Hot starting

[JBPILOT]

Carb float. It slowly smoothes out. Are you running the elect pump all the time, or for sure during warm start up ? ? ? ? ?

[tomkk]

It's probably worth checking it again but I just had them off recently because I did have one bad float and replaced it so I doubt that's the problem. If that was the problem it seems like it would cause a problem with all starts, not just hot starts after sitting for a few minutes. Worth checking again, though.

[tomkk]

It looks like insulating the fuel lines from the spider on top of the engine down to each carb has fixed the problem. I've had half a dozen hot starts since then with no problem at all.

[Bill_H]

One other thought - open your oil door when you shutdown. Helps exhaust the heat in there quickly. Also helps keep the voltage regulator cooler.

[tomkk]

I guess I spoke too soon - I'm still seeing this problem repeatably. It starts fine if I restart right after shutting down, it restarts fine after sitting for 30 - 45 minutes but starts real hard if I try 10 - 15 minutes after shutdown. I say "hard start" but it's actually more of a rough start. The engine runs but is very rough for 60 seconds or so then smooths out and runs great. During that rough running period the left EGT comes up pretty much normally but the right EGT doesn't budge. After 60 seconds or so the right EGT comes up and the engine smooths out and runs great.

I've put a reflective thermal wrap on the line from the fuel spider to each carb. I've also just finished wrapping the exhaust pipes from the heads to the muffler with thermal wrap.

I open the cowling oil access door when I shut down.

I didn't notice this before but, after I shut down after one of these difficult starts, I found a wet spot that smelled of gasoline on the concrete under the exhaust pipe. That would seem to indicate flooding, not starvation like I'd expect of a vapor lock that I'd been assuming was the problem.

I've tried various throttle settings from 1/8" to nearly full throttle. Full throttle seemed to work best but still not great.

I tried shutting off the fuel valve to starve the engine at shut down rather than just turning off the Ignition switches as someone suggested earlier. That didn't correct the problem but did cut down the time to when the right side started running from 60+ seconds to about 10 - 15 seconds.

The choke on both sides does go to full close when the choke lever is pushed in.

Any thoughts? I really don't want to restart after a short shutdown anymore - it's just too hard on the engine, mount, etc. This has really got me baffled ...

[Ueli N]

It sure seems like your problem is the right Carb, like other said, problem with float sinking.
Or the float valve not seating properly causing carburetor to overflow with residual gasoline flooding the right cylinders.
That also explains the good start after 30 minutes when the flooded gas had time to evaporate or the better start when you turned off the fuel valve before shutdown.

[Bill_H]

ROTAX GUYS:
NORMAL START: OK, you start the Rotax. For the first 5 seconds it runs rough. Then the soft start module "kicks out" and the idle rpm goes up and it smooths out.

Is it possible that the soft start system is not working correctly here - when hot maybe staying on for much longer?

[Mich48041]

Is it possible that the soft start system is not working correctly here

That would not account for one half of the engine running rough as indicated by the EGT.

[BigJohn]

Total time on engine?
Type of fuel used?
Any previous carb work done?

[tomkk]

About 70 hours total time. 93 octane auto fuel (with ethanol) used almost exclusively. 100LL used on one long trip. No previous carb work other than replacing the floats with the new style before the first engine run and to replace one of those new style floats that failed (sunk) after about 25 hours.

This week I think I'll pull the carb and see what I can see as others have suggested. I was so focused on vapor lock that I didn't really consider the carb itself until I noticed the fuel spot on the concrete.