Topic: '75 CB550 #1 + #2 run great, #3 + #4, not so great...

[1timduke]

This is my first post so here it goes...   
'75 550 motor, stock 4 in 4, new rings, valve job, gaskets, etc...   Still trying to find stock airbox, currently running on creatively modified pods (a little more restrictive, more like the airbox).   Problem is, cyls #1 and #2 run well, clean exhaust, no backfires.   Cyl #3 has a weak, cold exhaust that smells of unburned fuel, when you pull the plug lead, nothing happens to RPMs (running about 1200 rpm).   Cyl #4 has same exhaust as #3, but when you pull the lead, the motor dies, what gives???   I had the motor work done about 400 miles ago, then I got a head gasket leak, pulled it again and fixed the leak, now after re-reassembly, it ain't quite right.   The symptoms of #3 and #4 don't seem to change at upper RPMs, so should I rule out slow jets?   I've checked the petcock tubes and they are clear, blown through both carb fuel lines (clear too), so I know I'm getting fuel to the carbs.   Puzzlingly enough, I don't get how if the exhaust is cold (no fire right?) that pulling the #4 plug would make the engine die    I'm fairly new to this and would appreciate any input from you guys.   And I promise to post pics as soon as I can figure out how (I still type with two index fingers, so give me a few days).   Thanks, -Tim   

[TwoTired]

The spark plugs fire in pairs.  The current travels from one spark plug to the other through the coil.  If one plug can't fire due to too large a gap (plug wire pulled) then the other can't fire either.  If you have a 4 cylinder bike running on three and you stop two others from firing, it usually dies.

If your #3 spark plug is carbon fouled, it has a circuit path (through the carbon) for current to flow.  So, #2 fires (it's coil partner), but the spark in #3 is shunted away from the spark gap and doesn't initiate the fuel burn.

Clean or replace your spark plugs (at least #3).

Then you get to figure out why #3 is running so rich that it fouls spark plugs. One possibility is that the emulsion tube for the carbs were not cleaned, another is the float valve is not shutting off properly or at the wrong level of fuel in the carb bowl.

So, take a look at your spark plug deposits.  It's your window into the combustion conditions, (or lack thereof).

Cheers,

[NGL_BrSH]

I guess my first steps would be, open carb bowl drains to verify fuel is in those.. then i'd check the tappet clearances, then pull the plug and see if they are wet with fuel..

Check spark by placing plug on outside of engine and watching for the spark.

Then if i have spark and fuel, make sure the choke isn't shut on those carbs, maybe check compression with a tester?

If you have all that it should fire.

[Hush]

Yeah, what he said.

[1timduke]

Thanks for the responses guys .   I pulled all 4 plugs last night just after shutting it off and they all were dry and medium tan, except for #1, which was a hair lighter than the rest (slightly lean, I assume).   Tappet clearances and spark test I'll check and post back in a few hours.    -Tim   

[1timduke]

I got a mercury sync gauge from a friend last week and was trying it out... and should have remembered to put the little screws back in on # 3 and # 4    I was really trying to pin all these problems down to one cause, turns out that it was my own oversight.   And it does still seem that #3 is not pulling at idle, and I'll bet that it's a plugged jet.   Thanks for all the help guys!   And to let everybody know, be very careful what you're holding on to when you conduct the 'live spark test', it seems the current travels quite well through sweaty hands!!!   Thanks again