83 XJ750 Wont Rev above 3000rpm and spark fails on cyl 2/3 Help.

Hi Guys,
I am now stuck, My XJ wont rev above 3k and drops the spark on cylinders 2/3, I have swapped the coils and the missfire remained on cylinder 2/3 so i don't believe it to be the coils and i checked the resistances and they were spot on at 2.8 for primary and 11k on secondary, i checked the resistance on the pickup coils and they 644 and 622 ohms, i checked the voltage on the orange and the grey wires going from the TCI unit to the coils and at the point the spark goes they are showing 13v basically charge voltage......It looks like a faulty TCI unit but i dont want to just change it and i cannot find any info on either output voltages for the TCI or any pin to pin resistance measurements....ant help would be good and greatfully recieved.

 

if it stayed on cylinders 2 and 3 change your spark plugs.
did you also ohm out the spark plug caps and trim the spark plug wire?
and swap caps

 

ive checked the caps 5k, trimmed the leads 10mm and the plugs are brand new

 

Ive got some videos of it with a color tune but cant upload them for some reason

 

Can you post the video on youtube and then put the link on here?

 

How is it that you know the spark is dropping out at 3,ooo RPM? What test equipment are you using?

 

When the tacho gets to 3000rpm the bike starts to misfire and the tacho backs off, i fitted a colortune and did it again and the spark disappears, granted the tacho is backed off and the throttle increased before it complete disappears. also it should be noted that the votage on the signal wire between the ignition coil and TCI becomes a sold voltage reading with no pulse pattern.

 

try disconnecting the tachometer.

 

Why? Haven't got one? can't afford/justify one? Want to try and repair yours? The 750 is scarce in the UK I think but there are several aftermarket options one from Czech I believe..

 

Have you checked valve clearance? Have you done a compression check? Low compression or valves to tight can cause issues .

 

Ok this makes me think that the TCI is at fault. The fact that the voltage stops pulsing tells me that it could be capacitors that have failed. I vote for opening the TCI for inspection

 

Why? Because I am not one to just fit parts to a bike until a problem is fixed....It is not cost effective and most componants can be tested to prove if they are still serviceable.

 

It turns out that the Regulator Rectifier was the cause of the problem.

 

You said initially you thought it was the TCI but didn't want to change it - I merely questioned the logic behind that statement. Most people can only test TCIs by exchanging for a known good one. Fortunately though you have found the correct root cause which is good.

 

The only way to test a TCI (without an oscilloscope) is to swap it to a running bike, or swap in a known good TCI.