50mm bbk with a9 cam an no power/top end

[peteyparado]

i just installed a 47mm a month ago with a9 cam 5g weights 1k torque spring,racing clutch, lighten bell, 20mm carb 95jet cdi hotwire/coil.. i got clocked at 50mph 37mph up hills..
THen one day a friend says get the 50mm bbk an i should be going 60mph+ so i did order the 50mm bbk. it came in put it in left everything the same.. an i cant get pass 30+35mph W_T_F..........help please tried timing, valves carb jetting...cant figure it out..

[90GTVert]

:welcome2:

Checked compression on the new kit?

[vvtr]

yep - compression is probably too high and youve done the head gasket within 5 mins....
not uncommon.

[peteyparado]

ok i will.
an what do you mean when i done the head gasket in 5mins?

[Deleted]

he means the head gasket may have not lasted long if compression was/is too high.

[peteyparado]

i should get the 47mm head or the 50mm head!!!! or is there other ways to lower compression?

[Deleted]

first what does the compression test say

[peteyparado]

What if the compression is low wat could b the issue?
An wat causes the high compression with wat I'm running
On my bike.. 50mm bbk with stock head an a9 cam ! An everything FIts good, all timed... It starts idles but no power 30mph
Top speed... Rpms are at 7600-7800

[larry001964]

Hi Peteyparado Did you do a plug chop to see if your too rich or lean ? What does the engine sound like when your losing power ? is it spitting and sputtering, or bogging down ?

[peteyparado]

larry - i did check the plug an it fine colors good.about the revving it just sounds it having a hard time getting to the high rpm's an it dont make any power with this 50mm bbk.with my old 47mm bbk it ran like a champ the first time i put it in. 45mph all day long. took apart put the 50mm bbk an lost everything 30mph worst than stock... i was a very angry person hearing friend say go big you will go faster more power,so i spent the money an crap.. im hoping its because of the stock head.... whats do you think?

[larry001964]

Mar 23, 2012 5:24:09 GMT -5 peteyparado said:

larry - i did check the plug an it fine colors good.about the revving it just sounds it having a hard time getting to the high rpm's an it dont make any power with this 50mm bbk.with my old 47mm bbk it ran like a champ the first time i put it in. 45mph all day long. took apart put the 50mm bbk an lost everything 30mph worst than stock... i was a very angry person hearing friend say go big you will go faster more power,so i spent the money an crap.. im hoping its because of the stock head.... whats do you think?

I'm thinking a couple of things Pete.
Going bigger won't really get you faster, as a 49 your engine probably didn't have the torque to make full use of the gearing you have. where as a 72 you do have more torque, so I betting your balance between torque and resistance was better getting you some speed gains, but as an 83 I'm thinking your not going to see much more gain in the way of speed unless you change your finial drive gear ratios.. You will have much more power for hills and wind, acceleration will be better but top speed should be about the same..

As others have said it could be a compression thing, so I'm going to take a different direction, I'm thinking that perhaps your exhaust may be part of this.. If your using the stock exhaust it could be creating a bottleneck... When I look at our builds, I only see a few differences, the Stroker crank, a 20 mm carb with 100 main, and our exhaust, and im using the stock airbox with a 39mm flex pipe from the rear of the carb to the airbox.. A 83 will have higher exhaust output so I'm wondering if that could be part of the problem with yours.

If your compression check shows a good reading, your plug color is good, then I would look at your air coming in and going out..

[peteyparado]

thanks larry

[Deleted]

Great points Larry. it has been my experience that the stock exhaust works fine on engines with stock size carbs and valves.
I put a Leo Vince exhaust on my 83cc bike that has the stock 18mm carb and stock size valves and saw a 0mph top speed gain.
From some charts i have seen an 18mm carb is supposed to be sufficient for motors up to 90cc. 150cc scoots the stock exhaust is not much bigger than a 50cc.
If you have a large valve head and perhaps a 22mm carb an aftermarket exhaust will probably be a good match. In this case I dont think the exhaust is the problem but sure its possible.

Just some thoughts

[peteyparado]

ok guys did a compression test on 50mm bbk with hoca big valve performance head reading was 90-95psi very low.. no bent valve check that an gaskets are good... could it b the rings? but the kit is brand new!!! defect or put the ring in wrong i dont think i did could my 47mm bbk ran good i did it the same way!!! what do u guys think?

[peteyparado]

heres another question will a hoca 50mm head with big valves work with a 47mm bbk? what to know if i have to scrap the 50mm. or order parts to fix it..