TOPIC:

Trying to figure out a really strange issue. If I'm cruising down the highway, and I roll past approx. 70 percent throttle to accelerate, the engine immediately breaks up, and the tach jumps to 10,000+ but the engine doesn't rev. In fact, I actively lose rpm and speed because the engine is breaking up so bad. If I let off to below the throttle position where it occurred, the tach drops back down, and the bike picks back up.
I've checked every electrical connection at this point, and ran it with and without the tach, and it exhibits the same running behavior.... I'm stumped, any ideas?

Does it only happen:
at one rpm?
in one gear only?
on the highway?
at a certain vibration level?
at a certain speed?
when did it start happening?
was there any work done on the bike?

It happens any time I am over 6500-7000 rpm and roll up towards full throttle. Probably 80% throttle and higher. It seems to get worse in the higher gears when you can stay higher throttle for longer. It's really noticeable on the highway just due to he high rpms and higher required throttle. The bike has a top speed of about 80 right now because of it, but you can feel it at the top of every gear as you accelerate.
It is an aftermarket tach, but I have tried 2 aftermarket tachs now, and also with the tach disconnected. It still breaks up at high rpms either way. It's just that when the tach is connected, I can see it ramp quickly up to 10,000+ rpms indicated. I have taken apart and cleaned every electrical connection, replaced plugs, rebuilt and balanced the carbs. I would have thought carbs if it weren't for the odd tach behavior.
It almost seems like it's trying to spark fast enough for 10,000rpm while the engine isn't turning that fast and just destroying the spark timing making it miss and stumble to the point of actually losing speed.

Are you running solid core spark plug wires?

Yes, stranded copper core. I don't know how old, the wires were on the bike when I got it 2 years ago.

Sorry,
Just brainstorming on what it could be.
Anything from AC voltage ripple from a leaky rectifier, to loose connections, to RFI from your secondary ignition(spark plugs, spark plug wires,cdi), bad cdi, bad coil.
I'm researching internet forums to get more clues.

Can you get it misfire in neutral?
How long has it been running like this?
What kind of work was done to the bike previous to this problem?

Thanks for all your help so far. Electrical is my weak spot. I can't get it to misfire in neutral, it only seems to do it under load. It has been running like this for about a month. The only work I've done this year is install and balance the rebuilt carbs, and install the aftermarket tachs (had an 82 tach on it when I got it, but I have no tach drive off the cam).
I hooked the first tach up to the stock tach trigger wire that splits off the black coil primary, but it had a tach filter, (capacitor and resistor deal) and ran terrible with that on it. I removed the filter and it ran fine but the tach was a little jumpy.
That tach died within a month (cheap China tach) and it was already running weird by that point, which was about a month ago.
I installed the new tach and have tried running it off the stock wire in the headlight as well as directly off the coil, with no luck. I also checked and cleaned all electrical connections, and checked the pickup coils, which look fine, but i didn't check resistance. Could that filter have fried the CDI maybe?

and....you don't have a cdi...just an ignitor. I just found that out.
which is fine.
I'll probably just run through some voltage checks to see if anything looks strange.

Grab your meter
Checking battery voltage
Bike running
meter to DC V
Red meter probe--Positive battery post(the post will be the part the battery cable attaches to)
Black meter probe--Negative battery post
Note voltage

Checking charging voltage at idle
move Red meter probe--Red/White wire out of voltage regulator(pink box)
Note voltage

Checking charging voltage at 4k
Raise rpm up to 4000rpm
Note voltage

meter to AC V
Checking for AC ripple out of rectifier
Red meter probe--same wire(Red/white wire)
Black meter probe---Yellow/Black wire(in the pink box--same connector as the red/white wire)
be careful--don't let the probes touch each other
raise rpm to 4000
note voltage


photo: www.classiccycles.org/

Sounds good, I'll get this checked when I can get a second pair of hands to help wrangle the meter and the throttle at the same time. Really appreciate your help.

Just to throw out a few possibilities in addition to what F64 is instructing...

You may want to look closely at the ignition switch contacts and kill switch contacts. Also a flakey main fuse could cause the same problem. When vibration makes and breaks the connection, the ignition is turning on and off at a very high rate, similar to running at high RPM which would trick a tach into registering a very high RPM.

Another remote possibility is the rotor and pickup colliding.
Take off the pickup cover and look very closely at the tip of the rotor and center poles of the pickups. Look for any sign of the rotor impacting the pickups. If it does, the rotor point will look to have scrape marks and the pickups will probably have some really small scrapes and little bits of steel filings on them.

To fix it you have to try to loosen up the pickups and timing plate to make sure everything is centered and that there is enough clearance between pickup and rotor.

Reviving an old post. It ended up being the cheap aftermarket tach, it was shorting and grounding out the coil. I replaced it with a different model and all is good!

Thanks for the update. made in china junk then?