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Spark Mystery

  • Thread starter Thread starter MisterCinders
  • Start date Start date
M

MisterCinders

Guest
This is a hangover from my rebuild thread.

I overhauled my top end, new rings, valve guide seals, gaskets, etc.

The carbs are completely cleaned, stripped, dipped, etc., new rubber, re-jetted for the pods and pipe. New intake o-rings.

The valves are in spec.

I replaced the points with a Dyna-S ignition.

The bike starts and is rough at idle, with some intermittent backfiring. Plugs are dark and sooty after idling so its a bit rich, but that could be from idling with choke. I haven't run the engine for more than a few minutes at a time, because I can't get it to idle well enough to dial in the air screws, vacuum synch, etc.

Why? Because it's running on only 3 cylinders. Header pipes for 1,2 and 4 are nice and hot. No. 3 pipe is cold, so it is not firing.

Cold compression tests show 120/120/95/120 - so No. 3 is a bit low, but has enough compression to fire. Indeed, initial tests after the build were 90/90/75/90.

No. 2 is firing so that should rule out coils/ignition.

I have tested the No. 3 plug by grounding it at the engine block, and it sparks.

I have swapped the plug connections for 2/3 and the problem did not migrate. Swapped the plugs themselves, still no change.

There is petrol in the float bowl, so it has fuel.

Why the f**k won't this thing fire?

I have one theory, but folks in the rebuild thread were not buying it. To be sure, they know better than I do. Nevertheless, here is the best that I can fathom.

When I rebuilt the top end, I had to fix a botched plug hole insert for No. 3. Some PO had done it before and the insert fused to the plug instead of the head, preventing me from replacing the plug. I used a new kit to install an insert. The instructions said to use loctite to secure the insert. My wacky theory is that I used too much of the stuff and managed to insulate the insert from the head. That would explain the fact that the plug sparks against the engine block for testing, but does not seem to spark in the cylinder.

I have a second cylinder head that I can swap in for this one, and a new head gasket in en route. Before I tear back into that, I wondered if the sparkies in this forum had other ideas on the mystery.

Is my insulation theory completely nuts? If it is, then swapping heads will not fix the problem.

Is there anything else to explain this non-firing cylinder mystery?

TIA
 
So just how can you see the plug electrodes while its in the head?? My bet is that you got something askew in that one carb. Pull them and concentrate on that carbs and verify the floats not stuck, the float height is correct, the emulsion tube and pilot holes are poked COMPLETLY spotless, ...you know the drill. Also run a bread tie wire all the way down the tiny hole in the pick up tube.. and check the clip position on that carb as well.
 
chuck hahn said:
So just how can you see the plug electrodes while its in the head??
Click to expand...

Um, I can't see the plug electrodes in the head. When I test it outside of the head -- i.e., connected to the cap and resting on the block -- it sparks when you turn the motor, hit the starter, etc.

I can pull that carb apart again, but when I unscrew the bowl plug, fuel flows out the bowl. So there is gas in there.
 
Just because #2 fires does not mean that #3 will, too. :-k

You could have a bad wire or plug cap on #3 wire.

You could also have a bad spark plug. Try swapping #3 with another one.

.
 
Steve said:
That is like the guy in another thread that "cleaned the carbs" by spraying the outside of the carbs. :-\\\

.
Click to expand...


Yeah, yeah. I am a noob, but know that drill. ;)

I stripped all the carbs, soaked them in dip for >24 hrs., sprayed out all the holes. Shined a light through to make sure the fuel screw hole was clear, etc. Indeed, I had to replace this carb body because the old one had a broken fuel screw tip in that passage. So sad to say that I am intimately familiar with this carb body. :confused:

All that said, I will tear down the carbs while they are off the bike again to make sure. It would delight me to find out that this is just a stuck float or needle jet, so I don't have to tear down the head again.
 
have you checked the carb boots for any leaks? I'm not guru by any means but could boot 3 possibly have a slight leak which could cause the slight loss of compression and not enough suction to get gas into it?
 
Have you pulled the plug after running for a bit, is it wet? If its dry you are not getting fuel, if its wet its not igniting the fuel.

Put a few drops of fuel into the cylinder when you have the plug out. Does it fire? If so you have a carb problem.

These things can be frustrating sometimes!
 
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