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BTC vs. ATC Timing Question

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Bhedbloom View Drop Down
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    Posted: September-16-2010 at 3:05pm
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?
Barry, South Carolina
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TRBenj View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote TRBenj Quote  Post ReplyReply Direct Link To This Post Posted: September-16-2010 at 3:10pm
Originally posted by Bhedbloom Bhedbloom wrote:

My only guess was that they used the same harmonic balancer for both right hand and left hand rotation engines. Could that be right?

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 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bhedbloom Quote  Post ReplyReply Direct Link To This Post Posted: September-16-2010 at 3:25pm
Thanks for the quick reply confirming the thought. I can breathe easier now! Happy Boating!
Barry, South Carolina
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Riley View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Riley Quote  Post ReplyReply Direct Link To This Post Posted: September-16-2010 at 3:36pm
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 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bhedbloom Quote  Post ReplyReply Direct Link To This Post Posted: September-16-2010 at 8:15pm
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
Barry, South Carolina
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Riley View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Riley Quote  Post ReplyReply Direct Link To This Post Posted: September-17-2010 at 5:25pm
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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Post Options Post Options   Thanks (0) Thanks(0)   Quote 8122pbrainard Quote  Post ReplyReply Direct Link To This Post Posted: September-17-2010 at 6:01pm
Originally posted by Bhedbloom Bhedbloom wrote:


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.
Barry

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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Keep it original, Pete
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Bhedbloom View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bhedbloom Quote  Post ReplyReply Direct Link To This Post Posted: September-21-2010 at 10:36pm
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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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rweichbrodt Quote  Post ReplyReply Direct Link To This Post Posted: April-10-2021 at 8:09am
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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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jonny Quest Quote  Post ReplyReply Direct Link To This Post Posted: April-10-2021 at 12:57pm
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.

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