1982 XJ650J Ressurrection & Trouble

Hi,

I am in the process of fixing a 1982 XJ650 Maxim and am having some difficulty: I cannot get cylinder #3 to come to life. It had not been used by the previous owner for 10-15 years or more (last inspection sticker was 96) and had been sitting outside for at least some of that duration. Here is what I've tried so far:

After doing some basic work (fluids, battery, etc) I finally dug in and removed the carbs. They were really a mess as the fuel had long ago turned to varnish. I split them and did a thorough rebuild/cleaning, completely disassembling them, soaking the bodies in carb cleaner and then using spray to ensure that all passages were zestfully clean. I did a bench sync, set the float heights dry and then wet, and reinstalled. Viola! The bike started and ran; seemingly very smooth. To my dismay however, upon inspection the third exhaust header remained cold while all the others quickly got up to temp. No 3 did not seem to be firing at all. I tried troubleshooting this in various ways, running at higher RPM to see if it would work off the main jet, checking for spark (good), replacing plugs with new (just in case), swapping the second and third plug wires (no change on 3, second cylinder operated normally), checking compression (1: 128psi, 2: 126psi, 3: 135psi, 4: 129psi), and removing the carbs once again to re-clean and ensure I didn't miss anything. To no effect.

Fuel is getting to the cylinder as the plug is wet when removed. And as I said I do get a good spark when holding the plug against the block. So I have spark, fuel, and compression... but no ignition. I am stumped.

Does anyone have a suggestion where to go from here/what to do next to at least narrow down the problem?

 

Try a different spark plug. Just becasue it's sparking on the head does not mean that it's sparking under compression.

Have you checked the valve clearances?

 

did you remove mixture screws? after swap/replacing spark plug 3 open up the mixture screw a little more.

 

Someone has done some reading and thorough research as in "zestfully clean" - kudos to you. One helpful addition would be did the rebuild include throttle shaft seals, mixture screw o-rings, and float needle valves as these are usually the bare minimum in bringing the carbs back to full service.

How do the intake manifold boots look, any cracking? Possibly #3 has a large air leak causing an issue. Or maybe an overly rich condition - #3 typically operates the petcock vacuum diaphragm so maybe try disconnecting the vacuum hose at the carb manifold and plug it and temporarily run the petcock in prime position.

The air jets can also be inadvertently reversed, and are labeled incorrectly in the manual - the diagram below shows the correct orientation. Since the bike seems to be running OK on 3 cylinders probably not the issue but included just in case #3 was accidentally reversed.

Courtesy of RickCoMatic

 

Today I swapped the spark plugs today and found no difference. I then figured I might as well check the air jets and low and behold, they were reversed! I swapped them (not so easy with the carbs on the bike!) and started the engine to find all FOUR cylinders firing away harmoniously. What a relief! (I did notice the exhaust on 1 & 2 got hotter than 3 & 4, but hopefully that will be resolved with a proper carb sync, throttle shaft seals, etc).

To address the other questions, I did replace the intake boots, mixture o rings, float needles and seats, however I did not replace the throttle shaft seals or check the valve clearances. I know I will most likely need to. I also realized I had the vacuum line for the petcock on the #2 boot instead of #3, however since I had it plugged I don't believe it was related to my troubles anyway.

Thank you all for your suggestions and for saving me from further frustration. It feels good to be able to move on to the next steps in getting this bike on the road.