jonnienumberfive
Scoot Member
just found this 1970 2 cylinder 2 stroke yamaha in a barn,been there since 1980
Posts: 87
|
Post by jonnienumberfive on Jun 14, 2012 13:56:08 GMT -5
I spent the morning installing a BBK..followed a thread on here(thanks 90GTVERT)..all is well,EXCEPT,the thing wont rev past 7800-7900...im up to a 95 jet,air box tube removed,low and mid range feel great..just wont rev past the 8K mark.reinstalling the air box snorkel causes severe bog. ..could it be the stock Keeway exhaust??(new LV 70 due here sat)...any suggestions welcome... :sos:
|
|
|
Post by seaquell on Jun 14, 2012 14:31:59 GMT -5
When I put on my BBK I was 1 tooth off on timing it still ran good but when I got it right it ran GREAT!!!
|
|
|
Post by teddy554 on Jun 14, 2012 14:51:15 GMT -5
is it a 4t or 2t i am guessing 2t, yes a stock pipe could keep your revs down on 2t not sure about 4t, also is your cdi a unrestricted cdi and if you got the stock airbox on wither its 2t or 4t a 95 main jet is to big of a jet,
|
|
jonnienumberfive
Scoot Member
just found this 1970 2 cylinder 2 stroke yamaha in a barn,been there since 1980
Posts: 87
|
Post by jonnienumberfive on Jun 14, 2012 14:57:05 GMT -5
its a 2T..started with an 84 main and it looked lean on the plug...so went up..cdi is unrestricted..now a 90 main..runs good low and mid range and seems to pull going to full throttle,,just wont rev past 7900..simply stays there then drops back as speed increases to 7400-7500 and just runs there
|
|
|
Post by teddy554 on Jun 14, 2012 15:11:45 GMT -5
I would say it is you pipe, they always recommend a sport pipe with bbk and there are different ranges in the aftermarket pipes some only let rev to a certain rpm from what i have seen sport pipes 9000-10000 mid race 9500-10500 race 11-13000 so i am certian the stock pipe is keeping your rpms down
|
|
|
Post by teddy554 on Jun 14, 2012 15:15:38 GMT -5
You can run a mid sport bbk with stock airbox and pipe with 70-75 main not until you add the pipe will you be in the 80-90 range also in my experience the having roller weights over 6 grams will also keep your revs down like that
|
|
jonnienumberfive
Scoot Member
just found this 1970 2 cylinder 2 stroke yamaha in a barn,been there since 1980
Posts: 87
|
Post by jonnienumberfive on Jun 14, 2012 15:36:47 GMT -5
thanks Teddy...weights are 4 gram...i suspect the stock pipe cant handle the volume of exhaust trying to escape...the exh port on the BBK is almost twice the size of the orig jug port..new pipe here tomorrow...so,saturday we'll see what happens...Jon
|
|
jonnienumberfive
Scoot Member
just found this 1970 2 cylinder 2 stroke yamaha in a barn,been there since 1980
Posts: 87
|
Post by jonnienumberfive on Jun 14, 2012 15:40:28 GMT -5
When I put on my BBK I was 1 tooth off on timing it still ran good but when I got it right it ran GREAT!!! ..... ...this is the 2STROKE forum...no teeth off..on my engine....but THANK YOU for trying to help...Jon
|
|
|
Post by teddy554 on Jun 14, 2012 20:24:15 GMT -5
yeah let know how the pipe works
|
|
|
Post by 90GTVert on Jun 14, 2012 20:28:33 GMT -5
I'd expect it to be capable of 8500RPM with the stock pipe. Not all stock pipes are created equal though. If the pipe doesn't do it, try lighter rollers or a stiffer contra spring.
|
|
jonnienumberfive
Scoot Member
just found this 1970 2 cylinder 2 stroke yamaha in a barn,been there since 1980
Posts: 87
|
Post by jonnienumberfive on Jun 15, 2012 6:02:18 GMT -5
Brent,the pipe is the Keeway expansion chamber look alike...all in all the scooter is slower now than before the BBK....hope the new pipe makes something happen...
|
|
|
Post by 90GTVert on Jun 15, 2012 7:06:31 GMT -5
Even with a stock pipe, I can't see how it should be slower than before if it's in tune. Maybe those fake tuned pipe designs are just terrible, I'm not sure. I always assumed they were OK as a stock pipe, just no one should think of them as anything more than that.
I don't even really think you should have to swap rollers to get RPM. The added power should push the revs higher with the same rollers as stock if all is well. If you are jetted wrong, that can certainly have some effect on RPM. The jet you are using may be correct. I've used very large jets on a 70 with a stock pipe before. Some just react different and of course elevation changes things. You said it looked lean with an 84. If you take a quick ride and check jetting by color on a new plug, it'll always look lean unless it's severely rich. Quick rides you have to check for the mixture ring, which is a real pain and expensive using a new plug every run. I tend to tune now by making test passes with different jets. I use the same exact route and run WOT. Best if done in a narrow time range to avoid changes in the weather, especially wind. Note max speed one way and then back (assuming you can use a straight stretch and you'll be turning around) and observe anything that seems good or bad with the tune. When starting out I'll kill the scoot from WOT and coast in so I can see if the plug is wet or dry if the tune seems far off. I keep a notepad handy and write down what jet, what speed, and any observations I believe may be relevant. Then I choose a jet a size up or down, depending on what I experienced, and take another ride the same way. If it improves, go another step in the same direction. If it gets worse, go the opposite way. Again, making notes from every pass. You should end up with a small range of jets that are giving the best results. Choose from those, and if you get the same result with a couple of jets, use the richer of them. You can always swap later should you see a problem, but it's safer to start rich. If the jet with the best results seems like it could be lean, keep a close watch on things when you ride. If there is any sign of detonation at or near WOT you may wish to move a step richer. If you experience a soft seize, you must go at least a step richer.
|
|
|
Post by teddy554 on Jun 15, 2012 8:47:52 GMT -5
how about squish clearance and i think u are jetted to high for stock bbk i have stage 6 street race bbk rs2 pipe and 88 main and still had to remove snorkel and drill 12 holes in my airbox (didnt have any smaller jets) to get it to run right and i am still a little rich
|
|
jonnienumberfive
Scoot Member
just found this 1970 2 cylinder 2 stroke yamaha in a barn,been there since 1980
Posts: 87
|
Post by jonnienumberfive on Jun 15, 2012 18:10:25 GMT -5
I put the LV 70 on tonight,,right away it will rev to 85-8600..and pulls hard..remember I'm 240 lbs also..a few test rides on the same 1/4 mile strech and it looks like a 95 main is where i see the plug ring golden colored..made 2 passes both ways 1 with a 92 and one with the 95.with a new plug each time.it needs a stiffer contra spring for sure...thats tomorrows job..the hole where the snorkel was is right next to the carb inlet i'm sure that is making it ask for more jet..I tried to run an 84 main with the snorkel in and all i got was severe bog...so for now it seems right?? red spring tomorrow...and the saga continues....THANKS ALL WHO ANSWERED...I'll keep you posted....Jon
|
|
|
Post by 90GTVert on Jun 16, 2012 6:55:58 GMT -5
Is this the Handmade 70? If so, that pipe is supposed to be good for high RPM... like 10-11,000 with with right cylinder. I'd definitely try lighter rollers or a stiffer contra and if you haven't done it already, check the squish and compression figures as Teddy suggested. Some BBKs aren't setup well out of the box and end up with wide squish/low compression, others are fine.
|
|