weak spark on inside #2 plug please help

[john78xs11]

HI I am new to the site (just found it yesterday) but am glad to see it I am having a problem with weak spark on #2, I have changed the plugs,caps,coil, and ignition unit but have had no luck does any one have a suggestion for me. I hope to hear from all of you. Thanks John

[skids]

I guess you are saying that No 2 is not firing because of weak spark? That will cause No. 2 head pipe to be cold in comparison, and make a fuel wet plug. Change the plug wires with No. 3 and see if indeed it is a spark problem...then, No 3 will be cold while No 2 will start firing. (those fire simultaneously off of the same coil and can be switched. ) If the problem remains at No 2, there must either be a compression problem or a fuel delivery problem.

[thumperjsa]

How come you are assuming a "weak spark?" If it is because you suspect that cylinder is not firing and it could be an air fuel mixture problem.

Try what skids says with swapping the spark plug cable. Each coil fires 2 cylinders at the same time. So if you have a weak spark on 3, you should have it on 2 unless it is a bad wire, spark plug cap or spark plug.

One coil fires cylinders 1 and 4, the other 2 and 3. So by swapping 2 and 3, if you have the same issue, it could be that you are leaking air after the carb.

Sometimes leaky intake manifolds can cause the issue of a cylinder not firing because it is pulling in air and not fuel mixture. You can tell this by spraying wd-40 or engine starting fluid and see if the idle speed changes.

[john78xs11]

Thanks Thumperjsa and Skids for the advice I have already tried engine starter on the intake and there was no change in the idle. the NO.2 head pipe is cold in comparison to the others and so I will try changing the wires and see what happens Thanks John

[Ken Talbot]

You might also want to use a compression tester. Something as simple as an exhaust valve hanging open could kill a cylinder. Better yet, a leak down tester would give you a better read of the mechanical shape of the motor. With all you've changed out already, I would suspect a fuel supply problem rather than an electrical problem. Have you discovered the wonderful world of 'triple clean' yet?

[skids]

If the header is cold and the plug is wettish with fuel or carboned, chances are that there is a problem with adjusting the throttle linkages ("synching the carbs") and/or the pilot screw being screwed out too far. I say this because you seem to indicate that the header is at least warming-up. Rich mixtures burn at low temperatures. Do she miss at higher rpms? Bad spark is not just a low idle thing, but bad carb synch is a low idle thing and so is mixture adjustment with the pilot screw. If you see spark at all when holding the cap with an old plug in it near the fins, it is probably not misfiring due to weak spark.

Originally posted by john78xs11
Thanks Thumperjsa and Skids for the advice I have already tried engine starter on the intake and there was no change in the idle. the NO.2 head pipe is cold in comparison to the others and so I will try changing the wires and see what happens Thanks John

[john78xs11]

I switched the NO 2 and NO.3 plug wire and 2 was still cold in comparison, also did a compression teste and 1,3,4 were at 115 but 2 was at 90 could this be fixed with a valve ajustment or is the problem deeper. will the engine still run with this difference in compression?

[john78xs11]

Yes it does miss at higher rpms

[thumperjsa]

Did you do the compression test with the throttle wide open?

Also squirt a couple of drops of car type or Marvel Mystery oil in the cylinder after you do compression test and then re-test. Does your compression go up on the second test and by how much?

[skids]

If the bike has been sitting a long time (year +) without running, it is possible to run a few tankfulls of gas through the engine and put some miles on her before you get too exited about a major engine restore. You should be able to get the cylinder to fire better at 90 psi compression (you DID hold the throttle open when checking compression didn't you?). I recommend, especially if the bike is new to you, to read that spark plug to see what is going on in there at idle. I think I would synch the carb linkages by equalizing vacuum, run the bike to color the plugs near idle, and then adjust the pilot mixture screw if needed. Keep track of its current setting and count the quarter turns of adjustment with a screwdriver. Unless you are very talanted, you will likely have to remove the tank to access the pilot screw. Continue the adjustment until the plug look right and the headpipe acts like the others. If compression eventually comes up (and you luck out) you might have to resynch the carbs and readjust that pilot screw. Sorry. Some have reported that adjusting valve shims helped compression a bit. If you have the time, it wouldn't hurt to check it first. You will need a shim tool, a gasket, RTV for the half moon seals, a source for shims, and a good feeler gage. If you do all that you will become very "intimate" with the bike! :-)

Originally posted by john78xs11
I switched the NO 2 and NO.3 plug wire and 2 was still cold in comparison, also did a compression teste and 1,3,4 were at 115 but 2 was at 90 could this be fixed with a valve ajustment or is the problem deeper. will the engine still run with this difference in compression?

[uk1.1s]

Same trouble...

Had the same thing a year or two ago...

I first made sure that the carbs / fuel system were working properly.. fuel tap vacuum and needle valve were both changed.

Things were more "willing" but still not right and 1 cold down-pipe.

Tried warming up the plug in the oven and injecting a little fresh fuel directly into the combustion chamber through the plug-hole just before I put the hot plug back in.

Got a hesitating runner - which eventually cleared up.....

Suspect that the plug was a but dodgy coupled with a fuel starvation problem.

So.....if you are sure of the fuel supply (and it's not too much) then try swapping / changing plugs too.