XJ650 - The Cylinder from Hell

Just when you think you’re out… they pull you back in.

So, it seems my cylinder #2 misfire has returned with a vengeance. I recently thought I fixed it. Replaced the throttle shaft seals in the carbs, and I immediately saw an improvement. From cold, the idle is smooth, and it levels out right where it should when the bike gets warm. Better throttle response, easier synching. All great.

But today, after a longer ride, the engine started to get quite hot, and the poor running returned. Rough idle, even noticeable vibration when decelerating with the throttle closed. When I got home, I checked the spark plugs: 1-3-4 all looked perfect, but cylinder 2 looked black and sooty. I could tell when riding that it’s not firing every time.

At this point, I’m out of ideas. Here’s what I’ve done so far:

• Replaced throttle shaft seals
• Checked carb fuel level, was 3.5mm. A little low, but within spec
  
• Checked valve clearances. Intake and exhaust both in spec
• Replaced spark plugs
• Checked spark plug cap and secondary coil resistances. Both in spec
• Rebuilt petcock, verified fuel flow while gas cap is closed

Any ideas what else could be causing this?

 

Compression???

 

I have not checked compression, but it’s now worth a try. Would poor compression make it run badly at high engine temperature, while it still runs well when cold?

Even if not, a compression gauge is cheap. I’ll try to check it off the list.

 

If you want to be really cheap you can often get one on a tool loaner program from an auto parts store.

 

I don’t think I’ll be able to check my compression before I leave for Niagara Falls this weekend, but I did fine something that may be a problem. The enrichment plunger on #2 is further down in its port than the other three.



Pushing down on the enrichment lever does not change the plunger position for the other three, but it does for number 2. Pushing down on the lever also changes how it idles slightly. I think this means that the #2 enrichment plunger has been slightly out of its socket while the others are fine.

How could this happen? Is there any way to adjust the position of the darned thing?

 

No, further down would be closed (not enriching at all).

Unless the plunger or the seat is damaged somehow (????)

Are you sure it's just not the position of the rubber dust boot things that is causing it to look funky? Looks like your #4 dust boot is not pulled down over the nut.

 

Yes, I am sure. Pushing down on the enrichment lever allows the plunger to move further down into the port. It’s hard to get a good photo, but with the choke lever all the way in, the little pawl is positioned near the top of the #2 plunger.

I wonder if the plunger itself is damaged. It’s just metal plunger in a metal seat, so I don’t see how “fully seated” could be lower than the other carbs.

 

Does look a little like the "fork" on #2 might be contacting the top of the plunger shaft. If it's not allowing the plunger to go all the way down, you'd basically be running #2 with the choke cracked open all the time.

There are set screws that hold the fork to the shaft, but they go into detents in the shaft, so not much adjustment there. You might have to bend the fork a little if seems out of alignment with the rest of them.

An easy thing to check before you take it all apart is to make sure there is enough slack in the choke cable. Mine actually bows the cable slightly when fully close to assure it's not pulling on the shaft at all. If it is keeping tension, it might be enough to raise #2 if its alignment is a little off without affecting the rest. More slack might let them all get in the "slop" zone.

 

Another thought if you think the plunger shaft itself looks lower than the rest, is the washer missing?

Or did a PO replace the plunger with one that isn't quite the right one...

 

It seems the top scenario is what has been happening. The choke cable always has a bit of play, hence how I was able to press down on the plungers. But it seems plunger #2 was definitely not being depressed all the way. Check out what I found tonight.

Plunger #2 is a little deeper in the hole when fully depressed.


Plungers #1 and #2 are identical.


Seats #1 and #2 are identical.


The port in carb #1 is a full millimeter shallower than the port in carb #2.



This is, unfortunately, the carb body I replaced with one from Len, since the old one had an epoxied float post. I would have thought one HSC32 carb body is the same as any other, but I’ll be darned. Maybe not.

 

The dreaded carb body replacement. I feel you there. See some of my other threads on carb fun with the 900.

So many carbs look identical but have slight variations. I think there was a lot of mid production tweaking that went on back then...

The question now is what to do.

I think I'd probably start by taking the set screw out of the fork so it can't lift the plunger and ride it for a while to see if that solves it. If it runs good, the simplest fix is probably to bend the fork a little to make up that 1mm. Beyond that look for a plunger with the same dimension and longer shaft if such thing exists, or find somebody with precision machining skills to either and a custom plunger or make all the holes the same.

You might check with Len too and get his thoughts.

Good catch on spotting that. Very subtle.

 

Just for grins I went out and measured one of my spares that was already stripped and easy to get to...I think it might even be a #2. It looks like the bottom of the hole is tapered so its hard to get consistent readings, but I saw about 21.7 against the wall. Were you up against the side when measuring both?

BTW: I think mine is actually an HSC 33 so maybe not accurate comparison. I don't know for sure where they originated since they came in a box with a bunch if other parts when I bought my Seca 750.

 

Yeah, I measured it against the side since I couldn’t get consistent purchase with my caliper against the very bottom. Just wanted to make sure it’s a consistent measuring point for my comparison.

The difference is so subtle that I can’t see a difference just from looking down into them. Only once the plungers were seated could I spot the height difference with the naked eye.

 

What washer is that? The threaded brass seat for the choke plunger does not use a washer between the seat and the carb body.......

Black & sooty could either mean lean or rich....although in this situation you are describing I would think "rich".

One time, many a carb ago, we came across an engine that was running rich on one carb, and couldn't do anything to resolve the problem, and finally after full dis-assembly we realized (somehow) that the starter plunger valve machined seat in the base of the carb bore has a gouge in it that was allowing fuel to leak past it even when the plunger valve was fully seated. Luckily it was a minor flaw and we were able to carefully polish the tapered seat with some 400-grit (I think....) paper.......because of the somewhat in-accessible location, it was --- as you can image --- a pain-in-the-plunger to do.

Of course, the flaw might also be on the brass plunger valve tapered seat (instead of in the carb body itself), but only a very close inspection with a strong light will reveal any secrets that might be lurking down in there.

P.S. I don't think there were any differences in the DEPTH of the plunger valve bore (or the machined seat) or the length of the plunger shafts for the HSC carbs (or at least, none that I have ever observed or known about).

 

In the photo it looked like a washer under the seat on #1, but I later realized it's just a flange. So, ignore.

 

So, this was my thought as well when taking them apart. I expected to find something wrong with the plunger or seat, which is why I set them side by side for comparison. Also, although I didn’t get a picture, I inspected the insides of the tapered part inside the carb body with a pen light, and nothing seemed amiss. For additional comparison, I did a measurement of the old carb body. Came in about the same as the other carb that was originally on the bike.



You can see my methodology in the photo. I held the extended part of the caliper flush against the aft wall of the carb port for my measurements. Here’s a photo of the bore on this carb too (can’t get pics of the current carbs until I’m back from my vacation.



My bike is a 1981 non-YICS model. What are the odds that there is a subtle difference between the carbs for YICS/non-YICS? I see literally no benefit to that for Hitachi or Yamaha though, so I’m basically spitballing here. Seems like a nonsensical difference.

 

What you need is a left-handed plunger stretcher.

 

Those are expensive.......

Well, for the 650 carbs, the easy tell for YICS vs. non-YICS carbs is the thread pitch in the pilot mixture screw port: non-YICS are coarse threaded, and YICS carbs are fine-threaded. But they all use the same starter plunger shaft, lifter brackets, plunger seats/dust caps, and plunger valves.

 

No need to stretch three plungers when you and shorten one using a metric plunger compactor.

 

Looks like you've found the problem, maybe the carbs were made on a different machine, different prod run, who knows?
So the fix is to adjust the setting of the lifter fork so you have slack when shut, up to you how...

 

Okay, back from vacation. I have an update. I bent the lifter for plunger #2 so that it properly bottoms out. I actually had to bend it pretty far. Just getting the plunger seated wasn’t enough. Because only one of the plunger springs is bringing it back to the bottom once the choke lever is all the way in, it has a tendency to stop short. I actually had to bend it a bit further so that the other lifters moved up a little. Now, the lifter comes roughly the same distance up each plunger. It seems to lift them all evenly, and they all bottom out when the choke lever is in.

And what do you know. When I put in the color tune plug, I got no misfires. The mixture was set super lean too, since fuel was pouring in through the enrichment circuit. Adjusted the mixture, synched the carbs. And when going on a longer ride, I did not experience any loss of power at idle.

Hoping I’ve heard the last gasp of the cylinder from hell.

 

Awesome! Now go ride.

 

Perhaps Chacal has them in stock .

 

We're all sold out of them............

 

Perhaps a better choice would have been a hole de-depthifier. Got any of those?

 

They're on back-order..............things are tough everywhere.