Right cylinder blows cold at low rpms and hot when revved please help!!!

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Hotlap72d

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Right cylinder blows cold at low rams and hot when revved please help!!!

Hey guys I have a 81 xs with bone stock everything motor and ignition wise. Here's my problem when I start my bike the right side pipe blows cold/misfires at low rpms but when I rev the bike up you can feel the exhaust warm up but then right back to cold once the rev is over. I have checked the cam chain, checked valves and everything is good there but the intake valve on the left side had to be backed off a fair amount more then all the other valves but the left side is firing. I checked spark and have good spark, swapped wires, swapped plugs and it still is dead on the right side no matter what plug or wire is there. One thing I have found is that my right carb isn't really pulling vacuum, haven't put a gauge on it yet but it is def lower then the left side. When I unplug the right side vacuum plug the bike continues to run but when I unplug the left side the bike shuts off almost instantly. Also I have gone through and cleaned both carbs and checked both diaphragms. The bike does have mikes xs air pods and straight pipes so I know I'm going to have to tune/rejet the carbs but I'm just trying to get it running. I also have pulled compression tests on both cylinders and they are at 120psi. Any help is appreciated.
 
Sync the carbs. It could be as simple as the right side carb is turned down so much lower than the left that the cylinder is effectively shut off at idle.
 
turn the screw in the middle of the carbs clockwise. this will open the throttle plate a bit, maybe matching the other carb. but in any case, the right cylinder should fire if you have fuel, air, spark.
Clockwise opens right throttle plate, counterclockwise closes it. it does nothing to the left carb, that is adjusted from the idle screw.
You need a yardstick, 20 feet of tubing, and a couple pilot jets(those cheap green air valves for fishtanks work too..
And please, get unifoams. those pleated air filters suck.
 
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I guaranee if you turn that screw between the carbs...i think clock wise that cylinder will pick up. Buy a proper set of sync guages

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You need to remove the jets and report here what size of jets you have. Once you know what size you have, you will likely receive some advice as to which larger size is needed for the pods and open exhaust.

I'd say the pilot circuit on the right side is not working much at all. Once you rev the engine, the main jet starts to work. You need that pilot circuit to work from idle as well as through the full range. Has the metal cap been removed, which allows access to the pilot (mixture) screws?

When you opened up the carbs, did you find a black rubber plug covering the pilot jet, on both carbs?

You should do a "bench" sync with the carbs. A piece of paper. about 3" long and 1/2" wide, is used as a feeler gauge. Adjust the throttle stop screw out of range so that the butterflys fully close. Slide the feeler gauge (paper) under each butterfly, and adjust the screw that is between the 2 butterflys, until you feel equal drag. The bench sync will get the sync very close, so no fluid will be drawn into the engine,and you can use a manometer later as a final touch up.

Edit: Hey Hotlap, whats up with running 2 identical threads......................that's not cool.
 
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I am not sure what air jet is in the carbs but the main jet is 132.5 and yes both black plugs were in the carbs and both plugs are out of the mixture screws leaving them open to adjust I have not bench synced the carbs but what has got me is that this is the second set of carbs I have put on this bike trying to figure out this problem. Also I found tha the side that is firing all the time pulled a 140psi compression test the the side not is 120psi I don't know if that matters. Also how do I check the pilot circuit.
 
Hotlap72d said:
I am not sure what air jet is in the carbs but the main jet is 132.5 and yes both black plugs were in the carbs and both plugs are out of the mixture screws leaving them open to adjust I have not bench synced the carbs but what has got me is that this is the second set of carbs I have put on this bike trying to figure out this problem. Also I found tha the side that is firing all the time pulled a 140psi compression test the the side not is 120psi I don't know if that matters. Also how do I check the pilot circuit.
Click to expand...

On the right side carb, you should remove the pilot jet to see if its blocked. Its under the rubber plug. Its a very fine orifice, so it takes only a small amount of debris to block it. You need to note the size of both pilot jets.

How many turns are the mixture screws set at now?

With the pilot jet and the mixture screw removed, you can shoot some carb cleaner through the small passage ways, and follow that with compressed air. The pilot passage ways go through to the small holes in the carb throat near the butterflys. Use safety glasses if spraying carb cleaner.
 
This is exactly what my bike is doing, same year, same set up, same carbs, pretty much the same problem. I would be curious to see if you have figured it out yet, and what the problem and fix was.
 
Yup. Everything except for taking out the mix screws and inspecting and cleaning them. The original brass plugs were in the bike when I bought it, I drilled them out and rejetted them. The right side ran before I took the carbs off and went thru them, both sides ran, but ran rich. Now the left side runs rich, but the right side wont fire at all at idle. Im going to do the mix screws tonight, and I bench synced them using the paper method described above, but Im not sure how close I got it, or if I even did it right lol. Gonna soldier on with that.
 
Okay, there's a tube in the carb bowl that can get clogged and keep the cylinder from firing at idle. It's bitten me before giving the same symptoms.....an easy fix but can reoccur if there's debris in the tank. Not a bad idea to replace the bowl screws with allen heads for ease of removal while the carbs are in situ.... HA!, yeah, right.
This is an easy check to do and may resolve the problem.
 
Well the tank is spotless inside, no rust and I blew it out and put a new screen and petcock gasket in when I got it back from paint. Already switched over to allen heads lol. So the tube you are talking about is in the bowl itself? If you had a pic of it, that would REALLY help me as this whole thing has me thinking its the coil, no wait its a stuck valve, no wait its the carbs, no wait I HAVE NO CLUE WHAT IT REALLY IS!!!!!! Im game to try anything at this point.
 
5twins has a pic posted in the forum somewheres.......mebbe in the tech section. I'm hoping for you that if you're not aware of that pickup tube it could be your problem......
 
Well I went thru his carb guide again and I think I figured out which tube you are referring to. Im going to give that a shot. I hope and pray that is it, cause Im stumped and considering taking it to a "gasp" vintage bike mechanic........
 
Thats the one I was thinking it was. Gonna give that a few shots of cleaner and follow up with compressed air. Plus take out the mix screws and inspect the o ring, and a few squirts of cleaner and air as well. Fingers crossed, I hope thats it, cause I REALLY dont want to have to do a top end rebuild.
 

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