Inspiration for next steps diagnosing poor running cylinder

Hey folks,

I've been chasing a running issue on cylinder number four for a while now and am interested to know if anyone has any suggestions for next steps. ('82 XJ650 Seca)

Quick history: the bike was off the road for around 8 years. I've recently rebuilt the top end, carbs, etc.

The issue:
I noticed that the bike has never run really smoothly, and more recently have seen that the exhaust from cylinder four doesn't get hot like the other three, so I suspect this is the culprit. I've really only had the bike "idling" in the garage, so things like reading spark plugs has been hard, since I haven't been able to ride it.

What I've tried so far:
- Swapped spark lead for #1 and #4 to see if the issue chases the lead, no change. Still #4
- Checked the spark plug, no issues
- Removed the carbs, cleaned again, repeated 3 times. Tried an entirely different carb body on #4, no change
- Checked the exhaust for blockages, seems fine
- Pretty sure the valves are OK (90%, but could be worth checking)
- Checked compression, reading around 125 with my shitty compression checker.

I'm thinking my next steps would be to check the valve clearances again, just to tick it off. Maybe try a different (higher flow) exhaust that I have (Not using this because I can't find main jets suitable for it online without buying an expensive "kit"). Any other thoughts on step to try?

Many thanks.

 

Welcome to the forum. Tight valves on four could be the issue. I would check them all, adjust as necessary and then synchronise the carburettors.

 

Did you sync/balance the carbs?

 

Blocked enrichment circuit jet in float bowl four?

 

Thanks very much to you both.

I bench synced the carbs and tried a running sync with a homemade "jar and oil" type manometer, but could probably do a better job of it. Part of the challenge there is not having a YICS blanking tool to effectively block off that passage. I've heard of people using an oily rag stuffed down the passage as an option, but seems like that might be prone to error.

I've spent quite a bit of time trying to ensure that this circuit is clean and I can definitely get carb cleaner to flow through these passages, but there is definitely a chance it's not clean enough. I'll put this on the list of steps to check.

For now, I'll do another thorough round of valve clearance checks and adjustments, tear down and clean carb number four again and see how we get on from there. Would love any advice on the YICS passage blanking if anyone has suggestions.

Cheers

 

How about the colortune/idle air mixture adjustment?

And regarding the cleaning of the passages - does that include the tiny enrichment circuit jets in the float bowls?

 

Before removing the carbs, etc. get some carb cleaner, start the machine and spray the ends of the throttle shafts, around the intake boots, everywhere on the engine side of the vacuum plungers to check for vacuum leaks.

 

Checked the #4 plug, or swapped the #4 plug with one from another cylinder? Spark plugs can work fine at atmospheric pressure but fail under compression.

 

Thanks again, all. I had a bit of time tonight, so I checked the following:

- intake and exhaust valve clearances on #4. (0.007/.178mm exhaust, 0.005/.127mm intake). Pretty sure that's in spec, but happy to be corrected if I have that wrong!
- took the exhaust off again and made absolutely sure there are no blockages. No issues.
-sprayed some throttle body cleaner around throttle shaft seals while the engine ran. No change in rpm. However, I did spray some into carb throats while running. No change on #4, but others bogged down
- I also checked the enrichment circuit on the carb body that was on the bike originally, and spraying carb cleaner into the hole under the enrichment plunger causes it to leak out of the pressed in jet in the bowl side

I'm a bit miffed at this point, but next I guess I'll try taking the carbs off for another thorough cleaning, triple check all jets, bench sync, and put a brand new spark plug in.

Any ideas on how to sync without a blanking tool? Oily rag time?

Anywho, thanks for all the ideas. Will keep plugging away at it and hopefully figure it out.

 

The overwhelming consensus on this site is the blanking tool is not needed to get an adequate sync

I guess that just means #4 is not contributing, which is why you are here. A new plug in that cylinder or swap plugs as k-moe suggested seems like a good idea. If no joy there, then perhaps get a better sync tool and make sure #4 is set correctly

 

If you are spraying into the jet with the carb cleaner red straw and it just leaking through the jet, open the jet up so that the carb cleaner shoots out.

 

To me, that would indicate a spark issue on that cylinder if there was no change with spraying starter fluid directly into the carb.

When you pull that plug on #4, is the plug wet?