Topic: Wet plug that fires

[Somemonkey]

So after searching the forums I couldn't find anything that addresses this issue:

I have a 1976 Honda CB550. The carbs are clean and have the correct sized jets along with all proper adjustments. When I start the bike, pipe number two doesn't get hot. (1 3 and 4 fire just fine) When I pull the plug the end is wet with gas. I tested the plug by pulling it from the head, holding it to the block and hitting the starter. It was firing as it should with a fat blue spark. How can it be getting gas and also be firing the plug without causing an explosion within the chamber? The bike has run before without this issue. This is the first time this cylinder hasn't fired. Could this be caused by sticking valves? Any help would be appreciated.

[Short Round]

I had the same problem with my #4 cylinder.  Everything was working fine (spark, gas, air, ect).  Did a compression test and found the #4 was poping at 48psi.  Also found that the rest of the cylinders were not getting the right compression either.  Do a compression test and see where you stand from there.......Hope everything checks out...........Chris

[OldSchool_IsCool]

Check your tappet adjustment.  If you have a tappet that is so tight that it's holding the valve open, then you won't get enough compression to ignite the mix.

You can rule out points timing as #3 shares the points & coil.

A quick check would be to pull the plug cap off the wire and see if there is any corrosion in there.  If you see any green, clean the spike in the cap and clip a small amount of wire off.

[Steve F]

Try swapping plugs?  Have seen plugs look ok, but have a crack in the insulator or maybe the cap is cracked, and won't short out until the plug is installed in the head.

[Somemonkey]

I had the same problem with my #4 cylinder.  Everything was working fine (spark, gas, air, ect).  Did a compression test and found the #4 was poping at 48psi.  Also found that the rest of the cylinders were not getting the right compression either.  Do a compression test and see where you stand from there.......Hope everything checks out...........Chris

Thanks for the tips!

I'm going to check the tappet adjustment this evening. Also, I'm going to use a brand new spark plug to rull out a broken one. I'd like to check the compression but the compression gauge I bought doesn't have a small enough adapter to fit the plug opening. Do any of you have an idea where I might find an adapter?

[Bodi]

Your tester may be different, mine uses a normal airline quick-disconnect fitting on the hose to attach the plug adaptor thingy, with the one-way valve in the hose end fitting somewhere.
I broke apart a spark plug (not easy, but I got finally the metal screwing in part separated) and had it welded to a quick disconnect nipple fitting I bought at Home Depot. Works fine.

[mattcb350f]

Spark plugs can act very differently under pressure. This is what Steve F was refering to. I've tested sparkplugs on a machine that allows the plug to be pressurized to about 120psi and seen good plugs fail. They may spark fine out of the cylinder, but when covered in gas and under pressure they may not.
 Matt.

[paulages]

did you recently rebuild your carbs? if so, are your float heights correct, and did you synchronize them? mixture screw is definitely set right?

[Somemonkey]

did you recently rebuild your carbs? if so, are your float heights correct, and did you synchronize them? mixture screw is definitely set right?

thanks for the tips guys.

I ended up having to replace the plug. I adjusted the valves thinking that might have been the problem but it turned out to be a bad plug. While it sparked up nice when outside the block, it wasn't getting any spark inside. I switched it out with another plug and now all my pipes are hot.

I did clean out my carbs and adjust the float levels recently but that only made things run better. (as I had wanted) Also, I swapped out my points and condensers with a brand new set from Z1. Doing that last bit helped me go from a weak orange spark to a fat blue spark. I've learned that starting with the basics can be a real time saver!

As for the adapter on my compression gauge: I'm not sure what I'll do now. I'll just try to find the right size adapter for it. Worse comes to worse, I'll weld some plug threads as the other gentleman suggested. I already have some threads available from where I broke them off in the head a couple weeks ago. Man, I thought I'd never get the threads out!