Cylinder not firing, please help

Far right cylinder seems to not be firing. I check by touching the exhaust headers, and the far right header isn't getting near as hot. It seems warm after a while, but I can still hold my hand on it. All plugs are brand new, and I do see spark when I test it. I opened the respective carb's drain plug and there is fuel in it. Could this problem be a valve stuck or something? The bike runs but doesn't have full power.

Also I've had instances where the bike seems to backfire and it feels like I get full power for a moment, and when I replaced the plugs all the cylinders were firing for a little while and a ton of black smoke was coming out of the exhaust. Now its back to the same old thing.

Any ideas would be helpful.

 

Idea #1: Inspect plug cap for too much resistance.
Idea #2: Swap complete coil units to see if symptom moves with coil.

 

When is the last time that those Carbs were off and gone-through, completely.

You might have a few things going-on that you need to correct.

Fouled Air Holes in the Emulsion Tubes causing a low volume of AIR to be drawn into the Fuel Supplied by the Main Jets.
The Fuel Stream isn't atomized right and it acts like its flooding-out.

Sticking or even a stuck Diaphragm Assembly.
They don't rise fast enough or get stuck and you have no power as a result.
Mal-adjusted Pilot Mixture Screws.
They are the FIRST thing somebody will tinker with when the bike isn't running in top shape.

Think you can Clean the Carbs?
Or is that something you'll have to Farm Out?

 

Just recently rebuilt carbs 1 month ago. I like the idea of the fouled air holes, I'll check it out. What do you mean by flooding out?

 

When the Air Holes in the Emulsion Tubes are clogged ... the Fuel Stream being drawn-up from the Main Jet is not broken-up by the Main Air Supply.

The Fuel getting drawn into the engine isn't atomized.
Rather than enter the Combustion Chamber like a spray ... it goes in like a wet stream, instead.

The resultin factor is an incomplete burn that causes a loss in power and a great deal of unburned gas in the exhaust. The bike might smoke some or stall.

The plugs are usually pretty black with a dry carbon coating.
Further complicating matters is that the carbon gets on the valves and the crown of the piston and can lead to burned valves or overheating because when the mixture is corrected ... the carbon will heat-up and eventually burn-off ...

Unless its on there too thick.

Then, it bakes into the hardest substance you ever seen, short of man-made diamond, and needs to be wire brushed off when you pull the head and do the valve job.

 

Very similar to what I had:

http://xjbikes.com/Forums/viewtopic/p=56254.html#56254

Bad coil/plug

 

Be very very careful touching the header pipes they are hot enough to cause a serious burn.

If you are away from "home" spit on the header if at home use a few drops of water.

If the water goes "sssss...." and boils away (there you are over 100 degrees centigrade) the cylinder is firing, if not then you know which cylinder has the problem.

 

I presently have exactly the same symptoms a Sweetirony 8O

#4 header is cool & I have a spark.

I recently rebuilt the carbs & thought I had been meticulous!

Switched around #3 & 4 plugs: still cool
Switched around the coils to get a different coil on #4: still cool
Checked cylinder compressions: #1, 110, #2 105, #3 110, #4 105. They may be a tad low but the're all the same (semi warm engine, throttle wide open & no plugs in).

I will try my Colourtune to see if I have any combustion at all on #4.

Then I will open up #4 carb & investigate.

 

Has anyone ever tried the "$1.98 Carb Tune-Up" on these bikes? Never heard of it? I used to do this to old carburated cars when they'd act like there was something stuck in the carb. What you do is rev the engine real high--like 4K rpm--then, with the air cleaner and it's stud removed, place your flat palm over the throat of the carb. This would stifle the engine of course, but the amount of suction would usual pull through any crap that was clogging the carb. I know these carbs are built completely different than an automobile, but I'm wondering if it would help in this situation to pop the boot off the #4 carb and give it a try.

Thoughts? Anyone up for an experiement? Reasons NOT to do it? :?:

 

Don't know that I'd want a finger to be sucked in there. There must be other ways to get a "feel" for the stage 3 ignition stroke.

 

My father has the same thing on his 86 x. I told him it is the carb on that end. Pilot jet maybe because above 3k rpms, it's golden

 

Whe would need to be 'Pulled' woild have to fit through the eye of a needle or smaller. You'd have to protect the rubber diaphragm from being pulled out of joint, shape or being torn.

That "Buck-ninety-eight" might blossom into a couple of hundred dollars of the Diaphragm split because it couldn't handle all the vacuum.

 

And that's why I asked for reasons not to do it. :wink:

 

Get yourself a Rubber Ear Syringe at the Drug Store.
Put the Spray Tube for the Carb Cleaner inn the end and fill the Rubber Syringe a little bit.
Pull the Pilot Mixture Screw and Insert the long, rubber end of the Syringe in thee and FLUSH (Back-flush) the Pilot Air and Fuel Passages.

(This works better Flushing from the Pilot Jet Hole ... anyway ... try it!)

Work the Syringe and "Toilet-plunger style" Flush the passage ... before putting-back-in the Pilot Screw and it associated parts.

There's NO guarantee it's going to work unless the Pilot Jet is removed ... but it's worth a shot.
Nothing to lose.
Everything to gain if clears-out crap keeping the passages clogged.

 

Thanks for the suggestion Rick, I'll be beavering away 'till I solve it.

Took the carb to head adapter off so I could see down the inlet tract & broke the lower Allen bolt.

Had to drill it out, retap & use a new ss bolt. Put a little gasket maker on the face of the adapter to ensure a good seal as the gasket looked somewhat the worse for wear.

I have all winter to puzzle it out. Have a Burgman 400 for pleasure rides. The '83 XJ750 is just to keep me occupied through the winter

 

Mine kept me occupied for two Winters. I got it when a guy had me haul it off his property after spending a few years outside leaning against a Pine tree.
Sap, bug nests, bugs, mold, mildew, flaking paint and whatever the gold stuff is that plated on the aluminum.

I took the Midnight off a Midnight Maxim ... stripped all bronze stuff and de-blacked the cases and jugs.

Two years later ... she's my number one ride ... rendering the 900 I have to fun on weekends and special outings.

 

Not to pour salt into a wound, and just to increase my understanding:

That allen bolt is the same one Rick says to leave alone when removing carbs, right?

 

I think he's talking about the ones that hold the rubber carb-mount boots to the head. Yeah, they're buggers.

 

The ones that hold the rubber boots to the head. 2 per boot.

 

Well, now I'm confused (again...). There is a allen manifold bolt that is better left alone when removing carbs. I'm not at all sure which one that is.

 

The Allen Headed Cap Screws that connect the Intake Manifolds to the Head.

When they are stuck ... they BREAK rather than come out. The Allen Bolts are made of soft steel and will just break-off instead of break-free.

Mucho headache and heartache when that happens because there isn't room to get a drill in there to do the drilling in situ. You sometimes are forced to remove the Cylinder Head.

I HATE when that happens.

 

Luckily mine was the lower one on #4 cyl. I still had to remove the battery & battery box & starter motor in order to be able to manoeuvre the air box out of the way enough to be able to drill.

I went out & bought a 90° electric drill this morning to give me better drill access for future drilling

I never did find a way to completely remove the air filter box :evil:

BTW: when I removed the carbs after I found #4 header cold, I drained the gas out of each. #4 had NO gas in it at all. I guess the float needle had stuck closed depite my careful cleaning!!

I did remove the idle mixture screw & passed carb cleaner through orifice. It was clear.

Carbs go back on later on today & I'm hoping for a hot #4 header

 

"Success" I have a hot #4 header

Redid all the tiny air & gas passages in #4 carb.

All 4 cylinders firing just fine now

Thanks all

 

That was some work did, there, Goatman --- nice going!