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Old 09-23-2009, 05:27 PM   #1
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Default What are the signs of low compression?

When do you change the piston/sleeve? Is there any kind of marking engraved on the piston (I dun think so, but I'll ask anyway)? How can you tell when the engine is running out of compression? Some guys report of using the same sleeve/piston till the 10 gallon mark...

Thanks in advance
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Old 09-23-2009, 05:48 PM   #2
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Do some searches. I started to have poor idling, followed by lack of pinch when turning the flywheel by hand. Engine runs fine when it's running and up to temp, but has a poor idle (cuts out, idles high).

No need to replace right away, though. You can have the sleeve repinched by a service like RayAracing. Cost is about $15-20, much cheaper than a new piston/sleeve. There are other guys who provide the service too. Like I said, do some forum searching.
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Old 09-23-2009, 05:57 PM   #3
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I use the 3 o'clock test.. With the motor out of the vehicle, hold onto the flywheel so the head is at 3 o'clock, if it flops down after making a complete resolution, then it needs repinched or replace. Someone else probly can exsplain this test better than i can.

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Old 09-23-2009, 06:07 PM   #4
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Like I said, do some forum searching.
Thanks, did that and the threads got derailed at every opportunity when I read it.

thanks to everyone, I was wondering how long I can tolerate my poor idling, just wondering how much longer I can run the engine
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Old 09-23-2009, 06:10 PM   #5
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Thanks, did that and the threads got derailed at every opportunity when I read it.

thanks to everyone, I was wondering how long I can tolerate my poor idling, just wondering how much longer I can run the engine
Idleing can be tuned out, you'll know once it does go! When you first fire it up when it's cold it will run fine then once it gets up to operating temp then the tune will go away!!!!!. How many gallons do you have one your motor??
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Old 09-23-2009, 06:34 PM   #6
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my osspeed is doing exactly what kuruption said [poor idling,high idle].she is at about 8 gallons now with not much pinch left in her.i finally put it away for a repinch and rod.i also am gonna pick another p/s for for later use!i will run this motor till there is nothin left.for 500 bucks i want to try for 20 gallons between 2 p/s,2 pinches.

for now im gonna finish my season up with my little 3port nova,another great mill
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Old 09-23-2009, 07:10 PM   #7
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I would say when it looses top end and the torque go away. This tells me time for a new sleeve and piston.
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Old 09-23-2009, 11:53 PM   #8
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Originally Posted by Gearhead racer View Post
I use the 3 o'clock test.. With the motor out of the vehicle, hold onto the flywheel so the head is at 3 o'clock, if it flops down after making a complete resolution, then it needs repinched or replace. Someone else probly can exsplain this test better than i can.

Tyler

just my .02. . This is the most useless test ever. The only time an engine will pass this test is when it is new. when you break it in the pinch starts to go. You dont want pinch you want compression and compression wont keep the engine up due to its just air getting pressed out
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Old 09-24-2009, 03:46 AM   #9
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just my .02. . This is the most useless test ever. The only time an engine will pass this test is when it is new. when you break it in the pinch starts to go. You dont want pinch you want compression and compression wont keep the engine up due to its just air getting pressed out
But doesn't pinch allow you to have compression??????????????
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Old 09-24-2009, 04:28 AM   #10
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Pinch is when you can have the glow plug out and the engine will lock or get stuck at a certain point before turning over the rest of the revolution.

Compression is like the ''seal'' between the piston and the sleeve through fuel and air and will gradually decrease as the sleeve ages and expands, sometimes running overly hot can cause this. The gap between the p/s gets bigger and there you have it, no compression.

You're left with compression normally after the 5 - 10 litre mark on most good quality engines if broken in right. This is when the engine will truly start revving hard and you can lean in on them a little more.

That's the way I see it anyways.....
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Old 09-24-2009, 04:37 AM   #11
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I always know when the engine will start easy when cold, and once it hits operating temp it shuts off, and it will NOT start until it cools down.
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Old 09-24-2009, 06:43 AM   #12
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Originally Posted by losi_racer View Post
just my .02. . This is the most useless test ever. The only time an engine will pass this test is when it is new. when you break it in the pinch starts to go. You dont want pinch you want compression and compression wont keep the engine up due to its just air getting pressed out
not so true - my nova Plus4 started failing the test at the 5Gallong mark... still works great though, but probably not for long anymore. It'll be pinched this winter

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Old 09-24-2009, 07:04 AM   #13
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Quote:
Originally Posted by losi_racer View Post
just my .02. . This is the most useless test ever. The only time an engine will pass this test is when it is new. when you break it in the pinch starts to go. You dont want pinch you want compression and compression wont keep the engine up due to its just air getting pressed out
I use this test on all my engines and I have 4 gallons on my vspec and it stil hold strong, now It dont hvae to stay at 3 oclock but it needs to hold for 4 o 5 seconds
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Old 09-24-2009, 12:17 PM   #14
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I use this test on all my engines and I have 4 gallons on my vspec and it stil hold strong, now It dont hvae to stay at 3 oclock but it needs to hold for 4 o 5 seconds
all my engines ive seen and owned never can do this, especially vspec. If you have pinch at 5 gallon mark um thats a long break in lol. My nova +4, my vspec both have about 2 gallons maybe 2.5 and they fail the "3 o clock test". I break them in pretty close as the break in bible states been doing that before that was posted. I broke my dads vspec in and his lasted 10 gallons on the original rod and bearings(his choice not mine) . I guess some ways work for others and some dont
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Old 09-24-2009, 12:33 PM   #15
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There are motors that run great with zero pinch and motors that wont run at all with lots of pinch. You really can't go by feel when cold. Now you can turn the flywheel as soon as you shut it off and it's still good and hot, plug in of course, and see how good the thump is. If it has a fair amount of thump (not talking pinch but actual compression) then it is probably still ok. If you can spin it easily with your thumb, it's probably shot. Bottom line, if it runs good it's fine, if it doesn't something is wrong.
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