No Compression on front cylinder. - Road Star Warrior Forum : Yamaha Star Warrior Forums

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Old 11-07-2019, 01:01 AM   #1 (permalink)
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Default No Compression on front cylinder.

Hello all, well I a proud new owner of a 2002 RS Warrior. well my front cylinder does not have any compression when I do the test. I unplugged the decompression solenoid, and had the throttle on full open to test and Zero psi. the back one is at 125 PSI. the bike was backfiring on start up and on throttle down. the front cylinder was cold unless I gave it throttle up to 2500 Rpms. it has new coil and spark plugs on it and the backfire is still there.
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Old 11-07-2019, 03:14 AM   #2 (permalink)
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Sea Level compression 171psi.
Min 145psi and Max 203psi (or 191psi for page 3-24). Allowable 14psi difference between cylinders.

In addition to defeating the decompression system, and disconnecting the spark plug wires, you need to hold the throttle wide open 100% for maybe 3 or 4 cranks.

If it's under minimum, add a teaspoon of motor oil (or various other optional materials) into the spark plug hole and quickly retest. If compression psi improved then piston rings are suspect. Otherwise valves are suspect. Assuming decompression system has been correctly managed.

I cannot explain zero psi.

It appears, if you tested properly, that one jug has a hole blown to atmosphere - and the other is well below minimum. Odd. For that reason I wonder if you might re-test in case a step was skipped. Remember to zero your gauge.
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Old 11-07-2019, 09:12 AM   #3 (permalink)
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Thank you for heads up. I will be retesting it today with the oil and see if it helps. I will also test my tester just in case.
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Old 11-07-2019, 10:40 AM   #4 (permalink)
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If no inspection camera or bore scope: Pull both spark plugs & bring the piston up to the top, shine a good light in one plug hole & look in the other hole you should be able to see the piston top. If the piston is damaged replace both pistons & rings. When the heads are off fill each port with mineral spirits to check valve seating. If no piston damage is visible pull the rocker shafts & check compression you should have some.

Any indication of previous motor work? If so the pushrods for the front cylinder may be operating the wrong rocker arm shafts. Consult the FSM for proper orientation. If the valves are out of time they will not hit the pistons & get bent.

Continued cranking during a no start condition will wash the cylinder walls as advised put some oil in the cylinders.

Another test is to fab an adaptor to screw into a plug hole & fill the cylinder with shop air when on the compression stroke, air noise from intake or exhaust pipe = bad valves, crankcase noise = piston / ring issue.

Good luck.
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Last edited by Churchkey; 11-07-2019 at 10:49 AM.
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Old 11-07-2019, 11:41 AM   #5 (permalink)
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Good point on the shop air test. A cheap leak-down-type compression tester can be had for twenty bucks. In fact the compression tester you are using might be leakdown design (uses an air compressor) you can Google it if uncertain. Silver lining in this case is that I suspect you are just feeling the effects of pesky pilot error it happens to everyone now and again no worries. Try again and let us know. Okay?
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Last edited by arizonawarrior; 11-07-2019 at 11:45 AM.
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Old 11-07-2019, 11:49 AM   #6 (permalink)
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It looks like you think your front cylinder is not running until you get it up around 2.5k rpms, is this correct? Do you feel it kick in and suddenly start running smoother?

I have to believe your test method is suspect. I have not done compression tests on hundreds of engines, more like tens... but I can only remember 3 with actual issues. One of which was an outboard that I then took apart. The top of the piston looked like someone shot a shotgun at it, and the cylinder wall was scrapped to ****. A bearing blew loose and the shrapnel was bouncing around the cylinder and bits were getting jammed inbetween the piston and the wall (it actually 'siezed' twice before I tore her down). I guess I should get to the point... even that disaster of an engine had registered some compression (something like 35PSI if memory serves me right). To register a zero seems excessive. Also, if you think the front cylinder IS running at >2.5k rpms then there is no way it has zero compression. Maybe something silly like you took both plugs out of the front and then put the tester in one of the plug holes?

Another anecdote for you; I recently sold an outboard engine. The guy came and checked the compression. The first two cylinders were 125-130PSI. He checks the last cylinder and it's 95. Tries it repeatedly and gets 95. He says, "I think you got an issue." I said, "I agree... unless your tester happened to just break... you want to put it back on the other two?"
He went back and tried cylinders 1 and 2. Both registered 95PSI exactly I'll be damned... his compression tester broke after he tested the first two cylinders... I wouldn't have believed it if I didn't' see it for myself. He bought the engine and texted me later that he tried it on two other engines when he got home... it hit 95PSI every time.
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Old 11-07-2019, 01:35 PM   #7 (permalink)
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Hello everyone and thank you for all your quick replies. just a bit about myself, I am just a regular married guy with two kids and I am not a mechanic, but, I am ok with mechanics and everything related to it, having said that all these ideas are great! so I did all the tests that were recommended here, first I tested my tester with the shop compressor and compared the PSI it was about 10 psi off so I purchased a new one and the readings were the same. then I ran the test again, disconnected the decompression solenoid at the connector under the seat, put in about a table spoon of motor oil in the back piston disconnected the spark plugs (all) and the test came to 135psi an increase of about 10psi, then did the same for front and again no pressure at all. After this I did the compressed air into the cylinder trick, the compressor was at 90Psi and still zero, but I notice that its coming out of the intake and exhaust... so now i'm thinking a bad valve or valves. is there a way to clean the valves without taking everything apart? and by the way I have achieved more in less then 4 hours with you guys then by myself in 4 weeks, thank you! I will also be doing the light test to see if piston is broken. will report my findings again.
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Old 11-07-2019, 02:05 PM   #8 (permalink)
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So I just did the light test and the cylinder is intact and it still has the oil in it from a few hours ago. Great I thought I was in big trouble with that one... as for if the engine runs smoother at above 25k rpms it does and the front cylinder comes back and there is a regular smoke coming from the front exhaust. ???
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Old 11-07-2019, 02:36 PM   #9 (permalink)
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Did you manually set piston to top dead center before introducing compressor air?
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Last edited by arizonawarrior; 11-07-2019 at 02:41 PM.
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Old 11-07-2019, 02:52 PM   #10 (permalink)
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Good luck friend, you got some heavy hitters helping you out here. That's why this is the best forum in the world.
How about some pictures of the bike. Maybe theyll show if the engine has been worked on.
Leaking air from intake and exhaust you said. Is there valve overlap on this bike? I dont know. What are the chances you had the cylinder at top of exhaust stroke not compression stroke?

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