SoCal Jet Boats
Tech Talk => Engine Mechanical / Electrical => Topic started by: Horizonjet on August 31, 2011, 09:50:00 PM
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I have a BBC and I have ordered the vacume advance block off plate for 8 bucks and was wondering.... :sly:
Do I leave the timing where its at now? or do I need to play around with it...It is now at about 34 BTDC with the vacume advance on and when I unplug and blok off it is around 14 BTDC. My guess would be to leave where its at and adjust the idle and test drive to fine tune.
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I have a BBC and I have ordered the vacume advance block off plate for 8 bucks and was wondering.... :sly:
Do I leave the timing where its at now? or do I need to play around with it...It is now at about 34 BTDC with the vacume advance on and when I unplug and blok off it is around 14 BTDC. My guess would be to leave where its at and adjust the idle and test drive to fine tune.
This wont come out right because I am half awake...But
The only timing point that is really important is above 2500 RPM, this should be the point where the ignition timing is "all in"...Typically on a BBC it is between 32* and 38*BTDC. The reason for even having an advance mechanism at all on a jet is for starting, a locked out ignition timing set up can be difficult to crank with that much lead off.
So to answer your question, once the lock plate is installed you will have to start over and adjust the total advance, or find someone with a distributor machine and find out at what RPM the advance stops, how much mechanical advance is present and then calculate the base timing...
Either way, you will have to check and adjust the timing.
One other thing, there is no reason at all to block off the dash pot on the advance mechanism. it is spring loaded and cannot move without vacuum applied... ;) You should however plug/cap the port on the carburetor... :thumbup:
GT
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Gotcha. Thanks for the quick info. I forgot that it is spring loaded so I will have to try it this weekend and just see if its worth doing. I can always go back if needed and I have just see a lot of others not runnign a vacume advance.
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Gotcha. Thanks for the quick info. I forgot that it is spring loaded so I will have to try it this weekend and just see if its worth doing. I can always go back if needed and I have just see a lot of others not runnign a vacume advance.
Engine loads are too great to benefit from a vacuum advance, trust me here, you wont go back.......Your doing good by eliminating it. :thumbup: My junk all runs about 14* of mechanical advance and that's it, a vacuum advance pot is typically good for about 8*, so I guess you could "do the math".... :-\
Good luck!
GT
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To late to do math right now. Thank you. Do you run a advance curve kit also? or jsut no vacume advance? I have an HEI i forgot to mention if that makes a differance. Engine not too tricked out but looking for as much performance as i can get.
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To late to do math right now. Thank you. Do you run a advance curve kit also? or jsut no vacume advance? I have an HEI i forgot to mention if that makes a differance. Engine not too tricked out but looking for as much performance as i can get.
In my experience, you only need to have a retarded enough initial timing to get it to start hot.
Once the engine is under a large load, there is not enough vacuum generated to pull in the pot.
HEI, points, CDI, the timing theory really never varies much.
Many guys out there (some on this forum) run locked out advance mechanisms, meaning the timing is the same regardless of RPM, they can be a real bitch to start without a separate ignition and starter switch....
Remove the vacuum parts and set the total timing at 2500-3000RPM for about 34-36* and she'll rock... :thumbup: :thumbup:
GT
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Awesome! Thank you for the info. :D