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4 stroke starts but won't run

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Old 11-22-2017, 03:47 PM
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Default 4 stroke starts but won't run

Hi guys,

I've been out of the hobby for a few years, back in flying electrics a year or so back, and now it's time to resurrect my higher performance nitro engined aircraft.

Apart from some gumming up, they seem to be OK, but one of my Magnum 90 four strokes won't run. It starts readily, runs for less than a second and quits.

I've tried flushing the tank with methanol, then flushing with fuel, made up a batch of new fuel, and changed the glo plug twice, but exactly the same problem. Other engines are running well on the same fuel.

I'm thinking that it may be the fuel system because due to space constraints, and CoG, I have a main tank and a header tank. The problem is that accessing the header is not easy, so I want to exhaust other options, if any, first.

Any ideas on what to look for/at??
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Old 11-22-2017, 04:38 PM
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tailskid
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Any chance you can hook up a second (temporarily) directly to the engine and bypass the other tanks?
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Old 11-22-2017, 06:04 PM
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Originally Posted by tailskid View Post
Any chance you can hook up a second (temporarily) directly to the engine and bypass the other tanks?

Thanks tailskid, and it's a possibility I had covered, but not enacted.

The more I think about the problem, the more convinced I am that it's the fuel system and not the fuel itself.

If the glow is OK, and it is, and there is fuel initially, sufficient to get started, and the fuel is good, then it almost certainly must be somewhere in the fuel system.
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Old 11-22-2017, 06:51 PM
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I would pull the head clean it and adjust the valves.
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Old 11-22-2017, 08:04 PM
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I assume this fuel system is a two hose setup. Take both hoses off their nipples. Blow into the hose that came off the muffler, If the fuel lines going to and from the tank are not clogged, you should get a steady stream of fuel from the other hose with little effort. If that is so, then you have a clog somewhere after the carburetor intake nipple, probably near the needle valve. The easiest way to clean that area out is to remove the carb from the engine, take the needle valve out of it, and dump the entire carb and intake manifold into a crockpot of antifreeze and let it cook at about 225 to 250 degrees for several hours. Use the old style green ethylene glycol for this. Then rinse the assembly with water and blow out with compressed air until dry. If there is the slightest hint of a clog in that area, this will clean it out.

Actually, when I am trying to resurrect an old, gummed up engine, I don't even mess with it. I simply disassemble it as much as I can, dump it in the antifreeze for several hours, rinse it off and blow it out, then reassemble it before I even attempt to chase a problem In about 95% of the time, the engine will then start and run fine without further ado. You now have a clean engine, inside and out, that will run as well as it ever will without replacing parts. You have eliminated chasing a hard to find problem and the engine looks good.

I just recently did a Saito .91 that had been sitting inverted with fuel in it for about five years. That thing was a mess. The intake valve was stuck fully open and there was gel inside it everywhere. I completely disassembled this one including removal of the cam from the cam box because the intake lifter was also stuck in its bore. I let that one cook for about 24 hours, rinsed it off and blew it out with air and reassembled it. I use Marvel Mystery Oil to lubricate parts as I am reassembling them. I hit that puppy with a starter and it was running in just a couple revolutions.

I find it easier to assume these things won't run if they have been sitting with fuel in them for any extended period of time and do the antifreeze thing in a crockpot to them before I even attempt to start them. I would prefer to begin with a like new engine than try to chase an unknown problem in a cruddy engine. Maybe a new glo plug will do it. OK, that didn't do it, maybe adjusting the valves will do it. Nope. Maybe spraying the inside of the needle valve with carb cleaner will do it. All right, that didn't work either. I know. I'll replace the fuel lines at the tank. Nope. Not that either. Maybe, maybe, maybe..........

Or I can disassemble it, dump it in the crockpot, and build whatever wing I happen to be working on while the engine cleans itself.
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Old 11-22-2017, 08:25 PM
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Assuming you've already changed the carb oring and aren't getting bubbles in your fuel line, I'd say JollyPopper has it right. The valves could have some gum stopping them from seating fully, or your needle valve could be blocked. A bath could be just the thing to fix it right up.
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Old 11-22-2017, 11:34 PM
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Great suggestions guys, and thanks a lot.

I took the plunge and pulled the fuel tanks out, and found a tiny split in one of the lines where it slides onto the aluminium tube through the bung, so it was sucking air. When I put my finger over the carb and pulled the prop through, it sucked fuel, but when the demand was increased by the running engine, air got into the line. All solved.

Thanks again for your help.
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Old 11-23-2017, 07:30 AM
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FWIW, I change fuel lines and stoppers annually as part of standard routine maintenance. Silicone does break down with time, so think of all rubber parts as consumables just like fuel, props, switches, and batteries.
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Old 11-24-2017, 07:17 AM
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Steve Collins
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Over the years I found that the pink fuel tubing held up much better than the blue. It is far less susceptible to splitting and pinholes that will drive you crazy. Unless you are already using the pink tubing, I recommend that you convert over to it.
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