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set timing with vac gauge
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Steve00136



Joined: 14 Jan 2019
Posts: 23
Location: nottingham

PostPosted: Wed Apr 10, 2019 11:39 pm    Post subject: set timing with vac gauge Reply with quote

Well i have read a lot about how you should ignore manufacturers settings which was based on a totally different 1970s leaded fuel and use a vacuum gauge to set timing and not a strobe. I connected my Gunsons vac gauge up after slackening the distributor clamp and as soon as started engine the gauge was showing retarded timing (even though it was set at the prescribed 10deg). I advanced the timing bit by bit. Vac went up and so did tickover speed until it levelled out at 20" so retarded it 1" on the scale which measures in inches. Disconnected and went for a run and a massive difference in acceleration and sounds smoother. On return out of interest i connected my timing strobe and found it was now 20 deg advanced. Seemed a lot to me but will run it and monitor for any pinking.
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steve
Nottingham
1975 MGB Roadster
blog: https://mgb.tips
1985 CI Sprite 400 caravan
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Farmer John



Joined: 18 Feb 2010
Posts: 167
Location: Manawatu NZ

PostPosted: Thu Apr 11, 2019 3:01 am    Post subject: mgb Timing Reply with quote

Hello Steve congratulations on being a true sports car owner! By that I mean having the desire and the confidence to alter settings, specifications or even parts. You might very well end up back at the same settings but where it ends up does not matter, you will have had a go, learned a lot, and will appreciate your car even more because it will be optimised.
Just remember that timing added at idle is carried through to maximum advance which might mean that the engine is straying into the region of detonation. The causes of detonation are over advanced timing, excess compression, too low an octane rating, and heat. So at idle, cruise, low speed, only briefly using full throttle, or using racing fuel, it is unlikely to occur.
To deal with each of these factors, you will know from your reading that max advance can be preset inside the distributor, a compression test will tell you how close you are to the limit of compression and I would be very careful if it approaches 180psi. Put some racing fuel in while you get it all optimised, and ensure your cooling system is spot on, be absolutely certain that your A/F ratio at full power is correct. Cooling and mixture both affect combustion chamber temperature.
I assumed you have an MGB with a cast iron head. It is worth investigating the squish (USA=quench) to see if you can get it at about 1mm (40 thou). That is the gap between the piston and the flat surface of the head at T.D.C.
Do the whole job right and you will get power and much better economy.
John
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Steve00136



Joined: 14 Jan 2019
Posts: 23
Location: nottingham

PostPosted: Thu Apr 11, 2019 6:48 am    Post subject: Reply with quote

Thanks for the advice John.
My engine is fitted with the 41610 distributor which has a max advance of 30deg at 6500rpm which i doubt will be going nowhere near see https://mymgbinfo.wordpress.com/2019/04/03/distributor-advance-curves/
but will take heed of your warnings re detonation.
I guess the first signs of prolonged detonation damage would be on the plugs which i will check regularly.
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steve
Nottingham
1975 MGB Roadster
blog: https://mgb.tips
1985 CI Sprite 400 caravan
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47Jag



Joined: 26 Jun 2008
Posts: 1468
Location: Bothwell, Scotland

PostPosted: Fri Apr 12, 2019 4:06 pm    Post subject: Reply with quote

Steve,

As a matter of interest where are the timing marks? If they are on a vibration damper then this could indicate that the rubber in the damper has sheared and the outer part slowly creeps backwards due to inertia. I used to see this a lot, especially on GM products when I was in Canada. I didnít use a vacuum gauge but a technique called power timing where you rev the engine to about 2000rpm and then advance the distributor until the revs start to die away, back off a bit and lock it down. Sounds like the same procedure but using your ear instead of a vacuum gauge.

Art
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Steve00136



Joined: 14 Jan 2019
Posts: 23
Location: nottingham

PostPosted: Fri Apr 12, 2019 6:34 pm    Post subject: Reply with quote

I thought something similar so checked TDC by taking no1 plug out. So far car running well not heard any pinking.
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steve
Nottingham
1975 MGB Roadster
blog: https://mgb.tips
1985 CI Sprite 400 caravan
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Steve00136



Joined: 14 Jan 2019
Posts: 23
Location: nottingham

PostPosted: Sun Nov 10, 2019 12:52 am    Post subject: Reply with quote

Updated info that includes how to set mixture as well as timing with a vacuum gauge at https://mgbtips.wordpress.com/2019/11/04/tuning-for-performance-and-diagnosing-faults-with-a-vacuum-gauge/
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steve
Nottingham
1975 MGB Roadster
blog: https://mgb.tips
1985 CI Sprite 400 caravan
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