RPM fall to 2000, hold then fall to idle??

[Hagan]

Hi, I'm new to the web site. I bought a 79 CBX about 5 months ago. Previous owner had replaced the airbox with K&N filters (pods), an aftermarket exhaust and gone through the engine and replaced all the old gaskets with old gaskets + silicone, stripping out a head bolt in the prcess. Leakeded oil like the Exon Valdez. $600 dollars of gasketsl ater, carb overhaul, tank cleaned and sealed the bike is tight and runs well but after revving the engine and letting go of the throttle, the RPM will stop at 2000 for about 6 seconds then drop back down to baseline idle. Any help would be greatly appreciated. -Larry

[EMS]

Welcome, Hagan!

Leak at the air intake between carbs and cylinder heads Or sticking throttle cables/mechanism The latter can easily be checked by taking the tank off and observing the movements, of course.
Leaks at the intake manifolds can be detected by spraying starting fluid (or WD40?) on them while engine is running. If the rpm picks up, there is a leak.

[spencer]

You are not the first person to have this happen. I have never seen a really good explanation of how to fix it, but when it happened to me, last spring, I synced the carburetors, and turned the idle mixture screws out from 2 to 2 1/2 turns, and reset the idle to 1000 rpm when the engine was warmed up. My problem went away and has not returned. I'm not sure which of the three things I did solved the problem, or if it was a combination of all three.

Randaak has a tip page. Although it was intended for GL1000 carbs, it has many items which cold apply equally to the CBX. Here is a link.

http://www.randakks.com/TechTip30.htm

When I asked Mike Nixon for advice, he gave me a list of possibilities. I don't think he would mind if I passed it on to you.

* Idle circuits not totally clear (very difficult to do, even for a pro)
* Idle mixture screws put in wrong (order of o-ring and washer primarily)
* Carb synch
* Tight valves
* Mixed up primary and secondary main jets
* Individual air filters
* Vacuum leaks at manifolds
* Vacuum leaks at throttle shafts
* Ignition timing anomalies

So, now you have a few ideas of where to look. Good luck!

[Rick Pope]

I had the same issues after cleaning my carbs. I've had generally good luck doing my own carb work, so naturally, I didn't make any misteaks, right?

Hah! Idle passages partially plugged, and idle mixture screws not completely seating before adjusting back out to 2 1/2 turns, will really screw up your idle speed.

Of course, it took a real pro to find my misteaks.

[alimey4u2]

Hello and welcome Hagan.... Nothing more to add I'm afraid as the previous posters have it well covered..

[Hagan]

Thanks for the help!!

[Ringnalda]

Check the air diaphragm on the side of the carbs, typically if this is perforated, the bike will hang on to high idle before coming down.

Jan