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Having recently fitted a Pertronix set up to my TR3a, I have been experimenting with the timing.

 

I indexed the front pulley when I rebuilt the engine so it is a fairly straightforward task to check the timing with a strobe - I don't have one of the fancy modern ones that allows you to read it off the light.

 

I have set the plug gaps to 32 thou and have found that the engine runs best and at its smoothest with a nice clean pick up from low revs and no pinking with the timing set to 10 deg. BTDC at idle (800 rpm) with the vacuum advance removed.

 

My calculated CR is 9.2:1 and I am using Shell V-Power fuel.

 

However what has interested me is the change in the exhaust note as the timing is altered. When I tried setting it to 15 deg. BTDC the sound became a much more aggressive sounding 'bark' and the engine wouldn't pick up cleanly from low revs, it kept misfiring. I moved it back progressively to the point where it ran best which was the aforementioned 10 degrees.

 

Does any expert out there know why the exhaust tone would change when the timing is advanced?

 

Rgds Ian

 

PS The Pertronix appears to do just what it says on the tin, timing mark is rock solid steady using the strobe and the engine runs as sweet as a nut with a nice even tick over.

 

PPS Whilst the mixture (as evidenced from the plug colour) is spot on at cruise, at idle it is too rich - I am using SM needles and red springs in HS6 carbs - what would be the options for addressing this?

 

 

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Hi Ian,

could it be that the extra 5' is giving the fuel more time to burn and so producing a slightly higher pressure.

 

When I did my valve timing on the engine rebuild last year it was a little too advanced and that caused a slight bark especially on the overrun.

 

It is now back to what it should be (with excellent guidance for the forum) and the sound is nice - especially through tunnels (Merlin engines come to mind)

 

Roger

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Ian,

Tick-over mixture should be rich, to overcome b888r all compression and residual burned gas carried over.

 

Does it need 99RON to accelerate without pinking? If so I'd measure the centrifugal advance and compare with book. It might be coming in too early due to weak springs or opened up eyes.

I'd also set the static timing as per book and see what the strobe then gives at tickover, for the same reason. IIRC static and 800rpm strobe shoud be the same - no c/f advance.

 

At 15 btdc the misfiring/stumbling suggests too me it is timed wrong. Againg I'd suspect the c'f advance swing is too early or too much, or both. If the c'/f adv is all-in too early then the bark could be caused by the sparks at say 3000rpm being too retarded and cylinder gas pressure being higher when the ev opens. Note that 25 deg of centrifugal adv at 2000rpm is not the same time-wise as 25deg at 3000rpm. At 3000rpm the adv gives less time for the flame than at 2000rpm

 

To me it all points to the centrifugal advance.

 

 

 

Peter

Edited by Peter Cobbold
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Hi Peter,

 

I use V Power because it seems to have a longer shelf life. Pinking doesn't seem to be an issue but the reluctance to pick up smoothly from low revs is. Where I have it set now at 10 deg. BTDC is in line with the manual and I am happy with the performance so my main question was what causes the exhaust bark if I over advance it to 15 deg. I suspect Roger is right, there is more time for the mixture to burn which leads to a higher compression.

 

With regard to the mixture, I looked at the Minty Lamb site but all the needles I looked at seemed to have about the same profile as the SM ones that I am using at the lower end and in any event, at cruise I would be operating at light throttle opening so weakening the mixture there would also affect idle unless there was a significant change in profile at the bottom of the needle. I suspect I will just live with it because at the moment when I drive it, the engine feels very smooth and pulls strongly from low revs.

 

Rgds Ian

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Hi Peter,

 

I use V Power because it seems to have a longer shelf life. Pinking doesn't seem to be an issue but the reluctance to pick up smoothly from low revs is. Where I have it set now at 10 deg. BTDC is in line with the manual and I am happy with the performance so my main question was what causes the exhaust bark if I over advance it to 15 deg. I suspect Roger is right, there is more time for the mixture to burn which leads to a higher compression.

 

Rgds Ian

NO its not in line with the manual - your are using strobe to set 10 btdc at 800rpm, the manual says 10BTDC static.

Lets be clear about why I say check the centrifugal swing. I suggest that what you think is 10BTDC according to your strobe at 800 rpm actually comprises 0 BTDC static with 10deg of advance caused by loose spring eyes. Then you open the throttle and the rpm build and the weights swing out to add another 25 deg giving you 35 BTDC at 2000rpm. That's too much, hence the stumbling. However 35 is good at 3000rpm and above.

Peter

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Interesting point Peter, Those of us with electronic "Points" cannot easily check the static timing, I tend to set to around 32° BTDC using a strobe while revs are 3K+, this does then (in my case) return to 5 - 10° at tickover.

 

Bob.

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My manual says 4 deg BTDC static so when I had points I would set the timing with the points just opening at TDC and then advance it by half a turn using the vernier on the dizzy. I haven't tried checking it at 3k revs with the strobe - presumably with the vacuum tube disconnected Bob? Maybe I'll try that next.

 

Incidently, there is a very good section on setting the timing on Macy's web site - follow the attached link.

 

Rgds Ian

 

http://www.macysgarage.com/myweb6/ign-timing.htm

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Interesting point Peter, Those of us with electronic "Points" cannot easily check the static timing, I tend to set to around 32° BTDC using a strobe while revs are 3K+, this does then (in my case) return to 5 - 10° at tickover.

 

Bob.

That's a good idea. Though I woudn't ditch the points!

Another complication is modern strobes dont simply flash with each HT pulse..

I prefer to keep it simple.

Peter

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My manual says 4 deg BTDC static so when I had points I would set the timing with the points just opening at TDC and then advance it by half a turn using the vernier on the dizzy. I haven't tried checking it at 3k revs with the strobe - presumably with the vacuum tube disconnected Bob? Maybe I'll try that next.

 

Incidently, there is a very good section on setting the timing on Macy's web site - follow the attached link.

 

Rgds Ian

 

http://www.macysgarage.com/myweb6/ign-timing.htm

 

This is a good way of setting the ignition but I am not convinced that half a turn represents 4 degrees. If you refer to the factory manual it says that 1 division on the vernier is 4 deg, that is more than half a turn so they can't both be right.

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