Only firing on 2 cylinders and backfiring on decel

Hi guys,

I've got a 1981 xj650, not the midnight, just the regular maxim. Had the carbs cleaned and synced last year, as well as the valves adjusted. It's ran beautifully ever since until last week when I loaded it up in my truck. I didn't have a ramp so I used the wooden top from my picnic table which is only about 5-6 feet long. Needless to say the angle was pretty steep. So steep in fact that I noticed the oil light came on when I was slowly driving it up to the bed of the truck. I'd say the bike was in this position for about 30 seconds or so as I was going very slow. Didn't notice anything strange during the loading process.

The next day when I unloaded and started it up, I immediately noticed it was running sluggish. By sluggish I mean that when I started off from first gear, the engine seemed to hesitate for about a second after I engaged the throttle. It was also noticeable when shifting to second. Once I was in the higher gears the hesitation wasn't noticeable. I also noticed it was having a difficult time idling. Almost seems like it wanted to die if you didn't give it some gas. That's when I noticed the tachometer didn't seem to be giving an accurate reading. When I was reving it to what sounded like 4k, the tach was only showing about 2k. Another symptom I noticed was that after I left off the throttle the engine would hang for a few seconds rather than returning to idle speed. And the backfiring. Lots of backfiring and popping from the carbs, particularly during deceleration. So, after I started it up yesterday and let it warm up, I sprayed some water on the headers and guess what, only 2 of the 4 cylinders were firing (or at least not firing as much as the water dripped off those pipes and boiled right off the other 2. Not sure which # pipes they are, but if you are standing in front of the bike it's the pipe all the way to the left and the 2nd from the right.

Here's what I've done:
1. Sprayed wd40 around all the carb boots and vacuum lines - no leaks that I can find.

2. Took the carbs off yesterday and cleaned out all the bowls, floats, pins and jets. I did not mess with the diaphragms.

3. Checked and cleaned all the plugs. They were a little rich looking but nothing horrific. It did seem to run about 10% better after I cleaned them.

I've now reassembled and the problem still exists. It's always worth telling you that last year I replaced the gaskets between the headers and the engine, so I'm doubting it's an exhaust leak from there.

The bike does crank up pretty much right away and seems to still operate and drive (not that i am). It's just that it's hesitaring, not idling right, funny readings from the tach and most alarming, only appears to be firing on 2 cylinders.

Under normal circumstances I would assume it's just a carb problem and is possibly out of sync. But since the problem started right after loading it on a very steep incline, I'm worried I really screwed something serious up.

Does anyone have any ideas on what could be the underlying cause for all of these symptoms? I'm no mechanic so I don't have a manometer or a way to twll compression readings. Any help, suggestion or advice would be much appreciated.

Thanks guys!

 

Have you check the oil level via the sight glass?

 

Yes, it looks normal. No discoloration.

 

Check for spark from both coils.

 

Yep, I'll do that. I'll keep you posted on the results. Thanks k-moe!

 

Holy crap man. I just doubled checked my oil through the window and couldn't see any. I just changed the oil like 2 weeks ago. My head gasket leaks just the slightest bit, there's no way it all came out there. Where could it have gone? There's no big oil puddles on the floor either??

Could it had flowed into the carbs or something when I went up that super steep incline?

I haven't put more oil in yet, wanted to hear your thoughts first...

 

The oil didn't go anywhere, but the gas did. Open the oil cap. Smells like gas dosen't it?

At some point the floats bounced around and let fuel pass by the float needles, and into the sump. It could have been an unlucky set of circumstances, or you may need new float needles and seats.

Drain the oil and gas from the sump, but don't refill.
Turn the petcock to prime and see if any gas drops out the oil drain.
If it does, tap on the float bowls with a screwdriver handle to see if it's just a stuck float.
If that dosen't work then you'll have to pull the carbs.

 

Thanks a ton for the help man.

Ok, so I took off the oil cap and it did have a gas smell. And I know where the overflow hose is located on the clutch-lever side of the bike. However, I'm not sure what you mean by draining the sump? Are you say that I need to remove the oil plug and drain all the oil, and suspected gas? Or is the sump something else? If so where is it located and what does it look like?

Keep in mind, I also just cleaned and reinstalled the carbs yesterday and only drove it around the block since. I'm not saying it's not the floats and seats, in fact it's very possible. But wouldn't have run fine for at least a minute right after I reinstalled? I know the float wasn't stuck down so it had to get at least 1 bowl of gas? I'm not doubting you at all btw, just trying to fully understand what's going on here. I'm just really nervous that it's only hitting on 2 cylinders. Praying I didn't do any real damage - even though it still fires right up! Lol

So talk to me some more about this sump thing that needs to be drained, please. Thanks again for everything bud.

 

I am saying that it is the floats and the seats. Your crankcase is full of gas and oil, which is why it looks empty but smells of gas. The only way for fuel to get to the crankcase is by overflowing through the carbs because the float needles aren't sealing.

It's only hitting on two because the other two are/were flooded.
The sump is the part of the crankcase where the oil resides when the bike is sitting. Drain the oil as you would for an oil change (including the filter), but prepare to drain at least 4 quarts.

Don't run the bike again until this is fixed or you could bend a rod, damage the bearings, or even make a big boom happen in the crankcase.


Also, save youself a lot of trouble the next tiem yuo have to load the bike in a truck, and park the truck in a low spot. I used to park partway in the street and use the lower height of the driveway skirt to my advantage (of course I eventually got smart and bought a trailer).

 

That makes tons of sense. I will drain the oil and gas right away. I'll also order some new floats, float pins and seats. After replacing all of that, and the oil and filter of coarse, should that theoretically fix the problem? Or will I need to replace the plugs or anything?

 

I wouldn't order parts until you know that you need them. Rap on the float bowls first like I said before. Now if the float needles and seats are still the originals, then it would be prudent to change those and reset the float levels. You will not need new floats or float pins. You will not need to replace the spark plugs, but you should pull them and let them air out.

 

So I went by the local napa auto parts and of course they didn't have the filter so I ordered one and it should be in tomorrow. Regardless, I did take off the filter and drain the oil (see pic below). The oil shown in the pic has no more than 25 miles on it as it was just changed and it looks black!

I also left the drain plug out and didn't see any gas coming through, so I'm not sure if the floats were stuck or not. Remember, I just pulled the carbs 2 days ago to clean up the floats etc so maybe they are functioning properly now. Fingers crossed anyway! I'll pull the spark plugs next and let them air out before I stalling the new filter and oil tomorrow. I'll also test to make sure all plugs are giving a good spark.

Assuming all goes well, I should be able to put everything back together and fire her up? Should I do anything else before trying to start her up?

Again, thanks for all your help and advice k-moe!

 

All that black is the sludge that was freed up by the gasoline. Your oil will not get that dark so soon under normal circumstances.
I think the floats were probably bounced around and one hung up at some point during loading, or transport.
When you checked did you have the petcock set to Prime?

 

Thanks for the info. Funny thing, my petcock isn't functioning properly so it's always stuck in prime. Probably the root cause of the problem. I'll get some hemostats to crimp the gas line when not in use from here on out. I get the new filter today so I'll keep you posted on any progress/results.

 

The petcock is easy to rebuild.

Rebuilding the Yamaha Vacuum Fuel Valve w/pics

 

UPDATE

-changed oil filter
-Put new oil in
-Tested to make sure I was getting a spark on the cylinders that weren't hitting - I did get sparks
-tapped float bowls with screw driver
-opened fuel drain screw on suspected carbs - gas did come out

After all of that it still didn't fix the problem. However, I have noticed this sort of chugging sound when I give it gas, mostly when I throttle it from an idle. So...I took the pod filter off the suspected carb and sure enough, the plug looking thing with the metal rod that's connected to the other end of the diaphram diaphragm seems to be vibrating to the same rythem as the chugging (or hesitation) sound I described early. It reminds me of the quivering lower lip of a child trying to hold back from crying. Lol. It also backfired through the opening where the air filter was a few times. It was more of a mist than a backfire, wasnt very loud. The plug looking thing I mentioned is what the guy in the picture's thumb is on.

Could it be a tear in the rubber diaphragm itself? If so, how likely is it that it's the problem?

Could anything else prevent that plug/sharp rod from lifting up when I throttle it?

The other question I have is how did both of these stop working or tear immediately after I drove it up that steep ramp? Could it have been a pressure thing?

 

Fuel will always come out of the bowl drain screws, which is why I reccomened that you test the floats with the oil drain plug out.

Those tears in the slide diaphrams were there, or almost there, before you ever got the bike. You'll need new ones; I've had no luck in repairing tears, just pinholes. Any holes in the diaphrams will keep the slides from moving correctly.

http://jbmindustries.com/HITACHI.html

Do a full carb rebuild so you don't chase your tail forever.

IN THE CHURCH OF CLEAN

Chacal has all the parts in stock. Start a conversation with him.

 

The picture of the torn diaphram wasn't a picture of mine. That's just a pic I ripped off from Google to show you the past I was talking about since I'd didn't know the official name. Apparently that's called the slide. I guess my question is, based on the symptoms I shared, does it sound likely that my diaphragms might be torn or have a hole in them? I haven't inspected them yet. Really didn't want to pull the carbs off again if you didn't think a tear or pinhole in the diaphram would cause those symptoms.

 

Pinholes or a torn diaphragm would keep the slide from rising, but so could a sticking slide. Look up "Clunk Test" should be part of the information overload hour, or the Church of clean.

Your not going to really know what you have until you take the carbs off and completely disassemble them. You never know what somebody else could have done to them good or bad. You can try trouble shooting specific symptoms or just pull them off once and do a total rebuild and be done with them. Most likely they have never been fully rebuilt and there could be several small things causing a big problem.

And once they come back from church make sure you protect them with an inline fuel filter.

 

Kind of odd that only two cyls are firing AND those happen to be " one side of one coil and the other sidevofcthe other coil".....

So MY question is----did some wires get switched that you haven't mentioned? Double-check, please, that the left coil leads ( as you're sitting on the bike) go to plugs 1 & 4, the right coil leads go to 2 & 3

Dave F

 

Dave, thanks for the reply. Yes I agree, it sounds like a coil went bad.. but as you eluded too, it would have to be both coils. However, I've since tested And am getting Sparks from all plug wires so it can't be the coils. I also took the Caps off last night to inspect the diaphragms for pin holes or tears - didn't find a Single problem in the rubber diaphragms seem to be in perfectly good condition as they were flexible and no dry rot whatsoever. I can also confirm that none of the wires have been switched since the bike was running properly. Furthermore I conducted the clunk test and the slides seem to be working properly when I manually push them up. However, when the bike is running in the Pod filters are off I can see both slides that correspond to the cylinders that are firing are really having a tough time lifting and seem to be stuttering which is causing the hesitation upon acceleration.

I'm starting to get concerned that this is not a car problem and might be something more serious. Again, the bike ran perfectly fine for over a year and literally just started acting up the day I loaded it in my truck from a very steep ramp, call it a 40 degree angle. I continued 2 inch the bike up the ramp for probably 30 seconds maybe a little bit more with the oil light on.Could I have possibly locked up those two cylinders in that short amount of time? I'm sure the bike was under quite a bit of stress as we were on such a steep incline. How would I know if I damaged the engine or those 2 cylinders? Is there an easy way to tell if those two cylinders are locked up?

I really thought It was going to be the diaphragms but was shocked to see that they were all in great condition. Any additional information you can provide or advice including different things I can test to weed out as the problem would be great. Just to recap the slides on cylinder 2 & 4 are hesitating to be pulled upwards when I throttle. I can see and hear the hesitation and if I manually push them up with my finger tip the revs hang for a bit then slowly comes back down.

Thanks in advance, and especially to k-moe for all his help and support.

 

I'm not concerned about the motion of the slides.

I AM wondering if you re-synced the carbs after having removed them and gone inside.

 

I thought about that too and it is certainly a possibility. However, after putting everything back together The bike is still showing the exact same problems as it did before I ever took carbs apart.

If they are in fact out of sync is there an easy way to diagnose? I do not have a manometer Nor have I ever used one. If I was able to get my hands on one would I be able to sync temporarily using one or will that only show me that they are out of sync? I know a bench sync is the best way but will a manometer at least get it up and running temporarily So I don't have to spend a bunch of time and money on a bench sync that might not even be the problem?

 

You're not understanding the sync process then--

Bench sync only gets you to a point that will let the bike start and run enough to warm up so that you can do a running sync.

Running sync is with a gauge so you can see how to adjust the sync screws.

I suspect that part, of not all, the issue is that you are way out of sync. I also wonder if you sucked in a bit of water or other stuff when you were at such an incline. The incline alone is not the cause. It may be what finally brought out whatever the cause is----we just have to find it.

Also, have you looked to see if you have another wiring connections that may have been loose and are now looser because of being jostled around.

Finally, if the floats, and needles were taken out, did you re-confirm float heights when you put stuff back in?

 

another simple thought--- have you tried a NEW set of plugs? Bayou could also just swap the plugs around and see if the problem follows them or not-----

 

+1 on all counts.