#3 not firing

[azr]

78 1000. Ok, I've about had it with so I'm coming for help from more sane people. I can't get my #3 to fire properly. In case anyone doesn't know the bike, everything has been done, carbs, electrical, etc, etc..everything. I put a new rack on the bike 3 weeks ago and she hasn't been running 'right' since. I rebuilt them last year for a guy with a 79 750. He sold the bike to me this spring. I put the carbs on because they had all new parts and mine was leaking a bit from a bowl. Pulled them off the first time because I forgot they had different main needles. Pulled them off the 2nd time because the bike wasn't running right, had to hit #4 & #3 with the mallet to get the cylinders to fire sometime, Dipped them and put them back on. Now at this point I started to think a little because I really didn't want to pull them again. Swapped plug wires across, did not fire #3. Changed plugs, did not fire. changed caps, no go. Pulled them the 3rd time put a guaranteed clean #3 carb in the rack. Same problem. ARRRRR......I'm starting to think I'm crazy. There's fuel on the plug. Weird thing I can get it to fire intermittently if I pull the cap off slightly. She's getting the same colour spark as all the rest and wakes me up well when I get zapped Could there possible be something happening in the valve shims that would cause this? The bikes pushing some blue from the valve seals and probably rings but I wouldn't think that would stop the cylinder from firing, it's not that bad and the plug is not fouled. I can once in awhile get the pipe to warm up but it takes a fair bit of time, unlike the other which are instant. I'm thinking....replace the plug wire and cap from #3, put in a brand new plug. From there I'm thinking look at the valve shims? As always any thoughts are greatly appreciated.

[rustybronco]

I worked with wallowgreen a few years ago trying to figure out his running rich condition on one carb. As best as we could figure, it ended up being a cracked carb body. We tried every thing to get it to run correctly. When he replaced 'just' that one carb body and the problem went away.

If you have a spare carb, I'd swap it with the one you are having troubles with.

[azr]

That's what I did on number 3 pull out of the carb rack. I put in a guaranteed clean carb with no change. Or was it number 4 pull out? oh boy...

[chef1366]

Have you checked the compression?

[salty_monk]

Sounds like a break in the plug wire or cap, insulation breaking down, something like that to me....

I had a Dyna coil that went bad inside the hole where one of the wires connects. The contact had basically spark eroded away, weird!

Have you tried running the engine at night & wriggling things around. Sometimes you can see where the spark is jumping to (usually to the fins or valve cover, sometimes to the frame up by the coil).

Good luck!

[Steve]

You brought up the subject of valve adjustment (shims).
When was the last time you checked the adjustment?
If you can't remember, it's been too long.
Probably not your problem, but won't hurt to check.


You mentioned crossing wires with no improvement.
You are sure that you have them all on correctly now?
The wires from the left coil should go to plugs 1 and 4, the right coil should power 2 and 3.
Have you tried a set of brand NEW plugs?


Let's also start at GROUND ZERO and make sure you are counting correctly.
#1 is on the left, under your clutch hand, #4 is on the right, under your throttle hand.
#3 is the one that should have the vacuum hose to the petcock attached. Is it attached? Is it intact?


What are your pilot screw settings?
I don't know if your bike is stock (as far as intake/exhaust goes), or not, so can't make suggestions about pilot settings.

.

[azr]

compression is good #1 150, 135, 135, 135. I swapped the wires back and forth between #2 and #3 to see if I could replicate the situation on #2 (to rule out plug wires and caps) but #3 still would not light. All the plug wire on on correctly, carbs are all set to stock set up, as the bike is stock. Valves were adjusted under 1000Km ago. I put in some brand new plugs with no difference. I think next I'll just put in a new wire and cap just for the heck of it.

[Steve]

OK, thanks for the reply.

With all that methodical swapping around (really the only way to do it), the only constant seems to be the carb.
Even though you said the spark plug appeared to be wet, it might be that there is not enough gas getting through the pilot circuit.

I recently had some problems getting my son's 650L to run correctly. Had just gone through the carbs with the full "strip and dip" routine, new o-rings, etc. Had all the mixture screws (CV-type carbs here) out my usual three turns, it just was not running right on all four. Gave TheCafeKid a call. By the time he got there, I had pulled the carbs, checked the float heights, and put the carbs back in. Still the same problem. He started playing with the mixture screws. After turning #3 out another turn or so, the bike just started humming. There is something about that carb that requires the mixture screw to be turned out more than the others. It's not really a "problem", it just points out the fact that you can not simply set the screws "X" number of turns out and call it good.

Any chance you have tried tweaking your mixture screws a bit?

.

[BigD_83]

You're describing a situation I had with #1 on my 650 recently. It was running beautifully, and then, out of nowhere I had no spark on #1.

In my case, #1 sparked when grounded against the head (or on my hand- ouch!), but when put back into the head, no fire. Swapping the leads around helped, but the problem didn't follow the lead.

I just snipped off another 1/4" of plug lead and put on a new NGK cap. Problem solved.

[tkent02]

#3 coincidentally, is the cylinder that the vacuum line to the petcock comes from. The one that sucks fuel in if the petcock diaphragm is bad. The one that will often be too rich to fire even with perfect carburetors. The one that most commonly fouls plugs.
And the one most likely to have problems like these.

Is it a coincidence?

You decide.

[azr]

I have tweaked the carbs prior to all this pulling apart 'again'. #3 and #4 were a fair bit different than 1 & 2, but it still didn't run 'right'.

Tried swapping plug caps with no avail. I even put in a new plug wire as mentioned by Dan but no difference.

You know, Todd mentioned the petcock earlier but it is not leaking, all the other carbs are firing fine hence getting fuel. I didn't apply vacuum to the petcock to make sure it's working fine but I can do that tonight. Am I missing something with the petcock in my thinking???

I've gone into the valve shims and surprisingly there were more than a few that were either too tight or too loose. I'm going to button her up after dinner and synch the carbs ONE MORE TIME...yeeeehaaa. Getting a bit tired of synching carbs, at least on the same bike. I'll keep my fingers crossed and hope for the best. Thanks for the help so far guys, keeps the insanity of it all a little further out of my head.

[Steve]

You know, Todd mentioned the petcock earlier but it is not leaking, all the other carbs are firing fine hence getting fuel. I didn't apply vacuum to the petcock to make sure it's working fine but I can do that tonight. Am I missing something with the petcock in my thinking???

What tkent was referring to is the fact that #3 is connected to the petcock. The vacuum generated by the running engine pulls on the diaphragm in the petcock to turn on the fuel flow. If that diaphragm has a leak, the gas that comes through will be going straight to #3 cylinder, making it run rather rich. You won't necessarily see any leak on the outside.

.

[azr]

Thanks for the clarification. I'll test the petcock for sure I even have an extra petcock from Z1, I can put on if the valve work doesn't rectify things. That would be pretty funny to find out I pulled my carbs 4 times, did another valve job, only to figure out it was the petcock that whole time, ha....ha.....ha.

[azr]

I tested the petcock, sucking on it while the tank was off, fuel would only flow when I applied what turned out to be very little suction, so it seems to be working fine. Put everything back together after doing the valve shims and #3 fired, go figure. It still was taking longer to heat up than the other 3 though. Played with the fuel and air mixtures and that improved things as well. Took it for a spin and the bikes running better that's for sure but not perfect. It still has an erratic idle, there's a surge here and there. Like a vacuum leak or something. Any idea's? I threw the colourtune on the plugs after I did a fastest idle tune and everything was pretty close. I've never done plug chops on this bike simply because everything is stock. I just put the fuel screws to the stock setting of 5/8ths out and tuned the air screws to that. Should I be doing plug chops? I did put some oil additive in the cylinders (thanks for the tip Todd) and man did it smoke. Also put some in the oil. Hard to tell if there is an improvement or not. I'm going for a ride with some fellas tomorrow, I'll ask them. I put a fork brace on that I've had for awhile, one of the older period correct ones, (thanks again Todd) and a new set of Hagon shocks came in the mail (thanks crapwacker). So with the progressive springs and fork brace up front and Hagons in the back she's handling better than the days she rolled out of the factory.

[azr]

As a follow up, went for a hard ride and the guys behind me said the bikes pushing a lot less blue smoke, none on acceleration and just a small amount on decel, and only on the right side. Fantastic tip on putting some oil additive down the rings, I guess you guys were right that some of the oil control rings were sticking. I've still got a bit of an erratic idle but other than that she's running great, and with my fingers crossed using less oil. I was going to do some plug chops but after yesterdays ride and not having any issues in 'any' spots, and I went to them all, I don't think I need to. Thanks again for the help guys.