Rough running bike

[tooserious]

My 750 runs rough on 1 side...irt back fires a little and is sluggish in mid range. I had the carbs rebuilt...the guy seems to think the valves need adjusted??? any thoughts...he also said it could be the air fuel mixture screw needs adjusting??? Any thoughts??? Thank you

[pat english]

my 350 twin had those symptoms..it was a bad coil..dam bike ran fine on one cylinder...do you have dual pipes?..put youre hand near the end of the muffler tips while its running..wait for it to warn up a bit..the side thats not firing will feel cold..

[LukeM]

This would make sense with a twin, but the CB750 coils fire cylinders on both sides: 1 & 4, and 2 & 3.
The coils fire their plugs once each revolution: once on the power stroke, and once on the exhaust. It's called a wasted spark system.

Agreed that touching the pipes with a wet fingertip should give you a good indication of combustion. You could even buy one of those IR laser gun thingies, if you need to be AR about it. :-)

You should download the FSM on the home page of our happy little forum here, and do everything in the Maintenance section: valve adjustment, cam chain adjustment, tightening of clamps and such, timing check, idle adjustment, etc.  THEN I would dive into what might be wrong.

Good luck, and keep us informed.  We'll help if we can.
Luke M

[sgtslag]

Year and model (K/C/F) would also help.  Cheers!

[tooserious]

1979 CB750K...the guy went tthrough the carbs.....cleaned and put carb kits........it didn't do that before he replaced but it was bad in the take off........

[pat english]

ah,right on.

[seestheday]

Luke, if the spark plugs are connected out of order, wouldn't that also cause rough running issues because of timing?  The PO of my bike had hooked up my plugs 1,2,3,4 instead of 1,4,2,3 and I swear that it started running better once I put them in the right order.

That said, I can't be sure because I also put in the Iridium plugs at the same time.

[Hellbilly429]

how would that even run like that? That's crazy, at least the po didn't do any damage to your bike like that sees, did it run hot on 2 cylinders and cool on the other 2?

[LukeM]

You can switch spark plug wire connections between 1 and 4, and between 2 and 3, but that's it.
Per the FSM, the firing order for the DOHC engine is 1-2-4-3.

Sees, I found your other posting from Jan 2012.  If you go with that firing order, the bike will essentially be running on 2 cylinders.  It will be low on RPM, and not accelerate well, but it will sort of run.

I remember reading somewhere that these engines will actually run if you spin the crank in reverse fast enough.  It won't run well, but it _will_ stumble along.

Luke M