xj650 won't rev past 6500!!

My '81 650 has had an "artificial redline" since I got it two years ago and I can't figure out why. I have cleaned the carbs, had them re-tuned, checked for vacuum leaks, cleaned out the tank and petcock filter, adjusted the cam tensioner, new plugs, filter, etc. Basically the bike runs perfect - idles smooth, good power around town, etc., but it winds to 6500 and falls flat on its face and will not rev further, and it will do the same under no load (revving in neutral). The cam chain was so loose before adjusting it was hitting the valve cover, but since adjusting it is tight and quiet (I thought that was it!!). All I can think is that there is some electric gremlin (TCI or old coils) or that the cam chain is still causing the problem. If anyone's had anything similar please advise if I'm on the right track or way off base.

thanks in advance

 

Possibly fitted with a rev limiter

 

I would suspect the TCI. If your bike has the electric tach and it becomes erratic at 6500 RPM then it is the TCI for sure. The tach doesn't always go erratic but very often does. Best way to check is to put your TCI in a known good bike and see if the problem follows along.

 

it doesnt need to rev over 6500rpm that is where all the power is just shift gears at 6500rpm

 

You kidding? Torque and Horsepower peak at about 9K. 6500 is where the fun starts.

I'm curious though - as you approach 6500 does it start to miss? Or does it just kinda bog down?

 

The bike doesn't miss, it just bogs down and won't wind any further. You can hold it wide open and it will just hover right at 6500. The engine note changes as it hits 6500 - doesn't tap or anything bad sounding but just sounds crappy - but still no discernable miss.

To answer a previous response this bike has a mechanical tach not a electronic one so there is not erratic behavior there.

 

I wonder if your TCI isn't advancing the spark. I'd pull the timing cover, slap a timing light on, and see if the advance is working.

 

how would I check this? I have set base timing on the bike but what would I be looking for exactly to see if it is advancing the timing or not?

 

At idle looking at the timing marks, they will be close to lined up. As it speeds up it should advance the timing, the Marks are going to move away from each other a bit.

 

The shop manual warns not to run in neutral above 6000RPM for more than 1 or 2 seconds.

Unfortunately the manual doesn't have an advance curve. I think though by 5000 RPM it should be very noticeable. If you don't see anything by 5000RPM let me know and I'll check mine.

 

Finally able to check the timing on the bike. At idle, it is perfect, and when accelerating it advances up to about 4000 rpm. After that, it doesn't advance at all anymore, and I notice if I hold it at WOT for a few seconds the timing will jump around ever so slightly. I would think that it should advance more past 4000 but have no way to be sure. If someone could check theirs against what I saw that would be awesome. Thanks.

 

I would quadruple check the cam and crank timing. If your sure you checked it 4 or 5 times, check again.
In 99 I did the head on my Datsun and had the cam one single notch off and i had great low end but no RPM above 3000. My ignition timing was dead on.
I checked and checked again and felt i was on, but it just wouldnt wake up.
Finally after dogging along for a bit into a 100 mile trip, I pulled over and reset my cam timing and my car came back to life.
Sometimes the silliest mistakes haunt the pants off us.

Steve

 

I had that problem with a Virago and thought it was electrical issue.
In troubleshooting, I discovered it had defective carb diaphrams.
Replaced them both and it screamed!

Rusty

 

I'd think it should keep advancing too. With a red line of 9500 there ought to be a lot of advance left.

I'll have a look at what mine does on Friday.

 

Just checked mine. Advanced to 4000 ROM and then stops.

Sounds like the TCI isn't your problem.

 

Would this also cause the motor to stay revved at about 4500 and slowly come back down and then die out? This is after I rev it past so. i have cracks in the carb boots as well, and that mgiht fix it, but im thinknig that may not totally fix the problem. WHen I had the carbs off, I noticed the plungers didnt work with the throttle plate while off... I'm nut sure if they are supposed to or not.

Thanks in advance!

 

Sounds like sticky slides. Not opening would cause it not to rev. Not closing cause it to idle high.

 

A hanging rev is more indicative of a lean mixture. Time to check the slides as Carl suggested and I would also check the fuel level. Sounds like it is starving of fuel which could be caused by low fuel levels in the float bowls or the cracks in the intake boots. Easy fix is to seal the boots first.

 

put another TCI in it anyways just to make sure - no change whatsoever. I thought that I should look at the carbs again because its been 2 years since I took them apart and I might have missed something. All diaphragms were in good shape, no tears, etc. Took the jets and emulsion tubes out and cleaned them (although they looked fine). Took pilot screws out, cleaned, reassembled, and set them at 2 1/2 turns out. Took plate under diaphragm off and removed jets? that are under it and cleaned (once again everything looked clean). Reassembled everything and tried to start back up. Cranks, tries to catch a little and then nothing. Stop, try to crank again and get a deafening backfire. I took the carbs back off to recheck everything - all looks fine and in the right place. Put it back together and the same routine. Checked petcock - works perfectly. Spark - check. Pistons moving up and down, valves opening and closing - check. Cleaned plugs - same deal. Noticed smoke from cyls. 2 + 4 after backfire and pulling plugs - re-checked pilot screws - all looks good. What the hell am I missing here??

 

just a quick question did you bench synch the carbs when you had them off ?

 

I never took them off their brackets or touched the adjustment - even if I did there's no way it would make this happen.