High compression and non-firing cylinder

[rockford]

#4 is dead and I can't figure out why. It has freshly cleaned carbs (same problem before cleaning), gas is getting there, strong spark, but it won't fire. I checked the compression and came up with 150-145-145-170 (on #4). I got exactly the same numbers about 2 years ago, except that #4 was 150.

I think the big increase must be carbon build-up on the piston, right? What else could cause it? Would that keep it from firing or did it result from it NOT firing? I'm stumped. Thanks for any suggestions.

EDIT: The plug has a strong spark when it's OUT of the cylinder and placed against the side of it but I put in a new plug and it looks like it's not sparking when it's INSIDE the cylinder. Could the high compression keep it from sparking?

[SqDancerLynn1]

I would swap #1 & 4 plug wires and see if anything changes. Have you changes the plug caps? Maybe at least remove the plug cap and trim the wire back 3/8" & reinstall the caps. If this doesn't help You may still have a carb problem.
Yes 170 lbs is probably due to carbon buildup, Just run a few tanks of cleaner thru it

[rockford]

Mahalo

Thanks for the suggestions. I've done them all. I payed special attention to #4 when I rebuilt them this time and it appeared to be perfect.

I'm wondering if the high compression could prevent it from firing? I also wonder if the carbon built up because it wasn't firing?

[tkent02]

If it's not firing at all it can't be growing any carbon deposits. Does the exhaust pipe stay stone cold? or does it fire sometimes? randomly?

How long has it been doing this?

What did you do to it right before it started doing this?

Has it ever hit on all four?

[rockford]

If it's not firing at all it can't be growing any carbon deposits. Does the exhaust pipe stay stone cold? or does it fire sometimes? randomly?

It does get hot but since it's connected to the engine, there's no way it could do otherwise. The bike seems to run fine at high speeds so it seems like it must be hitting on all 4 then.

How long has it been doing this?

A couple of months but it hasn't been ridden much during that time.

What did you do to it right before it started doing this?

I rode it.

Has it ever hit on all four?

Yes, for years. It's almost always seemed to run perfectly. One of the major problems (aside from rust) here is that you can't go very fast. I'm going to take it down to Waianae and back and try to run it fairly hard in the lower gears.

[Nessism]

Sounds like the pilot jet is clogged or there is no fuel in the floatbowl. I'd pull the drain plug do see if fuel is in the carb and if so, you might want to check the pilot by holding it up to a light to make sure it's open.

[Dave8338]

Problem in the pilot circuit or a weak plug/wire.

[rockford]

Back from down the road

Sounds like the pilot jet is clogged or there is no fuel in the floatbowl. I'd pull the drain plug do see if fuel is in the carb and if so, you might want to check the pilot by holding it up to a light to make sure it's open.

Jet's were clean when I removed the carbs to rebuild them and clean when I put them back together. Plenty of fuel in the float bowls.

[rockford]

Sparky

Problem in the pilot circuit or a weak plug/wire.

Good looking, clean carbs and a really strong looking spark; at least as good as the other 3. #1 and #4 wires were switched, as already mentioned.

[tkent02]

How about the tiny passages these pilot jets feed?
They are very small. Did you spray carb cleaner through each one to ensure that they are all clear?
The tiny holes in the choke pickup tube clear too?
And did you ensure the passage in the float bowl itself that feeds the choke pickup is clear?

[rockford]

Good job

How about the tiny passages these pilot jets feed?
They are very small. Did you spray carb cleaner through each one to ensure that they are all clear?

And did you ensure the passage in the float bowl itself that feeds the choke circuit is clear?

I think so. I'm satisfied if I'd paid a shop to do it they wouldn't have done as good of a job. I've done it about a dozen times in the past on this bike and others and I'm really good at it. I'd be amazed if the carbs are the problems but I may stick on a second set if I can't figure it out. (Yes, I have another perfect set all ready to go, but I refuse to think that's the problem.)

[tkent02]

I think so. I'm satisfied if I'd paid a shop to do it they wouldn't have done as good of a job. I've done it about a dozen times in the past on this bike and others and I'm really good at it. I'd be amazed if the carbs are the problems but I may stick on a second set if I can't figure it out. (Yes, I have another perfect set all ready to go, but I refuse to think that's the problem.)

So spend ten minutes swapping them out and know for sure.

[Dave8338]

I think so. I'm satisfied if I'd paid a shop to do it they wouldn't have done as good of a job. I've done it about a dozen times in the past on this bike and others and I'm really good at it. I'd be amazed if the carbs are the problems but I may stick on a second set if I can't figure it out. (Yes, I have another perfect set all ready to go, but I refuse to think that's the problem.)

Try the second set and report back...

[Nessism]

Wish I had a dollar for every time someone says their carbs are "clean", only to learn they weren't.

Fuel or spark. You tell us.

[rockford]

10 minutes?????

So spend ten minutes swapping them out and know for sure.

You've obviously never switched out a set of carbs on a motorcycle.