Learning to set vacuum

Mr Comet

Well-known member
The new plugs seem to have solved the rough running and this morning went for a short drive with 3 stops and it seems to run just fine. Questions linger, like why the plugs fouled in the first place? When I do a vacuum reading the vacuum appears to get better as I open the needle past 2 turns upwards to 2.5 turns it will bounce between 19-20 on the gauge. Is this OK or is this why it was running too rich? Was my issue of fouled plugs more caused by the plugs only being gapped at .035? Timing was at 15 btdc with distributor vacuum disconnected. Now it is set at 18 btdc as recommended by Faron Rhodes. I have the plugs at .045. Any suggestions? How often should I check my plugs?
 
Rather than get into specifics there is a nice simple how-to over here:
http://classicinlines.com/vacuum.asp

With the DS2 you can run a .050 gap. And 18° BTDC sounds very high.
I think I am at about 12 -14.
Are you running a ported vacuum source to the ds2, or manifold?
Should be ported, although opinions vary, not going there... ;)

You could also map the vacuum and timing in neutral, and find out what it reads at a range of rpm from idle up to say 3600, where your advance it should be all in.
A timing gun with an advance feature works well for that.
 
Denny, You should set the carb's mixture screw to its lean best idle, It may take several try's to get it right on! After you have now set your timing and plug gap and with engine warmed up good with a Tach (you can do the same with a vacuum gauge looking for highest vacuum) set the mixture screw to its highest idle RPM. Next turn the screw in to lean it 1/4 turn. Then set the curb idle to spec with the park brake on or with someone holding the brake pedal down with the car in drive. Next you should repeat these steps a second time or another until it's right on. When everything is adjusted correctly the car when warm will start without touching the gas pedal. Have you determined why the plugs fouled out? Ie oil or fuel? Good luck (y)
 
Jackfish

Vacuum advance is ported.

Bubba

I am not sure of myself and vacuum tuning. Also do not have enough experience reading the plugs. I do not see any blue tint and the buildup appear dryer which I think leans to fuel more than oil. 1 and 6 have more buildup than 2-5 which makes me think fuel. I'll keep trying your instructions.

Thanks

Denny
 
18 degrees initial is fine if the centrifugal advance has been limited to prevent over advance.
Let me know if further help is needed.
Bubba is correct to adjust the idle A/F on the lean edge.
 
Oh i forgot to mention I have a t5 tranny. I need to find a tack i can use in the engine bay. Thanks guys
 
Ok with a T5 or any stick trans it's even easier to set the curb idle with the trans in neutral. Try 650 to 750 RPM to see what your engine likes. Plug color might start to show after you have 500 or more miles you are looking for a nice tan color if everything is right. If they are more sooty or black it's to rich yet and you might want to recheck your float level is set correctly. Good luck (y)
 
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