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RSRBOB, Technician
Category: Motorcycle
Satisfied Customers: 1078
Experience:  Former Factory Service Rep, Dlr Line Tech, Service Manager, General Manager, Store Owner
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I recently purchased a 2008 Yamaha Big Bear 250 that registered 60 psi. I installed a new piston, rings and valves. The compression went up 140 PSI. I rebuilt the carb and had trouble starting it without starter fluid. I confirmed a strong blue spark with a spark tester. I eventually purchased an aftermarket carb to try and rule out the carb and am experiencing the same hard starting. Once started it will only run with the choke on and has no power. Once in gear it will barely move a few inches. The spark plugs are continually fouling black. I believe the quad was previously swamped because there was a lot of water in the oil as well as a few pieces of grass debris. I changed the oil five times and now have clean oil. Please help get her running.
Thanks, Tim
Submitted: 1 year ago.
Category: Motorcycle
Expert:  RSRBOB replied 1 year ago.
Hi and thank you for your question.The first thing that comes to mind would be to double check your cam timing. If it is off a tooth or two, it can have a major affect on the performance of the engine, including starting. Confirm the piston is at top dead center with either a flashlight or plastic straw down the spark plug hole showing piston movement. Once you establish the piston is at TDC, make sure the T mark on the fly wheel is at the indicator on the case cover where it should be. This lets us know the key for the fly wheel has not sheared and the fly wheel agrees with the piston movement as to TDC. Once you have verified the crank and piston are at TDC together, double check the reference mark on your cam shaft for cam timing.If those two items prove to be correct, my next suggestion would be a close inspection of the rocker arms, rocker arm shafts and cam lobes. We are checking to make sure there is no signs of excessive wear that would limit intended valve lift.If all of that looks good, the last thing would be check the valves sealing in the seats. If you have access to a leak down tester, this will help tremendously. That way you can confirm that something is leaking down and also what it is (intake or exhaust valve, head gasket)If you need any more help let me know.ThanksRSRBOB
Customer: replied 1 year ago.
Thanks. I will double check timing. I have done a low compression (15 psi) leakdown test and don't have any noticeable air leaking past the intake or exhaust valves. I do hear a hissing sound when doing the leakdown test from around the cylinder not sure if its coming from the cam chain area bease I have the cover off to confirm tdc. Is that normal? My leakdown tester is a pretty Crappy Harbor Freight Tester so I'm not sure about the accuracy of the percentages it shows me but it does seem to show it failing the leakdown test. Would a blown head gasket have these symptoms that I'm experiencing and still have 140 psi of compression? Not to get ahead of myself but how could I isolate it to the headgasket?
Customer: replied 1 year ago.
I guess we can disregard my previous post. I did a leakdown test and got 20 -25% consistently. If I crank up the pressure the only air I feel is going into the crankcase, maybe because the rings haven't seated... I have attached a pic of it at tdc. If the rocker arms or cam lobes were bad would I still be able to pass a leakdown test?I also forgot to mention that when the engine is running I hear a slight knocking sound. Not sure if this is coming from the bottom end or not but assuming the worst and it was bearing or rod knock would that impact performance to the degree that I am having now?
Expert:  RSRBOB replied 1 year ago.
Thank you for the additional information.If the rocker arms or cam lobes are wearing out, yes, it would still pass a leak down test. The reason it would pass is because the valves are not being compressed by the rocker arms or cams and the valve springs have the clearance they need to close the valves. If you had leakage past either valve, it would be due to not sealing. That could either be insufficient valve clearance, pitted valve seat, burned or bent valves.The reason I mentioned worn rockers or cam lobes is the performance issue. If you are not getting sufficient lift and duration of the valves, the cylinder either will not get a full intake charge limiting power production or will not be able to fully exhaust the burned gases from combustion, diluting the incoming fresh charge. This was prompted by your statement that the spark plug always comes out black. If the engine is not able to fully clear the burnt gases on exhaust, it is going to make the spark plug black.I am not sure exactly how you are doing your leak down because the norm on that is you keep dialing up the regulator until the needle is at zero, then connect to your cylinder and read your % of leak down from there. Most leak down testers I have worked with require at least 90 psi to function properly. I have never done a leak down test at partial pressure so I have no idea how accurate that is or is not.I will agree that the rings may not have seated yet allowing a bit more blow by now than once it gets broken in and the rings fully seat.Your cam timing is not exactly spot on. It is slightly advanced, about 1/2 tooth. This is not normal either. I suspect the cam chain has stretched and you had a choice of either 1/2 tooth advanced or retarded, but could not get the line under the cast arrow on the head. That having been said, I seriously doubt 1/2 tooth off on the cam timing is going to create the symptoms you are describing.Lastly, the knock you hear. I would suggest either using a mechanics stethoscope or a long screw driver (used as a stethoscope) to attempt to locate the source of the noise. If you suspect you may have a big end bearing going out, you can check it as follows: Remove the spark plug, use a plastic straw down the spark plug hole to track piston movement and turn the crank until the piston crests TDC and starts its down stroke. After it starts moving down in the bore, stop turning the crank and leave it there. Now, take a long screw driver and push down on the piston (through the spark plug hole) to see if it moves down a little bit without the crank turning. If you aren't sure, you can repeat the test as many times as you need to confirm whether there is play in the big end of the con rod or not. The key to doing this test correctly is make sure the crank pin is pulling down on the rod by pushing on the bottom of it. The rings will hold the piston up in the cylinder so the crank has to pull it down. If it is not pulling the piston down yet, it will be pushing on the top of the rod and you will never feel movement. Just make sure you turn the crank past TDC (does not have to be the compression stroke either) when you get ready to push down on the piston.If you suspect the head gasket might be leaking, you can spray soapy water on the head gasket and look for bubbles.Also, I am curious why the head of the bolt for the cam sprocket is so chewed up. Did this thing throw a cam chain at one time? Although the damage itself is not problematic, it is highly unusual.One other thing, are you also using a Harbor Freight Compression gauge? I have definitely seen them be way off on readings, but usually low, not high. The service manual says your engine should have 128 psi compression, so if your gauge is accurate, you are in good shape.Have you double checked the crank TDC mark with the piston location to make sure the key hasn't sheared on the fly wheel? Getting back to the basics of spark, fuel and compression are what is needed to make an engine run, it sounds like you have the compression part covered, so that leaves spark (at the correct time, since you stated you have a fat blue spark) or fuel. Check for fuel flow from the petcock all the way through the float bowl, testing at each joint or break in the fuel line all the way down. The last test would be to crack the drain screw loose on the float bowl to make sure you have adequate flow into the bowl. Yamaha does not provide a spec for volume of fuel flow, but when you test it, it should be obvious it has a lot of gas available to the carb. If it appears to be restricted, start checking for sources such as debris in the tank or petcock, fuel lines that have deteriorated internally or a mis-adjusted float assembly. The statement about not wanting to run without the choke on makes it sound like it is starving for fuel for some reason. Make sure your fuel screw on the carb is at least 3 turns out for starters. That may not be a final adjustment on that, but it should be sufficiently rich to allow the engine to start, run and idle.Have you tried starting it without the air filter to see if it changes anything? If not, it is worth a shot. It will NOT rev up without an air filter because it will be too lean, but at least you have affected the problem and know where to look.The opposite of that, and this is starting to get fairly obscure, would be if the exhaust system has a restriction in it. I did have one that the internal pipe closed down almost completely restricting exhaust flow, but it was not hard to start and would sit there and idle. The quick easy test for that is take the pipe off at the cylinder head and start it without the pipe to see if the symptoms disappear. This is going to be a bit loud but you will only need to run it for a few seconds to determine if it helps or not. If you don't notice an immediate improvement, that isn't the problem.If you have any more questions, feel free to ask.Thanks,RSRBOB
Customer: replied 1 year ago.
Thanks for the detailed response. I never thought to look at the rockers and cam lobes but after researching symptoms of worn cam lobes online it seems to be the same symptoms I am experiencing. Both my cam lobes and rockers appear to show wear and if my eyes are correct the cam lobes show uneven wear. I have attached a few pics if you don't mind giving me your opinion. I really appreciate the assistance!
Expert:  RSRBOB replied 1 year ago.
The only thing I can tell from the pics is there is heavy scoring on the base circle of the cam. The rest of the pics have no perspective for me to be able to evaluate. The easiest way to see wear on the cam lobes is looking straight down on them. A side view doesn't show much. If the pad for the rocker arm has wear it would have 2 distinct curvatures. The ends of the pad would show the original curvature and the wear would appear to have flattened that curve out.Glad to know the internet agrees with me.