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Honda- No Compression (What?)

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1972 Honda XL250 K0. (2,334 miles). Bike would not start after complete carb. and fuel tank rebuild. Checked coil and had to replace. I got spark and fuel. Still no crank. Removed header and carb. Checked timing with flywheel "T" aligned with indicator (piston @ TDC) both sets of valves are closed and valve lash is correct, still no crank! Ran a compression test with gauge without header and intake on and showed NO COMPRESSION! It seems to be sending compression back through intake. What is wrong??? The shop manual says the valve timing should be checked on compression stroke. Isn't whenever the "T" is aligned with indicator I am on a compression stroke at TDC? I have even checked the piston manually with dial indicator for TDC. Mark is correct. Where do I go from here?

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jd your bike is a 4 stroke......on one TDC  is  the top of the exhaust stroke and where the exhaust valve is just closing while the intake valve is just starting to open....one more full  turn is the compression stroke when both valves are closed and this is where compression is....this is also the same point where the ignition points would open and fire the sparkplug.  It takes two revolutions to make 4 strokes (compression, power, exhaust, intake)  so there are two Top Dead Centers that happen every two revolutions, but there is only one when the valves are both closed and the ignition fires.

 

It sounds like you are setting your valve clearance on the exhaust stroke and that isn't gonna work.

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Try pouting a tablespoon of engine oil down the spark plug hole and taking another compression test.

Possible stuck rings due to carbon/varnish build-up/lack of use.

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If you guys are confident that I have stuck rings, looks like a tear down is in order. I would rather it be the timing but I assume you are talking about stuck rings on piston and not where it seals the bore? Why would it be sending compression back through intake and compression test showing no compression with stuck rings? If I am not on compression stroke will valves be closed at TDC at both strokes? I am confused... Bike did sit up a long time before I purchased. 72' model with 3,000 miles, it had to sit up. No bike history from previous owner (deceased). I'm running blind. Bike exterior is pristine and like it was never in dirt. With over 17 vintage bikes, I can tell when one was cleaned up for sale. This bike was not touched up.

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Sorry to add another.

1. Aligned Flywheel to TDC

2. Checked cam to TDC

3. Checked piston at TDC

4. Checked valve lash at TDC

ALL at TDC and correct. Am I 180 degrees out? How do I recognize compression stroke?

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Jdd, if you have the valves set correctly and you can hear compression leaking out the intake port, something is wrong with the intake valve...it may be burnt, broken or what sometimes happens after a layover the valve is stuck open.  Sometimes if an engine is stopped and a valve is open, and the bike is stored, the valve sticks and won't fully close.   You can try to free it by squirting penetrating oil on the valve stem, through the intake port, and then cranking the bike....but if your camshaft and valve clearance are set properly, and the compression is still leaking, nothing beats removing the head to have a look.  Then with the head off, you can  check the bore and fix the valves.

 

You can tell the compression stroke when both valves are closed for the entire upstroke and downstroke of the piston.   On the other cycle the exhaust valve will be open as the piston starts to rise and close at the top of the stroke just as the intake valve starts to open.

 

I hope that helps.

 

patuca

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Finally! Thank you. You have told me what I suspected. I agree with the disassembly. At lest it will make me feel better. While engine out, I will replace all exterior seals with the kit I ordered. Thanks, I'm an old 2-stroke guy, change is hard. I'm on a learning curve.

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Like other fellas said, rings or valve stuck from sitting. I'd squirt the oil in kick it a few times, and if it still won't fire, get it on a hill and bump start it. I have an unknown mileage and history engine in mine now, and it does the same if it sits over a month. After it starts up once, it runs fine, has plenty of power, and starts one kick. My newer low mileage doesn't do this after sitting, feels like way more compression on starting, but oddly, seems to have close to the same power, maybe a hair more. So tear down may not be needed, and may not be much of a gain.

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I agree with the comments about a stuck valve.  Having sat for a while, and some varnish on the valve stem = stuck valve.  You can try penetrating oil and/or carb cleaner.  Whatever will cut that varnish.  JUST MAYBE you won't have to take the head off.  BTW - I had exactly that bike. A '72 XL250

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Yeah, those little 4t motor need to be revved and downshiffting too far is often disasterous.

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Whats downshifting do, icky? I only downshift mine until it skids :p of course the different engines seem to behave differently.

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Let me get this straight, inject a teaspoon of oil into spark plug hole and spin engine over a few times. After this replace plug and try to crank? Does the penetrating oil also go on valve stems or just down cylinder? Hard to reach valve stems with head on. What bothers me when rotating engine by flywheel I can see valves moving. When checking timing at TDC the valves are properly adjusted. If a valve is stuck part open would not the valve lash be incorrect? Would the valve not come into contact with piston? If this thing doesn't crank this time, everybody beware! You will see a white Raptor pulling a silver Honda by your home on the way to Japan. I'm tired of this one.

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Probably just the rings unseated, just roll that sucka down a good hill and shift up to 2nd.

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