QuestionQUESTION: i got this bike used, its got 110 and 115 compression but it should still run. its got spark on both sides, i cleaned out the jets with compressed air as well as the rest of the body of the carbs, every little nook and cranny that i could find after completely dismantling them. ive worked at a motor bike shop before and did this many times on other bikes, so i should know what im doing. well, its got spark, air and fuel. ive set the timing a few before top dead center to be set properly later. its got a vacuum fed fuel valve that seems to be working fine, (tested it by unplugging the vacuum hose and blocking it and hooking it back up and opening the float bowl drain screw. ive adjusted the floats up and down for to much or to little after the stock setting didnt seem to help. there are no tears in the DIAPHRAGM and the needle is set in the middle. so far everything is doing what it should be doing but it wont start, it just wont start. the battery is good and its hooked up to my running car battery to make sure we have enough battery power and to top the off, we give the kick starter a try every now and then. why wont it start? i beg you please to help me. the only possible thing i can think of is that maybe the needles and seats are not seating properly and letting in to much fuel. it cant be to little as the fuel does drain from the float bowls as previously mentioned. it could possibly be air leaks but ive had air leaks before on other bikes that still let the bike start just not run very well after starting. any ideas?
ANSWER: Tom, 110-115 compression is really marginal. Either the rings are worn out, valves are too tight or the camshaft timing is off a tooth. Ignition timing should be set at the F mark alignment, after you determine that the spark advancer is functioning properly.
After cranking over for 15 seconds a few times, remove the spark plugs to see if they are wet from fuel or not. If no, then keep after the fuel system. If yes, then spark timing is incorrect or weak coils or bad condensers.
You really need to fix the compression issues first. Should be about 150-175psi.
Bill Silver
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QUESTION: Hi Bill Silver,
I want to thank you for your try at this problem. it didnt work but still you tried. it turned out to be something most wouldnt even think to look at. the wires from the ignition to the coils were reversed. i never thought of that because having had two other two cylinder bikes, both only had one coil to both plugs firing on the top of the compression and exhaust. and i didnt solve the problem myself but only after reading a few hundred other posts from any sight that google would come up with did someone mention something similar and that was their solution.
so it started, great. but now i have another problem. a very loud clattering in the valves. in my attempt to find the first solution, i took the valve cover off the to check the timing chain is set properly, it is. maybe i didnt put it back on properly or maybe the noise was going to be there anyways as i was not able to start the engine until after i took off the cover and didnt hear it run before. i set the valve clearences to the books specs to no avail, still noisy. so i went online again and looked for similar problems. although i did not find any solutions, i did find other such problems where one other set the valves and only then got the same described noisy clatter. im thinking it may be related to having taken off the cover as the rockers are in the cover and that separated them from the valves. the other posted problem only had the owner setting the valves of which he claims there was no noise before. is this a common problem for this bike? when i had the cover off, i did not see anything wrong, everything seemed to move properly without any slack. there was no slack in the timing chain. the book says the chain is automatic and i could not find anything external to check the adjustment, still, it did not seem to have any slack.
AnswerTom, a Honda has color-coded wiring, so the coil wiring issue would have been more noticeable. Actually, the wiring usually can't be reversed on a Honda, as they use male/female connectors at the coil. Good for you to have sorted out an unusual problem, though, Reversed coil polarity does make for a weak spark at the plugs. Ah, the joys of Yamaha.... :>)
Noisy top ends are generally due to one or more of the following:
No/low oil flow to the top end.
Loose cam chain, whipping back and forth inside the cylinder block.
Loose camshaft sprocket bolts.
Valves adjusted on the wrong stroke. Setting them on overlap instead of compression will make them REALLY loose when the bike is running.
Adjust the valves at the TDC stroke for each cylinder, then turn the engine over ONE complete turn. If you set them on overlap stroke, then they will be very loose on compression stroke. Once you set the valves, you can turn the engine over a quarter turn at a time and keep checking the clearance, until you have turned it through 2 rotations. If the valves never loosen more than what the specifications are, then you can count out valve adjustment as a noise source.
If those causes are ruled out, then perhaps the rocker arm shafts are worn or turning in the top cover. If the cam timing is off, then you can hear the valves tapping the pistons, lightly, if you are just one tooth off. Cam timing issues would explain the low compression readings.
Bill Silver
