250 GS6 Throtltle Sticks Open Only in 6th

[Idorace]

The throttle cable is nice and free.  Doesnt get stuck open in any other gear.  It only sticks in 6th gear when the rpms are way up there, not at lower rpms.  Its almost like there is a vaccumm holding it open.

GPS showed 70.46 mph today.  Still had a little bit left.  The carb is a Mikuni, not a Bing.

Jeff

[brian kirby]

I would bet on a air leak, but its hard to say. I had the carb (Mikuni) on my Can-Am stick open like that once in a race, only after WOT for several sustained seconds. It was in a CC race and it did it at the same two spots two laps in a row. On quick wide open then closed like normal off road riding it would never do it, only on extended WOT. I took the slide out and never found anything obvious wrong with it, and once I put it back together it never did it again. Never did figure out what caused it.

Brian

[Idorace]

quote:Originally posted by brian kirby

I would bet on a air leak, but its hard to say. I had the carb (Mikuni) on my Can-Am stick open like that once in a race, only after WOT for several sustained seconds. It was in a CC race and it did it at the same two spots two laps in a row. On quick wide open then closed like normal off road riding it would never do it, only on extended WOT. I took the slide out and never found anything obvious wrong with it, and once I put it back together it never did it again. Never did figure out what caused it.

Brian

Brian:

You described it perfectly.

[brian kirby]

The only thing I changed that day from before or after was I had the needle clip in the bottom (richest) slot. It should not be possible for the needle to hang up in either the top or bottom slot, but after I pulled out of the race and took the slide out, I went ahead and put it one slot up. I dont believe that was the cause, but other than time it never did it. It was definitely not an air leak, something was hanging up because I could feel the throttle go slack when I shut it off. It was weird, and like I said, I never did figure out what happened. That was the first race Saturday, I raced it that afternoon and twice Sunday and it never did it again that weekend or any time after.

Brian

[Mike Hufnagel]

I'm still trying to track down the problem I have. Only when I miss a shift does this occur. No air leaks and I checked the slide & bore for burrs. None found.  Still have no idea.

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

[454MRW]

I am not sure that this would have anything to do with it, but what lubricant does everyone use on their carb slide? I use Slick 50 spray lube, even on the return spring so there is no possibility of coils stacking and hanging up (like coil bind) on a hard wide open throttle application might possibly cause. Mike

Michael R. Winter
I enjoy rebuilding and appreciating Pentons!
1974 250 HS Pentons-1980 KTM 175-400'S
1975 Can Am 175 TNT & 77 250 Black Widow
1979 Husqvarna OR250 & OR390
1976-78 RM & 77-79 PE Suzuki's
1974 CR250M 07 CR125R

[Mike Hufnagel]

Hmmmmm, never occured to me to lube the slide. Never had. But will give it a try. The chrome is good on the slide and I did put a nice small bevel on it when I filed it down, so I'll give it a try or quit missing shifts!

Thanks

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

[TedG]

Seldom do Mikuni carbs need the slide lubed. They just need to be clean. Make sure the slide moves up and down smoothly and there is no excessive wear especially around the locator pin that keeps the slide from rotating, check the slide and the body.  Lubrication on the slide can attract dirt and cause excessive wear and or make the slide stick.
Have you checked the seal under the stator? At high rpms and load it can suck in air.
So you need to go through the following checklist.
1. Check for leaks around the cyl, carb, exhaust. Make sure the cyl is properly torqued.
2. If the top end was ever removed did they properly set the deck height?
3. Crank seal on stator side, should be replaced periodically.
4. Is the engine properly torqued? Are the motor mount spacers in good shape? Sounds silly but what happens is if it is not torqued properly, the engine will vibrate. Mikuni carbs are more sensitive to vibration than the Bings and the fuel can froth causing a lean condition. That is why most Mikunis are rubber mounted. And why the Bings had what looked like a filter around the main jet, that is an anti frothing device.
5. Is the float level next to perfect? Again Mikunis like things correct.
6. The pilot jet and idle air screw need to be correct. The air screw needs to be at 1-1/4 to 1-3/4 turns out when the pilot jet is correct. If this is not the case the pressures are messy and things can go lean.
7. Is the engine properly timed? This usually won't cause what is happening unless it is way out of spec. But if it is too far advanced or retarded it can cause excessive vibration.  
8. Is the carb mounted correctly? If it is rubber mounted, make sure that it can move. And if not make sure it is not loose.
9. Did this happen after a needle adjustment? If so check two things,
first is the needle and clip under the special washer. I have seen where the needle was on top and things seem to work but...And also check to see if at WOT the needle doesn't come all the way out of the needle jet, if so the needle can go off center and make the slide stick.
Most of this stuff is easy to do and not taken for granted that things are correct.
If it just started doing this, air leaks are usually the culprit. If it has always done this from a point of a change, well that change needs to be looked at.

Also you should replace the seal behind the stator, if you don't have the proper timing equipment, consider going to Harbor Freight and buying a dial indicator and coming up with a way to hold it. A gutted spark plug with a set screw in the side will work.
Oh yeah, excessive vibration can make the slide stick.  

[Mike Hufnagel]

Everything seems fine. Timing, carb, etc. Sprayed the crank seal, stator side, nothing. Of course I didn't do it at a very high rpm, so I'll replace it. Couldn't hurt.    I wonder if a slightly restricted air box to carb boot may be part of the problem?

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

[TedG]

Any restrictions in the incoming air just makes it rich and it will want to blubber at 3/4 and above. That would not make it stick.
Let me ask a few pointed questions.
1. When it sticks how far is the throttle opened? 0-1/4, 1/4-3/4, 3/4 to WOT.
2. How many turns out is the air screw?
3. When you jetted the carb, did you run up the main jet till it blubbered and then back down?
4. When it sticks can you blip the throttle and have it come down?
5. How is the carb mounted?

[Mike Hufnagel]

* WOT, and only on missed shift
* 1.25
* No. Did it by doing plug chops until plug looked good, then going back and jetting for 1/2, 3/4 etc., throttle.
* Does not stick when just blipping, and comes down fairly quicky.
* Rubber mounted at both ends.

On another note, I really need a 2.0 slide, but nobody had one, so I took a 1.5 slide & filed it smooth with a small bevel to a 2.0 slide. It does have GEM reeds and an ACE pipe (the pipe really killed the bottom end)

I've never had this problem before so it's driving me nuts!!!!!

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

[tomale]

I am wondering if the vacuum of the motor reving like that, is holding the thottle open. I was told when I mounted up my Mikuni to my MC5 not to shorten the throttle spring because of this very problem. When the trottle is held wide open for extended periods the vacuum can actually hold the slide wide open. I am not sure what to do to solve it..I for one have set up my bike with a bit too rich Main jet. I race my bike on a MX course and so I rarely ever really get onto the main jet.
I wonder if the problem is cause by being too lean, if you did your plug chops on a dry warm day and the problem happened on a cooler day then that alone could create a lean problem.
This weekend I race my 74 Maico and it ran great in the morning but as the clouds moved away and the temp got hotter I started having a mid range problem, it was sputtering. the problem was too rich of a needle jet.

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]

The thing to determine is what is happening. By that I mean is the throttle really sticking MECHANICALLY?? Or is the thing just holding rpms?
If the throttle is actually sticking, you can roll the grip in both directions with no resistance. If it feels like the spring is working most likely the throttle isn't actually sticking, but it is holding rpm because of an air leak. But you have to determine which it is. The throttle actually sticking mechanically is obviously the easiest to fix, an air leak on the other hand can be very hard to find.
Another place to look for an air leak is the drain plug on the crankcase. They have fallen out, and they aren't easy to see.
Another thing that can happen is the throttle actually sticks and you back off, then what happens is the cable end comes out of the slide and snags the edge and doesn't go back in it's hole. That will make the slide not go all the way back down and blipping the throttle usually makes it seat.

[derek martin]

The original RH side case on my 74.5 250 had a crack that if memory serves began at the crank bearing fit and ran out and around and behind the cylinder fit. When I bought this bike off a fellow POG member he had had the motor rebuilt by Chicago Jerry who had replaced the case with one he had on hand but he gave me the original crankcases with the bike and I've got the cracked case out in the barn that I can look at more closely to give you more detail if you think this might be your problem. A crack of this nature could possibly produce an air leak when the motor was under load.

[Mike Hufnagel]

Mechanically the throttle is fine. Nice & smooth!  So I do believe a vacuum is holding it open. When I hit the decomprssor or kill switch, it brings it back down with no problems

I had  safety wired the drain plug, put a new sealing washer on it& then silicone, but I'll check that to make sure.

What I probably could also do, is when I replace the seal & remove the stator, I have a crack detector spray kit that I could use.

Hopefully I can solve this before MidOhio

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