Another carb/motor problem

[Mike Hufnagel]

Finally got to ride the bike and work out any kinks. Got most of them solved except one.

If I hit a nuetral on an up shift, the motor screams and won't come back to idle unless I hit the decpression lever or kill switch, then it idles just fine. When reving it up in neutral without going wfo there's no problem. Throttle is smoooth, does not stick or feel sticky anywhere only when I hit a false nuetral when acclerating.    My guess is I have an air leak some where. Any thoughts?   Also when kind of aerosol spray should I use when checking for air leaks? Starter fuid?

74 Yamaha TY 250
75 Jackpiner w/ 250 eng.
80 Ktm Mc 400
82 KTM GS250II (import)
82 KTM 495
2) 84 Yam RZ350
98 KTM 300mxc
00 KTM Duke 2
06 KTM 950 Adv.
08 Suzuki DR70Z

[joe novak]

Are you sure the carb slide is returning to the closed position?   Look for a burr in the area where the idle screw positions the slide.  Always adjust this screw when opening the throttle slightly.  joe

[brian kirby]

Sounds like you have an air leak. You can spray carb cleaner, contact cleaner, really anything that will get sucked into the intake and change the mixture. I forget, do you have a Bing or a Mikuni? If its a Bing, check that the metal insert that the slide groove rides on is not leaking air, they can leak there.

Brian

[tomale]

Mike, An air leak would be my guess,  several weeks ago we had the very same problem and it turned out to be a blown base gasket. It has been awile since I looked at a sach's motor but I seem to remember that the sealing surface around the transfer ports is a bit narrow. I have seen motors that have been dimpled around that area to get the gasket to hold better. I am not saying you should do that only that this could be were the problem is..I have a maico cylinder that is warped enongh that it does not lay flat against sealing surface of the bottom half of the motor. when I put the motor together, I use Susuki gasket sealer on both sides of the gasket and tighten it down. I let it cure at least a 24 hours before I run the bike. It is not the way I want it but it does work. eventually I will have to pull the cylinder and have the cylinder milled flat. This is more information than you probably need or want and I would start with simple stuff like the intake and carb but if this does not fix the problem, it could be a bit deeper..

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)
74'250 hare scrambler (project)
74' 1/2 440 maico
78' 440 maico
72' cr125 Husky (project)
74' TM125 suzuki
93' RMx 250 suzuki

[TedG]

One of the biggest places for air leaks is the crank seal behind the stator. Older bikes especially. Always replace periodically.

[tomale]

Very true, and I like to do a leak test after I have rebuilt a motor, these old motors, there is no telling what they have endured and what inperfections that will show up, even after 30 years. I have seen leaks develop in castings that were never there before, like pin holes in the transfer ports...

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)
74'250 hare scrambler (project)
74' 1/2 440 maico
78' 440 maico
72' cr125 Husky (project)
74' TM125 suzuki
93' RMx 250 suzuki