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Lost the pinch in my Jett 50

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Old 04-09-2010, 12:17 AM
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still4given
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Default Lost the pinch in my Jett 50

Hi Bob,

I had a warbird race a couple of weeks ago. My usual turn key SJ50 was all of a sudden acting up. The first round I had to richen the needle quite a bit and chocked it up to change in altitude. Took off OK but started surging in the second lap and so I shut it down and landed. For the second heat, I richened in up to where it was falling off the pipe. Took off and made it to the 8th lap before it started surging again. I backed it off to abour a 1/4 throttle and was able to finish the race by just barely keepin git flying. When I went to check the glow plug it had backed out a couple of turns. Aha I thought. In my quest to not overtighten the glow plug and risk turning the cylinder, I must not have tightened it quite enough. So, I checked the tightness of the head bolts, which were tight, and installed a new glow plug. Next heat, I again ran it so rich it would barely stay on the pipe. I took off and didn't make it one lap before it started sagging. I shut it down and landed. So, I get out my backup plane which is also a turnkey plane. I fire it up and it too is making a strange sound. I richen it up as much as I can and still keep it running. I take off and low and behold, it goes lean as well so I had to shut it down and land it. So much for Bronze class.

Well, I get home from the race and a couple of days later I decide to open up the engines and see what I can see. I don't see any damage except that usually, if I rotate the crank with the head off, the piston will push the cylinder out of the case due to the pinch at the top. Well, it doesn't do that anymore. The pinch seems to be gone. It still runs OK but is slightly down in rpms. Do you think it will come back?

Oh, by the way, I figured out why I had a problem at the race. When I went to check out the fuel back at home, I removed the six-shooter pump, and right there on the side of the can, behind the big strap, was the 60% I had written on the can when I filled it with some of the 60% fuel I had mixed up for my big YS engines the day before. I ran out of cans so grabbed an empty 15% can and didn't think to cross off the label. Just wrote on the side. Doh!

Thanks, Terry
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Old 04-10-2010, 07:59 AM
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Default RE: Lost the pinch in my Jett 50

One thing you can try....... toss a tiny prop on it ..... (QM size prop) and let it scream a bit. Take it up to 20K and let it run (if it will). Just a brief run around 3-4 min, and then put the piston at BDC and let the engine cool off for a while. See what happens.

Otherwise, maybe something just went away and its piston time. Usually the liner is ok. But they are a matched set.

Bob
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Old 04-26-2010, 04:57 AM
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Default RE: Lost the pinch in my Jett 50

Thanks Bob,

I'll try that. Probably have to clip a Q500 prop since I don't have any Q40 props. I ran it this weekend in our warbird race and it seems to run fine. I'm going to be putting is a sleeker frame and hope to turn a Q500 prop in the 19,000 range. Do you think that will be alright? I was running a Nelson Q500 in it before. Hoping to get somewhere close with my SJ50.

Thanks, Terry
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Old 05-01-2010, 05:01 AM
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Default RE: Lost the pinch in my Jett 50

Hi,

sorry for the delay

Yes.... sounds like it willl work fine. Let me know how it works out.

Bob
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