There's a 2KRPM vibration I just can't quite find the cause of..

[cjard]

Some fantastic help from here, I've chased down most of the running issues with the bike now.. All but for this one: there's a bit of vibration at around 1800rpm, and then it goes away, returning at 2100 rpm to about 2600rpm, then starts to clear again. The vibration seems more prominent on the overrun than on accel

On the stand you can hear something like one of the cylinders misfiring and I'm sure I've caught sight/sound of the occasional backfire through one of the carbs at lower revs

Work that's been done trying to chase it down:

*Removed float bowls and cleaned
*Soaked pilot jets in carb cleaner then given a light prod with a bit of copper wire
*Carbs balanced by a mechanic I trust, who's done the operation thousands of times - I havent asked whether he did anything with the pilot air screws but his boss says "hey probably will have.. he's very thorough"
*Plastidipped diaphragm transparent points and one pinhole I found, tested for leaks by filling them (upside down) with petrol and checking the level of it after 15 minutes - no dia was appreciably lower than any other
* Checked mainjet holes that they were still round and weren't worn oval
*Blown compressed air through all carb jet and airways
*Checked choke isnt sticking/is sealing properly


I havent done anything electrical yet, will check the coil voltages etc today - I was really thinking it was carb related but I've kinda run out of things to try other than swapping the pilots for the next size up

A mechanic friend says he doubts it's pilot circuit as at the rpms it happens it's getting into butterfly/diaphragm/mainjet territory.

Is there anything I've missed checking, or should look at again?

[cjard]

Forgot to mention:
* Float levels.. checked and all seemed within spec, though the haynes manual wasnt really clear where the measuring point was.. One of them looked a little lower than the others visually so that was adjsuted to be the same

[Arnie]

Why are you riding at 2KRpm ?  That is way too low for this engine.  Keep it at 3KRpm or higher, with the idle at 11-1200 Rpm - you and the FJ will both be much happier.

That said, you don't mention what year or how many kms/miles your FJ has.
Some other items that could cause vibrations include:
Engine mounts (need freeing and greasing)
Lower frame rail bolts (broken or loose)
Worn chain or sprockets
Worn or loose cam chain
Wheel out of balance
Tire scalloped
Inner exhaust header pipe rattling (early bikes only)

Probably a few other vibe causing items, but I'll let others add to this list

Cheers,
Arnie

[Pat Conlon]

I will take a guess....(as Arnie points out, assuming that it is a engine shake)  Spark plug boots?
The oem FJ plug wires are wire core and so they very durable, but sometimes the resistor plug boots develop issues.

[cjard]

Engine mounts unseized, cleaned and regreased.. that solved the 4000rpm vibrations..

This 2K vibration is happening while it's on the stand, and there's the occasional pop/bang from the engine that leads me to think one of the cylinders isnt happy with either its spark or its fuel, at around 2krpm..

I'll try a new set of plugs and HT leads (actually there's an XJ550 next door, i wonder if I can borrow some bits off that for testing..)

[SlowOldGuy]

Sounds a lot like an idle circuit is still dirty. 

I had a similar problem until I cleaned the carbs well and installed #40 idle jets.

Now everything is much more better and funner (how's that Pat :-)

DavidR.