Any Idea What Causes This Engine Quirk?

Debo · February 11, 2015, 03:20:57 PM

I thought this might be a fun exercise and I might learn something in the process. There's a quirk that happens whenever I shut down and try to restart my 8V-71N after only a few minutes. After the initial shutdown, I press the start button to re-crank the engine. It acts like it's going to run, then stumbles and cuts off. A second press of the start button, and it fires off and runs just like normal. It's a strong-running engine, and after it's been sitting for a while (longer than just a few minutes) a single press of the start button fires it up in what seems like one revolution.

What causes this stumble on restart? I'm thinking it might have something to do with the fuel shutoff. Any guesses?

luvrbus · February 11, 2015, 03:34:33 PM

Fuel pressure switch

Rick 74 MC-8 · February 11, 2015, 03:38:03 PM

the safety shutdown system is turning the engine off because of lack of oil pressure. after its been sitting you probably have a pretty severe air leak that bleeds the system down after a few minutes. No air pressure no shutdown. Most MCI' s have a pressure switch on the fuel filter. if that switch is still functioning you can just hold the starter button down as soon as it gets fuel pressure that switch will disengage the starter and holding the start button bypasses the safety shutdown

Rick

Debo · February 12, 2015, 03:29:43 AM

Hmmm. Hey Clifford, why would the fuel pressure switch shut it down on a restart, but not an initial start? I'm just trying to understand this...

Rick, same question I guess. If the oil pressure switch is shutting it down after a restart, then why not after it's been sitting? I know I don't have any severe air leaks. It takes a week for my pressure gauge to drop 10 pounds, and the piston on the shutdown cylinder stays extended for days.

Thanks for the answers so far guys. I want to make it clear too that this isn't a problem that I need to get going or anything. She fires up immediately on the first press when it's been sitting for even an hour or two. It's just more of a quirk that happens with my engine that I want to understand.

bevans6 · February 12, 2015, 05:24:11 AM

My fuel pressure switch (when it was connected) would cause the engine to not crank when fuel pressure was up. Cranking, almost starting and then stalling doesn't sound like that. The engine shut down systems all cause the engine stop air cylinder to activate, that is how they cut off fuel and cause the engine to stop. If you start the engine from the rear you can watch and check to see if the engine stop air cylinder is retracting all the way, or if it is getting pushed out/hanging out to cause the engine to stop.

The only other things that can cause a mechanical 8V-71N to stall is lack of air or lack of fuel. If it's running and has air and fuel, it will keep running. Lack of air can be the emergency stop flap doing something silly, if you still have that connected. Lack of fuel can be a lot of things, from the obvious low fuel pressure/worn out pump, to a plumbing issue to a governor tune-up issue.

Something to try - what happens if you do a hot restart that would normally fail, but you advance the speed/throttle lever to make the engine run at around 1,000 rpm instead of idle? That would make me want to adjust the start assist (if it has one, I don't remember on non-turbo engines) and check the buffer setting and probably the gap setting. The start assist is what sets the injector racks to a kind of half open setting so the engine will start, then as soon as it's running the idle spring takes over control of the racks. The buffer stabilizes the idle, if it is set wrong the idle is unstable and the engine could stall, same with the gap. Starting the engine with the speed/throttle control open over-rides all that stuff. If you don't know how to do a tune up on a Detroit two stroke, either read the book a lot before you start, or get someone who does know to do it for you. It's easy to do right, but it's really really really easy to do wrong.

Brian

luvrbus · February 12, 2015, 07:19:00 AM

Just disconnect the shut and overrule system then if it still does it then is not electrical some time the PO got creative with wiring some buses like a Eagle and MCI I have replaced the fuel sensor because of the wrong sensor they had to sense fuel pressure or they would start and die the first few tries

eagle19952 · February 12, 2015, 07:58:34 AM

Quote from: luvrbus on February 12, 2015, 07:19:00 AM
Just disconnect the shut and overrule system then if it still does it then is not electrical some time the PO got creative with wiring some buses like a Eagle and MCI I have replaced the fuel sensor because of the wrong sensor they had to sense fuel pressure or they would start and die the first few tries

does this motor have jakes ?

TomC · February 12, 2015, 08:02:51 AM

The only thing my fuel pressure switch does is kill the push button so not to engage the starter after the engine is running. That happens at 10psi. It sounds like you're loosing fuel prime. But usually fuel prime is lost when an engine is cold. Good Luck, TomC

Debo · February 12, 2015, 11:53:07 AM

All very interesting stuff. Thanks for the replies. BTW, the engine does not have Jakes. I still have the emergency air supply cutoff flapper, but it functions normally and is in its retracted position.

I think I'll do as Brian suggested and start it from the back and see what's happening with the fuel shutoff air cylinder. I'm thinking it's not retracting exactly right after reading these posts.

Oh well, probably not worth spending any more of the boards collective brain power on it. It's just a curiosity and I almost half expected someone else to chime in and say "mine does that too." Thanks again.

bevans6 · February 12, 2015, 12:15:22 PM

Nope, mine's never done that, not once, nun huh, never shut off as soon as air pressure came up either, never left the priming pump on so the fuel pressure cut-off wouldn't let it crank, never started it with an oil line off and all the oil fell out, never had the air flap close on it's own, never started it when an air line was undone so all the air came out in a giant screaming noise and had to figure out how to shut it down manually neither...

Brian

HB of CJ · February 12, 2015, 03:26:53 PM

IF ... you have the old fashioned manual shut down lever and NO electronical shut down and safety interlock stuff, then make sure the "PULL TO STOP" and "PUSH IN TO START" levers are all the way out or in. This MIGHT be the fuel rack not completely working either in the run or stop position.

If the engine cranks OK but does not start until the second try, it is probably NOT a continuity problem with the heavy cables to the starter relay thing. If it is the actual starter not cranking properly, then it might be dirty cables, plus the first paragraph above already said. Good luck.

HB of CJ (old coot)

lostagain · February 12, 2015, 06:00:12 PM

I usually do my first start of the day from the back and observe what's going on. And to be able to catch something wrong and shut off if necessary.

JC

Jim Eh. · February 12, 2015, 06:21:24 PM

Possibly the amount of air build up at that particular moment in time? Just kinda on the fence and make the shut off hang up or stick?

luvrbus · February 12, 2015, 06:45:08 PM

Could be the starting aid out of adjustment one never knows the old engines have their special little quirks sometimes

eagle19952 · February 12, 2015, 08:35:54 PM

Quote from: luvrbus on February 12, 2015, 06:45:08 PM
Could be the starting aid out of adjustment one never knows the old engines have their special little quirks sometimes

Years back my old Master mechanic would offer this advice, as will I today...

Let er develop... yup, let her develop.