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Indy 400 - unreal low compression

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  #1  
Old 01-06-2011
mikey007
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Default Indy 400 - unreal low compression

A 1986 indy 400 was given to me with a rebuilt top end. It has spark and is getting fuel. It has (tops) 30psi a side and I know the tester is good because I tested my running sled with the same tester 5 minutes before. The cylinders were honed and it has .010 over pistons in. The engine will not fire due to the low psi. What would cause this low of psi?
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  #2  
Old 01-06-2011
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I had an 86 indy 400, I miss that sled. Those things can really scream. Look in the spark plug hole, are the pistons new? Shine a light in there, do the cylinder walls look glazed or newly crosshatched(ground)? Have you seen it run at all? I don't think fresh pistons have alot of compression until they've ran, but seems like it should have more than 30psi.
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  #3  
Old 01-06-2011
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I took the head off and the pistons are brand new and it looks like the cylinders were just honed as well. I have not seen it run, yet. Any other thoughts?
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  #4  
Old 01-06-2011
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Did they put the rings on the piston? only thing I can think of if there is no holes in the piston. LOL Even broken rings should give more then that.
Pull a carb and look through the port.
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  #5  
Old 01-06-2011
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I looked at the rings through the y-pipe and they look fine from what I can tell.
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  #6  
Old 01-06-2011
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Is it possible that the rings were put on upside down?
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  #7  
Old 01-06-2011
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can you poke then and see it it "springs"?

Not sure if your pistons have a pin to keep the rings from rotating around the piston, if the do it is hard to put them on upside down. Should still give more compression. what is the piston to wall clearance. Feeler gauge?
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Old 01-06-2011
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Put some car motor oil on them to seal the rings and see if the compression come up?
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Old 01-06-2011
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Explain how it ended up this way so I can understand. Someone burned it down and rebuilt it, but couldn't get it running so gave it away?

There are arrows on the pistons, which way do they point?
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  #10  
Old 01-06-2011
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Should have way more than 30psi... Just rebuilt mine 110psi dry before fire.. check piston or rings possible std and bore .10 over....
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  #11  
Old 01-06-2011
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If the rings are there, then I vote standard rings on .020 pistons and bore.
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  #12  
Old 01-06-2011
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Quote:
Originally Posted by Gotmud View Post
Explain how it ended up this way so I can understand. Someone burned it down and rebuilt it, but couldn't get it running so gave it away?

There are arrows on the pistons, which way do they point?
A friend gave me his old 88 400 and 86 400. The 88 has been running for years so that one is fine. The 86, he bought to get going as well. When he got the 86, the pistons were seized so he unseized it and had it professionally rehoned with new pistons and rings but never had time to fully put it back together so he gave me that sled as well. The pistons are .010 bored over as that is stamped on the top of the piston. I have had it all together and it has hardly any compression.

I took the cylinders off tonight to look at the rings. The rings look fine but I pulled them off anyways and I cannot see if they were put on wrong to begin with or not. Is there a wrong way to put them on? There is not a single mark on either ring to identify the top from the bottom. There is only ONE ring per cylinder, is that normal? Any other ideas?
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  #13  
Old 01-06-2011
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on a stock style 400 piston yes...
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  #14  
Old 01-06-2011
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Do you have the headgaskets in?
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  #15  
Old 01-06-2011
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Quote:
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Do you have the headgaskets in?
Yes, I do.
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  #16  
Old 01-07-2011
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most rings can only go in one way, if you put a ring on, then try to compress it into the piston like you were going to put the cylinder on, if the ring gets stuck then its upside down. also look close at them, they might have a slight mis shape to them, you want to install them so the air ontop of the piston will push the ring out.

does the carb slide move when you sqweeze the throttle?
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  #17  
Old 01-07-2011
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The carbs were taken off and cleaned. The slides move fine. I am not trying to be rude, I am just learning, why would slides being stuck cause a loss of compression?
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  #18  
Old 01-07-2011
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Do you have some calipers? I'd measure the cylinder, something doesn't sound right.
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  #19  
Old 01-07-2011
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I am at a loss. The rings are good, the pistons are good. It must be the cylinders. I can pour gas on top of the cylinder and it disappears in seconds. Maybe they weren't honed or bored properly.
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  #20  
Old 01-07-2011
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I think you're going to have to take your cylinders off; take them and the pistons to a machine shop to get measured.
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  #21  
Old 01-07-2011
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^^ I think so, the cylinders must not be bored the same as the .010 over pistons.
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"Do not go where the path may LEAD, go instead where there is no path and leave a TRAIL"

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Originally Posted by Rubi View Post
Nobody on this site likes Ski Doo except Dirty Harry, and he's better at making babies than buying or fixing sleds, so you don't want to listen to him.
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  #22  
Old 01-07-2011
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Would the wrong ring cause too large of a ring-end gap, and all the compression is leaking through the gap? My Indy 400 pulled about 108 right after the rebuild, before it was run.
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  #23  
Old 01-07-2011
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Too large of a ring wouldn't compress enough to get into the cylinder, I suppose I should say I don't think it would. I'm thinking either the cylinder is out of round or someone bored it over too much.
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"Do not go where the path may LEAD, go instead where there is no path and leave a TRAIL"

Quote:
Originally Posted by Rubi View Post
Nobody on this site likes Ski Doo except Dirty Harry, and he's better at making babies than buying or fixing sleds, so you don't want to listen to him.
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  #24  
Old 01-07-2011
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I was kind of thinking if the ring was too short it would leave a large ring-end gap. Maybe too small a ring wouldn't even fit on the piston though?
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  #25  
Old 01-07-2011
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Too small of a ring would probably still fit into the piston but wouldn't make contact and like you said leave a gap.
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Rich
"Do not go where the path may LEAD, go instead where there is no path and leave a TRAIL"

Quote:
Originally Posted by Rubi View Post
Nobody on this site likes Ski Doo except Dirty Harry, and he's better at making babies than buying or fixing sleds, so you don't want to listen to him.
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