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Hot starting

tomkk

tomkk

Well Known Member
Seems like there would have been a thread about this previously but I couldn't find one.

I've only shut down a hot engine and tried a restart a short time later twice but both times I had a real hard time restarting. Hard time getting it to fire at all and, when it does, lots of vibration for a short time before it smooths out. Engine ran great before shutdown and ran fine once I got it restarted. Also fires right up when cold.

Don't remember the first time but this last time I ran the electric fuel pump for probably about a minute before the restart attempt if that's relevant.

When I restarted, I had the choke full open (pushed in) and the throttle cracked. I also tried throttle full open on the chance it was flooded but that didn't seem to do much.

So, what's the trick to restarting these things when hot? :confused:
 
On a 914, no pumps, no choke and just a touch of throttle. Anything else it would not hot start. Using choke and pump might be the issue. Hopefully others will chime in for the 912.
 
TroyBranch said:
Using choke and pump might be the issue. Hopefully others will chime in for the 912.
Click to expand...

He said choke fully open / in... in the context of a real choke (which it is not) that would mean choke off. The fuel pump on makes no difference.

I have found that the 912 hot starts better if the throttle is opened a little more than just cracked. Try with the throttle open about 1/8" movement of the throttle control.
 
Mine hot starts immediately. Don't use the choke out except for the first start of the day unless really cold with several hours "off." Hot start with a bit of throttle - the 1/8 inch mentioned is good. Fuel pump has never made a difference for me.

I do a fair amount of giving rides. Stop, shutdown, all electrical power stays going, passenger climbs out, new one climbs in, mags on, hot start. No problems. (They all get the walk around and basic briefing earlier.)
 
OK, thanks. I'll try a bit more throttle next time and see if that works better.
 
At least 1/8".

I do lots of Young Eagle flights-- start about 5-10 minutes after shut-down. Choke off, throttle 1/8 as the book says, normal start sequence, and she fires right up.

Wayne 120241/143WM
 
Still having a problem ...

OK, so this engine runs great. Starts great cold. Starts great hot if I start it right after shutting down. If I let it sit for 10 - 15 minutes or so, though, it starts with difficulty. Usually initially fires pretty much OK but with a LOT of vibration for 20-30 seconds or so. During that 20-30 seconds or so I can keep it going by playing with the throttle then it smooths out and runs normally. The left EGT ran cooler than the right RGT until a minute or so after the engine smoothed out. It seems to be very consistent - it's happened pretty much the same way every time I've tried to start it after letting it sit for 10-15 minutes or so.

I've tried various throttle positions from the 1/8" suggested, to full closed to fully open and in between, all without success. The 1/8" open is the best - that's the only way I can get to to start at all.

I took a video of my last try: https://youtu.be/VlJ4isGpqRc. For some reason I can't get YouTube to show the actual video but you can hear the sound. Sounds like my grandfather's old 2 cylinder John Deere :(. It smoothes out at about the 30 second point.

There's definitely something going on here but I'm not sure where to start looking ...
 
Could it be flooding out do to a high carb float level or sunken float(s)? Might want to look at that.
Alex
 
I'm with Alex - -

Carb float. It slowly smoothes out. Are you running the elect pump all the time, or for sure during warm start up ? ? ? ? ?
 
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.
 
Last edited:
Just to close th loop on this

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

It sure seems like a carb issue to me. Since you are in Florida maybe it is not out of the question to drop off that carb at Lockwood and have the experts go through it. Just a thought.

John
 
Yeah, I might end up having to do that. If I can't see anything when I pull it this week that'll have to be my next stop..
 
tomkk said:
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.
Click to expand...

Do you have the same problem when you're using 100LL or Mogas without ethanol?

Jim
 
Good question but I don't know. Fuel stops when traveling are long enough that the problem wouldn't exhibit itself and I didn't think to try a start after a short shut down.
 
I presume the float chambers and gaskets are definitely fitted correctly - torch and mirror is useful to check all the way round.

Also check that the float chamber ventilation tubes are correctly placed and open.

I'm not saying the above might causes the fault but it's worth checking everything.
 
Thanks, Jerry. All that's what I'll be checking this week. I'm actually hoping to find one of those things :) At least that'll get me past this problem that's been plaguing me ...
 
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