Backfire under load – problems at speed

Over the past year I have been dealing with an increasing difficulty not dying in idle especially when hot without the idle cranked up.  So I have been reading.  Suspected vacuum problem.  Was having to have idle too high and not getting response when adjusting idle screws.  When I rebuilt carb I only replaced the gasket under the carb, not under the spacer.  So tackled this first.

I got the correct 2 new gaskets and replaced under the spacer and carb.  Immediately I could adjust idle down to 650 or lower without it dying.  Ended up setting about 710 in driveway.  Vacuum gauge I got up to 20.5 inch Hg.  Timing at 10 degrees. 

Note: it took a while to start after reinstalling carb.  I did get one backfire through the carb right as it started, then no other problems while adjusting. 

Tow to lake, start adjust idle on trailer.  – I did not bring timing gun or vacuum.  Maybe this is the problem. 

Boat initially ran better than it ever has.  Never got to 40 mph as fast.  Nice and responsive.  Drop to idle, idled nice and smooth, no problems with any dying.  Bravo big smile on my face. 

Windy day, so crossed the lake and boated around at a slower speed.  Boat still running great.  

 

Combing back across the lake like 30 minutes later I had some smoother water so I dropped the throttle.  Bam, at 36 MPH I get a backfire and hesitation.  Backed off, accelerated again and bam another backfire at 36 MPH. No more 40 MPH.   

Cruised around a little at slower speeds, and now it kind of wanted to die in idle for a while, then this abated.  Headed back to launch tried to accelerate again, now its sputtering more and I am not even hitting 32 MPH with a backfire.  Also, its now wanting to die at idle. 

Once started it will sputter and backfire under load, it will rev in neutral perfectly smooth up to 4000 RPM. 

I had to start and rev to get into gear to get on trailer.  A couple of times it did not want to start until I backed off on the key then it would start?  Never had that before.  

I have a new coil and an old electric conversion.  It has a 2.5 power valve (carb had a 2 staged one from National Carb rebuilder prior owner when I rebuilt).  New wires, cap, rotor, plugs from Nautique parts within the last year.

 

Only other thing I can think of, is I had added a full tank of gas prior to our adaptive ski clinic and then did not burn it because my boat was acting up.  Shell gas.  So for about 6 weeks it has sat with that gas.  I did dump a half a bottle of lucus stabilizer in on the way to lake.  I would not have thought that would cause issues as it made it from the tank to carb, but I am wanting to put everything out there. 

So I am stuck. 

 

If the timing is off from being set in driveway vs under load why did it run so sweet for the first 20 minutes? 

I guess I should check the cap and rotor, make sure the distributor springs are fine, then re-do timing and vacuum in water under load?

Should I swap out power valve if I was getting 20.5 inch hg to the 6.5 vs 2.5? 

 Should I replace the gas?  I have never had a gas issue before in our climate with name brand gas.  But the contrast from the beginning of the day to end makes me feel like something changed – gas, timing, cap, distributor advance?

 

Any thoughts would be appreciated.  I plan to do a bunch of reading before I go out to the boat again. 

What should I be searching on the forum?   

 

 

My module is the first, the Prestolite.  Installed in 1998 when the engine had 648 hours, now at 972.  So its 21 years old with some 300 hours on it.  

Rebel, it depends on how highly you value your time on the water. I had similar issues a couple of years ago with a failing Prestolite module and vacuum leaks under the carb and spacer. Went back to breaker points briefly before upgrading to a DUI. Boat now runs like new. That's not to say it hasn't had other issues, but they have been minor and easily solvable. If you upgrade to the DUI, be prepared to face the low voltage gremlins in your boat in order to get 12V or more to the unit. All in all, it has been well worth it. Good luck.