Topic: Flat Spot in 3rd & 4th

[Scotty J]

I'm asking for a technophobic friend, so please allow me some leeway for not having ALL the facts...

He has a '72 750 that'll spin up to redline with no problems in 1st & 2nd, but it falls on it's face at 3k in 3rd & 4th.  Confusing, as garage logic dictates that the motor doesn't know what gear he's in... it should be the same in all gears...?

[flybox1]

I'm asking for a technophobic friend, so please allow me some leeway for not having ALL the facts...

yeah...ok    well, tell your friend it might be a slipping clutch?

[iron_worker]

The carbs don't know what gear you're in but it does know load on the engine ... sort of. The carbs only see throttle position. In first and second, the load on the engine is light and probably doesn't load the engine much. As the gears get higher the load to the engine increases since the torque militiplication is dropping while the wind resistance is increasing... due to this you need more and more throttle opening to attain higher speeds.

Each circuit in the carbs controls 1 chunk of the throtle position range.  Obivously these ranges blend together but below shows where each circuit is dominant:

Idle circuit - Idle to 1/4 throttle
Needle jet - 1/4 - 3/4
Main jet - 3/4 - WOT

There isn't really enough information to diagnose but a good guess would be that he is running lean in the mid range or main. Is his bike stock? Stock airbox? Stock exhaust?

IW

[iron_worker]

I'm asking for a technophobic friend, so please allow me some leeway for not having ALL the facts...

yeah...ok    well, tell your friend it might be a slipping clutch?

Also possible.

Does it rev out in the higher gears but not increase speed? If so, this would be slipping clutch symptom.

IW

[Scotty J]

Nah, for reals.  My bike runs awesome (because I bought it like that).

I should have been more clear:  By "falls on it's face", I meant the motor.  It bogs.  If it were a slipping clutch it would just spin up & go nowhere...

Hey I-dub!  Thanks for stopping by.  I thought you'd be out riding the beautiful resto with the ugly tank!  And thanks for the info; I'll pass it along.  He's running velocity stacks and a 4-1.  His bike's been set up like that as long as I've known him, but he's only recently been #$%*ing about it.

[iron_worker]

Well I rode the ugly beast to work today. Nice to get back into it.

I believe with a lean condition if you roll into it and it bogs but cleanly recovers as you let off the throttle again. Whereas, with a rich condition it will stutter and misfire as it goes too rich and then "burble" back to life as you let off the throttle ... since the engine takes a few revolutions to clear the excesss fuel.

Anyway, more info is needed!

IW

[Scotty J]

Yeah, I know more info is needed.  But the dude's traveling this week, so I won't hear anything from him for a while.  I'll pass the info on...