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BTC vs. ATC Timing Question |
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Jonny Quest 
Grand Poobah 
Joined: August-20-2013 Location: Utah--via Texas Status: Offline Points: 2979 |
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 Topic: BTC vs. ATC Timing QuestionPosted: April-10-2021 at 12:57pm |
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Each engine will have a specific timing need. The "stock" timing is what most people will want to use as a starting point. Most engines will have a BTDC timing requirement as they don't run well ATDC. In the marine world, you can have a Reverse-Rotation engine. If the RR engine uses a standard rotation harmonic balancer, then your timing marks may appear to be ATDC but they are, in reality, BTDC.
Per the 1970 H.M. manual in the reference section, a 302 SBF shows 12 degrees BTDC timing at idle. There are hand-written notes indicating 20 degrees total advance at 2,000 RPM and 38 degrees total advance at 4,000 RPM. Is your distributor mechanical advance working properly? The initial timing setting is only part of the equation. You really need to see the timing curve at idle and on up to 4,000 RPMs and that is dependent on how the distributor advance is performing.
JQ
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2003 Ski Nautique 206 Limited Previous 2001 Ski Nautique Open Bow 1994 Ski Nautique Open Bow Aqua skiing, ergo sum |
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Rweichbrodt
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Joined: March-20-2016 Location: Long Island, NY Status: Offline Points: 3 |
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Posted: April-10-2021 at 8:09am |
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Good morning, I’m in the process of restoring a HM302 and I have such little information on it. I came across your post about the timing, specifically that the balancer should be set at 12Atc (or 12btc) depending on rotation. I was going to set timing today and thought the mark TC was the goal, but I guess not. Thanks for the tip, if you can think of anything else I’m all ears!
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Bhedbloom
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Joined: December-01-2004 Location: United States Status: Offline Points: 116 |
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Posted: September-21-2010 at 10:36pm |
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FYI-I know I am way off of the engine topic, but I posted some pics in the diaries section at pbbrianards request that show the stringer layout of the San Juan. There are a lot of them!!!!! Tabbing the bulkheads back in is going to be a job and a half.
Barry |
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Barry, South Carolina
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8122pbrainard
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Joined: September-14-2006 Location: Three Lakes Wi. Status: Offline Points: 41045 |
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Posted: September-17-2010 at 6:01pm |
Barry, Yes, get some pictures posted. I'm sure I as well as others would enjoy seing one of the larger hulls being worked on. Be real careful with inspecting the stringer condition. The outward apperance appearance can be very deceiving. I suggest taking some core samples with the hole saw. The pilot drill for the actual wood and the hole saw to remove the glass so you can do a visual. I'd hate for you to have a problem someplace in the wood and put it all back together as is. Also remember that the factory did a poor job at best on the glassing plus the polyester resin used is hydroscopic and moisture does go through it. |
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Riley
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Joined: January-19-2004 Location: Portland, ME Status: Offline Points: 7954 |
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Posted: September-17-2010 at 5:25pm |
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We just finished our Mustang which took 3 years and are now getting back on our 62 Classic. I'd love to find a 66-70 San Juan someday just like yours. I don't want a hardtop, flybridge or even a bow rail. Just a big comfortable inboard for a big lake.
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Bhedbloom
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Joined: December-01-2004 Location: United States Status: Offline Points: 116 |
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Posted: September-16-2010 at 8:15pm |
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Riley,
Thanks for asking! The 68 was moving right along but I am a Residential Builder and everyone knows what that is like now. I have it torn all the way down to the stringers. They are in good shape so that is where I will start the rebuild. I was going back with a 454 but I might reconsider and go with one of the 315hp 350's out there. I will post more photos when I get further along. I think it will be a fun boat. They are big and comfortable. It's just slow going right now. Groceries come first! 
You working on any right now? Barry |
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Barry, South Carolina
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Riley
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Joined: January-19-2004 Location: Portland, ME Status: Offline Points: 7954 |
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Posted: September-16-2010 at 3:36pm |
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Good to know. Never heard that before.
Larry, how's that 68 San Juan coming? I really like that boat and would love to see pictures of it. A San Juan like that is on my list of boats to get. |
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Bhedbloom
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Joined: December-01-2004 Location: United States Status: Offline Points: 116 |
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Posted: September-16-2010 at 3:25pm |
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Thanks for the quick reply confirming the thought. I can breathe easier now! Happy Boating!
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Barry, South Carolina
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TRBenj
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Joined: June-29-2005 Location: NWCT Status: Offline Points: 21186 |
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Posted: September-16-2010 at 3:10pm |
Bingo, you got it. My '71 Skier has a LH balancer as well. I discovered this the first time I tuned it up... the engine does not run very well at 12 ATDC! |
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Bhedbloom
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Joined: December-01-2004 Location: United States Status: Offline Points: 116 |
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Posted: September-16-2010 at 3:05pm |
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I hope y'all can shed some light on this for me. My 1970 Mustang
HM302 right hand rotation manual says to set at 12BTC. When I checked it with the timing light it was reading 12ATC. Why would this be the case? Thanks! My only guess was that they used the same harmonic balancer for both right hand and left hand rotation engines. Could that be right? |
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Barry, South Carolina
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Bhedbloom wrote: