Interesting problem with BS V Twin

700 hour 40R577 BS V twin riding mower used without the deck as a Gator get around vehicle. Has the aluminum intake manifold. Recently started having intermittent loss of the right cylinder.
Compression 145 both sides, new premium fuel, filter, does it with the air cleaner off, I keep the oil clean, was running Iridium plugs and have changed back to the Champion Y12s and NGK 7938 V Power, all 3 would dry carbon up, grey insulator on a new plug while the good side was pure white.

Its totally random in too rich a mixture on the right bank and causing the plugs to foul with dry soot. Seems to be real sensitive to choke now as compared to normal when I push the lever over to full choke. It'll run normal for a time, maybe as long as 10-15 minutes and then start limping on one cyl. Just when you think your chemical treatments of fuel and through the air intake have done something, it does it again. Pushing against the governor on the throttle lever and it has no problem revving to way over 3600. Checked intake manifold and all suction hoses and no loose fittings, cracked hoses or any clues. Being overly rich one wouldn't expect an air leak anyway......that would lean out the mixture.....

I can't determine if I am chasing fuel or spark, or maybe a sticking intake valve stem....but the pattern doesn't seem like a stem sticking, besides with honey colored syn. oil in it all the time how would it get gummed up in the first place.. I checked the gap on the mags and put a timing light on both plugs. When it bogs I don't see any difference in the light patterns as when it's running ok. I am tending towards fuel due to the over rich condition causing the sooted plugs (opinion). I guess I need to remove the intake assembly and go through the carb (I did remove the shut off switch and drain the carb and what came out was clear with no sign of debris...it has lived on E10). If it were float/needle valve related, the intermittent nature of the problem doesn't support something that I think would be a constant menace if applicable. Its haying season and I don't want to tear it all apart just now.

Thoughts appreciated.

I think I'd check the valve guides. There have been instances of the guide moving up/down in the head and causing weird stuff like that.

Is there gas in the oil?

How clean is the air filter? A plugged up air filter will make it run rich.

The air filter is like 10 hrs use and I have the problem with it off while troubleshooting.
----------
A valve problem has been in the back of my mind as backfiring has accompanied this occurrence. With suitable compression, and the compression numbers being identical on both cylinders, and only 700 easy hours, and keeping upper cylindeer lube in the fuel, I really didn't expect a valve sealing problem.......buttttt who knows.

Since I relate backfiring to fuel being in the intake manifold, or a valve (function) not sealing off the intake when the plug fires and the explosion touches the gas, or has an exit path there and hence comes out the carb. I'll have time this week to pull the valve cover off and have a look. As far as timing being early causing the problem, it's too intermittent for that and I have been over the mags and spacing and mounting and all that. There is no deck on the machine so I can't suspect a partially sheared Woodruff Key... due to a blade impact with an immovable object type problem, which would be a constant problem if the problem.

Thanks for the replies.

Sounds like a coil problem as it fails when hot. Does it have a separate coil for each cylinder?
Dave

Yesterday I swapped the coils and set the gap to .014 where it had been at the low recommended end of .010, looked them over, cleaned off the mating surfaces and the gap poles. Meanwhile I am checking plugs for fowling....haven't run it enough to see if the fouling moved over with the coil, and have been shooting the intake with carb-choke cleaner. After half a can of that, I pulled both plugs and they were whiteish (spray deposits I assume) on the tip and the top 1/8 of porcelain nice and clean, indicating to me that both plugs are firing equally.

So the saga goes on. I am reluctant to pop the valve cover but that's next....makes such a mess to trouble shoot that area but it's next.
Thanks

The right side initially fouling the plug had the intake valve gap set a little sloppy, reset to specs. Everything else on both sides looked ok. Valve movement was smooth in and depressing the valves, each felt smooth and even. Engine is running somewhat better but still a gotcha out there lurking. Not fouling plugs.....yet. Guess when I get tired of putting up with this I'll rip the intake manifold and all associated parts out and have a look. The saga continues.......

Side note: As I said I bought this engine new and put 700 hours on it. The area under the valve cover and the underside of the valve cover were basically spotless, zero crud, zero sludge, nothing but shiny metal parts. Wallyworld 10-30 synthetic did a good job at keeping things slick and clean.

Well as it turned out Dave BN nailed it. As I mentioned, I ordered a carb but ordered a dual throat like all my 40R777s have.....well when I started pulling the intake manifolds off to install it,I realized that the 18 Hp V twin is a 40R577 and it is a single throat carb.....so no way that carb would make a single cylinder act up.

That left the coil as the only variable. Today I received a new one and the problem is solved. That turned out to be quite an experience for me since I can hold my own working on things and with several BS V twins around I have had my wrenches on them more than once. I guess I just had a senior moment ordering a carb from the wrong model number....but now I have a spare for the 777 engines.