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modified L18 timing questions


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I'm running an L18, .040" oversize, with L28 flat tops and skimmed V912 head - compression about 11 to 1. Two questions:

1. The Nissan owner's manual gives timing as 8 to 10 degrees BTDC. But that is for a stock motor. Should I not retard my timing - because the mods I have done on my L18 will shorten combustion/burn time?

2. When I rev the motor to about 2500rpm with a stroboscope timing light connected, the timing seems to advance very little - maybe to 12 or 15 degrees BTDC. But Honsowetz says total timing should be around 35 BTDC. thanks for any advice

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In theory you have a 10-12 degree static timing dialed in that never changes and is always there at all RPMs.

In the distributor there is a mechanical advance which adds up to about 20 degrees of advance starting above idle and maxing out at about 2K. This advance is totally dependent on RPMs


There is a vacuum advance that can add 15 to 20 degrees of advance and is totally dependent on manifold vacuum. Naturally at full throttle the vacuum is low and advance from this is almost non existent. This would leave static plus mechanical


Because the vacuum advance can skew the readings at anything below full throttle you should unplug the hose and plug it. Now as you rev it up you should read the static, (10-12 or whatever you set the distributor at) plus the mechanical advance rising in response to RPMs. Certainly by 2500 to 3000 it should be maxed out for a total of about 32 degrees.


If you are reading only an additional 8-12 degrees as it revs up, perhaps the mechanical advance weights are stuck or a spring has come off under the points plate.




The 3 ball bearings under can get dirty or rusty, sometimes the plastic cage that holds them breaks and the advance will stick. This one is metal but some are plastic.



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With the vacuum advance disconnected connect the timing light and confirm your static timing of 10-12 (or what ever you have set it at at idle. Increase engine speed and see how high it goes. Should be 32-34 at 2,500 to 3,000. If you get pinging under load when driving,you may have to retard the static timing down from 12

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The factory 620 manual for 1974 shows the advance to be 7deg. vacuum and 20deg. mech at 2150.


The manual for 1978 has instead 12.5 vacuum and only 11deg. mech.


The first would be performance oriented and the second is clearly designed with emissions as the primary concern.

Models other than the 620 may be different.


In the pre-emissions era, most multi-carb engines, as Volvo, Porsche, and the various Brit types, had mechanical only advance ignitions of around 30deg. total.

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Vacuum advance is strictly for part throttle performance and economy. Anything less than full throttle means the cylinder is not completely filled with gas and air. The less gas and air that is compressed and ignited the longer it takes to burn and so the spark must be advanced slightly to have it at maximum pressure as the piston begins going down on the power stroke. Vacuum advance on the carb is directly related to throttle opening. At WOT the cylinder is filled and vacuum is very low or none and the initial and mechanical advance totals about 30-35degrees. Which is ideal for 2,500-3,000 and up.


The factory 620 manual for 1974 shows the advance to be 7deg. vacuum and 20deg. mech at 2150.


Throw out the vacuum and add the mechanical plus the 12 degrees static timing and we have the ideal (or close) 34 total degrees at maximum advance



The manual for 1978 has instead 12.5 vacuum and only 11deg. mech.


Again throw out the vacuum advance as it isn't really a factor at WOT and 3K RPMs. The 11 degree mechanical is likely at the distributor which translates to 22 degrees at the crank ... which added to the 12 degrees static timing comes to ...34 degrees total



The biggest smog related changes to the distributor was in the late 60s when manifold vacuum was replaced by ported vacuum from the carb. Research shows that the longer time and higher the combustion temperatures the higher the emissions produced. Remember I said that part throttle does not fill the cylinder and the less filling the more advance is needed? Well idle is the least filling of the cylinder so it should have maximum advance but to reduce idle emissions there is no vacuum advance from the carb vacuum port at idle. Just above idle the port is exposed to vacuum and the advance jumps up for better performance, but at idle there is none just the static 12 degrees. A retarded ignition at idle reduces the maximum time and temp inside the combustion chamber. It reduces pollution but much of the energy is wasted as heat in the exhaust port and crappy idling. Next time you set your timing at 12 degrees advance it to 20 and see how much better and higher it idles because much more of the wasted power is captured.

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