Backfire and Misfire Help

[nitroxwzrd]

I'm having issues with the left side cylinder missing and stumbling from start to about 3500rpms and would like some guidance on where to look next. I have read and reread the tech section and searched for hours but have not been able to come up with anything that works everything I find seems to be talking about both sides having issues or one side just totally not working. I haven't been able to find anything on a constant issue with only one side.

Situation:
About 2 weeks ago I totally lost the left cylinder when heading home. Before then I had no issues to speak of. Bike ran relatively fine but I have been slowly tweaking and fixing things on it as I could. Since then I replaced both coils, wires, and plugs. When I put everything back together and fired the bike up the right side ran fine but the left side would backfire and misfire no matter what I did with the throttle. I have since cleared the carbs, switched the coils and wires from one side to the other, and put in new plugs again. None of that helped.

Bike:
1977 XS with twin Dellorto PHF36 and pod filters. Before the cylinder issue I was running 145 mains and what looked like bored out 65 idle jets. In the process of cleaning the carbs I went to 140 mains and new 65 idle jets. In this whole issue I have chanded the jets back and forth but that doesn't seem to have any effect. Bike also has drag pipes with and oldschool thumbscrew for baffles.

Any idea on where to look next would be Awsome.

 

[littlebill31]

Have you checked for air leaks? Did you adjust the cam chain tensioner, then valves, then timing, and then the carbs, in that order?
Synced the carbs?
And when you switched plug wires the left still didn't work?

 

[WacoBrian]

sounds like carbs to me, opps those are aftermarket, im a stock guy, like Bill says carbs or valves,

 

[nitroxwzrd]

No air leaks that I can find, adjusted the tensioner and valves before this issue came up, have not adjusted the timing and have not synced the carbs since I've been swapping jets. No change when the plug wires and coils were swapped. Originally when I lost the cylinder when I would wiggle the plug boot it would fire fine, but I figured since it was the original coil and wires I should change both. Since I've owned the bike the right side would always fire and run fine no matter what I did with the carbs but the left side has always seemed weaker. Could there be a grounding issue with the left coil.

 

[littlebill31]

So, when you switched the plugs, from left to right, the left still doesn't fire? Like not fire at all, or is just weak?
And a brand new coil and wires didn't help?
That timing needs to be checked/adjusted first. Always adjust the timing before going to the carbs.
And a compression check needs to be done.
Also, pull a plug, with cap on, and ground it to the cylinder to check for spark on both sides.

 

[grizld1]

1. Inspect points, set gaps, and check timing with strobe. If no joy,
2. Check compression. If no joy,
3. Check leakdown; a leaking valve can sometimes cause symptoms like yours.

In future, remember that combustion=ignition+compression+fuel, and that troubleshooting, in the absence of evidence to the contrary, should be performed in that order--carbies last.

Edit: I thought this only happened with 5twins; but I hit the "send" button and there you were, littlebill. As a friend of mine used to say, "Great minds run in the same circles."

 

[WacoBrian]

have not adjusted the timing and have not synced the carbs since No change when the plug wires and coils were swapped.


DO You see what your saying?

 

[grizld1]

As Bill already said, make sure everything else is dialed in before you fool with synch adjustment, or you'll run the risk of trying to compensate for one defect by introducing another. And I see exactly what I'm saying. There's no indication in what was posted that points leads had been reversed to fire the right cylinder from the left point set. Ignition has not been eliminated as the source of the trouble.

 

[nitroxwzrd]

Maybe I'm missing something but if the timing was an issue then wouldn't both cylinders have a problem and not just one, but I can conceed that it needs to be done. What I don't understand is why the timing would be affecting things now and not before.

I can ground the plug and get a spark. The cylinder will fire, just not consistently, and without staying on the throttle the bike will die; but that side will miss and backfire. The otherside has no issues.

 

[grizld1]

Yep, you're missing something, and you're assuming too much. You have one set of points per cylinder. Timing on one set is adjusted by the main plate and the other is on an independently adjustable subplate. Gap is also, of course, independently adjusted. Refer to your manual or to the factory manual at www.biker.net.

To check if timing or points condition is an issue, switch the leads to the points from left to right and right to left, then switch the plug wires left to right and right to left. If the misfire moves, you've found the problem.

 

[WacoBrian]

No air leaks that I can find, adjusted the tensioner and valves before this issue came up, have not adjusted the timing and have not synced the carbs since I've been swapping jets. No change when the plug wires and coils were swapped. Originally when I lost the cylinder when I would wiggle the plug boot it would fire fine, but I figured since it was the original coil and wires I should change both. Since I've owned the bike the right side would always fire and run fine no matter what I did with the carbs but the left side has always seemed weaker. Could there be a grounding issue with the left coil.

No change when the plug wires and coils were swapped.

 

[xjwmx]

In the op you're implying it used to be ok. The couple of times I've had your issue it was a flooding carb, cured by replacing the float valve and seat, and once it was a petcock leaking a stream of fuel to the outside, starving one carb enough to make it sporadic while the other side ran okay.

 

[grizld1]

WB, you're assuming the "swap" was done in such a way as to alter which point set controlled which cylinder. It's not obvious that this was done. You can "swap" the coils and plug wires so that left points fire the right coil, and hook the right coil wire to the left cylinder. That will tell you whether or not a coil or plug lead is bad, but it won't eliminate the left points or their adjustment as the source of the problem. To do that, you have to "swap" the points connections and the plug wires. I shouldn't have to explain this.

 

[5twins]

I don't think that will do it. You'll still have the left points and timing running the left cylinder, just through a different coil-plug wire-plug cap assembly. That will test the coil-plug wire-plug cap assembly for you but not the timing/points. If you do want to switch the points so they fire the opposite cylinders, I think you'd need to move the advance disc or points cam on the advance rod 180°. But ..... that's all too much work. It's much easier just to check the timing and points gap, check that the points are clean, etc.

What hasn't been mentioned are your condensers. The one powering the left cylinder may have gone bad. It's not common but does happen. A simple test for this would be to switch the condenser leads where they plug into the double orange and gray wires up by the top motor mount. If the misfire moves to the right cylinder, you've found your problem.

If you have an oil leak inside the points housing, that leaking oil may have contaminated your left points set. It is the lower set so would be more susceptible to leaking oil. It doesn't take much to foul the points faces and cause misfire problems.

 

[grizld1]

I got 360* reversed with the reversing. Right you are, the orientation of the ignition cam has to be changed at the timing disk for the test to work. Easier just to service the points (inspect, sand/file if needed, gap, hook up strobe) than to fool with the ATU.