Rpm limit 4000

Hey folks!
I've got an 1982 XJ750 here and I have this intermittent issue with the bike.

The RPM seems to be limited at 4000. When I go above 4000rpm, the rpm just drops and it feels like a misfire, without any backfire. This happens in all gears, including neutral.

Example: Let's say I am in 5th gear on the highway doing about 85 km/h at 4000 rpm. I turn the throttle grip, RPM goes to about 4100 and then starts going down FAST, bike is still rolling at the same speed. It sounds like it's misfiring, very similar feel to a car's rev limiter. When I release the throttle a little, RPM goes back to steady 4000rpm.

This issue happens randomly. I can have this happening for 3 days. Then one day I start the bike and it runs perfectly fine to high RPM. Then 3 days later the 4000rom issue stars again.

Oil is new. Carbs are super clean. spark plugs are brand new.

Any ideas?

Thanks

 

Perhaps the TCI doesn't compensate for higher rpm's anymore?
A coldshut somewhere in the TCI?

 

Are there any potential issues with the air inlet becoming partially blocked.

 

The airflow in and out is a lot less in neutral than on load, which leads me to believe its a tci issue. But, I'd still check the air filter, inlet etc. Exhaust is a potential limiter, but not so easy to check.

 

is it the correct tci?.
maybe check the charging system see if the voltage output is droping off when this happens.

 

Hot and humid is the enemy of electronics, especially electronics from the 80's and before.
The Philippines have plenty of both.
My bet is that the TCI needs repaired or replaced. Hopefully jsut a corroded trace or solder joint that failed, but the capacitors may have failed.

 

Basics first - check, clean and tighten all the connections to the TCI unit and the coils.
Nasty green corrosion grows out of sight and suddenly things don't work too well.

 

spray some water on the coils and wires to see if you can make it happen

 

Thanks for the reply sometimes the bike runs perpectly then suddenly the problem starts again i cant rev it to a high rpm when i check also the volt meter the numbers are flactuating and what is tci? Tci and cdi are the same?? Thanks I’ll wait for your reply

 

The coil are brand new and i have check the resistant it is pretty factory standard

 

Last time i lost spark and i soldered the cdi then it came back to normal then the misfire happen but sometimes when the bike is running for 1 hour it comeback to normal conditions

 

I believe it is the tci because before i lost spark then I soldered the tci then it came back again so maybe i need to solder again all the parts inside the tci

 

What do you suggest do i need to solder the cdi again??

 

TCI.. and Yup.
There seems to be a method to check whether there is a coldshut and that appears to be to smack it on some flat surface by hand.
Read it here somewhere I believe but I'm quite reluctant to do so..
Remember.. everything is almost 40y of age and like amplifiers and such.. the soldering can get brittle and crack.

 

the yamaha uses a TCI but many refer to is as a cdi.

there is a site that shows how to rebuild them
https://www.xjcd.org/general-electrical

more info

http://www.jetav8r.com/Vision/TCIRebuild/TCI_Rebuild.html

http://www.xs650.com/threads/tci-ignition-capacitor-and-transistor-replacement.26925/

you may want to consider coating with conformal coating after repair it seals the board against moisture
https://en.wikipedia.org/wiki/Conformal_coating

 

Thanks

 

Thanks