tickover, leave it as is. If you care about how the engine runs when you're actually driving it, do what Eric suggests...

Dick


Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

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Re: Dynamic Ignition Timing
#5
Borotosh Avatar
Borotosh Andy C
Christchurch, Dorset, UK   GBR
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1973 MG MGB
I still wonder if the ideal position on tick over is when achieve the highest revs

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Re: Dynamic Ignition Timing
#6
ghnl Avatar
ghnl Eric Russell
Mebane, NC, USA   USA
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1961 MG MGA "Calvin"
You need to know the total advance. I've seen distributors with >20* of total advance so a setting of 14* at idle should give 34* when the advance is "all in" (typically ~ 3500 RPM). But I've also seen distributors with only ~ 10* total advance. So an idle setting of 14* would only give ~ 24* maximum. Resetting it to 34* gave an idle timing too far advanced making it hard to start the engine (it'd try to push the piston back down before it got to TDC). Conversely, if the distributor has too much advance then you could be in a danger zone at higher RPM.


Eric Russell ~ Mebane, NC
1961 MGA #61, 1981 Alfa Romeo GTV6, 1984 Alfa Romeo Spider, 1991 Honda ST1100

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Re: Dynamic Ignition Timing
#7
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Crispin Crispin Allen
London, UK   GBR
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1970 Triumph Herald 1200
1970 Triumph Spit 6
1978 MG MGB
As an experiment, turn the distributor till it runs the best (adjust the idle as you go), then check the timing, it will be about 30°BTDC at 750rpm. The engine will be running at maximum efficiency, however you cannot have the centrifugal advance curve starting from 30, as when you press the accelerator, extra fuel is added to the mix. The flame front travels faster. The engine will pink almost immediately. Optimum advance curves generally start from 10-15 at 750 and increase to a maximum of about 32 at 3000rpm. (this is ignoring later emission shallow ignition curves eg 41610). NB: You can have 30 at idle, by using manifold vacuum advance which will immediately drop off, as you press the accelerator to follow the centrifugal curve.


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Re: Dynamic Ignition Timing
#8
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oleanderjoe Platinum AdvertiserAdvertiser Joseph Baba
Fresno, CA, USA   USA
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1962 Triumph Spitfire 4 (MkI) "SPITTY"
This IS Normal, regardless of the Make, Model or year. It is a balancing act between Idle RPM, and degrees of timing. I feel 15-18* BTDC and about 975 RPM are ideal.

In reply to # 4784186 by Borotosh Having just fitted a new electronic ignition kit and setting my timing with a strobe light, I found that if I went past my advised setting clockwise (advancing) I found that the revs increased, makes me think with all the variables on an old car would it be better setting the timing by ear/rpm?


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Re: Dynamic Ignition Timing
#9
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Arizona Shorty Greg McC
Prescott, AZ, USA   USA
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1968 Jeep CJ-5
1975 Yamaha MC RD350
1977 MG MGB
1985 Jeep CJ-7
"I still wonder if the ideal position on tick over is when achieve the highest revs"


No.




GMc


Wait for me at the gate Tuffy, because without you they're never gonna let me in.

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2008 - 2022

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Re: Dynamic Ignition Timing
#10
Borotosh Avatar
Borotosh Andy C
Christchurch, Dorset, UK   GBR
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1973 MG MGB
I have timed the engine with an accuspark gun, I turned the distributor until my 14 deg mark came around to the TDC mark.
Now I realise that the gun has a digital dial back function which defaults to 10 degrees.

Q1. With this gun can you no longer time the car in the traditional way as I have done, can you turn off the dial back.

Q2. If I set the gun to 14 degrees do I turn the distributor around until both TDC marks align?

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Re: Dynamic Ignition Timing
#11
Windmill Jeroen G
Penang, Malaysia   MYS
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In reply to # 4784417 by Borotosh Q2. If I set the gun to 14 degrees do I turn the distributor around until both TDC marks align?
Yes.

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Re: Dynamic Ignition Timing
#12
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geezer Silver Member charles durning
Magee, MS, USA   USA
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1967 Morris Minor 1000 Saloon (2-door) "Marvin"
1974 MG MGB GT "Foghorn Leghorn"
I have a low compression (8-1 compression ratio) 18V engine in my GT. It runs best with the timing at idle at 18 degrees. All in is 38 degrees, vacuum disconnected. No knocking, pinging, or run-on on 87 octane. Not sure I would do that with 8.8-1 compression or higher.


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Re: Dynamic Ignition Timing
#13
YKMini Paul Arthorne
Witehorse, YT, Canada   CAN
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1973 MG MGB GT
I installed an electronic ignition last year in my 73 MGB GT and found that the timing was happy slightly when more advanced than stock. The standard for MGB's is to have the timing set to 32 degrees total advance, but I found mine runs sweet at around 33 all in. This is running on premium 97 octane fuel. I drove it down to NAMGBR last year from Whitehorse Yukon to Calgary and back (6000km) and it ran great.

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