timing advance...

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  #1  
Old 05-11-07, 06:49 AM
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timing advance...

I've got an old ski boat with 1000 hrs on it... (not much for a big engine actually) - but it's 17 yrs old. 351 Ford Windsor with a prestolite mechanical advance screw down cap distributor. I just changed it over to electronic for the points/condensor system. When I set the timing - I have no issues - however, I notice that, when I ramp up the rpm's - the advance jumps initially from the proper 6degrees BTDC to around 25degrees BTDC - then drops back quickly while the rpm's are held high (4000+). As I'm getting some pre-detonation (backfiring) when I initially "hit it" - I'm wondering if my mechanical advance system isn't working up to snuff. Should the timing advance stay "up" at rpm - or is dropping back to the lower numbers proper?

If I "hit it" then quickly back off on the throttle and reapply throttle - there are no issues.. but I obviously want a smoother ramp up for hard starts. thoughts/suggestions??
 
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  #2  
Old 05-11-07, 09:12 AM
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does it have any vacum operated advance?
a mechanical advance should stay advanced as long as the rpms are up and start to return to the base timing once rpm is lowered.
is it backfiring through the carb?
 
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Old 05-11-07, 10:57 AM
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No vacuum advance - purely mechanical. I though the timing should stay advanced at rpm - but wasn't sure..... it drops soon after reaching higher rpms right now.

On hard acceleration - right after "hitting it" - it will backfire through the carb. Right after that, it will catch well and power up nicely. If I smoothly , but slowly accelerate - there are no issues..... Leading me to think it's the advance mechanism... old springs, etc....
 
  #4  
Old 05-11-07, 06:30 PM
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I think an advance mechanism problem would show up as a problem getting the distributor to advance rather than to hold the advance. The pinging would likely be related to advance spark rather than lack of advance

The small block 351 was a little more prone to ping than the big block Cleveland as I remember it. The latter being a trimmed down 400.

It seems like your distributor would have something to retard the timing under load conditions though - which is what the vacuum jobby provides for. On current engines the MAP sensor does that as well as providing a fuel mixture change.

Without something to handle the loading issue all I can think of to do is retard the base timing or work with higher octane fuels. You can also experiment with a different advance curve by changing the weights/springs = lighter/stiffer. That would tame the rate of advance. If the ping was occurring before the distributor hit full advance that could help. But I can't say if that would fix it completely for you or not.

Hope this helps,

Bob
 
  #5  
Old 05-11-07, 06:55 PM
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its back firing through the carb due to lack of advance its not due to detonation it should have no trouble with detonation even at 25 degrees btdc at any given rpm, you generally need to get up above 28 degrees btdc before detonation is ever a problem due to ignition timing.
there is something wrong but it almost seems electronic if it does advance initially and retards by itself would probably look at the conversion electronics used to convert it over from points as it seems like the mechanical is working.
 
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