Cylinder 2 and 4 doesn’t work

Hello,

last weekend I started my xj750 and noticed that the exhaust of cylinder 4 didn't get hot. All the spark plugs are new and the first thing I tried was switching them. This didn’t work. After that I switched the spark plug wires of cylinder 1 and 4 but this also didn’t help. The carburators are ultrasonic cleaned so the change is small there is some dirt in it, but I removed and opened them to make sure the carburators are totally clean. When the carburators where installed again I started the motorcycle and noted that cylinder 4 and cylinder 1 didn’t work. My first thought was that it was an issue with the ignition coil. But when I started the xj750 again today, cylinder 2 and 4 didn’t work..... Is it possible that this has something to do with the ignition timing? or does anyone have other suggestions?

thanks in advance!

 

What did you use for solvent in the ultrasonic cleaner?
Did you break the carbs down before cleaning (removing all parts, including the rubber parts)?

Cylinders 1 and 4 are fired by one coil, 2 and 3 by the other, so unless the plug wires are routed incorrectly it's not likely to be a spark issue.

 

Hi k-moe,

Yes I removed all the parts before the ultrasonic cleaner, so that can't be the issue. I don't exactly know which solvent I used because the cleaner belonged to someone else, but he always cleans his carburetors in it so I think that also can't be the issue. Is it possible that there is something wrong with the CDI igniter? and is there a way how I can check that?

 

Spark plug caps?

 

Unlikely it is the TCI. You would need another unit you know is working to see if that fixed the problem.

 

That seems to be a recurrent problem nowadays (with old bikes).

 

ohm the plug caps ,trim back the plug wires enough to get a good wire contact.
enough fuel in the tank?

mixture screws set to 2-1/2 turns from soft bottom?
did you do a bench sync of the carbs before reinstalling?
fuel level of wet set?

 

I just ordered new plug caps and ignition wires. they didn't look new anymore after 30 years so they can be replaced anyway. Because of the lockdown in holland, the delivery time can be later so let's hope I will have them by Saturday.

 

When I installed the carbs I checked the float fuel level and it seems alright.
I haven't synch the carbs yet, so that's something I can look into.
regarding the fuel level in the tank, the fuel floats from carb 1 to 4 right? So in my thoughts this can't be the issue of a failure of cylinder 2?

 

just asking about fuel in tank because it happens alot.
bench sync is not same as running sync but you should do 1 or the other to eliminate it as an issue. if carbs can be removed easy i would start with bench sync.
fuel float level is a starting point you need to do a wet test and get them in spec .
Setting the fuel levels

 

It could be an issue IF (and only if) the original fuel strainers in the carbs are still in place and crud from the fuel tank is blocking the strainer for #2. The strainers are under the float needle seats.

 

just a word about "ignition wires" , i assume you mean H.T. leads ?
not sure about the 750 , but most original XJ's . coils, the wires are not easily replaced, unless you cut and splice new ones in. or dig into the coil to get at the beginning of the lead.
there's a thread about dooing it ,on here somewhere
stu

 

Or get Honda CB750 coils. The leads are easy to replace when necessary.

 

Meh.. got plenty of caps and coils and leads lying around not far from where he lives since NL isn't that big with appr. 33K2 km's in surface area.. lol
No need to buy new. He could even make the caps resistorfree by himself.
Works great on the SecaRat.