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engine, intake, piston, rings, valves

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  #1  
Old 14th June 2012
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Default Lost compression

I pulled off to the side of the road yesterday for a smoke and when I went to leave I only had one cylinder. There was no pop or any kind of warning. The bike was running a little differently but I figured it was from rejetting my carb the day before. The bikes never really ran right, was sucking air at the manifold and carb, front cylinder plug wire was arcing on the head...just fixed all this over the last couple weeks. When I had the manifold off I felt the valve and it was incredibly crusted up with carbon. I got it from an old man who let it sit around a lot, bad gas I assume...so I had been running some additive in the gas to help break it up. There's only 8000 miles on this bike. What could cause a loss in compression like this? the front cylinder runs fine. I had changed my oil recently and the resovoire was way over filled. Everything is stock on the bike exept a kn air filter I installed recently. Should I not havechanged my jets? I didn't drill the bottom one out when I tuned the carb, just left the stock one. Can't think of any other useful info right now....I had let it get quit low on oil bfore changing it and th bike got hot a couple times. Also I had checked my oil whe I lost the cylinder and it had the consistincy of water but the tank wasn't that hot
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Old 14th June 2012
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Lost compression (rings gone, valves not closing, hole in piston, etc.) or one cylinder not firing?

If not firing, what is present condition of the plugs, plugs wires and coil?

Last edited by sportsterdoc; 14th June 2012 at 21:12.. Reason: hold corrected to hole
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There's very litle compression in the rear cylinder
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Old 14th June 2012
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A leak-down test will pinpoint the source of the leak. You'll hear the air flowing out of either the breathers (rings are leaking), the exhaust (exhaust valve leaking), or carb (intake valve leaking). Or in rare cases past the head gasket.
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Quote:
Originally Posted by dc96815 View Post
There's very litle compression in the rear cylinder
If it barely blows your finger off the plug hole, yet the piston is intact, most likely an open/burnt valve or worn rings. Squirt some oil in and if the reading comes up, it is most likely rings.
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Old 14th June 2012
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Quote:
Originally Posted by sportsterdoc View Post
Lost compression (rings gone, valves not closing, hold in piston, etc.) or one cylinder not firing?

If not firing, what is present condition of the plugs, plugs wires and coil?
Exactly, we need to gather more information here.

First thing since dc is certain it's loss of compression is to do a good compression test and write down the results, then back that up with an oil reading (squirt a few squirts of 30 wt in the jug and test compression again to see if it goes up any.

If it is bad compression since using the additive, it's possible that some of that crap on the valve stems got broken loose and is holding the valve open.

In California we get or used to get, (not sure if we still do), engines from Japan that had low mileage and when you buy one most of the time there is no compression from the corrosion in the valve ports that happens on the boat ride over here. So we spray kerosene in the ports, let it sit, and in a day or so it seals up just fine. So some variation of that could free it up.

While the plugs are out, post pictures of them being careful to show us what they look like inside on the metal inner wall and the porcelain all the way to the bottom if you can.

dc... you talk about rejetting, what jets did you end up with in there? Did you clean that carb thoroughly? I can't see the year of the bike in this window, so also, is it a diaphragm carb like the CV? If so, are the diaphragm's good? Did you find air leaks? Are you sure there are no air leaks now?
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Thanks...should I do the test or just take the valve cover off and see since I'm gonna have to anyway?
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OK, it's only got 8000 miles? Then unless running it low on oil hurt something badly (which is unlikely), then it's a stuck valve, and that will be easily determined with the aforementioned compression testing.

You probably don't have a way to do a leak down test at home, but the compression test will tell the story anyway.
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No, not the valve cover, the problem will be in the ports where you felt the grunge and buildup on the back of the valve and the stem. Assuming I am right here.

And yes, you always start with a compression test to make sure you are not chasing your tail, and to be able to record improvement, and for future reference after this is fixed.
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Not sure what jets they were. The old man handed over a carb tune kit so I used most of it. Its a 98 sportster. The carb was leaking before I changed the gaskets. The daphrams seemed good. The float looked a little worn but still funtional
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