br4inl3ss
Scoot Enthusiast
Posts: 451
Location: Québec
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Post by br4inl3ss on May 19, 2022 18:28:33 GMT -5
hi all. alright so i installed my NCY 81cc bbk on my 139qmb.
bbk been installed, wasnt starting. it was a timing issue ( 1 tooth off ). no big deal. also had a leak between cylinder and base. fixed. verified exhaust leak, we cant hear any leak. but it has this strange noise... i dont know how to explain... it happen ONLY on FAST throttle twist ( not even going 1/2 throttle.. not hard just fast ). after disassembling the engine for the 204858394759457 th time, we couldnt see anything wrong. not even a crankshaft play. there is a play on the crankshaft, but ti slide right to left. not pivoting and no play push/pull. i tought it was the crankshaft making this noise ( or connecting rod.. whatever ) but after verification it seemed fine. called a shop, the only one that accept to help people with chinese scooter, ans he said it was all fine, even that "slide" play. so i reassembled everythign again... still same thing. ok ima trust the " no loose anywhere " and that guy, and take a short road test. after 5 minute of riding ( even if it was hot before the test ride ), it started to make a metallic nosie and lose power. power get back after releasing throttle for 2-3 second. but will happen againa t a certain speed ( 50-60 km/h ). we dont even need to go full throttle to hit that speed so it has good compression, so i guess its not a ring problem either ?
the only other play i found 10 minute ago before eating, was attaching visegrip to the crankshaft ( on the variator nut, without the variator ) and try to find a play. there was one but extremly small. it was barely noticeable. so much that i had to wiggle it about 10 minute before noticing it. i couldnt film that, we wont see anything on video. yup its that small ( talking about the crankshaft play here ! )
no bent valve ( just a minor hit that happened while hand turning it with bad timing ), piston seems fine. never seen any trace on cylinder either. it was fine with stock 50cc. no weird noise at least. i tried to film the metallic noise, but couldnt. too much wind noise. tried to get that metallic noise on center stand, impossible, not even at 8k rpm ( not holding it ).
i find it weird that it was fine stock and the BANB BANG BANG BANG noise as heard on the video appeared right at first start.
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br4inl3ss
Scoot Enthusiast
Posts: 451
Location: Québec
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Post by br4inl3ss on May 19, 2022 20:05:36 GMT -5
10 views no help yet... so went to risk making other videos hoping it helps. in the second video we can hear the metallic noise. its barely noticeable tho.
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claydoh
Scoot Enthusiast
Posts: 172
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Post by claydoh on May 20, 2022 7:54:14 GMT -5
Not sure, but the sound from your first post's video sounds a lot like the one I had when installing a BBK. It was the sound of valve-to-piston contact. Mine was much worse, and did not clear up like yours seems to.My problem was solved by using a different head. The 'performance' one had noticeably less valve clearance than the stock one . Maybe try a different head gasket, or double up on the base gasket to give the piston a little extra room, maybe? i find it weird that it was fine stock and the BANB BANG BANG BANG noise as heard on the video appeared right at first start
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br4inl3ss
Scoot Enthusiast
Posts: 451
Location: Québec
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Post by br4inl3ss on May 20, 2022 9:15:46 GMT -5
thanks for the answer. i forget alot of thing... but now that you mention that, i remember that i tested valve clearance. i dont see any issue there. the engine has turned over at least 10 turn by hand on this test. before the test i wanted to get a second gasket to lower the compression just so im on the safer side, but unavailable anywhere. only from the racing-planet and that would cost me like 60$ CAD here. 80$ for a gasket ? no way. i checked for a head when i ordered the bbk. all backorder/out of stock everywhere but from racing-planet. and they abuse hardcore on shipping to here unless you order alot of stuff loke for at least 120$ else the shipping alone cost almost more than the order itself. i have to deal with what i have or any other solution that doesnt involve ordering another base gasket for the 81 or 88cc ( same cylinder sleeve diameter ) or a head. i have the old 50cc metal gasket that i could enlarge to be the same as the 81cc one, if needed and if it could help. also is there any logic behind the fact its only on FAST throttle twist at any rpm and less than 1/2 throttle only ? edit: can a bent valve cause this ? i will have to check if it leak. i noticed it run very rich even tho i didnt change the jet yet. before bbk it was on the 13.5-14 AFR, with bbk its between 10 to 12 AFR. i visually dont see anythign wrong but maybe its not visible....
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Post by GrumpyUnk on May 20, 2022 11:46:35 GMT -5
Bent valve would cause poor compression, difficult starting, and if started, lack of power.
I listened, and thought of loose valve clearance or something not tight on the top of the cylinder head. You could take the cam/rocker cover off, and start the engine(it may rustproof your chassis...) and see if you can discern anything loose or out of place. I make the assumption that the cylinder head, cam, and rockers are the originals... and not some mish-mash of 64/69mm valve stem lengths & rockers... Anyway, I would look closely at the cylinder head. Pull the plug, and rotate by hand, with fingertips on the valve springs, rocker arms, etc trying to sense impacts with 'something'. Do at least two full crank rotations. tom
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br4inl3ss
Scoot Enthusiast
Posts: 451
Location: Québec
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Post by br4inl3ss on May 20, 2022 12:15:18 GMT -5
ok ill check that. as long as my crankshaft seems fine, im happy. i could test with old 50cc cylinder but i'm done disassembling and reassembling that thing. it is actually hard to start, but seems to have alot of compression/power ( beside my transmission problem ofcourse ). the electric starter and acceleration shows its very fine there oh and yes everything else is stock on the engine.
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br4inl3ss
Scoot Enthusiast
Posts: 451
Location: Québec
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Post by br4inl3ss on May 20, 2022 17:29:41 GMT -5
EDIT problem found. i followed 2 website for installation instructions. both were saying the arrow or "IN" ( for INtake ) should be at the top when installed. W R O N G si it was isntalled upside down. everythign seems fine but a valve mark on the piston. everything else look good. even the valve itself is barely marked. i filled the intake port with fuel, no leak at all for 15 minute. i call it ok.
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Post by GrumpyUnk on May 21, 2022 7:50:39 GMT -5
If the IN mark on the piston does not belong at the side of the head where the intake valve is located(upper) then what in the dickens does it mean? The EXhaust valve is at the lower end... so IN should not go there... In other words, WTF. Could it be a mis-marked piston crown? Where some fellow had a liquid lunch at the factory, and thought, WTH, I think I will mess with the customer... ?? I know of zero pistons where the IN would be installed otherwise, and I have done a few, not a dozen, but close. As is said, sumsing wong... tom
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br4inl3ss
Scoot Enthusiast
Posts: 451
Location: Québec
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Post by br4inl3ss on May 21, 2022 11:02:59 GMT -5
exactly what i tought. the arrow point in the intake direction. and some place i read also says arrow AND "in" are the same so both should be at the top. anyway. i noticed it was upside down by looking at the valve ( i tought we could trust marking without checking valve........ my bad ) so i turned it. runs way better. but still have that noise on fast throttle. i guess its a exhaust leak then. hard to tell with that wind from the fan... while there, the base gasket was done. no way to get it out with a blade. it was so compressed that 90% of the gasket was out. no idea what happens ( could be cheap gasket... its 2022 where TRUE quality became rare ) but what i think is it got burned by something i cant see what. by the oil ? nope. so i dont know. so i got a free carboard gasket part from extremly friendly yamaha dealer nearby, and made 2 gasket since everyone say to double it. no oil leak, no metallic noise anymore, only that "maybe" exhaust leak.
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br4inl3ss
Scoot Enthusiast
Posts: 451
Location: Québec
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Post by br4inl3ss on May 21, 2022 11:14:29 GMT -5
exactly what i tought. the arrow point in the intake direction. and some place i read also says arrow AND "in" are the same so both should be at the top. anyway. i noticed it was upside down by looking at the valve ( i tought we could trust marking without checking valve........ my bad ) so i turned it. runs way better. but still have that noise on fast throttle. i guess its a exhaust leak then. hard to tell with that wind from the fan... while there, the base gasket was done. no way to get it out with a blade. it was so compressed that 90% of the gasket was out. no idea what happens ( could be cheap gasket... its 2022 where TRUE quality became rare ) but what i think is it got burned by something i cant see what. by the oil ? nope. so i dont know. so i got a free carboard gasket part from extremly friendly yamaha dealer nearby, and made 2 gasket since everyone say to double it. no oil leak, no metallic noise anymore, only that "maybe" exhaust leak.
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Post by GrumpyUnk on May 22, 2022 9:37:38 GMT -5
If the last picture is your piston, then if that large arrow is marked into the piston, I would have assumed that was the direction of flow... The IN valve is larger than the EX vavle. The valve cutouts seem to follow, IMO. If the arrow pointed towards the ground, the large valve cutout would be at the top, towards the IN valve, matching notch size to valve size.
So, is the above pic the current installation or original?
If the base gasket was the light tan paper version, they are not too sturdy. What head gasket style did you use? Personal preference is for the 'composite' gaskets with the steel metal ring around the combustion chamber. The composite in my mind will accept imperfections in the cylinder head surface moreso than the stamped sheet steel gaskets. I also think they conform better. They also allow for a slight decrease in compression ratio from the steel gasket. tom
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br4inl3ss
Scoot Enthusiast
Posts: 451
Location: Québec
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Post by br4inl3ss on May 22, 2022 10:08:46 GMT -5
thanks. last pic was original install, according to the guide i screenshot'ed.
and a forum post said the same thing, so i went ahead and installed it this way.
it didnt seems to hurt anything after ( beside a valve mark on the piston ), since it got the same monster power. and no constant noise.
the base gasket was a dark soft cardboard'ish gasket, replaced by high quality cardboard gasket ( same color as standard cardboard ). the cheap gasket was done after first installation. its what most shop around use and also where i got a peice for free ( yamaha dealership ). seems to work very good, no leak at all. while there i used 2 gasket to be on the safe side for compression. head gasket is the metal one. no leak, high compression, nothing hit ( by hand, idle, throttle on center stand ).
both noise are back, but i think the one when stepping on throttle is an exhaust leak. the metallic nosie is another story and this one is a real problem since when it happen it have no power and rpm decrease.
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br4inl3ss
Scoot Enthusiast
Posts: 451
Location: Québec
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Post by br4inl3ss on May 22, 2022 19:48:09 GMT -5
well i installed stock 39mm cylinder back and everything is fine. no noise, no lack of power when hot, nothing wrong.
so its either a bad kit or it really need the cylinder heads despite what they say.
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Post by GrumpyUnk on May 23, 2022 11:16:20 GMT -5
I can say this: The valve is NOT supposed to hit the piston crown. ALSO. The intake is larger than the exhaust, the intake valve is at the upper end of the piston, and as shown in the picture, the intake valve RELIEF(large one) is at the bottom. THAT PISTON IS UPSIDE DOWN. FWIW. That may be why there was valve:piston contact. In theory, the pistons both should have the same distance from the wrist pin to the top of the piston crown. If they differ, there is much more likelihood of piston:valve interference and contact. Not good. Glad it works with the 39mm, but I bet you miss the power. If you want to try again, put the piston on the connecting rod with the large relief at the top(towards the carb). I would also suggest a fiber head gasket, which will add a few thousandths to the head:piston:valve clearance and lower the CR just a bit.(you have enough compression already) tom
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br4inl3ss
Scoot Enthusiast
Posts: 451
Location: Québec
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Post by br4inl3ss on May 23, 2022 11:42:09 GMT -5
I can say this: The valve is NOT supposed to hit the piston crown. ALSO. The intake is larger than the exhaust, the intake valve is at the upper end of the piston, and as shown in the picture, the intake valve RELIEF(large one) is at the bottom. THAT PISTON IS UPSIDE DOWN. FWIW. That may be why there was valve:piston contact. In theory, the pistons both should have the same distance from the wrist pin to the top of the piston crown. If they differ, there is much more likelihood of piston:valve interference and contact. Not good. Glad it works with the 39mm, but I bet you miss the power. If you want to try again, put the piston on the connecting rod with the large relief at the top(towards the carb). I would also suggest a fiber head gasket, which will add a few thousandths to the head:piston:valve clearance and lower the CR just a bit.(you have enough compression already) tom thanks. the last test with the bbk the piston was installed properly, arrow pointing down. everythign fit. still had that metallic noise. so i guess most people were wrong and it need the 81cc cylinder head too. ill order it later today hoping they have it in stock and i dont have to keep the 39mm for months. people were sayign there is a decent difference in power with 50mm. "decent" is not the proper word. the difference is HUUUUUUUGE. literally comparing a stock civic to the heavily modded 5.9l dakota i had 2 years ago. IF i have to wait the head for months, can i use old 39mm head gasket with the 50mm one to there is 2 metal head gasket so there is no more contact ? PS i have dual base gasket.
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