engine missing

[brancher]

First post on this forum. I looked around but found no answers.
My scooter missing if you go wide open throttle from a stop. If you very slowly pull the gas and get to 20mph then go wide open it doesn't miss and runs really good. It didn't do this until i did the 70cc kit, exhaust, and jet. I already had the clutch and var. on.
-Tried a 105,100, 95, and 85 jet with no success. It had a 70 stock.
-Tried adjusting the pilot jet needle clip too with no success.
-Tried spraying 2 plus 2 around to look for intake leak but didn't find one.


I have have a Yamaha 50cc 2 stroke on a sin-ski scooter.
Athena 70cc kit and variator with 5g weights.
Leo Vince HM 70 pipe
malossi clutch
stock carb with 90 jet (carb measures 18.5mm) carbon reeds.
stock air intake with neck removed. (tried strait k&n filter and it keep bogging down)

what do you all think?
should i take the engine cylinder/head back apart?

[90GTVert]

:welcome2:

You never mentioned adjusting the mixture screw, so I'd start there and see if it improves or not.

[tango]

I too would have checked the carburettor situation as well, but then I recalled something. Have you checked/changed your plugs? I had a very similar problem a while back and the solution was to change the plugs.

[brancher]

thanks for the replys.
i haven't tried the plug but will.

i did adjust the mixture screw everytime i switched the jet and it was a pain in the butt everytime. i'm tired of doing that and cleaning the carb everytime.
thanks again

[brancher]

Ok, i tried the plug with no success.
So my friend has a beamer. We took the coil off of his and put it on mine, now it runs great. For some reason I never felt this until i did the 70cc kit i guess.

2 questions
my compression is 120psi, what should it be?

my squish is .053, I read it should be 0-90 right?

I ordered a hoca coil and a bpr7hix plug.

[90GTVert]

.053" is about 1.35mm. You'll probably want to tighten it up somewhere under 1mm. A. Graham Bell says 0.6-.8mm, I'd say 0.6-1.0mm. Not sure how much you'll be able to do with your kit. If it uses a flat metal head gasket, try removing that and using copper spray on the sealing surface instead. See what compression figures you get. Should be a good bit higher. 120psi is low. I'm hoping you'll see at least 140-150psi, and expecting more, if you can tighten up the squish clearance. If you get it in tune and it spark knocks or heats up and starts to spark knock, it's too much compression. I haven't found a magic umber for how much compression these things like, it varies with different setups to some degree. Some of mine need to be lower like 140-150psi and some are just fine 180psi or above. What gas you use (regular-premium) will limit you if you're trying to get the max out of it too. I always use premium 91-93.

EDIT : Just in case you don't have a way to measure in mm...

0.6mm = roughly 0.024"
1.0mm = roughly 0.039"

[brancher]

Aug 24, 2011 10:50:48 GMT -5 90GTVert said:

.053" is about 1.35mm. You'll probably want to tighten it up somewhere under 1mm. A. Graham Bell says 0.6-.8mm, I'd say 0.6-1.0mm. Not sure how much you'll be able to do with your kit. If it uses a flat metal head gasket, try removing that and using copper spray on the sealing surface instead. See what compression figures you get. Should be a good bit higher. 120psi is low. I'm hoping you'll see at least 140-150psi, and expecting more, if you can tighten up the squish clearance. If you get it in tune and it spark knocks or heats up and starts to spark knock, it's too much compression. I haven't found a magic umber for how much compression these things like, it varies with different setups to some degree. Some of mine need to be lower like 140-150psi and some are just fine 180psi or above. What gas you use (regular-premium) will limit you if you're trying to get the max out of it too. I always use premium 91-93.

EDIT : Just in case you don't have a way to measure in mm...

0.6mm = roughly 0.024"
1.0mm = roughly 0.039"

thanks a lot.
mine does have the metal head gasket and will try and remove it.
I do run 93 in mine.
I also thought about taken the gasket out from under the jug and using the metal shims instead. this might mess with the timing some. I read this 49ccscoot.proboards.com/index.cgi?board=yamajog&action=display&thread=353
but don't fully understand what i'm setting (maybe i'm slow). Do i have to do this if i take the gasket out from under the jug?
thanks still learning!

[90GTVert]

If you are only removing a base gasket, it's a minor change. Unless you plan on doing port work or filing the piston, there's not a lot of point in checking. Start with the head gasket anyway, and see where that gets you. That won't remove any port timing so it's the best option.

[brancher]

Aug 24, 2011 20:04:01 GMT -5 90GTVert said:

If you are only removing a base gasket, it's a minor change. Unless you plan on doing port work or filing the piston, there's not a lot of point in checking. Start with the head gasket anyway, and see where that gets you. That won't remove any port timing so it's the best option.

ok thanks
great info.

[brancher]

took head gasket off and now i got
.9mm squish
150 psi compression

thanks. Seems to be running great now. I got a little dead spot at about 1/4 trottle but I think i just need to mess with my carb.

What plug do you all recommend?
I got BPR7HIX is what the yamaha shop told me to use.

[90GTVert]

Re-adjust your mixture screw and then try a different needle clip position and you'll probably eliminate that dead spot.

That plug should be fine. I usually go with an 8 series, just slightly colder.