weird problem

[ergojack]

I am stumped.

Had been riding my 84 FJ1100 about 10 or 12 miles, when it suddenly started skipping and sometimes backfiring through the exhaust.
I rode home & started checking it out.  I found that while holding the throttle steady at 2,000 RPM, & pulling off the plug wires,
all cylinders would cause a RPM drop & stumble except for #1, which had no change.  Switching plug wires #1 & #4 had no change. 
Problem is still on #1.  I checked & then replaced all 4 spark plugs - no change.  Spraying fuel (carb cleaner) directly into intake #1
through the vacuum port had no change.  Compression test showed 120 on #1, 120 on #2, 110 on #3 & 110 on #4.  Pressurizing
#1 cyl. with air pressure revealed no air leaks from intake or exhaust.  Pulled the valve cover & checked the cams, valve clearance &
operation of the valve train. No problems found.  I have an extra set of coils, so I put them on just for the heck of it. No change. 
What's up with this bike?

[fjfool]

crack in your manifold boot between carb and head possibly or some other air leak after the carbs
the fact your #4 spark wire made no change directs attention away from the ignition(still an assumption at this point)
your carburetors could of gone out of sync
a jet in the #1 carb is blocked by debris, i believe the bike is a gravity feed fuel and does not permit use of a fuel filter
whats the condition of the inside of the fuel tank?

[Mark Olson]

you have a fuel problem ....sounds like a plugged carb / jet .
does the header pipe get hot? careful don't burn yourself.

[Flynt]

no fuel...  that's all it can be at this point.  Or too much fuel I suppose.

Frank

[cclase]

The slowoldguy helped me out recently with a similar issue.  Couple of thoughts:
-might be easier to use a spray bottle with water to spray the header and see if you have a cold exhaust pipe
-a clogged idle circuit in my #1 had the bike down to 3 cylinders at lower rpm
-popping/backfiring has been totally eliminated by tightening carb clamps

Chris

[balky1]

Had a similar problem after removing the carbs for needles and seats replacement and it turned out to be one of the rubber manifolds leaking. It wasn't cracked, just too stiff.

Ivan

[ergojack]

Everyone seems to think fuel problem. Remember that I introduced fuel directly into the intake track, which should have gotten a response - pickup in RPM.
It did not.  Just to be completely sure, I pulled the carb bank this morning and removed #1 float bowl.
It was full of fuel
I removed and examined the main jet and pilot jet -  both clear
I removed the needle seat & checked the inlet screen - clear
I closely examined the intake boot - no cracks or issues
There are no vacuum leaks. Even if there were, riding at speed and/or high RPM would reduce or eliminate this as the problem.
Also, remember that this issue started instantly while riding at cruise speed. That would tend to rule out a fuel clogging problem.
It skips consistently from idle through full throttle riding, so that would seem to eliminate a particular jet or carb circuit.
I rebuilt all the carbs about 250 miles ago and have not had any further issues with them. My fuel tank is clean - no rust or scale.

[Pat Conlon]

Ok, so it's not fuel....

Based on your description, because this happens to only the #1 cylinder, electrically we can rule out:
1) bad ignition pick up (it would affect 2 cylinders)
2) bad coil (it would affect 2 cylinders)
3) bad plug wire/boot (swap with #4 no change)
4) bad plugs (new)

You have confirmed by a visual, the lack of spark at #1, correct?
Pull the #1 plug, reconnect the wire, rest it on the head and crank the ignition...no spark, correct?
When you swapped plug leads and plug with #4, did you get a visible spark?
Are the quality of the sparks different? (should be a nice fat blue spark)

[ergojack]

It has spark.  #1 plug wire fires #4 spark plug just fine.  #4 plug wire fires #1 spark plug just fine.
This seems to rule out ignition. 
So to sum up, I have:
Compression - 120 PSI
Spark - good, fat spark on all 4 plugs - verified when doing three separate compression tests on each cylinder
Fuel - alternate supply introduced, plus carb has been checked
Proper operation of the valve train
Clear path from the carb to the intake valves. Verified by inspection while the carbs were off (Found a piece of a rag wrapped around an intake valve stem on a car once)
What the hell else could it be?
Obstructed exhaust pipe maybe?  I've seen this on cars, so I'm going to check this as well.

[Mark Olson]

Try a couple things.
ya know ,,,, the 1100's have a double wall exhaust that can rattle apart inside ... you may be onto something.

or
Is the header pipe on cyl 1 making any heat? you gotta check soon after you start engine or heat will transfer from head anyway.
use starting fluid at carb 1 throat after engine is running. not carb cleaner. did the cyl come to life?
what is your idle speed ? has it changed since this problem started. or just misses only when out trying to ride or accelerate.
Are all the vent hoses to the carbs still in place and cleared of any debris?

swap the main diaphragm from no.1 into another carb and visa versa. you may have a minor hole in it. this can cause symptoms similar to yours.

with the carbs installed , spray starting fluid around the intake manifold boot . this will check for air leaks. causing your carbs to freak out.

If you have fuel to the cyl then the spark plug will be wet with fuel , you have not mentioned this yet.

after all that, it is possible to have an intermittent electrical snafu in your kill switch. it can be cleaned upon disassembly ....less likely because only one cyl is affected.  

check for broken springs on the carbs where the carb sync screws are.

If you ride it and take it over 6krpm does the cyl kick in and start working?

switch your fuel to prime and see if it goes away.  

The whole thing sounds weird , so keep at it and you will find it.   

[copper]

What did the spark plug look like compared to the others? Air box or air filters?send pictures of plugs

[Bones]

Where's the Leopard when you need him, his diagnosis might just crack it.

[fj1289]

Where's the Leopard when you need him, his diagnosis might just crack it.

[ergojack]

Any kind of electrical issue such as a faulty kill switch would affect all cylinders, not just #1
This bike won't idle at all, it's only running on 3 cylinders
It skips at any RPM and/or load condition. Running at 2000 RPM in my shop with no load, or
heavy throttle and 80 MPH. 
The carb slide is working fine.  I use pods and they are off now while I'm testing in my shop.
I can turn the throttle and watch the slide rise just as the other 3 do.  I'm using a testing fuel
container, so that rules out any tank problems.
I plan on bringing an I.R. sensing thermometer home from work tomorrow to check the exhaust temp,
but since the cylinder is not producing much if any power, I expect it's temp to be low anyway.

[racerrad8]

I can turn the throttle and watch the slide rise just as the other 3 do. 

Well, if the slide is moving that tells you there is compression and/or vacuum to raise the slide.

When the slide rises do you see fuel coming out from the shroud of the brass emulsion tube? You should see a mist spray.

If you see fuel then you have narrowed it down to spark.

If you can't see the fuel then you have a carb issue.

Have you made sure the spark plug boot still has the resistor inside where it goes onto the spark plug?

Randy - RPM