rpms stay up after throttle is released

hi, i have a 81 xj650 maxim i recently bought.9,000 miles.garage kept.had the usual probs,intake boots ,dirty carbs,fuel line.replaced the boots,gaskets and gave the carbs a serious cleaning,after some adjusting i have her almost dialed in she pulls very strong but when i release the throttle the rpms stay were they were[scary] the slowly fall back to idle.no linkage or cable probs... any ideas??? thanks in advance for your replies

 

my seca does that after a run at speed. it doesnt take long for the rpm to drop, maybe a second or 3 but it does have some lag time...

ive HEARD its a dirty carb issue but if youve gone through yours then i dont know...

 

I know you say no cable or linkage problems, but something is stopping those butterflys from closing real quick, get to the end of the cable with a screwdriver and see if you can get some movement, it don't take much!

 

The Diaphragm Pistons are probably sticking.

Search: "Clunk Test"
By: RickCoMatic

You have to get rid of the Aluminum Oxidation in those Diaphragm Piston Bores.

If they are sticking you aren't getting the right performance out of the Carbs.

 

thanks guys,i am going to make sure the pistons and sleeves are clean.this is my first yamaha and i love it,i have had suzuki gs,kaw H1,and a few hondas. i really appreciate the help and look forward to spending some time here,great forum :!:

 

2nd...

Check to make sure your diaphrams on your carbs are not leaking and burnish the slide bores to eliminate ant sticking...

good luck....

 

If you want to get the most out of that Bike you need to spend the time bringing the Carbs right to their hairy edge.

Step One: Clean. Superbly clean and protected with an inline filter.

Step Two: Clean-Tuning. The Diaphragm Piston Bores need to be superbly clean. Refinish them if they haven't been treated in over 10-years.
Remove the Aluma-oxidation and as a Special Tuning Option ... polish the Bores to a mirror finish.

The job is done when the Diaphragm Pistons can Rise and Fall within the Bores with NO drag and NO Friction.

Good Bench Sync:

Quarter inch strips of Business card or 3X5 Card used as Feeler Gauges set the Butterflys to a PERFECT Mechanical Sync.

Vacuum Sync right on the money.

Pilot Mixture Screws:

An understanding of the Pilot Mixture System and how the adjustment is made helps to Dial-in a Perfect Fine-tuning.
Knowing that the Ideal Mixture is going to be found within about the width of a Nickel during the Tweaking of the Screws will help yoi in the final Fine-tuning.

Don't turn the Pilot Mixture Screws "A Half Turn" or "A Quarter Turn" when making the Fine-Tuning adjustments.
The window is within a few degrees.

This means you might have to almost NOT move the screw at all.
The tolerance is that close.
Having a Tweaking Screwdriver that fits the slot of the Pilot Mixture Screw so you can get the "Feel" of the small amount it needs to move is essential.

Once you have gotten the Plugs to where they are within the coloration of a good burn ... a few degrees of Richness or Lean-out will determine how well the bike will perform.

 

How do you recommend going about polishing the pistons? What polishing compounds would you use?

 

Not the "Pistons" ... the "Bores"!

If they are really dull and the Pistons are sticking or hesitating even the slightest ... you have to refinish the Bores.

For severs cases, Wet sand with 800, 1000, 1200 and 1500 Wet-O-Dry using WD-40 as the wetting agent.

In less severe cases, uuse ScotchBrite Pad. Medium (Gray) and dothe whole Bore until the Bore shines.
For Polishing, I use a Dremel Tool Polishing Kit and Jewelers Rouge #5 & #6.

The Rouges will put a mirror finish on the Aluminum Inside Diameter.
The Pistons will rise and fall without friction after the Inside Diameters are Polished.

The polishing is a Race Prep procedure. It allows the Piston to rise and fall with the manifold Vacuum giving you brisk acceleration and excellent engine braking.

If you like zooming around and keepinng the power on in turns and getting heavy handed in the twisties ... polishing the bores is for you.
Your engine response is immediate.
Matching revs for downshifting is precise.

Getting out-of the hole and getting through the gears with the power on is fun and you'll have enough acceleration to keep up with your sportbike companions because you get to the top-end of the tach with astonishing guickness.

Going fast is a treat. Especially when you need to slow down and want the engine to help you lose speed. With the bores polished, when you let-off the throttles, the Pistons fall and are pushed closed by their springs to make deceleration happen immediately.

Your control over the power of your engine is precise.
You have precision to hold a tight line in the turns.
You get big, fast power-on to pass and catch-up with traffic.

The Bores are easy to polish and it only takes a few minutes to do each one.

 

Rick,

Where can one find the super fine sand paper and the Jewelers Rouge that you talk about. I have been to several stores and the finest I have found is 600. Do I have to hit up a body shop?

 

Body Shops will have it.
Body Shop Supply Stores will have it.
Some very well stocked Hardware Stores have it.

Jewelers Rouge? Home Depot and Lowes.