If my motorcycle slowly accelerates fine, but boggs down when throttle is suddenly opened, what should I do?

Question

I have a 1986 Yamaha FZ750 with Mikuni carbs, which I recently changed the main jets in. I also adjusted the needle jet clip position. The bike runs well at every throttle position, but if you suddenly open the throttle, it hesitates. If you then ease back off the throttle, it will catch up and begin accelerating. I changed the main jet, because someone put very large jets in it, and it was flooding itself out. It would run much better with the air filters off. It has aftermarket exhaust, as well as K&N air filters. I changed the main jets back to the stock #105, and it ran lean, so I moved up to a #110, and moved the needle jet clip position to the top notch. I have tried everything I know, and now I'm stuck. I'm thinking I may need to adjust the accelerator pump, but I am not sure how. If I adjust the a/p s***w counter-clockwise, will this then inject enough gas into the carbs when the throttle is opened to make up for the increase in air flow?

Answer 1

Check the rubber diaphram on top of the cylinder that the needle goes through...I would guess that it has a slight tear in one or maybe both....If the bike has ever set for a few years then I would say they need replaced....That peice is called a vacuum cylinder I think......should run about $80.00 each.  What actually happens is that the sudden vacuum that occurs when you quickly open the throttle, pulls the cylinder up but since the tear allows it to loose vacuum it will fall back down cutting off your flow of fuel...

OR you may have pinched or not got a good seal with the rubber gasket when you put the carbs back together....You can use a very small amount of vaseline to put around the edge of the rubber on the vacuum cylinder to make sure the lip goes and stays in the groove around the top of the carb.  Then replace the top cover.  

Good Luck

Answer 2

Mr J,,

The Simple Answer,,,

Buy and install a Factory Pro Jet Kit.

Follow the Instructions.

You'll think ya got a New Bike.

BUT,, if you need Help getting it Razor-Sharp Tuned after installing the jet kit---FP's "Help" is probably THE BEST you'll get anywhere.

And they're Happy to do it

http://www.factorypro.com/Prod_Pages/pro...

You may even Contact them Beforehand,,,talk with them about >YOUR< Bike,,,and they'll tell you exactly what you need to get it Beyond just plain 'ol "RIGHT".

......................................

Air filter mods just murderlize those engines,,,it's an effort to get them right.

And You have your needle position in the opposite direction it should be---needle needs to be raised/Clip Lowered

Actually,,they Need different Needles and Pilot Jets.

.........................................

I RARELY suggest "spending money on a jet kit"

Usually ,Most People can get 80%~90% of the Results of a $100++ Jet Kit,,,,

for $20~30 and a little effort.

But Most People ain't trying to dial in a FZ750.

You'll come out WAY ahead,,,and be Past satisfied with the Results of Factory Pro's jet kit.

Good Luck,,let us know how it turns out.

Answer 3

id say its time for a new spar kplug