engine miss

[Andrewsteward]

I need some feedback please in regards to an engine miss.
At 1/4 throttle that seems to co-inside with 2800-3200 rpm I am getting an engine flutter /miss.
I have:
swapped leads and coil lead position.
checked valve backlash.
removed bowls and checked both are getting plenty fuel
checked carb diaphragms.
cleaned plugs which are fairly new.

its an 81 so has electronic ignition.

thanks

[Barry]

The only thing that jumps to mind about that rev range is it's just where the carbs come on the tapered part of the needle. I know you have already checked the diaphragms but by way of experiment you could raise the needles one notch just to see if it's a fueling issue.

Has this come on very suddenly and if so was there any work you did on the bike prior to the symptoms appearing ?

[montmil]

Do you still have the OEM black & gray coil fitted?

There is a lot of truth in this old Airhead proverb-
     When you think it's a carburetor problem, it's electrical.
     When you believe it's an electrical issue, it's the carburetion.  

[Luca]

+1 on the coil.  Mine showed it's failure during a rainstorm on my way home from work.  The bike acted like it was out of fuel unless I kept the revs above 3.5K

Took Monte's advice back then and checked the coil, which was cracked at the front.  Replaced it with a dyna brown from motoelekt.com and replaced the plug wires for good measure.

[Andrewsteward]

Thanks for the input, yes first thought is carby but agree with montmil that its very possible the opposite.
will have to check if its the oem coil, not sure.
Barry what do you mean or how do I raise the needle one notch to eliminate the carbs?
Im not sure if its a sudden thing as I have only just got the bike.
Riding today I noticed its up to the 3500 range but only if I hover at that 1/4 throttle range. Its on cue at that 1/4 throttle mark and not an issue if i power through it.
Just on another note. The damn neutral switch on the underside of the gear box started leaking today.
Any fix without taking gearbox out? Looks pretty damn tight under there!
thanks again for the comments.

[Barry]

I was thinking of the possibility that there was a severe flat spot (period of very weak mixture) just as the carbs come up on the needle. The needles have 4 positions.  1 provides the weakest mixture and 4 the richest. To move the needle up one notch you twist it through 90 Deg and push gently it will then click into a new position. It's good practice to measure the exposed length of needle on both sides so you know where you are starting from. It's not unknown to find the two carbs set at different positions.  Here are my measured needle lengths for each position. Each step is approx. 1.8 mm so it's easy to differentiate between one position and another even if your measurements differ slightly.

1      42.37
2      40.57
3      38.79
4      37.10
  

[montmil]

The neutral switch can be removed with the transmission in situ.

You will need to remove the aft motor mount bolt and the large aluminum tube spacer that the bolt passes through... after you have secured the gearbox with a bottle jack or similar to keep the engine from slipping down.

Unplug the two wires and then use an open-end wrench to unscrew the switch. You did drain the gearbox lube, right?

You have two choices for a new switch: The OEM part -which is the same design as your leaker and has no guarantee that it will not begin leaking again soon after. Or, Motobins sells an Italian-made switch that is cheaper and superior to the aluminum and plastic original. My '81 has The Italian Job and is dry after a number of years. Use a new crush washer and be gentle in snugging up the new switch. Don't over-torque that puppy. Refit the electrical connections.

The only PITA chore is getting that aluminum spacer back twix the frame rails. I ground a small chamfer on each end and that made the refit much easier.

[Andrewsteward]

Thanks Monte, I will get onto ordering a switch before I have a crack.
Much appreciated, I thought it was a gearbox removal for sure.

[Andrewsteward]

Thanks Barry, I think you might have been on the mark. Rhs was position 2 and left side position 3. I also found the choke was slightly on both sides so rectified.
Seems the left was over fueling. ( black plug) so I have positioned them both at 2 and although still generally slightly out of tune, all seems balanced now.
Both plugs look good.
Thanks again Barry. Not something I would have picked up on.

[arvo92]

The neutral switch can be removed with the transmission in situ.

Thanks for that info. I too have passed the neutral indicator switch replacement into the wintertime as I did not want to remove the gearbox and go through all that hassle for only that switch. My wires are hotwired at the moment and the green light on the dashboard is constantly on.

[Luca]

Arvo, the switch can be removed without taking the transmission out of the bike.

I'll be replacing mine soon, as the leak is getting worse.  I think I'll try putting the aluminium spacer tube in the freezer before I reinstall it to see if that makes the job less troublesome.

[montmil]

...  I think I'll try putting the aluminium spacer tube in the freezer before I reinstall it to see if that makes the job less troublesome.

I also tried the overnight freezer idea for the reinstall of the spacer. I quickly discovered that the Beemer chassis is an excellent heat sink.

[smiley=wall.gif]

Off to the grinder we go...