4000-5000 RPM glitch

[JPaganel]

A couple of days ago, a strange thing started happening.

While cruising at 4-5K I get an occasional hiccup. The engine feels like it's losing RPM, but the tach actually jerks up. This is quick, about as much time as the blip of throttle while making a shift.

This never happens at idle, or any RPM lower than 4000.

Any idea on what this might be?

[FJmonkey]

The last time my engine and tach seemed out of sync my clutch took a dump. Then I learned it was after market and less than OEM quality. I have a happy clutch now and a shitty 2nd gear...

[movenon]

 +1 As noted, it sounds like your clutch is letting go under load / slipping.  The good news it is easy to fix.
George

[JPaganel]

Must be the infamous spring losing tension..

[movenon]

Must be the infamous spring losing tension..

You should probably have on hand a clutch cover gasket and at a minimum a new diaphragm clutch Spring.
This is what I used http://www.rpmracingca.com/proddetail.asp?prod=Clutch%3AClutchSpring.  I did the double spring deal and went back to the single new spring.  Inspect the disks and steels if it has been slipping while the spring is off.

Here is a good thread to read about the clutch and spring.  http://www.fjowners.com/index.php?topic=1808.0
George

[Pat Conlon]

I dunno guys, if this hiccup happened during roll on acceleration I would agree about the clutch.
But a hiccup at steady cruise? I'm not convinced it's the clutch.
I would check the coil grounds, the quality of power to the coils, and the resistor caps on the plugs before opening up the clutch cover.

[JPaganel]

It occurs to me I don't remember when the plugs were last changed. Can bad plugs cause intermittent stumble like this?

[ribbert]

While cruising at 4-5K I get an occasional hiccup. The engine feels like it's losing RPM, but the tach actually jerks up. This is quick, about as much time as the blip of throttle while making a shift.

Any idea on what this might be?

From your description, this has the symptoms of a typical electrical interruption. The speed, sharpness and irregularity of it and the constant throttle combined with the tacho having a moment would suggest an electrical gremlin.

What that might be is a curly one.

A while back I had a similar problem on an FJ, although a little more consistant, but it would only happen when riding, I could not duplicate it in the garage.
There is a wire running behind the battery box with an in line fuse holder. It is unsupported for a couple of inches. The weight of the fuse holder was moving around in the breeze once under way and over the years had caused the wire to break inside the insulation, no visible clue. It was causing an intermittent total loss of power to the engine. As soon as the bike jerked with power loss, it would re establish contact and surge forward again.

Not saying this is a likely cause of your problem but just an example of how tricky they can be to track down.
Occasional electrical hiccups can be the hardest things to find because of the difficulty in duplicating them in the workshop.

An old mechanics trick is to push, pull, poke, twist, etc any wires or bunches of wires you can find while the bike is idling and see if you can make it stumble or cut out. If it does, it gives you a location to start with and systematically work from there. This has surprisingly good results when looking for something that more often than not has no visual clues.

A slipping clutch will have entirely different symptoms and all 4 spark plugs are unlikely to die momentarily only to come back to life a second later.

Any suggestions? yes, plenty, but no one more likely than another from the information you posted.

Noel

[JPaganel]

 I'm not sure it's all 4 plugs dying. The engine doesn't stall, it might be one or two plugs occasionally misfiring.

I think I'm going to start with new plugs and see if anything changes.

[JPaganel]

Changed plugs. Bike runs smoother than ever.

Still stumbles intermittently between 4-5K.

Wiggled wiring harnesses. No effect.

I wonder why this is so RPM specific.

[movenon]

Changed plugs. Bike runs smoother than ever.

Still stumbles intermittently between 4-5K.

Wiggled wiring harnesses. No effect.

I wonder why this is so RPM specific.

If it is a carb problem.  What is your measured throttle position when it happens?  1/4, 1/2, 3/4 etc.. Are you snapping the throttle on or just cruising and rolling it on?  That might give some of our carb experts more information.  RPM is a nice number but for troubleshooting a carb problem the throttle position is better information IMO.  


Loosing RPM by feel and the tach jumping up is perplexing to me. I guess if all else I would pull and inspect each terminal at the TCI/DCI/CDI, pick up coils, ignition coils and check out the condition of the run/stop switch ect..  Just inspect every connector.  I mean pull and clean front and back side.
George

[JPaganel]

If it is a carb problem.  What is your measured throttle position when it happens?  1/4, 1/2, 3/4 etc.. Are you snapping the throttle on or just cruising and rolling it on?  That might give some of our carb experts more information.  RPM is a nice number but for troubleshooting a carb problem the throttle position is better information IMO.  

It would be the normal position at 60-65mph cruise, which would make it 1/4 to 1/2. No snapping, slight roll maybe.

Loosing RPM by feel and the tach jumping up is perplexing to me. I guess if all else I would pull and inspect each terminal at the TCI/DCI/CDI, pick up coils, ignition coils and check out the condition of the run/stop switch ect..  Just inspect every connector.  I mean pull and clean front and back side.
George

I did that with the coil connectors when I changed pugs. Clean and shiny.

I almost  think it's an ignition box issue. Doesn't the tach feed from there?

[movenon]

Yes the tach signal is sent from the TCI/DCI box. I would clean those and the pick up terminals just because it free to do... Also check you run stop switch for any corrosion.  On my FJ the tach wire is grey.  At least that is what I would do. What year is your bike ?

You are probably closer to 1/4 throttle.  When you mark the grips you will be amazed at how many RPM you can get and only be at 1/4 throttle just coming off the pilot jets onto the needle. You are probably at a fairly high vacuum while in cruse mode.
George

[JPaganel]

Bike is an 86.

I just got a set of pilot jets from Randy. Maybe I should put them in and see what happens.

[JPaganel]

Paid attention to throttle position today. It's 1/4 or less.

Also, apparently it also does this over 5K, too.