Rebuilt T-420's in 04, new rotation assemblies to make them 496's. Both engines run fine over 2000 RPMs.... the problem occurs when I idle the Port engine for 10 mins or so, when I take off the engine sounds like one cylinder is filled with gas ( making the engine cough and sputter/ backfire ) until is clears up. (take s about 5 mins ) I have checked all the spark plugs, cables, no cracks in distributor.... Haven't tore into the engine Yet. Is there something im totally missing ? When I shut the engine off to idle, no coughing or sputtering of any kind....when i get back into it. Thanks for any responses in advance,

ave you tried to switch the carburators from one to the other engine ? Just to see that its not a fuel flow related issiu ?!
Do you have a fuel press gauge ? And you did not mess up the firing order on that one engine ? What rotating asembly is it that u used ( compcams) ? They make one where the #7+5 is switched ?!
Just trying to figure this out !

DD, I have not switched carb's. Running demons that are sequential in numbers. I do have pressure gauges but not hooked up, So not really. Yes, I am using Compcam... you pose a good point. but only one engine is having the problem. perhaps i need to dble check the cables on the other engine. (make sure they are similar) Ill have to wait on the carb check til' spring, winterized. thanks for your insight,

I was going to say check your accelerator pump, but you state that it takes 10 minutes to clear out...could be a stuck float, but then it would idle rough and black smoke. Because the time to "clear" is so long, I would idle it for however long it takes to create the problem, then pull plugs to see which cylinder(s) is fouled, then go from there. Start on the bad cylinder(s) with a compression test, if all normal, move to a leakdown, that is IF you need to go that far.

Dave

I had the stuck float problem on my Demon and it was exactly the same scenario.

X77, thanks for your comment. I did have adjustable jet plates installed over last winter, (both carbs) think they could have tweeked something to make the float stick ??? thanks to all for your help, this kind of advice is priceless.

I would also say ck the floats themselves real closely, as sounds like you got one hanging coming on plane..I had one coming off plane early this year....poured fuel in as soon as boat came off plane....