Question about 1976 Ducati 860 GTS

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I have an 860 GT and want to know the ideal compression pressure

I have an 860 GT and is about due for a top end rebuild. Prior to dismantling what should the compression pressure be in an 860 GT, for example 155 PSI

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You're asking a BAD question in my opinion. I'll explain, and give you a FAR better solution... Compression checks in my opinion are fine for checking from cylinder to cylinder ONLY. If you're within 10 psi plus or minus, you're fine. However, cylinder LEAKDOWN will tell you if you've got an issue. You may have good compression, but any compression readings will not show leaks in the valvetrain unless the leak is pretty bad. I would instead test cylinder leakdown both cold and hot using a leak down tester, a good air compressor with a good regulator, and if you have anything worse than 85% in a leakdown test, use your ears to find the air leak. If the leakdown says for example 70%, listen in the exhaust and through the carbs. If both are quiet, listen in the oil fill hole. Now not only do you KNOW how bad the issue is, you know right where it's at. Intake valve, exhaust valve, or piston/cylinder. the Stick

Posted on Dec 10, 2009

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There is always a better alternative than the dealership as far as costs. They want to put a new motor in a 2003 model? That is ridiculous to me- cost-prohibitive. Find a reputable independent shop. Look for references and customer satisfaction. A used, low mileage replacement engine is a better alternative, probably less than half the cost of a new engine.
There may be an even better alternative, where you refurbish the top end of the engine-the cylinder head and valve train. This is dependent on that a wet-dry compression test supports rebuilding the top end. What I mean by this is a compression test of the cylinders reveals low compression in one or more cylinders, okay. Next, they would squirt some oil (about a tablespoon) in the low cylinders and check compression again. If compression doesn't change, it points to the valves and valve train (the valve guides which wear and could let oil into the cylinder). But if compression is higher in the wet test, then the piston rings are worn and losing compression, and oil gets past them-this would indicate the engine needs a complete rebuild or a replacement is called for. The reason for the wet test is that the oil will temporarily seal around the piston rings and improve compression if the rings are the problem.
If the test shows the valves are the problem (and it is very possible on an engine with only 120K-your bottom end could be fine), then a top end rebuild of the engine is a much cheaper route than any other choices. You may not know this, but often a little oil burning in the cylinders at start-up is nothing more than the valve stem rubber seals are deteriorated-a real cheap fix. The dealership probably won't tell you this. However, with the loss of power, well, that points to something more severe than valve stem seals. So I would recommend the wet compression test, just to check if it is the top end or the bottom end. If it's the top end, you could have the cylinder head reconditioned and motor along for a few more years, trouble free.
Similar to a compression test, a "leak down" test of the cylinders is actually a more accurate method of finding where compression is lost, and it's only a one step test. They hold compressed air in the cylinder and analyze where it escapes to pinpoint compression loss. Loss of compression translates into loss of power. Have either test done before you decide. Find a good mechanic and trust his advice. (Stay far away from dealers. Greed is their motivating factor.)

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Do a compression test and check against the acceptable figures from your manual or a dealer.
If the compression is to low you will need to do a top end rebuild.

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Assuming you have spark and know you're getting proper fuel, next we have to suspect compression. Assuming you don't have a compression gauge at this point, check your valve clearances. You may end up finding some out of spec, but you are specifically looking for valves that have no lash at all. If you find any like this, you will have likely found your problem. Adjust those valves and check it out. If it works, keep a check on that valve. If it keeps tightening up, you may need a new valve.

Next, again assuming you don't have a compression gauge, remove both plugs. Hold your thumb over the hole on the side that does hit and crank the engine over. Notice how hard it blows your thumb off. Now the other side. Does it feel as strong? This will only get you ball park, but usually when a cylinder is completely dead due to lack of compression, you can pretty easily tell a difference. If there is a difference, it's time to rebuild the top end.

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Not starting


Hi there
  • Low compression, and lots of blow-by forcing oil into the air filter via the the crankcase breather. Sounds like you need a top end rebuild. If you can get your hands on a compression tester, then check it. If it comes in under 120, then certainly time for a tear down and top end rebuild.

  • If you put a quick squirt of oil down the spark plug hole and if the compression pressure goes up on a second compression test, then rings/piston/bore are worn. If it does not come back up, then it may be leaky valves, or a blown head gasket.

  • do the compression test first. That will tell you where to start. Or, if you have the tools, tear the top end down and inspect. A motor re conditioner/machine shop will help to measure up and tell you if piston, and bore are ok. May just need new rings to freshen up.

Happy to answer any questions that you may have.
regards robotek

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