Hey folks,

After hours of digging today, I can't find the cause of a misfire.. it's cylinder 2 and it gets ugly around 4,000rpm and then clears. The idle sounds ok but the 2nd header is just not getting hot. It was after a whole new bank of carbs it started so I thought it must be carb related, and the problem persists even after swapping the carb body AGAIN!

I've tried;

new carb boots (Honda)
new plugs
swapped leads for 2/3
swapped carb body no.2 which went in the little ultrasound
checked timing (spot on)
balanced carbs
checked cam chain tension (manual cct)
swapped CDI
Valves all in spec last year
Everything stock apart from a Pipercross air filter

What the hell else could it be? It doesn't get any worse or better!

This stupid bike has fought me every step of the damn way. Had enough now, it's always a broad issue with a million possibilities.

Thankyou folks

 

How do you know it's #2 cylinder?

 

Hello mate,

the only cold header pipe is cylinder 2

 

So #2 is at same temp as others at idle and gets cooler above 4000rpms? Or is it cold at idle as well?

4000rpm coincides with Helmholz resonance effect where there's triple-carburetion due to flow going back out and coming back in again. But this should affect all cylinders evenly.

Measure usual suspects:

- vacuum at sync-ports on all cylinders at idle and 4000rpms, inHg= ???
- float-levels on all float-bowls, drain each bowls into separate containers and compare volumes. Along with measuring float-height.
- supply voltage to all coils, volts = ???
- continuity of all coil trigger wires to ignitor connector, ohms =???
- impedance of all coils, ohms = ???
- inspect spark-colour of #2 compared to others at idle and 4000rpms, colour = red, yellow, blue, white, etc.?

Tricky part is to somehow measure fuel-volume/mass delivery to #2 cylinder at idle and 4000rpms to compare to other cylinders. As test, spray some starting-fluid into #2 directly and see if engine revs up past 4000rpm cleanly. Or it may be worse. This will narrow down to fuel-flow issue. If it still breaks up same, then something with ignition perhaps.

 

Hey, had to research the Helmholz resonance..

Wow Ill get the tools back out.. one thing I didn't mention was I replaced those little rubber covers for the balance ports, maybe there's a vacuum leak there.

I will get the multimeter out and report back! The header is cold at idle too, revs cleanly through but when you 'cruise' it misfires.

Thanks for sharing your expertise as always buddy!

 

SOLVED.

Thankyou for all the suggestions DANNOXYZ, all the coils, leads and spark inspected, all good, was getting that familiar feeling of frustration and disappointment but glad I kept going....

The culprit was the cap for vacuum port 2, it was mis-shaped just enough for air to get in and the carbs were so out of balance it was idling very rough anyway after swapping carb 2. After a balance and some air/fuel tweaks with the magic tool it runs better than it ever has. Sodding vacuum leaks are such a pain.

If you balance carbs, don't forget the little clip for the rubber caps!

 

Yay!!! Awesome job!!!