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setting ignition timing

ciffer

ciffer

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what is the procedure for setting ignition timing on a '72 350? the motor is not originial to the truck, so i don't know whether or not to trust the sticker on the header panel.

the motor is out of a '72 class C motorhome, it is completely stock and has about 25k miles on it. it has the originial vacuum-advance distributor. points are new, i have no reason to suspect any malfunctioning parts, just need to check the timing and make sure it is correct.
 
You need to set the dwell to about 30 degrees first. Then pull the line off the vac advance and plug it. Set the base timing to whatever the sticker says and plug the advance back in. 10 degrees would be a good starting point.
 
the sticker on the truck is for the 305 sitting in my garage with a rod sticking out the block. should i ignore the sticker and set it to 10 degrees?
 
Many guys advance timing till it pings under load which will depend on which fuel octane also. Id start at 7 BTDC which is probably closer to factory settings than 10 BTDC. Make sure the vac can advancer diaphram is still good after all these years and also if your gonna stay with a points type dist check out Pertronix setups,there cheap and work pretty slick. The last points running truck we had went thru points like it went thru gas cause most of the points avaliable these days are junk it seemed.
 
i set the timing. i hooked the vacuum advance to a vacuum source that shows 20 in/hg at idle, at idle the ignition timing is the same whether or not vacuum is connected to the distributor.
 
The vacuum advance may be shot. You still need to get the dwell right before you do any of this.
 
Carbs are designed with vac supply ports above the throttle plate and below the throttle plate for various needs. You have the dist vac advance line hooked up to the wrong place which is giving full advance at idle and when you crack the throttle the vac drops off and it falls on its face probably. You need to connect it to another source above the throttle plate so you basically get little to nothing vac wise at idle and as RPM increase so does the venturi effect thru the carb pulling the advancer up even higher. A couple checks you can make is to disconnect the vac from the dist nd with it running and a timing light hooked up and pointed at the timing marks increase the rpm`s and see if the timing mark on the balancer advance outta sight which indicates the mechanical advance is working. Then again with it running and using a vac pump or mighty vac or even your mouth and a section of vac line long enough suck hard as you can and the rpm should increase,if no vac can be held or no advance is happening that would indicate the diaphram is shot. Typically advancer cans can be had at any AP store pretty cheaply.
 
another option, once dwell and all that has been figured out. you can set base timing by vacuum. vacuum gauges are not very expensive, and its an easy setup. connect the gauge to the vaccum port on the carb, rotate the dizzy till the vaccum reads at its highest point. then adjust your idle screws, then adjust the timing again for highest vacuum, and then adjust the idle screws. adjusting for max vaccum reading will raise your rpms. use the idle screws to lower it back down. then adjust again, and again till you level out.

you should work down to equilibrium pretty quick. once youve got that set, record the setting in degrees, and use that from then on. you should have around a 750rpm idle.
 
apache said:
Carbs are designed with vac supply ports above the throttle plate and below the throttle plate for various needs. You have the dist vac advance line hooked up to the wrong place which is giving full advance at idle and when you crack the throttle the vac drops off and it falls on its face probably. You need to connect it to another source above the throttle plate so you basically get little to nothing vac wise at idle and as RPM increase so does the venturi effect thru the carb pulling the advancer up even higher. A couple checks you can make is to disconnect the vac from the dist nd with it running and a timing light hooked up and pointed at the timing marks increase the rpm`s and see if the timing mark on the balancer advance outta sight which indicates the mechanical advance is working. Then again with it running and using a vac pump or mighty vac or even your mouth and a section of vac line long enough suck hard as you can and the rpm should increase,if no vac can be held or no advance is happening that would indicate the diaphram is shot. Typically advancer cans can be had at any AP store pretty cheaply.
Click to expand...

i have had vacuum advance connected to a port that sees 20 in/hg at idle and one that sees almost zero vacuum at idle. no difference. i have taken note of making sure the vacuum advance is on the correct port.

with the vacuum advance disconnected the timing does advance at rpms increase.

i took the vacuum diaphragm off the distributor, i can suck or blow into the vacuum port and it does not hold any pressure or vacuum.

now for a few questions. curly, larry and moe put the engine in the truck, i don't know what distributor they used: the one that was on the original engine ('85 305) or the '72 donor vehicle. it has the coil mounted on the distributor cap. i am also unsure of whether or not the distributor has breaker points or not (i am thinking not). here is a picture.


edit - i am aware that the vacuum diaphragm is removed; i removed it.
 
that makes my job a little easier. i assume this means that the distriutor is from the 85 motor.
 
Not necessarily. It could be from any V8 Chevy engine from 1974 or 5 clear into the 80s. Need more info. How many terminals are on the ignition module?
 
i would have to check to make sure but i think there are two. one that leads to the bottom of the distributor, which i think was black, and one that leads elsewhere, which .i think was a white connector.
 
Okay, the ignition module is the little black piece bolted in the distributor with a plug in on each end of it. It will either have 4 pins or 7 pins.
 
The distributor he has is a 4 pin module (you can see it in the pic) which means the distributor is 1975-1979ish maybe as late as 1980. The 7 pin module distributors did not use a vacuum advance that i'm aware of. Those distributors were the beginning of the ECM controlled distributors.

Now he also needs to make sure that someone replaced the distributor wire with the proper 10ga wire used with HEI ignitions and that there is not a resistor still being used.
 
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