Theres a fella I work with with an 84 GS550 thats only firing on two cylinders. From what he told me all 4 plugs are getting spark. He also said the carbs were just replaced and this problem was happening before he replaced them. He doesnt know much about bikes and I dont really know what it could be either. Any ideas? Thanks in advance!

Compression test wouldn't be a bad place to start....

+1

+2

Question: If all four plugs are firing what gives him the indication that only two cylinders are actually combusting?
After doing a compression check and it's good I'd prime all four cylinders and try starting with no throttle added. If it fires up for a split second then your going to have the fun task of finding which two carbs are not supplying fuel to the combustion chamber?

Yea OK which 2 are running ... if its 1 & 4 or 2&3, its spark related. Yea I know it has spark ... but maybe its late ... early, like a whole stroke early.
Where is the vacuum line hook to ?
Does it have cross plenum synch (yamaha did in the 80's).

Oh I forgot, maybe its a GS500 not 550 ... most of our bikes fire only on 2 cyl most of the time y'know. So you dont see us complaining.
I even have a 1000 that fires only on 2 cyl ...
Cool.
Buddha.

If I'm not mistaken the GS 550 has 2 ignition coils, so one bad coil (or connection to or from it) will make 2 cylinders not run.

A compression test is an excellent idea.  How many miles are on it?  If the mileage is under about 40,000 the compression is probably fine unless someone never changed the oil and/or beat the piss out of it.  Even then the compression on all cylinders would likely drop in that scenario so it wouldn't explain the problem.  I'm leaning toward an electrical problem with the info we have here.  Does the bike look like it was cared for?  I would say an old bike that looks like it was cared for without excessive mileage would have an electrical problem as electrical problems become more common as a bike and all its connectors and components age.

im just putting this out there but if plugs arent gapped right the spark could actually be blowing out instead of igniting the fuel.

if adjacent cylinders are not firing it could be a headgasket leak. (again, compression,so +4 i think?)

alwaysgo back to basics when the bike wont do what you want it to.
air, fuel, compression, and spark. all these must work in order or your pushing it home.

and for those who didnt know how to figure out which cylinder isnt firing:
take a cold or cool bike, feel the exhaust pipes (wisely of course)
now start the bike
feel the exhaust pipes near the head (sometimes with a rag as a condom, or with your gloves). on twins you can check both cyls at a time. the cylinders that are firing will heat up QUICKLY, the cylinders that arent firing will not. even if you dont want to touch the pipes you can just get your hand close, and that will be a good indication.

No, Josh.  Plugs can't "blow out" as you describe.  It's a spark, not a flame, so it's immune to the compression and swirling gasses.  But yes, plugs are the first place to look when you have ignition problems.  Sometimes a cylinder can stop running and you pull the plug and find a piece of carbon bridging the gap, preventing the plug from sparking.  I've seen that in lawnmowers and minibikes.

Alot of insight so far guys thanks! I may or may not go to check this bike out. I really dont want to have to get into another project but if I check it out and its something obvious and simple then Ill serious consider it. I found an 89 CBR600F streetfighter in PA for a real good price so I'm trying to pursue that. I'm really trying to get a bike for my girl so if I get this CBR, sh'ell be riding mine.

Am I missing something here?

He said in the beginning that all spark plugs get a spark when turned over, so that doesn't eliminate electrical but it does suggest a carburetion issue first.

If the spark plugs are getting voltage but they are out of timing and such it would back fire like a Harley Pig on Thanksgiving. Now he did not describe that as an issue so that leans toward 2 carbs on the same feed line being blocked or having the same sort, or different clog not allowing gas into the combustion chamber does it not?