Topic: Throttle response Getting better Tuning/jetting/ims

[Tdubsy]

Hey guys,

Trying to get the bike tuned a little better,

Right now from at Idle when I give a large throttle blip there is little response/ the bike almost dies. I cant be a bit more gradual and its ok. Same for once I'm at a bit higher RPM.  Fuel accel pump is working.

What could you recommend in terms of jetting or adjusting IMS to help the bike out?

Should I also be checking the carb sync after changing IMS screws?

Video attached

[seanbarney41]

get it running on all 4 cylinders

[evinrude7]

what have you done so far?  did you bench sync and set floats yet?  is the bike cold when you try to rev it that fast? 

[Deltarider]

what have you done so far?  did you bench sync and set floats yet?  is the bike cold when you try to rev it that fast?

Set floats??? Why would you set the floats? Don't get me wrong: checking is always OK, but setting? You make it sound it is a regular routine. Well, it is not. This float madness keeps on ghosting in this forum and everybody is repeating everybody. Sad. Floats are always right. Look at them, their simplicity and you'll understand there is very little chance they can go of. Floats have one force to fear however: the owners fingers.

[evinrude7]

what have you done so far?  did you bench sync and set floats yet?  is the bike cold when you try to rev it that fast?

Set floats??? Why would you set the floats? Don't get me wrong: checking is always OK, but setting? You make it sound it is a regular routine. Well, it is not. This float madness keeps on ghosting in this forum and everybody is repeating everybody. Sad. Floats are always right. Look at them, their simplicity and you'll understand there is very little chance they can go of. Floats have one force to fear however: the owners fingers.

we don't know what condition the bike or the carbs for that matter are in.  maybe he just bought the thing and floats are all over the place.  or he cleaned his carbs and didn't set the float height after complete disassembly.  seems like a reasonable question to me. 

[Tdubsy]

This is what I've done so far,

- Rebuilt carbs (set floats, Accel pump verified working, INCREASED IDLE JET TO SIZE 40)
- New Carb boots
- New air filter
- New dynatech electronic ignition
- New 5 Ohms coils
- New spark plugs
- Timing chain auto adjustment set
- Set timing
- Set valve spacing
- Carb sync done with vacuum gauge

[flybox1]

what intake and exhaust setup are you running?

[Tdubsy]

what intake and exhaust setup are you running?

Stock Airbox and stock 4 to 4 exhaust

[flybox1]

40 pilot jet and stock main will do fine.
Do an idle plug chop to get your IMS dialed in.
if you have dial carb sync gauges, calibrate them.
do another round of vacuum syncing

[Don R]

  I bought aftermarket needles and seats, they required a float adjustment. Then they worked fine. I'd go one size up on the mains, these were smog motors and came lean when they were new. Mine was an F1 though, it didn't have an accelerator pump. Try a 1/4 turn at a time on the screws.
 And that's all I got, mostly from reading and trying it.

[TwoTired]

Where does it say what bike model is being worked on here?

Cheers,

[Tdubsy]

Where does it say what bike model is being worked on here?

Cheers,

1978 cb750 k8

[TwoTired]

Had same throttle response hestation on my 77 cb750 f model after carb cleaning. Had the accel pump working, but it wasn't working well enough.  The diaphram "mouse ears"  were swelled and narrowed the passage which reduced the volume pumped.  There was also a leaking check valve in the pump, reducing pressure.  Redoubled efforts to clean the pump system, which nearly doubled the accel pump volume, shooting fuel into the throats a good six inches on any throttle advance below one half throttle position. No more stumble at any throttle advance.
Bike had stock exhaust and induction with a K&N air filter.

Make sure your accel pump works as intended.

Cheers,

[flybox1]

Any swelling of the o-ring sitting on the top of carb bowl #2 (accel pump check valve) can completely block the circuit.
Viton or Buna-n o-rings here, only.