Bad Running, Poor Acceleration (Carb/fuel problem maybe?)

[David Allaband]

Hi All! Thanks for reading and any help!

I just bought my first FJ after years of hacking apart 1970s BMWs. Really like this bike, excited to ride, but there's a problem.

The Problem:
Sounds to me like it's hittin' on only two pistons but lend me your advice. The bike has a hard time idling. When I advance the throttle response is sluggish, then it catches up after a second or two chugging like a train. Once it gets up to speed it seems to run more smoothly. On deceleration it backfires and chokes. Needs a delicate wrist to keep it from stalling at idle or during acceleration.

What I've Done:
I cleaned the carbs. They were a little bit cruddy but not gummy. For the life of me I couldn't get the needle valve seat out of the body. Apparently there's an inlet screen on the hidden end of this? I wonder if that might be cruddy and restricting fuel flow? So I'm thinking I might need to completely rebuild the carbs now because if I have to force that needle valve seat out I'm sure I'll destroy it. All four are stuck like cement. Maybe I should change the fuel filter too? How about the fuel pump? Old BMWs don't have fuel pumps so I know nothing about them.

Any help would be great! I live in Brooklyn so I'm pushing the bike back and fourth across the street for street cleaning! I want to ride!

Thanks!

[andyb]

Strikes me that you need to know if it's not lighting all the cylinders rather than guessing, ya?  Feel the head pipes (or spit on them, wet rag, etc) and find out if everything's lighting.

The float needle seats can feel like they're welded in sometimes.  If you stick the butt end of a drill into them (and it has to be a near-perfect fit), then you can grab on with pliers and not worry about distorting the soft brass much.

What year FJ?  Not all have pumps.

[David Allaband]

Sorry, good point (two of them).

The bike is an 1993 FJ 1200.

I think it's a carb problem because the PO let the bike sit for a year and a half. It ran fine before that. So I figured carbonation is probably the issue because that's the one that goes bad without use over time.

Good idea on feeling the headers. I want to do the check where you pull off the spark plug leads one at a time and see if it still runs the same. I heard that's a problem on newer engines because you can blow a diode. Can I do the thing where I pull the wire, stick a spark plug in it, ground it, and then see if the bike runs the same?

Thanks for your help!

Dave.

[andyb]

Did you know the guy who owned it prior, or taking his word about how well it ran from a year ago?  Just sayin.

Assuming you have a decent path to ground and use a spare plug, don't see why you'd have a problem checking spark on one at a time and listening to things change.  At least, haven't heard of anyone blowing things up via doing it.

[David Allaband]

Yea, I know the PO. He's to be trusted.

Checked spark, I get nice thick spark on every line, but that's with a new plug. I can't pull the old plugs to check them because I don't have a socket of the proper size yet. I'll get that soon.

I checked heat on the exhaust manifolds like you said (very useful for a 4 cylinder engine!). Very hot on #1 and #3. Not so hot on #2 and #4. So I'm pulling on two cylinders. Guess that makes it a 600cc bike!

Engine still chugging up to speed, then holding steady for a moment. If I continue to hold the throttle at about 4K rpms the engine will start to sputter down and level out at 1.5 rpms without my permission. Backfires softly on its way.

I pulled the rack of carbs again. I'm going to try that drill bit trick you told me about. If it fails, I have to replace to needle seats anyway. I've looked around and there are MANY places to get rebuild kits for these carbs. I've seen gasket sets for $20 that include the devious needle seat, and I've seen complete kits for $35 that include jets and everything. None of them include the rubber vacuum diaphragm (I have an "almost" crack on one). Any recommendations on parts sources or specifically on carb kit places? Four of these things will run pretty pricey!

Thanks a lot for all your help Andy!

[andyb]

Well, get a socket   That'll help you discover if there's an obvious problem rather than a carb problem, and give you some history of that cylinder, but you knew that.

Sounds to me like there's something wrong on the pilot circuits.  Once you wind it up past 3k or so and are comfortably over half throttle, the needles start coming in and it should run relatively well, yes?  If that's the case you can dismiss the idea of a fuel starvation problem within the bowls--if they were running low/dry it'd more likely show up under hard use after a few moments as they drained because of not having the flow to keep up.  If they were dead dry (total occlusion somewhere) you'd have caught it already, most likely, and you'd have two very cold cylinders at all times.

Also do consider the possibility of air leaks causing a nasty lean condition (functionally the same as not enough fuel getting there is having waaaaay too much air, yes?).  Inspect the rubber boots between the head and carbs for cracks.  Please note that you may not be able to see the cracks necessarily unless the carbs are installed and forcing them into shape.  If the carbs are currently off, two screws will suffice to pull each of them in turn.  There's a large oring between each of them and the head, and those could be shot as well.

If a needle seat is anything but exactly round, the float needles can and will get hung up in them, and not move properly to let fuel past.  Make sure if you're cheating with a drill (or the butt end of a ream or whatever you use), know that it absolutely has to fit tightly.  In a pinch you could try to shim the bit to get exactly the size you need, but that's probably overly complicated.

The other possibility is that those two cylinders have far too much fuel, but you won't know until you check the plugs, really.

Float height can play a bit of a role as well, if you've got two really different than the other two.  Or a float that doesn't (float).  Etc.

You'll get there, just takes time and patience.  And probably pulling the carbs a whole buncha times.  You'll want to check synch at some point as well, though I'd think it'd have to be comically far out to give you this kind of issue.  My money is still on a plugged pilot or idle mixture circuit from what you're describing, but I've been known to be wrong.  Just ask my wife.

[David Allaband]

Thanks again for your advice Andy.

I forgot to mention something, I always have to have the choke on for the bike to run at all.

Actually the fuel starvation symptoms you mentioned do happen. "Engine chugs up to speed, then holds steady for a moment. If I continue to hold the throttle at about 4K rpms the engine will start to sputter down and level out at 1.5 rpms without my permission. Backfires softly on its way."

Lots of news, none of it good.

I finally pulled out the needle seat and found none of them had a screen at all! Guess there's no restriction problem there! Is that screen important? I'm assuming it's just a precaution, meaning I should have screen but they're not the cause of my current problem. The needle seats say "1.5" on them. Is that correct?

I Pulled the plugs.

#1 is mostly amber with some very light carbon fouling.
#2 is carbon fouled and soaking wet with gas!
#3 is carbon fouled.
#4 is amber brown, the healthiest of the bunch.

Pistons #2 and #4 are the cold ones.

So carbon fouling means the mixture is too rich, right? So there's plenty of fuel but not enough air? The air filter seems okay but it's hard to tell, it's a K&N so it's always dark, clean or dirty. I think I'll put the bike back together minus the air filter box (hardest part to put on anyway) and see if it runs right. If my whole problem was a restricted air filter I'll be pretty upset with myself.

Thanks again for all your help! Any ideas on this new information would be greatly appreciated!

Dave.

[David Allaband]

Oh yea,

What happens to the plugs if the fuel mixture is too lean?

I checked all four floats, they float very well.

[David Allaband]

No Airbox Results:

Didn't help. Bike runs the same without airbox on.

Update on holding the engine at 4K rpm. So I can get the bike up to 4k rpm with the choke off. I have to tickle it, baby it. Once it gets up to 4k it runs fast, then dives. I can keep it running with throttle fidgeting, but it keep trying to dive.

I took all the plugs out and checked their spark again. Same as a new plug I was using for reference. But THIS TIME there's less carbon on them! Could that be from the airbox removal?

Exhaust manifolds 2 and 4 are still the cool ones. Could this have something to do with piston 2 and 4 sharing the same Coil?

Going slightly crazy here! What the heck is going on?!?!

[racerrad8]

...Could this have something to do with piston 2 and 4 sharing the same Coil?

They don't share the same coil...

1&4 - 2&3

Randy - RPM

[RichBaker]

Thanks again for your advice Andy.

I forgot to mention something, I always have to have the choke on for the bike to run at all.  1st clue that the pilot jets are clogged!!

Actually the fuel starvation symptoms you mentioned do happen. "Engine chugs up to speed, then holds steady for a moment. If I continue to hold the throttle at about 4K rpms the engine will start to sputter down and level out at 1.5 rpms without my permission. Backfires softly on its way."  Another clue that the pilots are clogged....

Lots of news, none of it good.

I finally pulled out the needle seat and found none of them had a screen at all! Guess there's no restriction problem there! Is that screen important? I'm assuming it's just a precaution, meaning I should have screen but they're not the cause of my current problem. The needle seats say "1.5" on them. Is that correct?     Correct for a gravity-feed model.

I Pulled the plugs.

#1 is mostly amber with some very light carbon fouling.
#2 is carbon fouled and soaking wet with gas!
#3 is carbon fouled.
#4 is amber brown, the healthiest of the bunch.   Could be due to the enrichener (choke) being on, or a sticking float.

Pistons #2 and #4 are the cold ones.

So carbon fouling means the mixture is too rich, right? Yep....   So there's plenty of fuel but not enough air? The air filter seems okay but it's hard to tell, it's a K&N so it's always dark, clean or dirty. I think I'll put the bike back together minus the air filter box (hardest part to put on anyway) and see if it runs right. If my whole problem was a restricted air filter I'll be pretty upset with myself.

Thanks again for all your help! Any ideas on this new information would be greatly appreciated!

Dave.

You definitely need to go thru the carbs, again.... An air compressor is a HUGE help when cleaning carbs, lets you blow all the passages out to be SURE they're clear.

[David Allaband]

Thanks to Andy, Randy and Rich!

It runs! Rode it around for a half our! Relief is immense!

This time I took apart carbs #2 and #3 (one functioning one dead) and compared the parts. The large holes on the Pilot Jets were all fine, but there's a hole at the tip that's the width of a human hair. I couldn't see it until I held up both jets side by side with a flashlight behind them! To clean it out I stripped a piece of electrical wire and pulled out one thread of copper. Poked out the hole, flushed it with cleaner, reassembled and it worked!

Thanks all you guys for your help and advice! I was in a pretty tough place, low budget, new bike, confusing problem. I was wondering if it was time to throw in the towel and give up biking for another couple years. You helped a stranger in need and possibly created a new devotee of the FJ. (Given time I'm sure I'll fall in love with it, but it wasn't too endearing when it didn't run!) You have my gratitude. I hope I can pass on the favor some day.

I'll post all of my new problems on a thread with a different heading.

[andyb]

This time I took apart carbs #2 and #3 (one functioning one dead) and compared the parts. The large holes on the Pilot Jets were all fine, but there's a hole at the tip that's the width of a human hair. I couldn't see it until I held up both jets side by side with a flashlight behind them! To clean it out I stripped a piece of electrical wire and pulled out one thread of copper. Poked out the hole, flushed it with cleaner, reassembled and it worked!

Noted as "A" here.  Filthy pilots!

Now, read this and use this for reference while you work.  Get the carbs clean, the fuel lines routed properly, the fuel filter cleaned, blah blah blah, before you start troubleshooting anything else with any aspect of how it runs.

[Pat Conlon]

Andy, as it has been said before....those pictures are soooo friggin cool!  They are such a great help. Thanks again!