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Post by gy6forlife on Feb 21, 2013 16:32:23 GMT -5
Hello, I have installed a BBK from Scrappy,a 49cc-80cc kit.To anyone who has ever installed one of these when you had the engine at TDC,and the cam gear perfectly aligned with the two holes,and lines were inline with the top of the valve cover head casting.The piston never did come all the way to the top of the cylinder,it stopped at about 3/16" and then started to go back down.Has anyone ever had this occur when installing your kit?Thanks for any help.Engine fired right up and runs fine up on kickstand,but will hardly pull up to 20mph?Valves are set at .004 ANY HELP IS APPRECIATED AS I AM OUT OF IDEAS. Kind Regards, Jason/gy6forlife
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Post by Rebel on Feb 22, 2013 1:52:38 GMT -5
you are not getting all the compression you need to take advantage of the larger bore and you are loosing displacement then.
Sounds like it is the wrong piston for your motor.
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Post by gy6forlife on Feb 22, 2013 9:46:27 GMT -5
That is what I thought as well,I have asked the Tech at Scrappy for the stock 49cc pin to top of the piston height and he keeps avoiding the question,all he wants now is a Compression PSI test to see how much there is,I am having a very hard time finding a gauge that works correctly,as some how my good Snap-on one with the air line chuck on the end so you can use the different adaptors is not working correctly,as it will not hold the pressure in the gauge,but just bumps the needle up to around 50psi,and MO the tech said that it takes atleast 90 psi to even start one of these engines.I do think they gave me the wrong piston,it fits the bore size,but the pin height has to be off,as the stock piston came all the way to the top of the cylinder and was "high dome"this one is the exact opposite,dished and doesnt go all the way to the top of the cylinder!Mo tells me that these engines are not like my drag race engines?But common sense tells me that it should go to the top like the other,that is why,if any one has installed a BBK I ask for there input PLEASE. Jason/gy6forlife
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Post by rocky88 on Feb 22, 2013 13:08:53 GMT -5
i always thought the piston measures 44.20 from top to bottom of skirt..
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Post by rapidjim on Feb 22, 2013 15:49:49 GMT -5
I have used Scrappydog's 80cc (72cc) kits and never had an issue. I have also used Partsforscooters and others.
3/16 of an inch really? Something just don't sound right. 3/16 of an inch converts to 4.8 mm, standard stroke of a 50cc motor is 41.4mm. So 3/16 of an inch would drop your stroke to 36.6mm. With a 47mm piston, that destroke would be 63cc. Even at that it should go over 20 mph, in worse case not even start. There is also a difference in dome pistons vrs dished, dome piston will be a higher compression ratio than a dished, but not pin height.
I have to ask, how did you measure this? Are you sure the jug was not sliding up with the piston when you rotated the crank? If you had that much space, it don't make sense to me why you even finished putting it back together.
I have never seen this is in all the 80cc (72cc) kits that installed. Then again I have never measured the piston to jug height either.
I assume you have this motor assembled, since you mentioned speed. A compression check and leak down test would help give us more info.
Jim/Owner Rapid Repair
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Post by rocky88 on Feb 22, 2013 15:57:14 GMT -5
all so jim cant he get a good figure by bringing the piston to top and get some sort of mesurement by the spark plug hole? i know if what he is saying by the piston being to short it dont make sence to me. i think it's a compression problem dont you? just put your finger over the plug hole and kick it over. it should blow your finger off.
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Post by rapidjim on Feb 22, 2013 16:03:05 GMT -5
all so jim cant he get a good figure by bringing the piston to top and get some sort of mesurement by the spark plug hole? i know if what he is saying by the piston being to short it dont make sence to me. i think it's a compression problem dont you? just put your finger over the plug hole and kick it over. it should blow your finger off. If he don't have a good compression gauge, I doubt if he has a dial indicator set for a 10mm plug hole. That is what I think too that he has a compression problem, not timed right, cracked ring, leaking head ect ect ect. Jim
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Post by rapidjim on Feb 22, 2013 16:05:52 GMT -5
Jason, what is your motor number?
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Post by Rebel on Feb 22, 2013 23:21:56 GMT -5
I don't know what you feel like doing but if the wrist pin is not in the right place, it will need to come out. If you take it out can you post a pic of the two side by side? Another thing that can ruin your compression is not having the rings on right such as upside down or the gaps being lined up.
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Post by suddethtray on Feb 23, 2013 4:47:39 GMT -5
OK Jason i just had same prob on my qmb139 and tried every thing finally replaced the crank now she purr like a kitten u need to break the casing but first take off ur head put piston on td put back together AND then line ur flywheel on the locating pin/mark to the t then look at ur cam hole my problem was no matter what i tried i was a half tooth off ehteir way
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Post by gy6forlife on Feb 23, 2013 14:42:13 GMT -5
OK, Here is the engine code=1P39QMB Timing check out at the first line above the "F" on flywheel,MO said that was perfect timing for it? After I installed the jug down on to the piston and the four studs,w/the gasket at the bottom of the jug,I very slowly squeezed the rings one by one,down on into the jug,and yes they are in there proper gaps in the rings turned to different positions on the piston,and with the correct sides upward on the rings. Yes the jug was all the way down on the gasket and sitting with the two little sleeves that go on the studs oposite of each other,I had mad a jig to hold the engine at TDC before taking the head,jug,and piston off,and checked it during the install to make sure it never moved of TDC,as I read and looked at a whole process of pictures,even held the timing chain with a heavy object as show,so that it did not come off the bottom inside the case.I rotated the engine over while holding the cylinder down tight and it never came all the way to the top?So I took my digital calipers and used the end to get a measurement,as I knew this was not right and was going to need a reading of just how far it was down in the cylinder,the engine will start up at the first crank of the starter after a few shots of the throttle and run what my tach shows as up to 10Krpm?and yes I break my engines in hard under a load to get the rings to seat good and fast.I am in need of a 10mm adaptor,as I have three compression gauges and none of them go down to that small.One is a Snap-on it has five different adaptors,none go that small,,a Milton,and some other older one a friend had,yes I do have a dial indicator and I did check it with that to get my TDC, and I have called every shop in town and only on place has a 10mm,but its in a kit with a new Compression gauge that I dont have 45 bucks just to use once.The cam went in with the little holes lined up with the valve cover edge,and the approx 3/8" hole exactly vertical to them,even check with a square that it was in line as well.I dont see where I have done any thing wrong,but it sure does not have hardly any power as with any engine the piston being down that far will make the CR so low that I cant believe it starts?It has me stumped.Thanks for any and all help,anyone have that adaptor for a 10mm Compression tester,gladly send you a brand new racing blue CDI I have a extra of to use it? Jason/gy6forlife
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Post by gy6forlife on Feb 23, 2013 14:44:09 GMT -5
Jason, what is your motor number? 1P39QMB
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Post by gy6forlife on Feb 23, 2013 15:02:42 GMT -5
all so jim cant he get a good figure by bringing the piston to top and get some sort of mesurement by the spark plug hole? i know if what he is saying by the piston being to short it dont make sence to me. i think it's a compression problem dont you? just put your finger over the plug hole and kick it over. it should blow your finger off.(It does blow my finger off,it has a good amount of compression)JASON If he don't have a good compression gauge, I doubt if he has a dial indicator set for a 10mm plug hole. That is what I think too that he has a compression problem, not timed right, cracked ring, leaking head ect ect ect. Jim
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Post by gy6forlife on Feb 23, 2013 22:37:44 GMT -5
Ok, If so tell me the way or correct way you want me to do all the item I have done,I am not a idiot and I underand whey thing happen,as its all mathmatics in the end right?,So when I measure the piston height,the tech MO,at Scrappdog.webistie,is not wanting to tell me,as I know he has a set of claipers,and as I do and most all of you reading this thread,10.55mm with a hight dome piston for a 49cc,as stated by a considered member,it has got me to about 6 to one ratio and yet,MO says it wont start with out 90psi?so I have the rinds broke in and good compression,I am asking for a new piston and,rings,wrist pin ,and all gasket's they can sent at least as he MO even stated that the timing was exactly perfect,at the time I oreded the $300+ parts from them they were a lot on back order,so I think that just maybe there was a mistake in the piston and ring,jug for my rebuild,as I have but engines to gather for over 25 years? Kind Regards, Jason
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Post by Rebel on Feb 24, 2013 2:26:18 GMT -5
Jason if the piston is not coming up to the top of the cylinder, the compression will suffer and yes MO is right there needs to be a good 90 PSI in the compression stroke, if my memory serves me the specs call for about 129 -125 psi.
Words are nice but photos reveal a lot.
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