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GT40 Harmonic Balancer slip - can't time to 5BTDC |
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Canuck-Surfer 
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Joined: September-14-2019 Location: Ontario, Canada Status: Offline Points: 222 |
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 Topic: GT40 Harmonic Balancer slip - can't time to 5BTDCPosted: August-22-2022 at 12:13am |
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Many of you have likely been following my other thread that started earlier this summer regarding my GT40 cranking but not starting up. Ended up being a combination of distributor (specifically likely the PIP sensor), and secondarlily the TPS sensor. LINK
During distributor replacement, I followed timing instructions to the letter from many posts here. For the life of me, I cannot get the timing 1000 rpm (with grey spout connector removed) to the spec 5 degrees BTDC. It running at about 10-12 degrees. Anything lower and engine gets rough and then dies about 6 degrees. My questions: 1) I feel like the outer ring on the harmonic balancer has possibly slipped over the years. It was not replaced during rebuild. Engine is likely timed correctly but appears off due to slippage. If this is the case, this would mean engine is not externally balanced correctly. How big of a deal is this and can it damage anything? 2) I am aware of Damper Doctor (and the other company Dudes). I am considering shipping mine down. But does anyone have a GT40 handy that they could align the 4 bolts into a cross and tell me where the spike aligns to on the outer wheel (or any other orientation you think makes sense). I'd love to compare to a working one and see if it is obvious its slipped an by how much approx. 3) The casting part number is F3JL-BA. I have searched high and low and cannot find a referene to this this HB online anywhere, even ebay. Any other ideas on the timing?
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1996 Sport Nautique GT40 EFI
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Jonny Quest
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Joined: August-20-2013 Location: Utah--via Texas Status: Offline Points: 2979 |
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Posted: August-22-2022 at 10:40am |
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Have you verified that the harmonic balancer is NOT oriented where it's supposed to be? If so, how did you verify the HB orientation?
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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Canuck-Surfer
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Joined: September-14-2019 Location: Ontario, Canada Status: Offline Points: 222 |
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Posted: August-22-2022 at 10:55am |
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That’s what I’m trying to accomplish here. If there’s a way to do this I am unaware of it. That’s why I asked if anybody had a GT 40 they could give me some measurements So I can do exactly this. How do you suggest I verify the orientation? |
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1996 Sport Nautique GT40 EFI
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MrMcD
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Joined: January-28-2014 Location: Folsom, CA Status: Offline Points: 3750 |
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Posted: August-22-2022 at 12:42pm |
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To verify TDC, Top Dead Center in an engine you use a piston stop in the #1 cylinder. spin the engine slowly by hand till the piston reaches the stop. Mark the exact location on the harmonic balancer. Now reverse the rotation and spin the engine by hand back 360 degrees till you hit the stop once more. You have to turn gentle so you do not do any damage to the piston when it touches the stop.
Mark the balancer again. There will be a distance between the two marks you make. The center point between your marks is true top dead center. TDC. This mark should match the factory line on your harmonic balancer for TDC. If it does not match something has changed, maybe wrong balancer installed at the rebuild? A piston stop is just a tool that screws into the #1 spark plug hole and reaches down into the cylinder so you can set a stop very close to TDC to do this measurement. Many retailers sell them including Amazon.
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KENO
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Joined: June-06-2004 Location: United States Status: Offline Points: 11112 |
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Posted: August-22-2022 at 3:24pm |
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 Here's a picture of a 351w harmonic balancer used on the marine 351w engines. This one is a 3 bolt balancer but that doesn't matter. 3 or 4 bolts, the keyway and the TDC mark will be the same in the boat application The long scribe mark at the lower left is lined up with the center of the keyway. The short scribe mark is lined up with the TDC mark on the outer circumference. Measuring along the outer circumference the distance is about 2.16 inches or in degrees the TDC mark is about 39 degrees to the right of the center of the keyway if you have a timing tape for a 6.375 inch diameter pulley. This is for a 6.375 inch diameter balancer which yours is, with the timing pointer at about the 1 o'clock position on the timing cover which yours is on a gt40 A balancer for a 351 w with the pointer in a different location like on many automotive applications would have the TDC mark in a different spot. You'd probably get good accurate numbers if you did all this with the balancer off the engine, but being a mid engine boat, if you take the lower pulley and the crankshaft bolt off off and get in the right twisted and contorted position you could get some decent measurements.
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desertskier
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Joined: December-19-2006 Location: Az Status: Offline Points: 1115 |
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Posted: August-22-2022 at 6:20pm |
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If you still want some measurements from a GT40 let me know.
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92 SN - Owned since 93
99 Pro Air 89 SN - Went to live on a lake in Texas 75 Donzi 16 - Sold in 93 |
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Jonny Quest wrote: