Cylinders 3 and 4 not firing??

[ACAL]

Hi, new to the forum here. Not sure if I’m doing this right but have a question regarding cylinders not firing.

Picked up an old 1980 xs1100 special and from what I can tell cylinder 3 and 4 are not firing. So basically the whole right side of the bike. Pulled the spark plugs and am getting spark, also feel compressions in the spark plug holes. Seems that cylinder 3 sometimes fires?? The carb has been pulled and cleaned, jets seemed to be fine although I could have missed something there. Just seems odd it’s the right side of the bike only. Any ideas?

Should say, I’m completely new to all of this, I just picked up a project bike to try to teach myself some things and have something to tinker with.

Thanks in advance

[DiverRay]

You don't have gas getting to the carbs on 3 and 4. Check for a kink in the fuel line or a blocked petcock if the octy has been bypassed. Also check the bell on the intake side of the carbs, it may be something blocking the vent for the float bowel.

[ACAL]

Diver,

thanks for the reply! I think I have fuel getting to the carb, i unhooked line going into carb and turned petcock on and fuel ran freely. So might be something in the carb, I’ll dig into that next.

[DiverRay]

If you remove the floats from carbs 3&4, and then turn on the fuel you should have gas going through both carbs. If not, or if it's really slow, check the screens in the carbs. Pull the seats and you will find a screen on the top of them. They may be plugged with junk.

[jkorn]

Not the ignition since it 3 and 4. Something in the fuel. If you pull the bolts under the carb bowls, does fuel come out? Catch it with something. A small bowl. That will tell if fuel is in there..

[cajun31]

As has been stated it is definitely a fuel delivery issue. What condition is the gas tank in? Is it rusting. Since you acquired the bike has it ever ran right? From your post I am assuming what you are experiencing is how you received it. Just thinking about it if the tank was really fouled up it probably would foul up all 4 carbs. But who knows for sure not being there. Inspect the tank. On the 80 specials there is a plastic fuel filter attached to each petcock. It is pushed into each one on either side. You probably can't see them without removing the petcocks from the tank. If not present at all then if you have debris in the tank it is getting to the carbs and could be blocking them. If it were my bike I would start with the carbs. There is a good tech tip under the maintenance area that gives you a step-by-step process with pictures to do a fairly thorough cleaning of the carbs. Check that out and perform that first. Question, if you know what type of gas has been used in the bike and how long has it sat without being ridden. The reason I ask is because ethanol in our gas today can be very destructive to carburetors. Sitting for too long become a science experiment in them. The alcohol breaks down and forms into water basically and can clog up your jets and float needles. Not too hard to clean up if it hasn't been sitting like that for too long a period of time. Spray can carb cleaner is a necessity. I always buy the Walmart brand because it is generally the cheapest and is just a effective as anything else I have ever used to get most everything cleaned up. I take the cleaning process a few steps further then the maintenance tip addresses but for you purposes it should be fine to at least isolate the problem. Keep asking questions. We are here to help, and we all enjoy helping folks to keep these old bikes on the road. By the way those fuel filters I spoke about are still available new from several different sources.

[ACAL]

I resealed the tank, it had some minor rust but went ahead and cleaned it anyways. Petcock and filters seem to be clean, next to nothing on them. Will check the carbs again and see if I missed something but I’m worried I may have low compression. If it’s worn piston rings or valves, when do you cut your losses and sell for parts? Top end rebuild seems expensive or time demanding so would it be worth it in the end in your opinion?

[DEEBS11]

I know you just picked up the bike but has it ever run correctly that you know of? The spark plug wires could be switched.

[jkorn]

Again, try the drain plugs on the carb bowls of 3 and 4 That will tell you if fuel is in each one and what it looks like with bike on center stand if it has one.

[ACAL]

So there is definitely fuel in the bowls. I did a compression test today and got 145 psi on 1,2,4 and 120 psi on 3. Will dig in to the carb this weekend to check jets again and the screens. Is a possible valve adjustment on cylinder 3 in the near future? Also, not sure if the carb is synced or not.

Bike ran very rough cold when I first got it but there was an obvious break in the vacuum line so fixed that and ran pretty decent once properly warmed up. Still not sounding 100% there though

[DiverRay]

I would check ALL the valves before going any farther. The valve clearance CLOSES as they wear, and will cause burnt valves if they are not taken care of.
Compression test should be done on a warm engine. I usually ride for at least ten minutes before doing one, as the numbers can be off due to cold oil not sealing properly.

[cajun31]

May not be accurate if someone changed the speedometer but how many miles does it show?

[ACAL]

It’s got 40,000 miles on the speedometer.

I got in the carb today and nothing jumped out at me, although I am a beginner. These were all the jets, tubes, and needles all seeming to be clear and open holes. Am I missing something else that needs to be cleaned?? There was plenty fuel in the bowls.

[bikerphil]

Make sure the passage in the bowl for the enrichment circuit is clear. There is a non removable jet in there. Spritz carb cleaner with the straw through. You can shine a bright flashlight in the lowest bowl passage and see the jet thru the top passage if it's clear.

[ACAL]

So I dug into the valves today and found something suspect. At TDC, the dots on the cam align with the arrows, but the lines on the back of the sprockets don’t line up. Could this be a problem?