Misfiring/Backfiring on Low RPM's

[Barry]

...I'm still getting a jumpy tach. This time it doesn't jump along with engine stumbles it was jumping even when the bike was running steady. More often I noticed it would jump when I left off the throttle and when i hit the throttle again it would try and even out

Well the tach just counts ignition pulse so even if the engine is running better unless the tach is faulty I would still say that  it's ignition related. The tach needs a reasonably clean signal so a couple of obvious things to check would that all electrical connections to the tach are good and clean and then I would look to see if you have the correct resistance in the ignition secondary circuit. No resistance at all would cause a noisy signal that might cause the tach a problem. The usual set up is plain copper ignition lead with no resistance built in, Resistor spark plug caps and ideally none resistor spark plugs.
 
Mario, I forget which year your bike is. It should be 5000 ohm caps for the electronic ignition bikes and 1000 ohms for the points ignition.

[mfrias2nd]

it's an 82. about 30k miles. to the best of my knowledge that is. All i know is it has those black NGK caps. Maybe it's about time to replace them anyway.

Bottom line is I'm getting a rich mixture in the left cylinder and for experiments sake I set the mixture for a really lean mixture and I was still getting a sooty plug. As soon as I rule out the carbs by replacing the floats and diaphrams and another cleaning I'm going to hit the electrical system.

Like someone said it's best to work one end of the bike first to rule it out instead of jumping around to different things half fast.

The tachometer reading works on a ground connection, correct? my wiring diagram shows a black wire from the ignition coils to the tach which if my problem is electrical would make sense. I disconnected the tach and didn't notice a major improvement.

[tvrla]

Causes of a rich mixture, besides weak spark, are float level high, leaky float needle, choke on or gasket leaking, jet needle too high, worn needle and jet, wrong size jets.

I can't think of any other reasons at the moment.

[mfrias2nd]

Well the bike was running nicely and then I replaced the float needles then it started acting up! I can't imagine the float level is high because before I adjusted the float to compensate for old/faulty needles. If anything the float level would be low.

I'm really looking forward to seeing how the bike reacts when I change the diaphrams and floats. I hope that solves the problem.

[tvrla]

Were the floats adjusted after this work? Does the fuel turn off when they're parallel?

One thing I forgot to mention - sometimes I've seen the float tang gouged out from the action of the float needle. After a while it gets bad enough the needle doesn't slide anymore as the floats rise. What happens is it jams, preventing the float rising and then of course the fuel goes high or overflows.

[mfrias2nd]

Hey everyone! thanks for your advice and tips. I replaced the diaphrams and floats and rebuilt both carbs again. The bike runs great! I took it on a 30 mile ride yesterday and didn't get a single misfire/backfire or bogging down at the start. I had smooth transitions from idle all the way through the powerband.

Only thing...The tach must have been a seperate issue because it's still jumping wildly. I'll try some contact cleaning.

[79beem]

Check the front coil mounting bracket. On my 79 its the ground for the electrics. When it broke I got all sorts of weird electrical issues, one being erratic tacho.

I cable tied the coil to the frame and extended ( soldered and shrink wrapped the connection) the ground to the rear coil mounting bracket.

Problem soldered! Haha.

Just a thought as no one else seems to have mentioned it.

[Bob_Roller]

The bike is an '82 model year, the dual coil setup was changed with the '81 model year, so the main grounding point isn't the front coil bracket, it was moved to the relay area under the tank, all of the brown wires are 'ganged' together at one connector and then bolted to the frame .

[79beem]

I stand corrected.  [smiley=deal2.gif]