SN 85 bogs at high rpm

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   Hey boys needs some help.  Seems there are many topics about this but not much resolution I could find.  My SN 85 351w runs great and starts up every time.  But after running high rpm for 20/30 minutes the engine begins to stall and lose power.  If not backed down to idle it will completely stall.  However, it will crank right back up and idle at low rpms for a slow ride home.   

It has the Pertronix 1581 electronic ignition Conversion kit and 1.5 ohm pertronix flame thrower ignition coil.  I take it this coil has an internal resistor and therefore the actual boat resistor should be bypassed?  I’m guessing once the coil gets too hot the spark dies and high rpms are not possible?  But it will crank immediately after it stalls and idle fine..

The carburetor is tuned and basically brand new.  Any insight much appreciated!!

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TRON Members Profile Send Private Message Find Members Posts Add to Buddy List Newbie Joined: June-07-2020 Location: NC Status: Offline Points: 3	Post Options Post Reply Quote TRON Report Post Thanks(0) Quote Reply Topic: SN 85 bogs at high rpm Posted: June-14-2020 at 1:03am
	Hey boys needs some help. Seems there are many topics about this but not much resolution I could find. My SN 85 351w runs great and starts up every time. But after running high rpm for 20/30 minutes the engine begins to stall and lose power. If not backed down to idle it will completely stall. However, it will crank right back up and idle at low rpms for a slow ride home. It has the Pertronix 1581 electronic ignition Conversion kit and 1.5 ohm pertronix flame thrower ignition coil. I take it this coil has an internal resistor and therefore the actual boat resistor should be bypassed? I’m guessing once the coil gets too hot the spark dies and high rpms are not possible? But it will crank immediately after it stalls and idle fine.. The carburetor is tuned and basically brand new. Any insight much appreciated!!

KENO Members Profile Send Private Message Find Members Posts Add to Buddy List Grand Poobah Joined: June-06-2004 Location: United States Status: Offline Points: 11112	Post Options Post Reply Quote KENO Report Post Thanks(0) Quote Reply Posted: June-14-2020 at 11:35am
	Tron I'm thinking, based on your description that your ignition system is OK and you have a fuel system issue like plugging of the suction line in the gas tank or the anti siphon valve at the tank outlet or a plugged tank vent line. Junk can get sucked up from a dirty tank and is enough to restrict flow at higher loads, come down to idle and it's OK. Then after the boat is off some of the junk can fall back into the tank waiting to get sucked up all over again the next time you use the boat. You could unscrew the anti siphon valve at the tank, check that it's clean and blow out the suction line or remove it to see if it's clean, put it back together and see what happens. Also make sure the tank vent is clear so you're not drawing a vacuum in the tank. A quick check here would be to loosen the gas cap before heading out so there's a vent path through the loose cap. Or run the engine from a portable tank hooked to the fuel pump suction to see how it runs. The portable tank needs to be vented so the fuel pump doesn't draw a vacuum in the tank and quit being able to pump. If the problem is gone, then you have the fuel filter, the hose back to the tank, the anti siphon valve and the suction line/tank to check out. I'd try this stuff to rule fuel supply related issues and maybe even check fuel pressure while it's running before the problem happens. If it was your coil, it wouldn't cool off enough in a few seconds for the boat to idle fine again most likely. I could be totally out to lunch, but ya never know

SNobsessed Members Profile Send Private Message Find Members Posts Add to Buddy List Grand Poobah Joined: October-21-2007 Location: IA Status: Offline Points: 7102	Post Options Post Reply Quote SNobsessed Report Post Thanks(0) Quote Reply Posted: June-14-2020 at 11:35am
	Could be fuel delivery just as well as spark. Maybe hook up pressure gage to the fuel pump & monitor when it starts stalling. To test spark theory, maybe get a can of 'cold spray' & douse the coil & dizzy when it acts up
	“Beer is proof that God loves us and wants us to be happy.” Ben Franklin

TRON Members Profile Send Private Message Find Members Posts Add to Buddy List Newbie Joined: June-07-2020 Location: NC Status: Offline Points: 3	Post Options Post Reply Quote TRON Report Post Thanks(0) Quote Reply Posted: June-14-2020 at 12:38pm
	Thanks for the input. I’ll disconnect the input gas line from the fuel pump and temp in a hose to an external gas tank. We’ll see if that remedies the problem. If so guess it will be time to take the gas tank out and do some cleaning

lakedog55 Members Profile Send Private Message Find Members Posts Add to Buddy List Gold Member Joined: November-11-2010 Location: Lake Weir Fl Status: Offline Points: 835	Post Options Post Reply Quote lakedog55 Report Post Thanks(0) Quote Reply Posted: June-15-2020 at 11:45am
	Much easier and safer to unscrew cap. This then you will no if it is the vent. If it still does it it could be the pickup in tank clogged.
	Lakedog55

lakedog55 Members Profile Send Private Message Find Members Posts Add to Buddy List Gold Member Joined: November-11-2010 Location: Lake Weir Fl Status: Offline Points: 835	Post Options Post Reply Quote lakedog55 Report Post Thanks(0) Quote Reply Posted: June-15-2020 at 11:48am
	Floats could be a little low. Could be water in your bowls on carb. I would drain them. Accelerator pump. Lots of stuff it could be
	Lakedog55

lakedog55 Members Profile Send Private Message Find Members Posts Add to Buddy List Gold Member Joined: November-11-2010 Location: Lake Weir Fl Status: Offline Points: 835	Post Options Post Reply Quote lakedog55 Report Post Thanks(0) Quote Reply Posted: June-15-2020 at 11:51am
	How high rpm.? 20 or 30 minutes ? Almost enough time for a charging system problem to show up. If it were fuel related it would do it in like 2 minutes
	Lakedog55

lakedog55 Members Profile Send Private Message Find Members Posts Add to Buddy List Gold Member Joined: November-11-2010 Location: Lake Weir Fl Status: Offline Points: 835	Post Options Post Reply Quote lakedog55 Report Post Thanks(0) Quote Reply Posted: June-15-2020 at 11:54am
	Should of read the whole post. Time would be long enough for vacuum in tank . Other than that I would look at charging system
	Lakedog55

SN 85 bogs at high rpm

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