Only running on 3&4. Help please

Hi friends. I used to be very active on this forum back in 2008ish timeframe. Just got another XJ and I’m back.

I’ve owned four XJ650’s and dozens and dozens of other jap bikes. I have rebuilt many carbs and got a lot of bikes running great that have set for 10+ years. While I am not an expert, I consider myself pretty good. But this one has me stumped. It is a 1980 XJ650 maxim that I owned about 6 years ago, sold in great riding condition in 2015, then bought it back recently from the guy I sold it too in ‘16. It had been sitting for 2 years when I got it. It was stored properly. But it only runs on cylinders 3&4. So after discovering that, I swapped spark plugs around from cylinder to cylinder to see if that was it, then put in new NGK plugs, rebuilt the carbs the right way. And I still have this exact same problem. I don’t think it’s the coil packs because they are not common between 1&2. I have not messed with anything else. I’ve honestly never messed with the ignition on these bikes beyond replacing coil packs. It can hold an idle running on 3&4. I know the other cylinders are not firing because the exhaust is stone cold, other side is fire hot. Bike is standing strait up and down. Not a fuel problem. I am getting fuel in 1&2. Spark plugs are wet. I did check and I have spark at all four. Must be a timing thing. But I have never messed with that. Any thoughts?

 

Hi mate,
it's got to be a fuel issue hasn't it?
Spark on all four but no fire. Could those two cylinders be flooding?

 

Thanks for the response. I don’t think I am getting too much fuel. But I will check. I’ll pull the carbs again and see if something doesn’t look right. Is there anything in the ignition system that would could the spark timing to be way off!

 

I think if the timing was 180° out there would be a lot of backfiring but on all four not just two.
There might be some broken wires between the pick up coils and the HT leads or somewhere like the TCI and the pick up coils or maybe a duff TCI.
Keep searching, it's there somewhere. Sorry can't be of more help.

 

if it runs on 3 and 4 and you swapped wires and still runs on 3 and 4 your ignition is working. it is a wasted spark ignition so when plug 1 fires so does plug 4 both cylinders are at tdc of their strokes. same thing for 2 and 3.

check shim clearances and do compression test.

plugs in 1 and 2 wet? then you are getting fuel maybe to much fuel. float valves may be leaking.

pilot jets and enrichment wells may be clogged

 

I failed to mention that I did a compression check on all four when I first started diagnosing this. That was good and even at 125 psi or more on each.

When it does run on 3 and four, it doesn’t backfire on 1 and 2. Obsoletely no igniting combustion activity on those two cylinders while 3 and 4 are keeping the engine alive

I pulled the carbs off today and ran it without them on by spraying starting fluid into each cylinder intake port. Kind of hard to do, but I got all four cylinders to fire. Enough to get pipes on 1 and 2 hot. So I think I have good hot spark at the right time, but getting too much fuel. Spark plugs are wet.

what / where is the enrichment well? There’s the single flathead screw on each carb at the top on the engine side of the carb. I what should those be set at? All were between 3 and 4 turns out

 

Perhaps time for a thorough carb cleaning: https://xjbikes.com/forums/threads/in-the-church-of-clean.14692/
So much of the running condition of these bikes comes down to all the details in the carbs. Sounds like you have good compression and spark, so this is the next place to invest.
BTW, the user "Hogfiddles" here cleans carbs on the side. He did mine and my bike runs so well!

 

Yeah it’s definitely something with the carbs. I’ll read that thread.

see attached pic. I shifted the carbs over to the left two cylinders. I put carbs 3&4 on 1&2 cylinders and it ran on 1&2 no problem. So process of elimination points to those to left carbs.

 

Wow, never seen that done! Definitely a great diagnostic technique. Could be something as simple as a blocked fuel feed to the left side, but as you know, with carbs of this age, it never hurts to clean them properly and replace *all* the seals.

 

Sometimes you have to get creative lol. Looks like I need to really go through these carbs with a fine toothed comb. That’s a great thread in that link. Long but good

 

in the bowls there is a hole in the side wall that runs to the inside of the carb. the enrichment rod the long bress thing goes into it. it picks up fuel to start bike using "choke" lever.
The choke is the enrichment circuit in the carbs it draws fuel into the carbs untill you sshut it off(lever) when bike is warmed up enough the choke is not needed and the pilot jets keep bike running.

those screws are the pilot air mixture screws 2-1/2 is base line better at 2-3/4 to start with untill you dial them in.
located top front of carbs. next to the enrichment plungers

from daves photo in https://xjbikes.com/forums/threads/owww-my-eyes.112790/

you may/should do a bench sync while you have the carbs off
that involves the screws and idle knob on back of carbs.

if when you cleaned the carbs you did not break the rack the fuel rail may be clogged there the tee and tubes go into carb bodies.

if you did not pull the needle valve bodies the little cap screens may be clogged
these are a generic photo but the caps are the same

you have shown that the issue is in the 2 carbs 1 and 2 with your test maybe try doing 3 carbs(that was cool)