Topic: Carb tuning question

[singlefan]

Im trying to get my 78 750f carbs tuned a little better, everything is pretty good, except from idle to about 2,000 rpm the engine doesnt want to rev freely, especially if the throttle is snapped open. It sounds like it is bogging a little.  It did this even after the carbs were synched. I have played with the idle air screws a little , but I cant seem to get it any better. The plugs show a nice light tan color, so I think the mixture is ok. Any ideas what the problem might be?

[jsaab2748]

Not familiar with later carbs, but think yours have an accelerator pump somewhere. Maybe the pump is in-operative, causing a lean condition during accelleration. Someone else more familiar will prob be able to elaborate. Might want to check all other areas of tune up proceedure as well, dwell, timing, valve adjustments, etc.

[singlefan]

Yeah, I made sure to adjust cam chain, valves, timing etc. If it is the accelerator pump how would I be able to tell that from a lean condition?

[TwoTired]

The 78 750F doesn't have idle air bleed screws.  They have idle mixture screws.  These control fuel, not air.

These carbs are sensitive to air filter and exhaust changes.  A description of those should be in your problem statement.  Without this information, and assuming stock configuration, I'd have to guess your accelerator pump isn't juicing the carbs enough when you snap open throttle.

Your slide are mechanically linked to the twist grip.  When you snap open the throttle the carb bore loses most of it's vacuum and all but stops drawing fuel from the metering jets.  The accelerator pump squirts raw fuel into the carb bore to compensate for this.  If your spark plugs are showing good color at individual throttle settings, your jets are correct.  If sudden throtle openings are unresponsive the accelerator pump is inadequate.

Whenever a performance issue occurs with these bikes, a full and complete tuneup review is recommended before any other corrective action.

[singlefan]

The bike is completely stock, I'll check the accellerator pump to see if it is working.

[eldar]

There is an arm on the throttle part of the carb that actuates the pump rod. There is a small gap between the arm and this rod, about .08 or something like that. You will want to take apart the pump and make sure the diaphragm is in good shape and that it is sealing properly. The spring should be between the cap and the diaphragm. I replaced mine with one from partsnmore for a good cost as opposed to the arm and leg most other places charge. One thing about the operation. Everyone describes it as a squirt. I suppose it is but it is very small. It only shoots a couple ml of fuel.

[crazypj]

Duh, reading between the lines a bit here but, why do you want to give it full throttle at idle rpm? If rpm wont come up at normal openings you probably want to look at pilot jets. If you had a CV carb you can give it as much throttle as you like cos slides  wont open until engine needs more air.
BTW, mixture screws, out is richer. Air screws, out is leaner
PJ

[singlefan]

After testing out the problem more, it seems to be a lean condition just off idle. I doubt it could be the accelerator pump because if I just crack the throttle a little it stumbles and builds revs slowly. Im going to keep making small adjustments and reading the plugs to see if that helps.

[eldar]

I would not leave out the pump.Its whole function is to get you from idle to speed. It is critical for proper operation of the carbs.

[anparkinson]

Had the same problem with my CB550. Turned out the idle jets had gummed up a bit (my bike had sat for 12 years before I rescued it). They should have been a .42mm dia so re-did them with a .4mm model drill by hand. White crumbly powder came out while I was doing it. Worked perfectly and problem solved.