throttle opening restricted from 3rd gear onwards

[pete.keys]

Hi all,

here is a weird one for you...

My commuter g/s (yes I didn't sell her): at stand still engine running or not the throttle opens 100%. In first and second gear.. bike runs as normal .. I can open the throttle all the way. However as I approach 70 in 3rd, the throttle stops opening at about 30% open - like its hit a stop. Its 100% consistent.

All the cables must be free for it to open at standstill and when I check it over. I am thinking that it might be a butterfly spindle and to do with one of the the butterflies at high suction from the engine getting stuck in the body?

Any other ideas anyone?

 

[fayeslane]

Definitely weird I reckon your theory is pretty good although if it is working OK in 1st & 2nd suction shouldn't increase with road speed. Might be vibration effecting the spindle/butterfly. Easy to check the butterfly screws to see if they are loose. They shouldn't come out because the end is peened. I know this because I forced one undone and rooted the throttle shaft Check also wear in the spindle.

 

[Shep]

Perhaps the 70 mph wind is dislodging the cable or tank causing the sticking certainly weird?

 

[Rob Farmer]

Torn diaphragm in one of your carbs give the same symptoms.

I take it your not hanging off the bike looking at the throttle arms but just getting to a certain speed and it won't go any faster?

The first time it happened to me riding normally the bike went through all the gears as you'd expect but wouldn't go past 70Mph, changing down a gear and trying to rev it more still wouldn't get it past 70.

be careful which diaphragm rubbers you buy. I always buy genuine as some of the pattern ones are made from a different material that doesn't resist the modern fuels. I had an old pattern one go all gooey and start to dissolve.

 

[pete.keys]

thanks every one

I take it your not hanging off the bike looking at the throttle arms but just getting to a certain speed and it won't go any faster?

Competent rider , yes but stunt man I'm certainly not

The first time it happened to me riding normally the bike went through all the gears as you'd expect but wouldn't go past 70Mph, changing down a gear and trying to rev it more still wouldn't get it past 70.

Symptoms are like this, plus the throttle just won't physically turn past about 30% open

 

[(RIP) maverick]

Had a similar thing when I bought a R100T, opened the top of the carb and the diaphragm was in bits. From memory the first part of the accelaration is performed by the cable and then further by the vacuum created in the carb. Strangely enough I had excellent fuel consumption until I replaced the diaphragm's