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MG MGA - For Barney ign. timing
I am not sure if I am understanding your recomendation to go to 20% BTDC on an A [[1500, good engine]. 12% gives me 33/34 max mechanical. 20% would run out to 40%. A lot of advance, under load, at say
4000 RPM ? ? I tried it, running a positive earth electronic distributor from Moss, and 98 octane petrol. Didn't ping ,felt a bit harsh and preformance felt much the same. The advance curve should be ok ,as these new distributors are dedicated units for these early B engines. I'm also running a 3.9 diff and drive between 2500 & 4000 revs, the engine feels happy to pull the higher diff. Thanks Sean
PS., I returned again and again to your site whilst refurbishing from the front to the rear bumper Thanks
|Obviously I'm not Barney, but this has come up a couple of times, in addition to my questions.|
The concensus seems to be to set the timing dynamically at 32-34 degrees BTDC with the vacuum pipe disconnected running at 3500 rpm.
My car pings slightly at 34 so I back off from that by ear.
Hope that helps.
|Yep, that's how I set mine up, like TM. A couple of provisos though, the balance weights are not shot and you have the correct mechanical advance cam, see here: http://www.jcna.com/library/tech/tech0015.html|
Suffered from both.
|My recommendation was for a stock distributor in good condition on a stock engine in good condition. You may never know what non-standard swap part some DPO may have installed, if someone may have changed the springs or weights, or if the thing is just worn out. So you should always check the advance function with a set-back timing light, especially the maximum advance at speed (3500 rpm or higher).|
Distributor vacuum is tapped from the venturi area of the carburetor on the MGA, just in front of the edge of the throttle plate. There will be no vacuum at idle and high vacuum with a small throttle setting (quick throttle response). At intermediate speed with a light throttle setting it still generates some vacuum advance (giving better fuel mileage). At high throttle setting (high power output) this vacuum source decreases vacuum to below the threshold for operation of the distributor vacuum unit, so the vac advance drops out at high throttle and high load.
You cannot produce sustained high throttle and high load condition in your garage unless you have the engine or the car on a dynamometer under load. So when checking for the specified max advance you need to disconnect and plug the vacuum pipe to simulate the full throttle low-vac (no vac advance) condition. Then you will be checking initial advance setting plus maximum mechanical advance. This is the number that should not exceed about 34d BTDC.
Initial timing at 20d BTDC allows for distributor mechanical advance of 7-8d (14-16d at the crankshaft), bringing total advance (no vac) up to 34-36d. Plus or minus a couple of degrees here is generally negligible (no perceptible difference), as long as it doesn't ping under load.
If you have a more aggressive mechanical advance doing say 11d at the distributor, 22d at the crankshaft, then you need to back off the initial timing setting (to 12d BTDC in this case) to limit total advance at high speed (12+22=34).
If it pings with full throttle under load around 2500 rpm, then try higher octane fuel or back off the timing a little. Setting timing by ping alone with no strobe light may not be feasible, as some engines (like mine) will not ping under any circumstances (moderately low compression).
Less spark advance at slow idle speed may result in a “sluggish” idle and poor throttle response. Conversely, more advance at low speed can give a lively feel with quicker throttle response. Too much advance at idle can contribute to internal hot spots and run-on after switch-off. If you cannot get the desired setting at idle along with the prescribed upper limit, then you need to rebuild/remodel the mechanical advance mechanism in the distributor.
|Yes Barney I understand exactly what you are saying. I didn't know the were distributors that only ran to 14% crankshaft. My Moss unit is brand new and runs approx 20% mech.. ,so my static at 12% fits the bill. Compression is 8.2 and like yours I couldn't make it ping on kerosene ! I'll give it a couple of degrees more to help the slow speed throttle response. My timing light lets me check check total advance.|
Thanks for your input Sean down under
This thread was discussed between 25/07/2008 and 26/07/2008
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